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FINAL REPORT THE ROLE OF BOAT WAKES @N SHORE EROSION -in Anne Arundel County, Maryland '77 co GB 495.5 M3 F491 ...... 1980 FINAL REPORT ON THE ROLE OF BOAT WAKES IN SHORE.EROSION IN ANNE ARUNDEL COUNTY, MARYLAND Chris Zabawa and Chris Ostrom, editors- with contributions by Robert J. Byrne, and John D. Boon, iii Coastal Environmental Asssociates Gloucester Point, Virginia Mark Alderson,,' Chris Ostrom, and Chris Zabawa Maryland Department of Natural Resources Annapolis, Maryland Thomas Burnett United States Naval Academy Annapolis, Maryland IDA Deborah Blades, Tristina Deitz, Michael Perry, and Rhonda Waller 0 Anne Arundel Community College Arnold, Maryland Prepared for Coastal Resources Division Dr. Sarah J. Taylor, Director Tidewater Administration Maryland Department of Natural Resources Tawes State Of f ice Building Annapolis, Maryland 21401 December 1, 1980 11y) U.S. DEPARTMENT OF COMMERCE. NOAA 11V@ENT OF COMMERCE NOAA @0 COASTAL SERVICES CENTER "RVICES CENTER 2234 SOUTH HOBSON AVENUE HOBSON AVENUE CHARLESTON SC 29405-2413 -C 294.05-2413 Qlj- I-- TABLE OF CONTENTS .Acknowl6dqements ... ..................... ....... Editors' Note ... ....... ........... ...... Executive:Summary ........ iv Chapter I. Introduction ............................. 1-1 A. Purpose of the Study ......... ...... 1-1 B. Contents of this Report ....... ..... 1-3 C. The Results .... ............ 1-5 D. The Conclusions ................... 1-6 E. Thoughts for Managers ...... o. 1-8 F. Suggested Future Studies 1-9 Chapter II. The Erosion Process ................. o ... 2-1 Chapter III. Sampling Strategy and Site Selection 3-1 A. Sampling Strategy ................. 3-1 B. Site Selection ............. o....... 3-1 Chapter IV. Observed Changes in the Shoreline Profiles from.october 1978 to October 1979 ....... 4-1 A. Introduction o .............. 4-1 40, B. Methods ............... 0 ....... 0 ... 4-1 C. Results ............ o ........... 4-4 *10 Si.te Descriptions Site A . ....................... 4-6 Site 13. ....................... 4-21 Site C . ........................... 4-36 Site D . ................... 0....... 4-50 Site.E . ........................... 4-65 Chapter V. Behavior of Shoreline Profiles at Additional Sites ............ o o..* 5-1 A. Introduction ...................... o 5-1 B. Methods o o ........... 5-1 Site Descriptions Site AA. 5-2 BB. ...... o 5-10 S Site CC. ....oo .............. o o. 5-16 Site EE. oo ........... ooo..-o..o.o 5-23 Site FFi ............... o.o...o ..... 5-29 Ch ter VI. Boating Freque- ncie.s and Characteristics 6-1 A. Introduction ................................ . 6-' 1 B. Methods .............. ............... 6-5 C. Results ...... ....................... 6-6 SITE DESCRIPTIONS SITE A . ................................... 6-8 SITE B . .................................. 6-10 SITE C . .................................. 6-12 SITE D . .................... .............. 6-14 SITE E. .................... ........ 6-14 Chapter VII. Comparison of Boat-Wake and Wind-Wave Energy Budgets ................. 7-1 A. Introduction .......................... I... 7-1 B. Methods ................................. 7-2 i. Boat.-Wake Energy Calculations-... ...... 7-3 ii. Regression Analysis between Wave Energy and Boating Frequency .... 7-5 iii. Average Hourly Boating Frequency and Wave. E,*ergy due to Boats ......... 7-10 C. Results ................................. 7-14 Chapter VIII. Waves Generated by Passage of a Boat 8-1 A. Introduction .......... 8-1 B. Field Measurements of Boat Passes ............................. 8-4 C. Suspended Sediments Resulting from Boat Wakes .............................. 8-18 Chapter IX. Discussion,,Conclusions, and Thoughts for.Managers .................... 9-1 A. Discussion ............................... 9-1 B. Conclusions ........... .................. 9-6 C. Thoughts for Managers .................... 9-8 D. Recommended Further Studies ........ ! .... 9-9 Chapter X. References Cited .......................... ApEendix A. House Joint Resolution No. 40 ............. A-1. Appendix B. Wind-Generated Waves ...................... B-1 A. Introduction ............................ B-1 B. Methods ................................. B-3 C. Results .................................. B-12 Appendix C. Shallow Water Wave Gauge .................. C-1 ACKNOWLEDGEMENTS We thank Lee Zeni, Owen Bricker, Moe Ringenbach, Kathy Fitzpatrick, and Suzanne Bayley for reading the manuscript and providing constructive criticisms. The design of the study benefited from valuable discussions with Randy Kerhin* of the Maryland Geological Survey, Paul Massicot of the Maryland Power Plant Siting Program, and Jim Meyers of,the Johns Hopkins Applied Physics Laboratory. Information concerning potential study sites was provided by the Anne Arundel County Office of Planning and Zoning, South River Association, Severn River Assock ation, and Magothy River Association. Abbie Ringenbach and William Bodenstein assisted in reviewing proposals and selecting a contractor to perform part of the study. Michael Carron of CEA and Kathleen O'Neill of DNR assisted in various field operations. Mike Green prepared much of the graphics, and valuable assistance in producing. the final report was also provided by Matt Norman, Pieter van Slyck, Marsha Miller, Dean Pendleton, Sam Cook, and Peter Lampell. We also thank Shirley Crossley, Karen Spencer and Donna Klein for preparing the manuscript. Special thanks go to the property owners at the five study sites for allowing ready access to DNR and CEA personnel, and for cheerfully participating in the study. The cooperation of the.DNR Marine Police in.conducting the experimental boat runs and providing information on boating activity is appreciated. EDITOR@' NQTF@ The environmental impacts caused by motorboats have been the subject of several recent studies. Much of the technical information which has been amassed so far deals with the effects of two-cycle outboard motorboat engines on water quality and the health of aquatic organisms (Jackivicz, et. al. 1973). A few studies have shown there are short-term changes in turbidity and other water parameters while boats are operating, but the effects are very temporary (Yousef, 1974, et'. al., 1978; Anderson, 1976; Moss, 1977; Liou and Herbich, 1977). None of the studies have concluded that these short-lived environmental impacts are actually detrimental to the ecology of isolated lakes'or small creeks and coves where boats operate. There have also be en several recent studies on the impacts of boat wakes, but most of the attention has been directed to the wake characteristics of large commercial ships, barges, and tugboats. Information has been gathered to address some suspected environmental problems where boat wakes travel out of shipping,lanes onto recreational beaches, and where wakes wash against levees in restricted channels and canals (Hay, 1968; Das, 1969; Johnson, 1969; Collins, et al, 1971). The technical studies show how different types of passes by these large-hulled craft can produce different wakes along the shoreline (Brebner, et. al., 1966; Johnson, 1957, 1968, 1969; Sorensen, 1967a, b, 1973;Das and Johnson, 1970). This document describes a study of the role.o*f wakes from 6maller boats in causing erosion along the shoreline in areas which are relatively sheltered from natural wind- generated waves. The Severn and South Rivers are two tributaries to the Chesapeake Bay near Annapolis which are popular for recr 'eational boating, along with the smaller creeks and coves adjacent to the main river channels. The information which was collected as part of this study was used to answer some important questions about the relation- ship between recreational motorboating and shoreline erosion in these areas: 1. What levels of wave energy Are associated with boat wakes in particularly popular areas, and-how do the wakes compare with the normal wind-generated waves as a source of energy for erosion and transport at the shoreline? 2. Can different types of boating patterns change the levels of wave energy in boat wakes which break. along the shoreline? 3. How do rates of.shore erosion during the boating season compare to,other times,of.the year? This study report desdribes measurements of boat wakes,. wind waves, and shoreline surveys collected over a one-year period which included a single boating season and a single winter storm season. Thus, the length of time over which the data base extends is limited, and the results are strictly applicable only to the shorelines at the study sites. Nevertheless, the limited data set and associated provide preliminary answers to the questions above .which will be useful to scientists, managers, decision- makers, and other persons who participate with interest in public forums and related discussions where recreational motorboating is regarded as a significant management issue. Chris Zabawa C h r i sOstrom December 1, 1980 EXECUTIVE SUMMARY In 1976, the Maryland General Assembly passed a reso7 lution requesting that DNR design and undertake a study to determine whether recreational mot orboat traffic is detri- mental to the ecology of small creeks and coves in Anne Arundel County. The data which are described in this report were collected at five popular areas for,motorboating to assess the effects of boat wakes on shore erosion. over a one-year period, wave energy in boat wakes at each site was compared with the energy in wind waves to show the increased potential for shore erosion due to boats. Erosion rates during the boating geason were also compared to other times of the year at each of the study sites. Finally, wakes were measured from controlled boat passes to determine the im- portance of different boat speeds and distances from the shore in producing different-sized wakes. Except at one site, the most important contribution to shore-erosion during the year of study was Tropical Storm David, which passed through Maryland in September, 1979, and was accompanied by the greatest changes in some of the shoreline profiles. Wind waves ranked behind the storm effects; and in all cases, boat wakes contributed less energy for erosion than. wind.waves. Only one of the study sites showed evidence of erosion during the boating season. This site also had the highest levels of wave energy from boat wakes, even though some of the other sites had higher amounts of boat traffic. But the boats,pas.sed particularly close to the shoreline at the site where erosion occurred during the boating season; conse- the wake energy did not dissipate before reaching the beach. Thus, the distance to which boats approach the shore is a very important factor for evaluating erosion due to boat wakes. Two other important fac tors for evaluating erosion in small creeks and coves are the physical nature of the sedi- ments, and the appearance of the shoreline profiles. The sites used in this study possessed physical characteristics which are representative of many other shoreline locations in Anne Arundel County, and the report discusses the par- ticular characteristics at each site which were important to the erosion process. Other factors were also studied because they affect the heights of*waves, and thus the energy, in boat wakes. Besides distance from shore, the energy in boat wakes varied with different boat speeds,.and with different depths of water. For the range of Water depths in small creeks and iv coves of Anne Arundel County, the largest wakes can be expected from boats travelling slightly faster than the six-knot speed limit which is posted in many places. This increases the potential for erosion due to boat wakes in areas where boats reduce their speeds to conform to posted speed limits, as well as in areas where boats.exceed the posted speed limit by only a small amount (1-2 knot.s). Chapter VIII of this report contains a table of calculations for estimating those boat speeds which will qenerate maximum wakes in creeks and coves with different water depths. These ranges of boat speeds are suitable for use in a review of existing State policy to suggest changes in boating speed limits based on both safety.and environ- mental reasons; but, this study shows that other environ- mental factors also need to be considered, especially the physical nature of the shoreline in any particular area, and the distance away from shore which motorboats pass. v INTRODUCTION Chris Zabawa, Chris Ostrom, Robert J. Byrne, John D. Boon III Rhonda Waller, and Deborah Blades A. Purposeof the Study Since the close of World War II, the population in the counties fringing the Chesapeake Bay estuary in Maryland has increased dramatically, i3articularly on thevestern shore. Along with this population increase, recreational boating activity on the waterways has also increased substantially. For example, between 196 8 and 1973, the number of pleasure boats registered in the State of Maryland grew at an annual rate of about 5% (from about 6 2,000 boats registered in 1968 to over 76,000 in 1973) (Roy Mann Associates, 1974). Approximately 40% of this increase in registered boats was. concentrated in Anne Arundel, Baltimore, Harford, Cecil,. Kent, Queen Annes, and Talbot counties. There has been increasinq concern that the wakes gene.ra,ted,in some of these areas due.to the heavier boat traffic may be accelerating rates of shore.erosion, particularly in the smaller creeks and coves. In 1976, the Maryland General Assembly passed a resolution requesting the Department of Natural Resources to undertake a study to evaluate whether recreational*motorboat traffic is detrimental to the ecology of small creeks and coves in Anne Arundel County, Maryland (Appendix A),. Potential impacts from motorboats in smalitcreeks and coves could include effects on the turbidity and mixing of the water, toxic effects of oil and gas emissions from boat engines, damage to aquatic vegetation, and increased shore erosion due to boat wakes. In response to the General Assembly resolution, DNR conducted a literature search.of previous boating studies with the cooperation of Federal agencies, and assessed the implications of existing technical information for small creeks and coves in Anne Arundel County. There were few pertinent studies found, and none concluded that boating impacts were actually detrimental to the ecology. The Environmental Protection Agency is presently engaged in further studies of some potential boating impacts in the South River (Williams and Skove, 1980). Several years' worth of data are required in many of these studies to obtain an understanding of cause and effect, since the variability of environmental factors can be fairly large from year to year. On the other hand, a one year study.of boat-wake energy and shore erosion can p rovide an indication of the potential seriousness of the problem for sites similar to those selected in Anne Arundel County. Therefore, in response to the General Assembly Resolution, the Department of Natural Resources elected to conduct a study to.evaluate the contribution of boat-wake energy to the erosion of the shoreline fringes of small creeks and coves. 1-2 B. Contents of-this Report This report describes the collection and analysis of data from five shoreline sites to t*est three hypothesas: 1. Boat-wake energy is a substantial contributor to the overall wave-energy budget at the study sites. .2. Erosion of the.shoreline.sites is higher during the boating season than at other times of the year. 3. Different boat designs and passage characteristics can change the levels of wave'energy in boat wakes. To test the first two hypotheses, measurements of boat wakes were collected during the summer of 1979, and measurements of wind waves were collected at all times of the year between October 1978 and October 1979. In addition, shoreline surveys at the study sites.were collected on a monthly basis between October 1978 and October 1979. The study.sites.constituted a representative cross-section of shoreline types.(including beach, marsh, and bluff) in Anne Arundel County. The principal sites a nd shoreline surveys are dL&scribed in Chapter IV of this report. Some alternative sites were chosen to be used by the consultants in case the boating patterns at the original five selected sites were not as anticipated. Chapter V contains descriptions of these additional sites. 1-3 TABLE 1.1 3 Average Boat-2 Yearly Wave.Energy Budgets Site Locationi Passes Per Day Wave Energy (ft-lbs/ft') Yearly% % of Wave WD = weekday Wind @Boat Boat-Wake Energy in Wakes WE = Weekend Wave Wake Enerq in'Boating Season A Vegetated sand 170.5 (WD) 5,450,816 118,100 2.2% 4.4% spit.on Lower 735.7 (WE) South River at Harness Creek B Steep bank on 91.9 (WD) 4,133,173 70,680 1.7% 3.6% Upper South 344.2 (WE) River near Goose Island C Marshy promon- 95.2 (WD) 3,823,991 3716,040 9.6% 20.4% tory on Broad 326.2 (WE) Creek D Bluff on lower 155.8 (WD) 6i969,310 247,660 3.6% 8.4% Severn River at 268.8 (WE) Severnside E Pocket marsh in 44.6 (WD) 3,181,249 15,650 0.5%- 0.9%. Maynedier Creek 69.8 (WE) 1. From Chapter IV. 2. From Chapter VI. 3. From Chapter VII and Appendix B. Milk ANO 2Sft 'AMMiik momi monk Ammis MOML MON's fionk OM104, moms, dammi 12@11 Aummi-w, Boating patterns at each of the five principal sites during the summer of 1979 ate described in Chapter VI. The wind-wave measurements are containr@d in Appendix B, and the wind-wave energies are comparec' to boat wakes in Chapter VII. -To test the third hypothesis Usted above, trial runs of-boats with two different hull d-siqns were conducted at one shoreline site, and the wakes from different boat passes were compared.for boat passes at dtfferent speeds'and distances from the shoreline. These data are described and analyzed in Chapter VIII. C. The Results 'Table 1.1 (opposite) summarizes the boating frequencies and wave-energy budgets which were collected for the study. At four of the study site:3,,there was no increase in shore. er3sion which could be at-ributed to boating during the summer. The most important contribution to shore erosion was Tropical Storm David, which passed through.Maryland in early September,1979,.and was accompanied.by the largest erosion in some of the shoreline surveys. Wind waves rank behind the storm effects in causinj the observed shoreline changes over the year of observations, and in all cases boat wakes represented lower levels of wave energy. It.is important to note that these results are drawn from a one- year data base and do not incorporate any variability which might be detected over.a. longer period of data collection. At onesite (Site C) there was considerable erosion.. of the fastland durinq the summer of 1979. The boat-wake energy at Site C is an important factor.responsible for the er ut the-ph setting could osi6n,.b ysical shoreline also be important. Since Site C is located at a narrow point on a creek, the boats pass particularly close to shore relative to the other sites, and wake energy does not dissipate before reaching the beach. Thus this site ,experienced the highest boat-wake energy during the summer of 1979, even though some of the other sites had higher frequencies of boat"passes. The results of the.experiment w.Lth controlled boat passes show different types of boats, and different modes of operation of the same boat, can.produce measurabl-e. changes in the wave energy contained in boat wakes. For the types of boats tested, maximum boat-wake energy-oc- curred when the boat spe ed was about 8 knots; &high- spee(I passage (20 knots) produced lower wake energy. The water depth in this.case was approximately 12 feet. For different water depths, maximum wake energy can occur at different speeds since the wave energy varies with both the Speed of the boat and the water depth. In water depths-of 6 feet or less, maximum or near-maximum wake energies can occur at boat speeds closer to 6 knots. D. The Conclusions one conclusion about boat wakes is tAe largest contribution to tfie total wave energy (and thus to the total potential for shor(.! erosion) from wakes can be anticipated where there is ahigh 1-6 frequency of boat passes close to a particular-shoreline site. The actual level of fastland retreat in response to recreational boating patterns at any particular site will also depend upon the nearshore change in slope on the shoreline profile, upon the composition of the fastland, and upon the supply of sediment carried onto the shoreline site from alongshore. The type of shoreline most.susceptible to erosion would have a combination of: o.exposed point of land in a narrow creek or cove; o fastland consisting of easily-erodable material- such as sand or gravel; o steep nearshore gradient on the shoreline profile; o location adjacent to a high rate of boating, with boat passes relatively close to the shoreline. The site which experienced the most fastland erosion during the boating season.(Site C had all four of the above characteristics. Three more conclusions about wakes can be. drawn from this. one-year*study for the range of basin depths frequentlv encountered in-.narrow.creeks.and coves in Anne Arundel County: 1. As boats reduce their speeds to conform to posted speed limits, they pass.through a speed range in which the hull generates a maximum wake.. 1-7 2;. If the approach to-a posted speed-control area is within a narrow creek, then the shores adjacent to the speed-reduction zone will be- exposed to the high wake energies. 3. Boat operators can unknowin,ily generate a near-maximum wake, while they are transiting a waterway if they misestimate their speed by only a few knots while their boat is in a posted speed-control. zone. E. Thoughts for Managers 1. The data collected for this study show that depth conditions are suitable at.some shoreline.sites in Anne Arundel County for maximum Y)oat-wake.energies to be generated from boats passing a posted 6-knot (or 6.9 mph) speed-limit zone. One of the products of this study is Table 8.3 in Chapter VIII which can be used to estimate the speeds at which maximum wakes would be generated in different areas which have different water depths. In some cases, posting a lower speed limit would decrease the wave heights in wakes which break on theshoreline, 2. Since boats which are slowing to approach a posted. speed-controlzone will pAss.through the ranqe of 1-8 speeds which generate maximum wake, speed-limit signs should be posted, when possible, in portions of. creeks which are so wide that wake energies will substantially dissipate before reaching the shoreline. 3a The data collected for this StUdy indicate that the greatest potential for boating to increase erosion rates above natural levels can be expected when high frequencies of boat passes occur, within a few hundred feet from the shore. F. Suggested Future Studies Further studies at other sites in Anne Arundel County .are not likely to show boat wake:3 contribute more wave ,.energy than wind waves for shore erosion. In the one-year period of observations, narrow waterways where boats passed closed to the shore held the greatest potential for increased erosion due to boat wakes. Further studies over a period of 4 to 5 years might show-this potential is highly va riable depending on,boat traffic and boating patterns. At Site C, about 80% of the boat traffic occurred at distances of 200 feet or less from the shore; in contrast, Site B had about 75% of.the boat passes.at distances greater than 500 feet. As a result, the wave energy due to boats at Site B was only about 20% of that expe.rienced.at the Broad Creek site. So boat passes between 2007500 feet.from shore 1-9 can appreciabl.y reduce the level of wave energy in wakes which break along the shoreline. Furthe r observations of controlled boat passes over awider range of distances at selected shoreline sites would permit a more accurate determination.to be made of the critical cre,-A width which is needed to produce negligible wake energy along the shoreline. The controlled boat passes which were conducted for the study described in this-report cover;d-the range of distances from 5.0 to 200 feet offshore at a single shoreline -site. This range of controlled boat passes should be' extended to at least 500 feet from.the shore. In addition, other areas with different beach and nearshore profiles could be selected for measuring wakes from controlled boat passes, and boats with different hull designs could be testel. 1-10 THE EROSION PROCESS Robert J. Byrne, John D. Boon III Rhonda Waller, and Deborah Blades Shoreline erosion is defined here as the loss of subaerial fastland to the iqueous environm6nt; it is-not necessarily reflected in short-term changes in the beach which can be measured by surveying the shoreline over periods of a few weeks or months. As an examplef consider a shoreline segment where the "fas-tland" is 'a bluff (FigUre 2.1). In the geologic setting of Inne Arundel County, there is generally a narrow sand beach at the base of the bluff shorelines. Excavation of' this sand beach would disclose that the sand layer was relatively thin. and that within a few feet below the beach surface the consolidated bluff sediments would again be encountered. Observations of shoreline profiles at such a site throughout the course of a year would quite possibly show that the width and depth of the sand lens on this beach var.ed, while the portion of the.shoreline profile on the blu-'f face,.remain'ed unchanged. This beach is a natural.feature which-is formed and reshaped.by the action of breaki@ig waves throughout the nearshore and particularly in the swash zone, where the undulatory wave motion is transformed into turbulent uprush and backwash on the beach slope. Once beaches are formed by wav,! erosion of the fastland sediments, the beach sediments 2-1 Fi gure 2-2 are in turn the most effective natUral absorber of wave energy alonq the shoreline. The volume of sand on the beach at iny aiventime depends upon wave conditions and water elevations over the previous several days. Since the beach profile is so affected by short-term wave events, the con,,Iiti(.n of a single beach profile is itself not a reliable i,ndicator of erosion. However the retreat of the fastland, which can also be measured on shoreline profiles through time, is an unambiguous indicator. The chief agents of fastlan d erosion are wave action against the shoreline and the elevation of the water surface both during the normal tidal cycle and during severe storms. Tidal currents in certain circumstances may also exert a significant control on shoreline stability. Finally, surface rain runoff and groundwater seepage may play a Particularly important role in eroding steep bluffs and banks (Palmer, 1973). During periods.of active rainfall or snow mel-t, surface runoff may trickle down the steep slopes of bluffs or banks, and incise small channels in the exposed sediments. A more serious impact on erosion is probably due to the percolation of rainwater into the sediments which form the bluffs or steep embankments. This seepage can subsequently discharge from the..face of an embankment and c.ause instability and slumping.. In those bluffs containing impermeable layers, the groundwater discharge can be more concentrated where the. opposite: Figure 2.1 Schematic drawing of a beach at the base of a bluff. 2-3 upper Surface of the impermeable.layer is exposed. This surface then becomes a slide plane offering little.support to the sediments above the layer. Overthe long-term, the slump inri of these sediments will form a talus deposit at the base of the bluff, and the overall gradient of the exposed bluff face will decrease. Vegetation may also'grow on the. bluff face and stabilize the'eroding sediments. However, with the presence of significant wave action, the talus material at the toe of the bluff is "transporte-I away from the site, leaving the embankment in an oversteepened configuration once again. in regions of the 'Chesapeake Bay basin where wave energy for s'iore erosion is generated by local winds, the levels of wave energy are dependent Upon the open water distance over which the wind blows .(fetch), the duration of the wind, and most importantly,on thE. wind speed. However, the changes which are produced in the shoreline profile at any site due to wave energy are dependent upon the level of the water surface on the profile. Several factors control this level of the water surface, and thus control the zone of application of breaking waves on shorelines. Besidf-s the "normal" semi-diurnal tidal excursion of about 1 foot in the small creeks and coves of Anne Arundel County, the long- term fluctuation in sea level is an additional factor which influences the level of the water on shoreline profiles. .Due to the.melting of the polar ice caps over recent. 2-4 geologi c time, mean sea level,has risen to its present location' over the past few'thousand-years. Within the Chesapeake Bay region the relative sea-level rise is presently about 1 foot per century. At A nnapolis, the.sea level ha's rise'n.at least 4 inches since 1929, when tide gauge records first began to be collected (Hicks, 1972.) (Figure 2.2).. While this rate of sea-level rise is slow, it is sufficient to drown low-lying.lands and to maintain a continual landward encroachment of the zone of application. of wave energy by natural forces on any shoreline profile. Short-term sea-level- variations due to large-scale atmospheric events also play a strong role in determining where waves will erode sediments on shoreline profiles. With the-onset and duration of a regional northeast storm, variations in regional atmospheric pressure.cause additional' water to be forced into some portions of the-Chesapeake Bay basin. Thi3 results in a super-elevation of the mean tide level, or storm surge, which may overtop a beach in some areas and allow the waves to expend their energy directly against the fastland. Storm.surge elevations of two to three feet above expected tide level are not uncommon during northeasters in Chesapeake Bay. As a storm center passes through the Bay region, the easterly winds shift to the no rth.and northwest and frequently become stronger and of longer duration. This may increase the wind-generated wave heights but.it also relaxes the storm tidal surge in the vicinity of Anne Arundel County as the water level in the 2-5 rivers fringing the westernside of the Bay is then depressed below normal. Under these circumstances, the wave energy is dissipated along the lower portions of'-the shoreline profiles on the foreshore of the beach or in the nearshore zone, and the.fastland is-relatively immune to direct wave attack. Even in the absence of major northeasters, several other factors combine to produce a measurable variation in. mean tide level throughout the year. These include an annual variation in oceanic water temperature,.and,normal seasonal-lifferences in regional atmospheric pressure. At Annapolis, the monthly variation in sea level due' to al I factors is such that between April and Octoberthe mean tidal level is higher than between November and March (Figure 2.3). The range in the annual elevation of mean tide level 1.@; about equal to the tide range. The importance of this,phonomenon is that the zone of application of wave energy LS generally at higher elevations on the shoreline profile during the recreational boating season, with the maximum elevations being attained in August and September.. opposit@: Figure 2.2 (top) Changes.in values for yearly mean sea level at Annapolis, Md. (after Hicks, 1972).. Figure 2.3 (bottom) Monthly.-variation in mean sea level at Annapolis (after Boon, 1978). 2-6 go, twil go as a* a* 00) 'so, im FEET SCALE jw r4.L Zm ;n m Z2 z CD CID 0 SAMPLING STRATEGY AND SITE,SELECTibN Chris Zabawa, Chris Ostrom, and Mark Alderson ,.A. Sampling Strategy To study the effects-of boat wakes on the erosion ,process which was d.i:scussed in the previous chapter, measurements of shoreline change-, were collected over a year at some selected sites in Anne Arundel County. Changes in the.beach and fastland in the time between surveys can be discussed in terms of the wind-generated waves which dissipated their energy on the shoreline all year, and in terms of the boat wakes which are concentrated during the summer months. The sites were selected principally because theymere in popular areas for boating and water skiing, but they also are respresentative types of shorelines which occur in Anne Arundel County, including beach, inarsh, and bluff. Some effor t was made to obtain sites which received wind waves from different.directions and fetches. B. Site Selection The site selection process ---nvolved the following steps: 1) Areas of intense boatinq activity were identified by several groups, including: a. Magothy, Severn, and South River Associations; b. Anne Arundel County Boating A dvi.sory Committee; C. Anne Arundel County Planning and Zoning office; d. Maryland DNR Marine Police. 3-1 2) Once areas of intense boatinq activity were identified, potential shoreline sites were identified from aerial photographs which accompany the county tax assessment maps; 3) Letters were sent to approximately 120 landowners explaining the purpose of the study and requesting permission to make.a' site visit; .4) Visits were made by a DNR-team to approximately 84 sites whose.owners had no objection to partici- patinq in the study. Sites were disqualified if the owner indicated that he hM applied for a. permit for erosion control structures, or was planning to install shoreline structures within-the forthcoming year. Owners were Also asked whether they felt their land was located adjacent to an area of high boating activity. During the site visit, other observations were made, including: a. shoreline and beach morphology: b. shoreline sediment.type; c.. evidence of -erosion; d. proximity of shoreline structures-; e. orientation into the wind and approximate fetch; 5) 15 candidate sites were identified from the site visits. From these, five sites were selected for study by geologists from Coastal Resources Division of DNR and Maryland Geological Survey,.together with the consultants. 3-2 The locations of these five sites are shown in Fiqure 3.1. The sites which were selected include: S i te A. A vegetated sand spit on the lower South River, at the entrance to Harness Creek. Site B. steep bank on the upper South River, near Goose Island. Site C. A broad, marshy promontory on Broad Creek off the upper South River. Site D. A bluff on-the lower Severn River at Severnside. Site E.' A pocket marsh near the entrance of Mayneider Creek, off the upper Severn River. The sites were chosen as being representative of varying physJographic conditions with respect to bank elevation, sediment composition, nearshore bottom gradient,-and exposure-to wind-wave activity. Chapter IV contains a description of the shoreline profile f; which were collected -at these sites by the consultants on a monthly basis from October 1978 through. October 19,79. Chapters V,I and VII describe the boating frequencies which were measured at these sites during the summer o f,1979, and the boat-wake energy levels. Appendix B describes the wind-generated waves which were measured at udy. these sites during the year of st opposite: Figure 3.1 Location map of the consultants' study sites in Anne Arundel,County. 3-3, 4, 76030 76020' G1, 0 KILOMETERS 0 2 4 6 8 0 1 2 3 4 5 NAUTICAL MILES J), C, E (D @0. c U,S. NAVAL ACADEMY A B CHESAPEAKE BAY NEAR ANNAPOLIS, MD. 76020' 6030 At the beginning of the study period,, two additional sites were selected by.students participating in a DNR co-operative work-study proqrain atthe Anne Arundel. Communi"ty College. One site (_AA) was located along a bluff and adjacent pocket marsh inside Harness Creek, in the vicinity of the cons ultantst site A. Another site (FF) was located along a beach and sandy marsh at Beard's Point on the upper South River. These sites were regarded as "back-upO-site s to be used by the consultants in the ca se that boating patterns at one of the initial sites A-E were less intense than expected. The AA Community College students also monitored shoreline changes directly adjacent to two of the consultants' sites in areas where a different type of shoreline (marsh and bank) was immediately adjacent to the principal study area. The description of these additional sites prepared by the students is contained in Chapter V. 3-5 IV OBSERVED CHANGES IN THE SHORELINE PROFILES FROM' OCTOBER 1978 TO OCTOBER 197� Robert J. Byrne, John D. Boon III, Rhonda Waller, and Deborah Blades A. Introduction The five study sites listed in the previous chapter were surveyed on a monthly basis for a one-year period to determine whether there 'were marked.differences.in the rate of fastland retreat during the recr eational. boating.season and during other times of the year.. The results presented in this chapter.show few effects are able to be attributed to therecreational.boating activity in a single -season. The greatest changes were noted after the passage of Tropical Storm David, which occurred on September 5-6, 1979. Other changes in the surveyed profiles were also measured through the year at three of the sites. But-only one site showed any important change in the shoreline profile-during the boating season. These shoreline sites could be resurveyed on a continuing.seasonal basis with the landowners'.continued permission to see whether trends appear in.the.profiles during successive seasons when boats, wind7waves,'and other factors affect,the erosion and transport of sediments B. Methods At each of the sites,,three profiling locations were selectel with a separation distance of 30 feet. Each. 4-1 profile was established by inserting two reference pipes.or stakes several feet apart on a line perpendicular to the beach or shoreline. The position of the six reference pipes was then surveyed with a.transit and rod from a fixed bronze survey marker set in concrete. When the shoreline profiles were surveyed each month, the ground elevations along each profile were referenced to that of the benchmark using a precision level and rod. The rear reference pipe was considered to be the origin for each profile. The ground elevations were surveyed at 3-foot intervals, and at all additional intermediate points where a slope change occurred. At the two sites with bluffs (Sites B and D) the profiles were extended up the bluff face from the rear stakes, and elevations were surveyed at-intervals up to the instrument height. In order to test the precision of the profiling technique, r-.@plications were made at Sites A and B(Figures 4.1, 4.2). Site A was replicated on 11/25/78 with a mean deviation of 0.020.ft. The maximum deviation was 0.075 ft. at the step of the foreshore. This difference could repre- sent a real change in the position and elevation of the step since about one hour elapsed bE-tween successive profiles. opposite: Figure 4.1 (top) Plot of duplicate surveys a@t Site A on 11/25/78. Figure 4.2 (bottom) Plot of duplicate surveys at Site B on 11/4/78. 4-2 PLOT OF DUPLICATE SURVEYS SITE A 25 NOVEMBER 1978 cr cr Uj Woo 6-.= V.00 JLW 1%.m ab.co A sLoo sLco ob-se 6-go SIL a 01STUCE IN FEET Figure 4.1 PLOT OF DUPLICATE SURVEYS SITE B 4 NOVEMBER '1978 .4 N cc dL us ac to cn d2 tLa A. iLn ob.n oLm 11242 INAFAT 'Lo Tigure 4.2 4 -3 The maximum deviation outside of the step zon e was 0.052 ft. The comparative plot is shown in Figure 4.1. Site B was replicated on 11/4/78 with a mean deviation of-04-16 ft. The maximum deviation was 0.069 ft. which occurred in the nearshore zone. The mean deviation of elevation within the bluff and beach zone was 0.003 ft. with a maximum d(@viation of 0.005 ft. The comparative plot is shown in Figure 4.2 The replications indicate that the- profiling method is precise enough todiscriminate chances in elevation as small.at 0.1 foot. The dates when profi.les.were acquired at all the study sites are shown in Table 4.1. C. Results For the year-long period of observations, fastland retreat was measured.at three sites (B.C.D). There were changes in the shoreline profile amounting.to a reduction in the amount of slumped material at the toe of the bluffs at sites B and D. At site D, some bluff retreat was also measured after Tropical Storm Davidin early September, 1979. only at Site C on Broad Creek did siqnificant fastland retreat occur during the boating season.. At all the sites except Site E, there-were .variations measured in the beacti elevations on@the shoreline profiles from month to month. These variations were largely restricted to the intertidal zone.. Only at 4-4 TABLE- 4.1 DATES OF PROFILES SITE Month A B C D E Oct, 1978 29 - 29 29 29 Nov 25 24 25 26 26 Dec 20 20 20 21 21 Jan, 1979 Feb. 3 3 3 3 4 Mar 10 11 10 10 11 Apr- 16 6 16 17 17 May- 25 26 26 25 26 June 23 23 23 .24 24 July 28 28 29 29 29 Aug - 18 18 18 18 Sep 15 15 is 16 16 Oct, 1979 20 20 20 21 21 4-5 Site D was there any significant change in bottom. elevations at points seaward of the low-tide line. A detailed description of the shore zone response to boat, wakes and wind waves at each site follows.- SITE DESCRIPTIONS Site A. A vegetated sand spit on the lower South River, at the entrance to Harness Creek This site is located in the region known as Hillsmere' Shores (Figures 4.3, 4.4, 4.5). The beach seqment chosen for monitoring is located on a.spit which trends north-south along the shoreline. The sediments which.form the spit were derived from erosion of an Adjacent bluff which forms a headland slightly downstream on the South River. This headland bluff is about 30 feet in elevation, and is composed,of interbedded sand, silt and clay deposits of the Talbot Formation (Pleistocene 'Age), with a th'in lower horizon of pebbly sand and gravel exposed (Glaser, 1976). The spit beqins about 500 feet upstream from the zone of active bluff erosion, and is connected to the bluff by a sandy t(-rrace of approximately 3 feet elevation which broaden!3 froin the base of the bluff to a width of about-30 -feet upstream at the point of spit Attachment, This sandy terrace is also-experiencing'retreat due to frontal erosion along the South River shoreline. The entire portion of the South River shoreline-near the spit is littered with fallen trees. The mean tide ranqe in the area is approximately 1.0 foot-, and there were no shoreline structures present along the reach during the period of study. The spit itself is about 400 feet in length, and the. distal end exhibits a strong recurve. Earlier episodes of sani transport and deposition along the spit have led to the formation of a lagoon on the back side of the-spit, and subsequent marsh growth has separated this lagoon from Hatness Creek. Downstream from the study site on the spit, there is a frontal scarp which is evidence of active erosion. At the top of this scarp, the qround surface slopes towards the land instead of the water (Figure 4.5);- this suggests that erosion and shoreline recession in this portion of the shoreline have already devoured the "spine" of the spit, which was the crest of the earlier natural beach or berm line formed when the spit was growing. older marsh sediments are exposed on this scarp near the point of, spit attachment to the adjacent bluff, and a shell-bed up-to next pages: Figure 4.3 (left) Location map showing Site A. Figure 4.4 (Lpper right) Aerial view of Site A. Figure 4.5 (lower right) Typical-profile of Site A ct.ober,1978, 4-7 Feor 17 19 cou y,v VAJ I 10 ZZ- V. A IA La SCALE 1.24000 low 1000 am 3m 40M 5WD sm 74W FEET KKOWTER Figure 4.3 4-8 flmlm 16 RIP Awl-, ... . . ...... 7-0, 4-, Z 14 7E w P',- .......... W-l",", W* A,--- @115 " 81, "at Figure 4.4 APPROX. MEAN SEDIMENT SAMPLE TIDE LEVEL NUMBER -22.5 -20.0 LEFT r 8) -175 STAKE m .......... > > -15.0 CENTER STAKE -12.5 0 4 Z 3 .,.* vv 10.0 RIGHTa------- Z STAKE -7.5 -5.0 m m IISITE A:j -2.5 --1 r 7 .0 70 60 50 40 30 20 10 0 PROFILE LENGTH (feet) Figure 4.5 4-9 1.5 feet thick is also exposed in the scarp. The shells have been eroded and carried all along the beach to form a pavement over the lower foreshore and the immediate nearshore (Figure 4.6a). The eroding shell bed is composed solely of the shells of the oyster Crassostrea virginica, .and:it apparently represents a shell dump left by earlier inhabitants of the area. The spit is densely vegetated with shrubs, grasses, and small cedar trees. Sand deposits on the shoreline profiles are very narrow with as little as six feet between the shoreface and the fringe of the vegetation. The profile locations are midway along the length of the spit, with the center profile situated 2,00 feet from the point of spit attachment. The profile layout consists of three transects.spaced 30 feet apart. Typical profiles (October 1978) are shown in Figure 4.5. In April 1979, the sediments. were sampled from the beach in the upper.1-2 inches of the shoreline profiles and,-,the textural characteristics of the sediments are shown in Table 4..2. The offshore zone is represented.by samples taken 69 feet from the profile origins, and Table 4.2 shows 0 sediments in this porti n of the profile are sandy muds. In contrast, the beach materials represented by the foreshore samples Nos.-3, 7, and 1 1 are sand containing up to 20% Next pages: Figure 4.6a-c Photographic view of the three profile loF-ations at Site A in October 1978, May 1979, October 1979. 4-10 .......... Al RIGHT PIP", CENTER L E FT SITE "A OCT. 1978 F i g u r e 4.6 a 4-11 RIGHT Mu CENTER LEFT S I T E "A" MAY 1979 P-N-Ow Figure 4.6 b 4 - 12 M "Somme so dam'so, pol NI RIGHT CENTER LEFT' SITE A" OCT. 1979 Figure 4.6 c TABLE 4.2 SWIMM. CHARACTERISTICS 1. SIT9 A Sample Profile. Distance Zone "Gravel 2.0 am) Sand (0.062mm to 2mm) silt MOO 9 to 0.062mm) Clay (,0.0039 mm) -No. From Origin Mineral OrRanic Mineral I Orsanic Mineral Organic Mineral Oraanic I Right 69.0 ft. Offshore 0.06 gm 42.49 gm 0.39 gm. 35.91 gm 0.26 gm 17.20 go 3.68 gm 3 Right 24.0 Foreshore 6.81 91.12 0.37 0.29 0. 01 0.30 1.11 4 Right 18.2 Fastland 2.90 93.75 0.57 0.57 40.01 1.29 0.91 5 Center 69.0 Offshore 0.20 47.87 0.39 28.71 0.03 _19.22 3.58 7 -Center 30.0 Foreshore 20.46 76.34 0.46 0.33 <0.01 1.31 1.09 93.63 1.04 8 Center 22.3 . Fastland 0.77 0.76, 0. 01 .2.74 1.05 9 Left 69.0 offshore 0.96 36.85 0.26 38.05 0.04 20.04 3.80 11 Left 27.0 Foreshore 12.63 84.09 0.34 0.04 0.02 1.43 1.45 12 Left 21.7 Fastland 0 1.72 93.53 0.66 1.54 0.04 1.86 1.66 The numerical values shown represent the fractional weight, in grams, of 100 grams of sample*,' thus the results may also be Interpreted &a psr@@ntage values. low gravel-size material. The boundary zone between the predominance.of sand and mud in the nearshore occurs between 30 and 50 feet on the profiles. The fastland samples taken from the erosion scarp show a composition of sand with minor amounts of gravel; but, this scarp is not considered to be the principal source of the gravels in the beach sediments at the study site. Theenrichment of gravels in the beach is probably due principally to the erosion of the downstream bluff, and movement of these.materials along the shoreline ont6the spit. The shoreface of the spit receives boat-wake energy from boats entering and exiting Harness Creek, and from boats travelling up and down the South River. Since the width of Harness Creek is only about 500feet in.the vicinity of the profile stations, boats entering or leaving the creek pass relatively close.to the shore where the study site is located. On the other hand, boats travelling on the South River commonly pass at distances greater than 1 000 feet from the study site. The boating characteristics at this@site are discussed in Chapter VI. The spit also receives wind waves.which approach with the longest fetches from the east, south, and northwest. The wind-wave climate at this site is discussed in Appendix B, and the wind waves and boat wakes are compared in Chapter VII for their relative importance in causing any changes in the shoreline profiles. opposite: Table 4.2 Sediment characteristics at Site A. The locations of the samples listed in the table are shown on the profiles in Figure 4.5. The fastland boundary at Site A was defined as the edqe of vegetation. This boundary coincides with a pronounced break in- slope formed by the upper foreshore of the beach. On . some. spring high tides durinq the year of observation, the wave uprush would reach the limit of vegetation to.form. a scarp._ Photographic views of the three profiles which are shown in Figures 4.6 a,b,c for the months of October 1978, May 1979, and October 1979, respectively, were selected from all the monthly photographs to illustrate the conditions at the beqinn.ing and end of the "non-boating season" (October .1978 May 1979), and at the end of the "boating season" (May 1979 October 1979). The complete monthly photographic coverage is on file at the Coastal Resources Division of the Maryland Department of Natural Resources. ,Profile comparisons between successive months are shown in Figure 4.7., The envelope of total change is shown.in Figures 4.8a and b. The combined profile overlay for the period.october 1978 - May 1979 (Figure 4.8a).clearly shows the modulation of beach foreshore elevations from month to month, and the virtual absence of any change in the nearshore bottom. The greatest total vertical change within the envelope was at the upper foreshore adjace.nt.to.the. fastland.boundary. The combined profile overlay for the period May-October 1979 (Figure 4.8b) shows that some modulation of foreshore elevatio6 occurs during the boating season. Therewas also measurable retreat of about 0.5 feet in the fastland boundary on the leftand right profiles during the boating 4-16 season, but this is due principally to a fastland retreat solely in the profiles of 07/28/79 and 09/15/79. The changes in the shoreline profiles during this time period- can be reasonably attributed to the influence of the passage of Tropical Storm David on September 5-6, 1979. During the passage of David, a storm surge of about 2.5 feet was generated in the vicinity of the study area along with strong winds from the southeast. Under these conditions, the entire spit was awash and subjected to wave and current energy. It is important to note that during the boating season (the profile period 05/25/79 to 07/28/79), no retreat of the fAstland occurred at any of the three.profile stations,.., in fact, very little difference in foreshore elevation is evidenced in the profiles for those months. I'n order. to emphasize the changes in the zone of the fastland',boundary, segments of the monthly profiles which were collected through time are shown "stacked" in Figure 4.7. The vertical reference lines represent-the-position of the fastland boundary (edge of vegetation) in October, 1978. Again, note the absence of scarp retreat between the May and July surveys.. Next pages: Figure 4.7 (left) Profile comparisons between successive months at Site A. Figure 4.8a (upper right) Profile overlay for Site A from October 1978 to May,1979., Figure 4.8b (lower right) Profile overlay for Site A from May 1979 to October 1979. 4-17 OCT 78 Nov DEC FEB MAR 0 APR 4s MAY co JUN JUL SEP OCT left 1, center right 30-00 10-05 2 0 - (11 0 30-91 :0.s, o.-,c 2n. nG 0 310 DISTANCE IN FEET FROM RROFILE ORIGINS SITE A F i gure 4.7 FEB MAY J@ N main dlli@ MONTHLY PROFILE OVERLAY OCTOBER 1978-MAY 1979 3i CL CENTER CE tn RIMIT !,..oo da ab.00 A w 00 Woo ob. w so.oc ob.oo an'racelill FEE I gure 4.8a 9 It MONTHLY PROFILE OVERLAY MAY 1979-OCTOBER 1979 SITE A CD -------------- Ln C3 cc Koo A." ls.m ab." A%.** ib.0 ILM qb.m a.00 lb.ft sko lb.m DISTRICE III VEET Figure 4.8 b 4-19 The evidence from the,monthly surveys-and-photographs. indicates there was very little change in.the position of the fastland-boundary-during theyear of observations.- The scarp between"-the edge of-the active vegetation and the beach foreshore varied in ellevation.and steepness through the course of the year along with a variation in the volume of foreshore sand at the shoreline site. This modulation in beach sand occurred in response to varying wave conditions and@water 'levels. The profiles collected during the "boating season" on-05/25/79, 06/23/79, and 07/28/29 are virtually identical. Thus, boat wake activity didnot cause measurable monthly changes at Site A. during that period. The fastland retreat which was observed in- the survey of 09/15/79 at two of the profiles is attributed.to the wave and water level conditions during'Tropical Storm David. 4-20 Site B. A steep bank on the upper South River near Goose Island. This site is located near the subdivision known as Glen Isle (Figures 4.9, 4.10, 4.11). The beach,segment chosen for monitoring is located in a b luff section on the south shore of the upper South River, about 700 feet downstream. from the mouth of Flat Creek. The sediments on the beach at the study site are derived principally from the bluff, but the f@_ner-!qrained sediments in the nearshore may be derived princJ-pally from the sediment-discharqe of Flat Creek. The bluff has a maximum height of about 30 feet,and a frontal slope of about 45 degrees. It is composed of semi- consolidated clayey'sands of the Aquia Formation (Eocene. Age) that contain impermeable lenses of sediment.:cemente.d into a sand.stone-type material. As the bluff haseroded, these limonitic deposits have fallen onto the beach at,the base on the bluff and form a rubble pavement on:the shoreline profile. The bluff extends for about 500 feet along the upper South River shoreline, from the mouth of Flat Creek to a marsh which.has formed at the mouth of a ravine..., There Next pages: FiSure 4.9 (left) Location map showing Site B. Figure 4.10 (upper right) Aerial view of te B. Figure 4.11 (lower right) Typical profile of te 8 in November 1978, 4-21 !T1 R SME 1:24WO .0 1 "ILE lOOD 0 1000 MW 4m ww sm 7M FEET LOMETER Figure 4..9. 4-22 4@, @Q` Figure 410 SEDIMENT SAMPLE APPROX. MEAN NUMBER TIDE LEVEL 22.5- B9 F_ 20.0- Ld LLJ 17.5-': B8 LL @2 15. 0 z LEFT B6 *IB5 STAKE B4 Z >- ,..I. 0 a<@ 10.0- 83 :.:.. ... x '.*-. --CENTER B2 7.5- STAKE B, <> Ld 5.0- RIGHT _J 2.5 1 STAKE JISITE::B]l -5 0 10 20 30 40 @o @o PROFILE LENGTH (feet) Figure 4.11 4-23 TABLE 4.3 SEDIMENT CHARACTERISTICS, SITE B Sample Profile Distance Zone "Gravel 2.0 0301 Sand (0.062mm to 211m) Silt (0.0039 to 0.062mm) Clay 60.0039 mm) NO. From Origin Mineral Organic Mineral OrRanic Mineral Organic Mineral Organic Bl Right 24.0 ft. Nearshore 18.71 gm 66..ll gm 1. 60 gm 4.23 Sm 1. 04 gm 6.15 gm 2.15 gin B2 Right 3.0 Foreshore 5.04 83.81 0.76 1.34 0.05 7.13 1.87 83 Right -3.1 Bluff 1.48 68.44 3.00 14.26 0.73 11.34 3.52 84 Center 24.0 Nearshore 22.17 65.08 0.86 4.43 0.30 4.84 2.32 B5 Center 5.0 Foreshore 11.92 82.26 0.41 0.82 <0.01 2.72 1.86 B6 center -2.4 Bluff 1.51 77.40 0.86 3.89 0.09 13.67 2.5A B7 Left 24.0 Nearshore 44.28 43.70 0.53 4.60 0.03 5.06 1.80 Ba @Left 5.0 @oreshors 2.84 $5.91 0.87 1.62 0.30 6.72 1.74 B9 Left -4.2 Bluff 6.40 78.71 0.80 0.4 2 0.01 11.83 2.26 *The numerical values shovn represent the component veight, In grams, of 100 grams of sample, thus the results may also be Interpreted as percentage values. low dooms" MENOMONIE dommosom MONSOON MONSOON& are mature trees and shrubs located on the top,of the bluff, and the upper portions of the bluff face are covered with a slumped and hanging mass of soil and roots. The mean tide range at the site is about 1.0 foot, and there were no shoreline structures present along the reach during the period of study. Sand deposits on shoreline profiles at this site are narrow, with as little as 10 feet between the shoreface and the toe of the bluff. The profile locations for the study, are located at the downstream end of the*bluff, where the land surface slopes into the ravine, and iust before the' beach joins with the marsh. The profile'layout consists of three transects spaced 30 feet apart. Typical profiles (November 1978) are shown in Figure 4.11. In April 1979, the sediments were sampled from the beach in the upper 1-2 inches of the shoreline profiles, and the textural characteristics of the sediments are shown in Table 4.3. The offshore zone is composed of soft, fine-grained muds which blend into a relatively firm sandy bottom about 30 feet from the shoreline. These sands on the beach.and in the nearshore possess a significant gravel content which represents-the lag deposit left on the shoreline profile as the bluff recedes. Sediment samples from the bluff are composed predominantly of sand, but with a significant Opposite: Table 4.3 Sediment characteristics at Site B. The locations of the samples listed in the Table are shown on the profiles in Figure 4.11. 4-25 fine-grained component. As the bluff.se-diments erode, the finer-grained sediments are winnowed from the talus deposits at the base of the bluff by wave action and transported into deeper,water. The shoreface of the bluff receives boat-wake energy mostly from boats travelling the South River at distances of more than 1,000 feet. Some localized boat activity is generated from a smaller number of boats which circle Goose Island and pass within 100-200 feet of the study site. The boating characteristics at the :3ite are discussed in Chapter VI. The bluff site also receives wind waves which approach with the longe'st fetches from the north-northwest. Regional winds from this direction can qener.ite appreciable wave energy which would focus on the site; but, these winds also tend to drive water' out of the rivers on the western shore so that the erosive power of.the waves tends to be expended at low levels on the beach and in the nearshore, rather than on the toe of the bluff. The wind-wave climate at.this site is discussed in Appendix B, and thewind waves and.boat- wakes ate compared in Chapter VII for their relative. importance in causing anychang es in the shoreline.profilese The fastland boundary at Site B was.defin,ed.as either the in-place semi-consolidated sediments forming the bluff, material which slumped from the bluff face. The reason for-considering the slumped material as "fastland" is that were it not for the removal of this material by wave 4-26 action, the bluff slope would ultimately be reduced and become stabilized with vegetation. The initial condition of the profile sites is shown in the photographs of October, 1978 (Figure 4.12a). The Right profile is at a position where the bluff elevation is low and the semi-consolidated sediments were covered with a soil horizon. At the Center profile, the fastland is slumped material with limonitic fragments at the base, along with a notch about 0.75 feet deep cut into the sediment. The Left profile is a near-vertical cut of the native bluff sediments with a toe of limonitic fragments. Figures 4.12b, b, and d show the condition at the profiles in May, August, and October, 1979 respectively. Profile comparisons between successive months are shown in Figure 4.13. The envelope of total change is shown in Figures 4.14a and b for the periods October 1978 - May 1979 respectively. The combined profile overlay shows there was no change in the fastland throughout the entire year at the Right profile, simply a modulation of the sand elevation in the beach at the toe of the bluff. During the period October 1978 - May 1979, the Left and Center profiles show an episode of slumping and reduction of the slump by wave, action. During the period May-October, 1979, there was some further modification of the slumped material at the Center Ilext pages: Figure 4. 12 a-d. Photo-graphic views of the three profile locations at Site B in October 1978, May 1979, August 1979, October 1979. 4-27 M25 CENTER Ap- LEFT S ITEI.' B"-'--iD-CT'- 1-9-7-8, F i g' u'.r. e 4.1 2'a 4-28 Mon mmmmm mmm M M M MM M MM M RIGHT CENTER LEFT SITE B" MAY 1979 Figure 4. 12b RIGHT Aw, CENTER A6 'Vol Al' , SIT E B AUG. 1979 LEFT Fi gure 4.12 c 4-30 At RIGHT' CENTER LEFT SITE "B OCT. 1979 F i g u r e 4.12d and Left profiles, and again no change in fastland at the Right profile. A different view of the fastland boundary changes is provided in Figure 4.13. This figure shows that no change in the Right profile occurred throughout the entire year, whilelthe Center profile showed no change in fastland until sometime between the February and March 1979 surveys'. when a massive slump occurred. The slumped material underwent some reduction by wave action until April-May 1979, and then only very minor modification until the passage of Tropical Storm David in September 1979. The Left profile shows minor slumping between October-December 1978, and a.massive slump between December,1978 and early February 1979. The slumped material was again reduced by wave action in February,.March, and April. However between the late May and late August surveys in 1979,,there was little modification. Tropical.St,orm David in September was accompanied by a substantial reduction of the slumped material, and Figure 4.12d shows.the native bluff material was again exp osed after David. All the evidence indicates there was little modification of the fastland at this site during the boating season. However, there was significant modification of two of the three profile locations with the passage of Tropical Storm David. Figure 4.13 shows the approximate level of,the storm tidal surge during the David episode. This site wag apparently not exposed to much high wave action since the 4-32 wind was predominantly from the southeast a nd south. Even so, the shoreline.profiles collected after David show the slumped material at the Center and Left profiles was reduced. The Right profile showed no change. In summary, it,should be noted that very minor changes occurred during the boatinq,season of 1979. Comparison of the month-to-month surveys gh(,ws this was the period of lea st response in the shoreline profiles..to wave activity during the year of observations., Next pages: Figure 4.13 (left) Profile comparisons, between successive months at S'ite B. _Figure 4.14a (upper right) Profile overlay for Site B from October 1978 to May 1979. Figure 4.14b (lower right) Profile.overlay for Site B from may 1979 to October 19,79. 4-33 OCT 70 mov DEC FEB MAP APR MAY JUN cl JUL sz suz AUG Sep 0 T conter fight -10-00 oloo 10.0c ;3.01 DISTANCE IN FEET MON ORIGINS -SITE B Fi gure 4.13. I-- OCT 70 ( @,-M-o @v No @D EC FEB M A APR JU N JU L AUG SEP 0 CT OCT .,It MONTHLY PROFILE OVERLAY OCTOBER 1978 -MAY 1979 ITE 8 13 LEFT cr CD 'c; U U is CENTER RIr MT oo reEse. cc ob.co yb.oc ob. ac DISTANCE T Figure 4.14 a MONTHLY PROFILE OVERLAY MAY 1979-OCTOSER 1979 SITE 8 .0 4Cj LErT -ci LA CENTER u, FZIIHT ci d@= Ir. lb., o.oc *.Go ob.ft 01STA-c'! INUT Figure 4.14 b LErT _NTE, 4-35 Site C. A broad, marshy promontory on Broad Creek off the upper South River. This site is located approximately 1.2,00 feet north of, the mouth of Broad Creek (Figure 4.15i 4.16, 4.17). The shoreline segment chosen for monitoring is located on a, promontory at the junction of a north-south shoreline reach and another trending east-west. The promontory is composed of a low, marsh-capped, alluvial platform which is surrounded by hills attaining elevations of up to 60 feet within 500 feet of the study site. This adjacent topography is sculptured,from sandy deposits of the Aquia Formation (Eocene Age). The sediments at the study site were derived partially from the erosion of these adjacent landforms. Various exhumed,debris from.along the shoreline near the study site indicates that the site may also be partially composed of artificial fill. Upstream of the promontory for a distance of about 100 feet along the shoreline are.the remains of a concrete wall which are columnar in section, and about one-foot-square. These remains no longer providean effective barrier to shoreline erosion as they lay on the bottom of Broad Creek several feet from the.fastland at the. study site. In the area where the profiles"were located, the promontory forms a portion of the Broad Creek shor:eline about 130 feet in length. The fastland at the study site is 4-36 relatively flat, with an overwash "levee" present on the marsh surface (Figure 4.17). The marsh itself is flooded at higher tidal stages, and is composed of Spartina patenst'. Scirpus., and Distichlis grass species. The mean tide range in the area is abodt 1.0 foot, and there were no shoreline structures present along the reach during the period of study. Sand deposits on the shoreline profiles are narrow with as little as 10 feet between'the shoreface and edge of the marsh. The profile locations are situated on the promontory Along the east side of Broad Creek-facing the west. The profile layout consists of three transects spaced 30 feet .apart. Typical profiles (October, 1978) are shown in Figure 4.17. in April 1979, sediments were sampled from the beach, in the upper 1-2 inches of the shoreline profiles, and the textural characteristics of the sediments are shown in Table 4.4. The sediments in the foreshore, nearshore, and offshore (up to 69 feet from the profile oriqins) are all predominantly sand size with E,mali contributions of organics. The composition of the terrace.sediments is shown in a boring sample Ml. The Lipper 2-3 inches.of the boring are ari organic soil.with an abrupt transition below to a,. sandy gravel. Next pages: Figure 4.15 (left.) Location map showing Site C. Figure 4.16 (upper right) Aerial view of Site C. Figure 4.17.(lower right) Typical profile of Site C in October 1978. 4-37 v x KI Pat* & ZERO SCALE 1:24000 to= 0 tOOD 2wo 3w 4m mw GM 7OW FEET 0 F t g u re 4..15 4-38 "R N! WME, IT Ni@, A Figure 416 APPROX. MEAN SEDIMENT SAMPLE TIDE LEVEL NUMBER -22.5 m 14 -20.0 13 m LEFT -17.5 STAKE ml 15.0 18 16 1?-.5 > z 0 CENTER STAKE (0.0 21 7.5 RIGHT -5.0 M STAKE m 2.5 ---1 10 @O @o @O @O @O 10 0 PROFILE LENGTH (feet) Figure 4.17 4-39 TABLE 4. 4 SEDIMEW CHARACTERISTICS; SITE C @&,@pie -Profile Distance Zone Gravel (<2.0 mm) Sand (0. 62mm to 2mm) Silt (0. 39 to 0.062mm) Clay (<0 0039 mm) No. From Origin Mineral lOrsanic Hinerall Organic Mineral OrgatUrt Kineral Orstanic IS Right 69.0 ft. Offshore 3.58 89.79 0.54 1.25 3.01 1.70 20 Right Nearshore 2.20 90.90 0.36 1.57 0.08 2.98 1.91 21 Right 30.0 Foreshore 7.49 87.92 0.26 0.75 0.13 1.51 1.94 16 Center 69.0 Offshore 2.10 91.26 .0.55 1.85 0.19 2.48 1.57 17 Center 42.0 Nearshore 0.84 93.66 0.28 1.00 0.38 2.14 1.71 1.59 18 Center 30.0 Foreshore 15.60 80.90 0.24 0.67 <0.01 1.00 Center 13.0 Marsh 47.49 46.94 0.19 2.14 0.40 0.88 1.98 Terrace 13 Left 69.0 Offshore 1.02 93.84 0.38 0.96 0.10 1.74 1.96 14 Left 39.0 Nearshore 1.59 89.90 0.45 1.94 1.04 3.13 1.94 =15 Left 30.0 Foreshore 0.78 94.82 0.48 0.57 0.33 1.40 1.63 The numerical values shown represent the fractional weight, in grams, of 100 gram of sample, thus the results may also be interpreted as percentage values. The shoreface o f the promontory receives boat-wake energy from boats travelling up and down Broad Creek. The site is downstream from a posted speed-control zone, and both high- and low-speed boat passes are encountered. Due ,to the relatively narrow width of the creek in the area, the study site is positioned particularly close to boats generating wake. The boating characteristics at this site are discussed in Chapter VI. The site also receives wind waves which approach with the longest fetches from the north. But.waves generated by these northern winds have to undergo considerable refraction to approach the study.site.from directly offshore, sothe wind-wave energy at this site is considered to be small relative to sites with similar fetches on the-South River. The wind-wave climate at this site is discussed in Appendix Band the wind waves and-boat wakes are compared in Chapter VII for their relative importance in causing any changes in the shoreline profiles.. The initial condition of the profile sites.is shown In 'the photographs of,October, 1978. (Figure.4.18a). The fast- land boundary,.was defined as the edge of the marsh vegeta- tion capping the sand and gravel te rrace,. At all three profile locations, collapsed patches,of the cap marsh,were growing on the intertidal foreshore of the narrow beach. 012posite:, Table 4.4 Sediment characteristics at Site C. The locations o?__tWe_ samples listed in the Table are shown on the profiles in Figure 4.17. Next Ea es: Figure 4.18 a-d Photographic view of the three protile!q-0osclations at Site C in October 1978, May 1979, August 1979, and October 1979. 4-41 RIGHT CENTER mod" SITE C OCT. 1978 LEFT Figure 4.18 a 4-42 MRAM RIGHT @kA e jr, CENTER LEFT SITE c M AY 1979 Figure 4.18 b 4-43 2of RIGHT "Z@ P 7M CENTER .. .. .. . ... ............. @K. 'UPWAffil '40 Ai '4TI'M 00,1 ....... 'PROP MIA, "Ill, 211 ftk LEFT SITE "C AUG. 1979 Fi gure 4.18 c 4-44 . ........... .. . ........... . .............. . 77'"17M pt ji- qz RIGHT CENTER LEFT SITE c OCT!@ 1979 F i g u r e 4.18 d The overlays of monthly profiles for the periods October 1978-May 1979 and May - October 1979 are shown in Figures 4.20a and b. Both the Left and Center profiles reflect the existence of the slight positive relief on the terrace due to an overwash deposit formed by wave action awashing foreshore sand onto the marsh surface during times of:high water. in addition, the different geometry of the profiles near the fastland should be noted. 'Comparison of Figs. 4.20a and b shows a rather dramatic difference in the fastland response between the boating and non-boating. seasons. During the boating season a pronounced retreating scarp formed at the Left profile. At the Center profile the preexisting scarp continued to retreat. The-Right profile .exhibited no fastland retreat throughout the year.. The details of the observed fastland boundary retreat are shown in Figure 4.19. The Left profile, within 15 feet of the downstream end of the marsh terrace, had a slight scarp at the edge of vegetation which was stable.in position until after the.February 1979 survey. Between February and May 19791'the edge of vegetation retreated 3.7 feet but1with only;sliqht scarp,formation. By the time ofthe June survey, a pronounced scarp had formed. By the time of the survey of 18 August 1979,.the fastland scarp retreated an additional 2.6 feet. Finally,, between 18 August.and 20. October 1979, an additiona 1 0.5 feet of retreat occurred. This loss includes the effects of Tropical Storm David. in total, about 6.8.feet of fastland retreat occurred during the one year period. 4-46 The Center profile had a pronounced'scarp at the fast- land boundary throughout the period. The scarp position was stable until after the March 1979 survey, and between that time and the survey of 26 May 1979 the scarp retreated 1.6 feet. Between the May survey and that of 18 August, the scarp retreated an additional 2.6 feet, most,of which occur- red between the May and June surveys. Between the June and July surveys, fallen bulkhead sheeting was exposed in the foreshore. Finally,, between August and October 20, 1979, an additional 1.0 foot of retreat was measured. The total Eastland retreat was 5.2 feet during the course of the year.- In summary, the pattern of fastlan.d erosionat Site C appears to be that of a smoothing process which is tending to round the exposed corner of the marsh terrace,(see Fig. 4.16). An important factor in this process may be. the physical setting of the site.. I.t,is impor tant tonote that there is very little sand supplied to the site from the more erosion-resistant upstre am banks. Were there sand available from.this upstream source it would tend to.maintain.a beach in front of the marsh. Instead, the local:intertidal beach is composed of mater ials eroded from the terrace, which is Itself, composed of a highly-erodible, loose sand and gravel. Next pages: FiSure 4.19 (left) Profile comparisons between successive months.at Site C. figure 4.20a (upper right) Profile. overlay for Site C from October 1978-to May 1979. Figure 4.20b (lower right) Profile overlay for Site C from May 1979 to October 1979. 4-47 OCT Nov DEC FEB VAR APR MAY OD JUN JUL Fallen -Bulkhead hosting I AUG Sep i r I @Skt do .00 2b oo 3b ib-oo h oc TO oc DIST"m MT now Immix ORIGIN - sm c Figure 4.19 ead ::7T2 momdow lm@ mohm. "on" SOON& MONTHLY PROFILE OVERLAY OCTOBER 1978- MAY 1978 SITE C ev N z cr ralle Bulkhead sheeting C= 9 1.0 Ica &.0 iLm &.%IS, an INV, Figure 4.20 MONTHLY PROFILE OVERLAY MAY 1979- OCTOBER 1979. SITE C ca ew 2 C3 IV Fallen Blk 'co, sheeting czmrm RIG" Ift CUP S.01 &W qLas d6U 0.0 gure 4620b 4-49 Since materials including bricks and old bulkhead sheeting were exhumed from this site during the course of the year of study, it appears that at least part of the site is constructed of fill material. Site D. A bluff on the lower Severn River at Severnside. This site is located on the north shore of the Severn River approximately 4,000 feet southeast of the Route 50-301 Severn River Bridge (Figures 4.210, 4.22 and 4. 23), at.the region called Se Vernside. The shorel ine reach in which the site is situated is about 4,500 feet in length extending from Brice Point in the southeast to a terminal spit in the northwest. With the exception of the vicinity of Brice Point and a ravine dra.inage,.the entire reach is composed of bluffs as high as 80 feet in"elevation. The'bluffs are composed of semiconsolidated cl.ayey sand (Aquia Formation) which.in places stands at the near vertical, Trees and shrubs-are present on the top of the bluff, and.vines and shrubs cover some potions of the bluff face. Some fallen trees And driftwood litter the shoreline near the study site, and the only shore protection.structure in. the immediate vicin ity is a short 20-foot, cement-block- rubble groin about 230 feet southeAst.of the profiles. This short groin has no influence on the shore behavior at the profile area,. 4-50 The mean tide range in the area is about 0.9 feet. The profile monitor sites are at a bluff section about 50 feet in elevation, which intersects the shoreline about 150 feet northwest of the ravine cut. At the base of the ravine itself there is a low, wave-sculptured terrace. The profile layout.consists of,three transects spaced 30 feet apart. Typical profiles (October 1978) are shown in Figure 4023 which also indicates the sites where sediment samples were acquired in April, 1979. Sediments were sampled from the beach in the upper 1-2.inches of the shoreline profiles and textural characteristics of the.sediments are.shown in Table 4*5,, All.the sediments at1the study site are predominantly sand size, but the bluff and talus slopes contain a.significant fraction of, silt and clay. These fine-grained materials get winnowed out in the sorting process under waveaction and are deposited in deeper waters offshore., As in the case of the bluff at Site B (on.the upper South River near Goose Island), fragments of.limonitic sandstone-type material litterthe toe of the bluff on the shoreline prof UP. These fragments represent the lag material from successive slumps of the bluff face which remain after the sand and mud are redistributed by wave action. Next pages: Figure 4.21 (left) Loc ation Map showing Site D'. Figure 4.22 (upper right) Aerial view of Site D. Figure 4.23 (lower right) Typical profile of rite D in October 1978. 4-51 sr AJE ti@ 1006 iew 3ow 4m 5m em 70W fu@ I-VADPWM Figure@ 4*21 .4-52, 'l-V, i@ll 7 77 ww 7 A Figure 4.22 APPROX. MEAN SEDIMENT SAMPLE TIDE LEVEL NUMBER J25 2- 2. 5 24 29 -20.0 m I- LEFT m -17.5 STAKE < 28 -15.0 ..3 CENTER 12.5 0 Z STAKE 32 26 10.0 RIGHT -7.5 -4 STAKE (30 5.0 m po rrl -77 FMT@. .5 7'0 @O @O @O 30 20 10 @o -100 PROFILE LENGTH (feet) Figure 4.23 4-53 TABLE 4.5 SEDUMNT CF-MUCTERISTICS; SITZ D Sample Prof ile Distance Zone *Gravel (<2.0 am) Sand (0.062mm to 2mm) Silt (0. 39 to 0.062mm) clay (<0.0039 W No. From Origin Mineral Organic Mineral Oritanic Mineral Organic Mineral Orsanic 33 Right -4.8 Bluff 9.74 - 61.95 1.65 9.26 0.53 14.04 2.83 Talus 32 Right 3.0 Foreshorc 1.73 - 92.39 0.93 0.70 'CO.01 2.63 1.62 30 Right 69.0 Offshore 1.86 - 91.42 0.92 0.88 0.09 2.91 1.92 29. Center -5.7 Bluff 7.66 - 60.31 0.57 2.10 0.12 7.41 1.83 Talus Center 6.0 Foreshore 0.13 - 93.75 0.76 0.20 <0.01 3.45 1.71 28 26 Center 69.0 Offshore 0.67 - 89.53 0.90 0.46 0.42 5.89 2.12 25 Left -3.0 Bluff 1.62 - 86.59 1.05 .2.59 0.03 6.01 2.11 Talus 24 Left 9.0 Foreshorq 11.00 81.92 0.83 0.62 -CO.01 4.12 1.51 22 Left 69. 0 Of fabore 3.61 86.61 0.87 0.97 0.31 5.77 1. The numerical Values shown represent the fractional weight, In grams, of 100 gram of sample, thus the results may also be Interpreted an percentage values. 100, MmiL mmok snob 16moni The beach at the study site receives boat-wake energy from boats travelling up and down the Severn River. Most of the boat traffic passes at distances greater than 1000 feet from the shoreline, but some localized boat traffic does pass closer to the shore and generates wakes which attack the shoreline profile. The boating character- istics at this site are discussed in Chapter VI. This portion of the Severn River shoreline also receives wind waves which approach with.the longest fetches from the northwest, south, and southeast. The wind-wave climate at this site is discussed in Appendix Band the wind waves and boat wakes are compared in Chapter VII for their relative importance in causing any changes in the shoreline profiles. As in the case of the other bluff site (Site B), the 'fastland boundary was defined as either the consolidated sediments of the bluff or the loose material slumped from the bluff. The sequence of photog raphs shown in Figure 4.24 indicates.that until some time after the July survey (07/29/79) the modifications of the fastland were, in fact, due to removal of slumped material.. However the passage of Tropical Storm David in early September resulted in complete removal of the slumped material as well as erosion of the Opposite: Table 4.5- Sediment characteristics at Site D The locations of the samples listed in the TaLe are shown on the profiles in Figure 4.23. Next pages Figures 4.24 a-d Photographic view of the three profile locations at Site.D in October 1978, May 1979, July 1979, and October 1979. 4-55 RIGHT CENTER SITE "D" OCT. 1978 LEFT ,;@; iiivh Fi gure 4.24 a 4-56 no im, no so so so low 111"i am, ow, so 01 . ..... ..... . .... ... .......... RIGHT CENTER LEFT SITE "D" MAY 1979 F i g u r e 4.24 b . ........... . . RIGHT IJ CENTER . . . . . . . . . . . . . . . . . SITE "D" JULY 1979 LEFT im F i g u r e 4.24 c 4-58 a* so low, IM, 4110 OW 'am sm J'Plu Oki QD RIGHT CENTER LEFT Ift SITE "ID" OCT. 1979 Figure 4,.24d consolidated bluff sediments. Thus in Figure 4.24d (October 1979) we see an exposed bluff (Left & Center) with a scarped terrace of sand in the backshore rather than slumped material. The comparative envelopes of change between the periods of October 1978-May 1979, and May-October 1979, are shown in Figures 4.26 a and b, respectively. Note in particular that the surveys in June, July, and August cluster very close to the post-Tropical Storm,David profiles of September and October 1979. The profile comparisons between successive months offer additional illustration that there.was little profile modification during the peak boating season of June, July and August. The details of the fastland modifications are shown in Figure 4.25 where,sequential profile segments are displayed. At. least two episodes of.slumping occurred at.the Left profile between the surveys of November 19781and that -of May 1979, Intervening surveys show reductionof the slumped material. Between the Mayand June surveys some additional reduction of.slumped material (talus) occurred, but during theperiod of June through August the profiles were virtually identical. While the close similarity in monthly profile positions on this dynamic shoreline does not necessarily mean there has been no significant change in the time between profiles, there is little likelihood that the slump surface angles would be similar if there had, in fact, been'significant changes between profile date.s. Thus, these profiles are interpreted as showing no signifIcant changes.' 4-60 The storm surge associated with the passage to Tropical Storm David was about 2.5 feet, as determined from strand lines at the sites. This elevation-at Site D, along with large wave heiqhts (estimated up to 2-3 ft. by local observers) was sufficient to cause direct attack on the bluff as well as to reduce the volume of earlier slumped material. The comparative profiles of August and September 1979 (Figure 4.25) show a displacement of the fastland of 2.5 feet, part of which is bluff-face retreat. These profiles also show that-the sand beach following David was considerably higher in-elevation, and both the photographic evidence and the post-David survey of September:show a scarped beach backshore. Thus between the first and third' week of September .19.79, Tropic,al,Storm David eroded the bluff which resulted in a pronounced thickening of the beach sands, and t h-.is,,.in tu rn, was followed by a,reduction,in beach elevation as evidenced by the backshore sand..scarp shown in Figures 4.24d and 4.24b. The profile histories at the Center and Right.profiles exhibit essentially the same patterns o f behavior as previously discussed at the left profile: slumps reduced by Next pages: Figure 4.25 (left) Profile comparisons between successive months at.Site.D. ,Figure 4.26a (upper right) Profile overlay for Site D from October 1978 to May 19179. Figure 4.26b (lower right) Profile overlay. for Site D from May 1979 to October 1979. 4-61 OCT 7s v DEC pan MAN cl APR. C3 in MAY JUN L J A@UL 9-bAvib* @AUQ ap OCT lefjr- center HIM -10-00 .0-00 10-00 -to-oo 0.00 ib-oc -'10.00 6-00 ib DISTARM IN FEET FROM PROME ORIGIN SITE D Figure 4.25. A@@ summ, 4mmmi. ftmmli@ dmmmil sommm, dobalk, dims misms ARMEN, MONTHLY PROFILE OVERLAY OCTOBER 1978-MAY 1979 SITE D C3 UA RZINT nl&a arm tbAD ALUI 1,jjfn=.1j1ft 110-01 Fi gure 4.2..6a MONTHLY PROFILE OVERLAY MAY, 1979 -OCTOBER 19 rg SITE D Cluster of Jun, Jul. AM, 1*79 co clue or of Jun. Jul, A",t IL9.,9 7 al in cy Cluster of Jun. Jul, A", 1979 in c3 c3 us n wo -Ism Asn -Low wwo'sAa mfm 4-m Figure 4.26 b czerr" 4-63 wave action during the late fall, winter, and early spring; relative quiescence during the peak boating season of June, July,and August. The occurrence of Tropical Storm David with a large storm surge and large waves from the southeast dominated the profile response. j In summary, the observations over the one-year period demonstrated the -role of wind waves in the reduction of the material eroded from face of the bluffs.. But the most important effect was a single storm event, with a large storm surge and waves that dominated the fastland.and shore zone response over an annual cycle. Finally, it should be noted that the profile modificationsduring the boating season, aside from the storm response, were very small the changes during the non-boating season.. 4-64 Site E. A pocket marsh near the entrance of Maynedier_Creek- off of the upper Severn River. This site, located near the mouth of Maynedier Creek (Pigs. 4.27r 4.28, 4.29) is a ravine-mouth-marsh,, approximately 175 ft. in width across the frontal margin. The marsh is predominantly clump growths of �2artina cynosuroides and Scirpus sp. which are tightly bound by root mass and soil, and virtually "floato on a substrate of very soft org anic "mush". While a *shaky" firm" footing,may be found on the clumps, a misstep leaves the observer knee-high "in the mush". The shoreline on the flanks of the marsh intersects a.thin veneer of sand overlying a plastic-tan clay which also forms the steepbanks with an elevation of about 4 feet. The nearsh.ore (and offshore) fronting-the marsh.itself is a very soft substrate.varying between sandy-silt.to.silty-clay The.orqa,nic content of samples collected along the shoreline profiles is high (Table 4.6). The mean tide range is about 0.8 feet. There are no shoreline protection structures influencing the area. The marsh receive's boat-wake wave energy from boats Next pages: Figure 4.27 (left) Location.map showing Site E. Figure 4.28 (upperright) herial view of Site E. Figure-4.29 (lower right) Typical profile of Mte E in October 1978. 4-65 'ell S6kLE -1@4606 0 two 0 IOOD 2ODD 30M 4m - -=-- 6M 70M FEET L--3. Fi gu.re 4-.66. ",-fW @ K Ey Awl "k, 1. "'14 V R3 4i, !, Slr, illYf; 1:11 5-1, V@X al N 4,4 Smll 5 q, w-T al Ao- P., WrA i,, `2,g Ti mo a . .. .... A q"N"j, @,O M AM d@ L i U @2 Figure 4.28 SEDIMENT SAMPLE-,,,,, APPROX. MEAN NUMBER TIDE LEVEL 20.0- 9 8 7 LLJ 17.5 - - - - - - - - - - - LEFT LLJ STAKE LL_ g 15.0 6 5 4 z 12.5- CENTER z wr 1 .0- STAKE r a: 2 t 7.5 RIGHT < > 5.0- SIAKE LLI -1 2.5- FS -ITT T U-1 0- 0 I@ 2'0 @O @O @O PROFILE LENGTH (feet) Figure4.29 4-67 Table 4.6 SEDIMENT CHARACTERISTICS& SITE E Sample Profile Distance Zone *Gravel,(<2.0 mm) Sand (0.062mm to 2mm) silt (0. 39 to 0.062mm) Clay (<O 0039 mm) No. Prom Origin Mineral OrRanic Mineral Orstanic Mineral Orianic BD3 Right 34.0 ft. Marsh 0.97 93.02 0.94 2.06 0.22 0.83 1.96 BD2 Right 36.0 Foreshore 1.00 96.04 0.19 0.06 '0.87 0.37 1,47 BDI Right 51.0 Nearshore 6.08 65.35 3.37 10.59 2.00 9.23 3.39 BD6 Center 25.0 Marsh 8.09 32.17 14.86 19.76 3.13 -12.95 9.03 BD5 Center 28.0 Foreshore 7.41 14. 50 12.96 19.80 1.88 12.92 10.52 BD4 Center 39.0 Nearshore 4.23 44.93 6.18 27.58 2.02 ..8.01 7.04 BD9 Left 19.0 Marsh 6.81 29.81 24.79 117.77 2.91 7.24 10.67 BDO e t 26.0 Foreshore 10.71 41.32 15.28 14.87 1.99 7.74 7.89 20.06 BU7 Left 36.0 NearshoreT 6.79 47.65 8.15 1.50 8.36 The numerical values shown represent the fractional weight, In grams , of 100 grams of sample, thus the,results may also be Interpreted as percentage values. entering and leaving Maynedier Creek. There is a posted speed-controlzone in the creek on weekends. Boats travelling near the study site cominonly pass within a few hundred feet from the shoreline. The boating character- istics at this site are discussed in more detail in Chapter VI. The entrance to Maynedier Creek is relatively protected from heavy wave action by Mathiers Point and the shallow bathymetry of Round.Bay to the east. The limited fetch within Maynedier Creek (maximum about 2,000.ft. to the south) precludes any.significant wind-wave generation within the area. .The wind-wave climate at this site is discussed in Appendix 8, and.the wind waves and boat wakes are compared in Chapter VII for their relative importance in causIng any changes.in the shoreline profiles.. Three profile stations, 30 feetapart, are established on the frontal face of the marsh. Typical profiles (October 1978) are shown in.Figure 4.29. Repetitive profiling and visual observation between October 1978-Pebruary 1979 indicated little-or no change in the marsh behind the shore. After February,, auxillary profile stakes were emplaced to Opposite.: Table 4.6 Sediment characteristics at Site E. ;@he location.of the samples.listed in the Table are shown on the profiles in Figure 4.29. Next pages: Figures 4.30 a-d Photographic view of the three profile locations at Site E in October 1978, May 1979, Auqust 1979, and October 1979. 4-69 Amid CENTER .......... . .. war Rl GHT L EFT SITE "E" OCT. 1978 Figure 4.30a 4-70 LEFT CENTER RIGHT SITE 11E M AY 1979 Figure 4.30 b 4-71 . ........ . ..... RIGHT CENTER SITE "E" AUG. 1979 LEFT Figure 4.30c 4-72 4f 4!b 4 CA k", If 'I, RIGHT CENTER LEFT SITE E OCT. 1979 Fi g-u r'e 4.30 d Minimize observer disturbance to the marsh system. The edge of the marsh vegetation was considered to be the fastland boundary. The series of photographic observations and the profile surveys shown below indicate there was no fastland retreat during the one year observation period. The photographic series (October 1978; May, Augustf October 1979) are shown in Figure 4.31 and the profile envelopes for October 1978-to-May 1979, and May-to-October 1979 are shown in Figure 4.32. The difficulties of surveying in a marsh composed of isolated, irregularly shaped clumps of vegetation are exemplified in the two sets of profile overlays. The measurement of the exact position of the fastland edge varied with.the tightness.of the measuring tape, and the.prec.ision of.the rodman staying "on line". The tape tightness necessarily variedwith the height and density of vegetation while straying off line could result in missing the edge of a marsh.clump. This is best illustrated in Figure 4..30c (Left) where it is to be noted that the tape,passes just to the side of a.marsh segment. Positioning the tape slightly different would result in the inclusion of the marsh-segment in the profile. in the Left profile sequence of Figure 4.31 this was the case in the surveys of February and October 1979. In. summary,,the.monthly photography provides unambiguous decumentation that there was no measurable. 4-74 retr eat of the marsh edge on the'Left profile. The profile sequence in Figure 4.31 for the Center and Right profiles demonstrate that there was no change at these two profiles either. Next pages: Fi2ure 4.31 (left) Profile comparisons between successive months at Site E. Figure 4.32a (upper right) Profile overlay for Site E from October 1978 to Nay 1979, Figure 4-32b (lower right) Profile overlay for Site E from May 1979 to October 19-79. 4-75 OCT NOV tr DEC F E 0 MAR APR MAV JUN. JUL AUG 8FP OCT left 4b.OG-2b-00 3b -oo 410 - 00 3b oo 4b-oo 5FOO 7o Go 30-00 VISTOCE IN Mn nW P207112 MaM 31-TR At Figure 4.31 MONTHLY PROFILE OVERLAY OCTOBER 1978 MAY 1979 SITE E UWr CE"ER U3 a W tn -I'm ca wn Mao lLn I)JAn Figure 4.32a MONTHLY PROFILE OVERLAY MAY .1979- OCTOBER 1979 SITE, E fm A C2 CENIM-1 uj R I l*rr Figure 4-32b 4-77 V BEHAVIOR OF SHORELINE PROFILES AT ADDITIONAL SITES Michael Perry Deborah Blades, Rhonda Waller, and Tristina Deitz A. Introduction .In addition to those sites described in the last chapter, additional sets of monthly profiles were collected by DNR student interns..from,.the Environmental ,Studies Program at Anne Arundel Community College. Some of these supplemental sites were adjacent to the consultants' profiling locations and represented d-ifferent shoreline types, (i.e. a marsh next to a bluff, or a bank next to a marsh). The supplemental sites also included two additional locati ons.which were initially selected as "back-up" s.i tes to.the consultants'- locations, and were to be.used in the event that boating patterns at one of the principal sites turned out not.to be as anticipated. The results presen.,ted.in-this cha pter show Tropical Storm David produced the qreatest changes in shoreline profiles. Some other sediment movement was measured during the year of study,.but no important changes at any of the sites took place during the boa.ting season. B. Methods At each of the sites, three profiling locations were selected with a separation distance of 30 feet. Each 5-1 profile was established by inserting two reference pipes or stakes several feet apart on a line perpendicular to the beach or shoreline. The position of the six reference pipes was then surveyed from a fixed bronze survey marker set in concrete with a transit and rod. When the shoreline profiles were surveyed each mon-th, the ground elevations along each profile were referenced to that of the benchmark.using a precision level and rod. The rear reference pipe was considered to be the origin for each profile.* The ground elevations were surveyed at 3-foot intervals, and at all additional intermediate points where a slope change occurred. At the three sites with banks or bluffs (Sites AA, CC, EE), the profiles were extended up the bluff face from the rear stakes, and elevations were surveyed at intervals up to.the instrument height. Site AA: A pocket marsh and_adj_acent bluff in Harness Creek off the lower South River. This site is located in an area known as Hillsmere Shores (Figure 5.1.) The beach seqment chosen for Next pages: Figure 5.1 (left) Location map showing Site AA. Figure_5.2 (upper riqht) Aerial view of Site AA. Figure 5.3 (lower right) Typical profile of Site AA in October 1978. 5-2 A., Hillsmere @7_ Sham CWar 6@@ -ve i, . -n 1.?47 X) Point-.-, 16 71 1 'A <:: NI.I. @@ '. * I .. .,-. 1. -,.' -@2@ 1 . -, I X if, w kAL.E 1:24000 0 0" 3WO 4001D Gow 70 OD FEET I MLONUER Fig-ure 5.1 5-3 Figure' 562 JISITE AA@@ VEGETATION APPROX. MEAN -15 TI DE LEVE -14 12 RIGHT STAKE -12 0 > 10 .9 > CENTER STAKE M 8 8 aC) .7 -ro a -5 4 LEFT STAKE- .3 -2 10 70 66'0 5'0 @O io 20 10 0 -10 PROFILE LENGTH (ft.) Figure 5.3 5-4 TABLE 5-1 SEDIMENT CHARACTERISTICS SITE PROFILE SAMPLE DISTANCE ZONE % % SILT FROM ORIGIN SA14D PLUS CLAY AA LEFT 2 0 ft. BLUFF TALUS 70% 30% AA LEFT 12 20 ft. OFFSHORE 80% 20% AA RIGHT' 8 5 ft. MARSH 98% 2% AA RIGHT 9 25 ft. OFFSHORE 89% 11% BB RIGHT 6 3 ft. MARSH 99% 1% BB LEFT 17 3 ft. OFFSHORE 90% 10% BB LEFT 11 26 ft. OFFSHORE 88% 12% Qn cc LEFT 4 5 ft. BLUFF 76% 24% 1 cc LEFT 16 15 ft. OFFSHORE 95% 5% cc RIGHT 13 8 ft. NEARSHORE 99% 1% cc RIGHT 14 18 ft. OFFSHORE 95% 5% EE RIGHT 1 12 ft. BLUFF TALUS 70% 30% EE RIGHT 15 28 ft. OFFSHORE 93% 7% EE RIGHT 5 17 ft. NEARSHORE 98% 2% FF RIGH .T 3 15 ft. FORESHORE 97% 3% FF LEFT 10 10 ft. MARSH 66% 34% FF LEFT .7 ft. OFFSHORE 98% 2% monitoring is on the southern shore of Harness Creek in an area where a bluff meets with a.pocket marsh formed at the mouth of a ravine. Site AA is in the vicinity of the consultants' profile site Site A, which was discussed in the previous chapter. The profile layout for site AA consisted of 3 transects spaced 30 feet apart. The right and center profiles are located on the marsh, which extends approximately 300 feet across its frontal margin and about 200 feet inland. The left profile is located at the base of the adjacent bluff' which is approximately 10 feet high. The vegetation at the right and center profiles consists of marsh grasses growing inthick compact clumps. The marsh grass ends at the shoreline in a sharp boundary.. bluff face on the left profile i The slargely exposed, eroding sediments. At the top of the bluff are mature trees., shrubs, and vines which extend up to the bluff face. The bluff is nearly vertical in.the upper portions and covered with exposed root masses. A number of trees have fallen over the bluff edge onto the beach; the entir e- shoreline surrounding the study site is littered with fallen trees and driftwood. opposite: Table 5.1 Sediment characteristics at the additTonal study sites. The locations of the samples listed in the Table are shown on the profiles in Figures 5.3, 5.7, 5.11, 5.15,'and 5.19. 5-6 SITE-A%A ICENTFR 'q E WO STA NOV. 6. 1978 water level OEC. 7. 1978 JAN. 5. 1979 ice so ice .0 MARCH 2.1979 ice=* N *--"\ice APRIL 18. 1979 ice no ice mo MAY 7.1979 JUNE 6,1979 JUNE 29.1979 AUG. 2. 1979 ZEPT 13. 1979 OCT. 17. 1979 OCT. 31.1979 4 3 2 0 0 FEET Figure 5.4 /00" Sediment samples were collected-from the beach and nearshore in the upper 1-2 inches of the shoreline profiles (Table 5.1). The sediments in the nearshore at site AA are principally derived from the erosion of the bluff. Table 5.1 shows the bluff sediments contain about 30% silt and clay. The nearshore.sample'contains a. similar portion of fine-grained material. The samples collected in front of the marsh and nearshore contained slightly less silt and clay than the offshore samples. The mean tide range in the area is,approximately 1.0 foot, and there were no shoreline.structures present along the reach during the period of study. The shoreline at site AA receives boat-wake enerqy mainly from boats entering and' exiting Harness Creek. Much of.the boat traffic. near the study site stays within the main channel which is approximately 500-1000 feet from the profile locations. The shoreline of site AA also receives wind-wave energy mainly from the northwest.. Normal winds from any other direction produce small waves.at the site. The fastland.boundary for the bluff at the left profile sta ke was defined as either the inplace semi-consolidated sediments forming the bluff or the material which slumped from the bluff face. The reason for considering the slumped material as fastland is that were it not for the removal of Opposite@ Figure 5.4 Comparison of monthly profiles collected at Site AA. 5-8 this material by wave action, the bluff slope would ultimately reduce and become stabilized with vegetation. The fastland boundaries for the center and right stakes were- defined as the edge of vegetation. These boundaries are composed of the compact root masses of the marsh grass. Profiles at site AA were collected monthly, and a comparison between successive months is illustrated in Figure 5.4. This comparison of successive profiles shows that the bluff located at the left stake experienced modulation.of sediments and. fastland retreat. The right and' center stakes located in front,of the marsh showed no important changes during the year of study, in either the boating or non-boating seasons. There are noticeable changes in the amounts of material wh ich have accumulated at the base of the bluff on profiles in January,,April,and September of 1979 (Figure.5.4 This material is usually reduced by the next profile date. These modulations of sediments on the profiles are thought to be due to slumping and subsequent wave action washing out the slumped material. The boating season was considered to start in May of 1979, and some of the slumped material from the previous month's profile -at the left stake is still noticeable at the base of the bluff. This material decreases slightly during the summer. In September of 1979, the storm surge during Tropical Storm David focused wave action directly on the bluff face at the left profile stake, and,more material accumulated at the base of the bluff on 5-9 the shoreline pr'ofile taken after David. Some reduction of the talus deposit occurred between t@e September and October profiling dates, but the profile at the left stake was slightly built up again by the end of October. The near- shore portion of the profiles at the left stake show a slight accumulation of material until September 1979, when David came through the area. In summary, Tropical Storm David was accompanied by the greatest ch ange in shoreline profiles at this site. These changes occurred in front of the bluff, where the slumped material on the beach was eroded. Smaller changes in this portion of the shoreline profile also were observed earlier in the study period. The two adjacent profiles in the adjoining marsh showed no change ineither the boating on non-boatinq seasons. Site BB: A pocketmarsh on the upper South. River near Goose Island This site is located near the Glen Isle. subdivision on the southern shore of the upper South River (Figure 5.5). The 'shoreline segment chosen for monitoring is a small pocket marsh 750 feet downstream from the mouth of Flat Next pages; Figure 5.5 (left) Location map showing Site BB. Figure 5.6 (upper right) Aerial view of Site BB. Figure 5.7 (lower right) Typical profile of Site BB in October 1978. 5-10 S a KAM SCALE 1:24000 1060 a I= a0w -*M 4m sm 6wo 7=0 FEEt Figure 5.5 5-11 _;', rn@ P" Wl z ,v Figure 5.6 15- VEGETATION 14- APPROX. MEAN ILS I T E J@B:]@ 13- 12- TIDE LEVEL@ F- 10- -LEFT STAKE 9- Ej- >- 8- 6 -CENTER STAKE Lli 6- _j 5- 0 4- RIGHT STAKE 2- -10 0 10 20 30 40 50 60 @o PROFILE LENGTH (ft.) Figure U 5-12 SITE-W I LEFT STAKE ICENTER S RIGHT ST V"Wation woler NOV. 6.1978 401/ 000*0010 w4el AAAAAAr DEC. 6.197 JAN. 5. 197%,@ ice so ice a* FEB. 2. 1979&,-\ \ ice G* ice W* ice Eno MARCH 2.1979,&,\ ice ice ise APRIL 6.1979 MAY 2.1979 JUNE 6,1979 JUNE 29.19T9 AUG. 9.1979 SEPT. 13. 1979 OCT. 13.1979 OCT. 31.197 FEET Figure 5.8 A" 9, 4 3 5-13 Creek. Immediately'u'pstream is the bluff which is the location of the consultants' Site B discussed in the previous chapter. The profile layout for site BB consisted of 3 transects spaced 30 feet apart along the entire section of the marsh. The-vegetation at site BB consists of marsh grasses Phragmites-communis and pcirpus oln=i, which extend from the water line landward to a zone of shrubs and small trees. Further landward, the marsh meets a ravine and the adjacent bluff which are full of mature trees, shrubs, vines and considerable undergrowth. Sediment samples at site BB were collected from the beach, marsh, and nearshore zones within the upper 1-2 inches of the shoreline profiles. The sediments collected near the profile origins were composed of sand with a relatively minor silt and clay content (Table 5.1). The sample collected for analysis also contained well-sorted gravel. -At the site, the.relati vely firm sand al-ong the foreshore blends into finer-grained, less consolidated sediment offshor e(Table 5.1). These nearshore sediments are considered to be derived partly from the erosion of the adjacent bluff and partly from finer-qrained sediment carried into South River from Flat Creek. The mean tidal range in.the area of site B13 is [email protected] foot, and there were no.shoreline structures present along the reach that were considered to opposite: Figure 5.8 Comparison of monthly profiles collected at Site BB. 5-14 interfere with sediment transport at the site during the year of study. The shoreline of the marsh receives boat-wake energy from boats travelling on the South River generally-at distances greater than 1000 feet. More localized boat traffic is generated from boats which circle Goose Island, and pass within 100-200 feet of the study site. A large percentage of these boats are towing waterskiers and tend to make multiple passages of the shoreline in a relatively short period of time. Boatinq traffic for the.shoreline reach where sites B and BB are located is discussed further in chapters VI and VII. The marsh also receives wind waves which approach with the longest fetches from the north-northwest. Regional winds from this direction can generate appreciable wave energy which would focus on site B9, bu t these winds also tend to drive water out of the rivers on the western shore so the erosive power of the waves is expended at lower levels on the beach and in the nearshore, rather than on the fastland boundary of the marsh. Monthly profiles collected at site BB are illustrated in Figure.5.8. The fastland boundary for site BB was defined as the edge of vegetation, and showed little or no retreat for,all 3 profile stakes during the year of study. Comparative profiles in Figure 5.8 do clearly show modulation of.beach face sediments, but the fastland boundary for all 3 stakes.remained relatively unchanged. 5-15 One major instance of-visible fastland retreat at site BB occurred after the passage of Tropical Storm David September 8-90, 1979. Figure 5. 8 shows the beach profile at the center stake experienced a slight fastland loss and a substantial loss of nearshore sediments between the profile dates of August 9, 1979 and September 13, 1979 after the passage of David. These nearshore sediments were presumably lost during the storm and were partially replaced by erosion from the adjacent bluff within one month. In summary, some modulation of nearshore sediments occurred at a 11 three stakes, but no notable fastland changes can be attributed to boating. The only notable fastland. change at.this site can be attributed to the passage of Tropical Storm David. Site CC: A bluff in Broad Creek off the Upper South River. This site is located approximately 1200 feet north of the mouth of Broad Creek (Fiqure 5.9). The shoreline segment chosen for monitoring is located at the base of a bluff which has a maximum-elevation of 60 feet. The profiles at this site are located aporoximately 50 feet downstream from the -marshy promontory which is the location of the consultants' Site C discussed the previous chapter. Next pages: Figure 5.9 (left) Location map showing Site CC. Figure 5.10 (upper right) Aerial view of Site CC. Figure 5.11 (lower right) Typical prof,ile of. Site CC in October 1978. 5-16 7-- R4 tA V2- e, 7i @kn SCALE 1:2AOOO I NILE Wow 0. adio mw im 4m 50M GM 7000 FEET 0 1 PLOMETER Fi *.gu re 5-17 . 4, AQ as 4 yk_@, 3. p A,: V1,@11Ngw t q7 Figure 5.10 15 jISITE VEGETATION -14 -13 APPROX. MEAN 13 TIDE LEVEL 0 10 14 LEFT STAKE- -9 m 8 3@p M CENTER STAKE--..- -7 M_ 4 -6 -5 16 RIGHT STAKE-.....%,--- -3 -0 70 60 5b 4 310 210 110 01 -10 PROFILE LENGTH (ft.) Figure 5.11 5-18 SITE- CC JAKE] ICENTER raT8KQ OCT. 31. 1978 Vegetation approx. w. later DEC. 4. 1978 DEC. 29,1979 FEB. 3. 1979 MARCH 2.1979 APRIL 19.1979 ice vo ice m* ice as MAY 4. 1979 JUNE 6,1979 JUNE 29.1979 AUG. 2,1979 SEPT. 13, 1979 OCT. 17, 1979 OCT. 31, 1979 2 F6M FEET 20 Figure 5.12 5-19 The.profile layout for Site CC consists of 3 transects spaced 30 feet apart. The right profile is situated at the base of a bluff that meets the beach with an approximate 30 degree sloping face. The center profile is located 30 feet further upstream along the bluff and gradually steepens to a 50 degree sloping face. The bluff continues to steepen until it reaches a nearly - vertical section located at the left profile site. The bluff face does not contain many areas of exposed sediments and it s vegetation is composed of many trees, shrubs, thick vinesand small undergrowth. The beach grasses which grow at the base of the bluff along the shoreline consist of Scirpus olneyi and clump growths of Phragmites communis In some areas of the shoreline between the center and right profiles, the grasses are isolated clumps tightly bound at the roots. Sediments in the near- shore contain submerged clumps of lead root.material. The entire shoreline contains a 2-Afoot wide section of small grass growth that begins beyond the visible swash line. Sediment samples at Site CC were collected from the bank.beach, andnearshore zones within the upper 1-2 inches of the shoreline.profiles (Table 5.1). The s ediments collected from the beach and nearshore zones are considered to be principally derived from the erosion of materials within the bluff....This bluff contains predominantly sand opposite: Figure 5.12 Comparison of.monthly profiles collected at Site CC. 5-20 and some well-sorted qravel between ILO-50 mm. Sandy sediments comprise the beach within 3-5 feet of the base of the bluff. The strands of grass on the beach at the base of the bluff must play an important role in trapping these sandy sedimen ts. This nearshore zone also contains well-sorted gravel which appears similar in character to the gravel collected from the bluff. The mean tidal range at this site is approximat ely 1 foot, and there were no shoreline structures present along the reach which interferred with sediment movement during the period of study. There is.a pier adjacent to the profiles, but this pier is not considered to have an.effect on shore-erosion. The sho.reface of the bluff at the study site receives boat-wake energy from boats enterinq and exiting Broad. Creek, as well as from boats travelling up and down the South River. Boats entering and exiting Broad Creek pass at distances betweeen 400-500 feet as compared to distances greater than 1500 feet onthe South River. The inventory of boating activity collected at nearby Site C shows.that 63% of the boat passaqes were waterskiers that were making multiple passes near that shoreline in a relatively short period of time. This site also experienced considerably more boating traffic on.weekends as compared to weekdays. The shoreface of this.site, being situated some'500 feet from the main channel.of the Creek, receives much lower levels of boat-wake energy than the adjacent promontory marsh. The pier adjacent to Site CC extends out 130 feet 5-21 and discourages most boats from making passes anywhere near the shoreface of the site. Even though the boating frequencies in this portion of Broad Creek are considered to be high, the boat-wake energy expended on the beach at site CC is relatively small. The shoreface of Site CC also receives wind waves with the longest fetches between the south and southwest. Porter and Adison Points at the mouth of Broad Creek protect Site CC from many of the regional winds,except those focused directly into the creek. The wind roses illustrated in Appendix B show the bluff and marsh block any wind-waves when regional winds blow from the northern fetch areas, and the.total wave energy expended on the beach at Site CC.is negligible. Therefore the total wind-wave energy created at Site CC can be considered to be small. The fastland boundary for Site CC was defined as the edge of vegetation for the right and center profiles. This vegetation line is also accompanied by a slight scarp which is composed of grass clumps tightly bound to the beach by root masses. The fastland boundary for the left profile was defined as either the in-place sediments forming the bluff,. or the material which slumped from the bluff face. Monthly profiles were collected at Site CC and a comparison between successive months-is illustrated in Figure 5.12. Profile comparisons of successive months indicate that only very minor changes occurred at all three profile locations. The small vertical bluff face.at the 5-22 left profile has a web of thick tree roots which has held the sediments tightly in place. The monthly profiles at the right and center stakes show some modulation of sediments on the beach and foreshore, but neither set shows any cons'iderable fastland'retreat. There was also a small slump between the inital profile in October 1978 and the next profile in December 1978. The profile on December 29, 1978 shows removal of--much of the slumped sediment from the profile at the center stake, but the subseque'nt profiles do not show any additional change. In-summary, only 'minor changes were observed at all three profile locations during the period of the study. This site is sheltered from some of the strongest winds which blow from the north-northwest. The site is located in a popular boating.area, but it is protected from close passages by boats due.to a pier which extends out from the shore. Site EE: A small bank and beach in Mayriedier Creek off the @12per Severn River. This site is located inside the mouth of Maynedier Creek, off the upper Severn River (Figure 5.13). The beach segment chosen for monitoring consists of a small bank and beach which is adjacent to the ravine pocket mars h at the consultants' Site E describ@d in Chapter IV. The profile layout for site EE consists of two transects spaced 30' apart. The left profile contains a 5-23 small vegetated beach which is 10 feet upstream from the edge of the marsh* Landward of the beach oh.the left profile is a heavily-vegetated region of mature trees, shrubs, vines, and much small undergrowth. The right profile contains a beach with considerable growth of beach grasses extending 6.feet seaward of the bank. Landward of the beach on the right profile is a steep vegetated bank. The vegetation at the top of the bank consists of mature trees, shrubs, and vines. Sediment samples at Site EE were collected from the upper 1-.2 inches on the shoreline profile (Table 5.1). The sediments collected on the beach and in the nearshore are princi pally sand and are presumed to be derived from the erosion of the bank., The sediments in the bank contain 70% sand and some pebbles 1-3 mm..i n size. The beach face in front of the bank,contains 98% sand. Sediments bollected from the nearshore zone approximately 4 feet seaward of the grass on the profile contained higher silt And clay content. The sandy nature of the sediments At Site EE sta.nd in sharp contrast to the mucky consistency and high organic content of the adjacent marsh which is the consultants' Site E (Chapter IV). Next pages: Figure 5.13 (left),Location map showing Site EE. Figure 5.14 (upper right) Aerial view of Site EE. Figure 5.15 (lower right) Typical profile of Site EE in October 1978. 5-24 17 Bit y nA it '3. X@ Point ROUND St Helena Island Point q 'e > @, J Ij -SCALE 1:24000 0 101010 0 low 2M 3W0 4m sm sm 7000 FEET Figure 5.13. 5-25 @ E ,a FQ ITT- Aiei t AU" A V, 4Z "i OWN 0 N 414'@o 4 la"6 Q Al W, U@t ru 49 -a A'm W f _4 @4g 'Av Wi ap IRisk 1, Al p w lk@ Figure 5.14 15- VEGETATION 14- _EZ 13- 12- APPROX. MEAN TIDE LEVEL 10- 5 9- LLI >- T- c<t RIGHT STAKE La 6- in cr_ 5- 0 4- 3- -LEFT STAKE Of -10 0 10 20 30 40 50 60 70 PROFILE LENGT-H (f 0 - Figure 5.15 5-26 SITE-EE LEFT STAKEI RIGHT STAKE opprox. vegetation OCT 30, 197 DEC. 4.1978 lnjVVVUlfV DEC. 28. We FEB. 3. 1979 MARCH 2,1979 APRIL 6. 1979 ice ice m* MAY 4,1979 JUNE 6.1979 JUNE 29.197 AUG. 9. 19 79 SEPT. 13.1979 OCT. 17.1979 OCT 31, 1979 3 2 0 10 20 30 @4O 50 FEET Figure 5.16 5-27 The mean tidal range at,study site EE is approximately 0.8 feet. There were no shoreline protection stru ctures present along this reach during the period of study which are considered to interfere with sediment deposition. There is a pier adjacent to the profiles, but this pier is not considered to have an effect on shore erosion. The shoreface at Site EE receives boat-wake energy from @boats entering or exiting Maynedier.Creek. The boat ing characteristics are discussed for the consultants' Site E in Chapter VI. This study site, because of its,extreme north- west location on the upper Severn River, experienced the. least amount of boat-wake energy of the study sites. During the weekdays, 53% of the boats were towing waterskiers and made multiple passages of the shoreline. Maynedier Creek has a speed limit on weeke.nds and holidays and 97% of all boats travelled.at speeds of 10 mph or less., The shoreface at Site E.E also receives wind-wave.energy which is fairly limited by Mathiers Point at the mouth of Maynedier Creek and.by the shallow bathymetry of Round Bay beyond. The limited fetch within Ma ynedier Creek precludes of any appreciable wind waves in the area, except at very high wind speeds. The wind-wave climate in this area is discussed in Chapter VII. Profiles at Site EE were collected monthly and a comparison.between successive months is illustrated in Figure 5.16. The fastland boundary at Site EE for the left Opposite: Figure 5.16 Comparison of monthly profiles collected at Site EE. 5728 profile is defined as the landward edge of beach vegetation. The fastland boundary for the right profile site is defined as the base of the bank. The profile comparisons indicate that only minor changes occurred at both the right and left stakes. Some of these irregularities on successive monthly profiles are due to the driftwood, falling trees, and logs which collected on the shoreline and which were not removed when the surveys were collected. For instance a comparison between the December 4, 1978and December 28, 1978 profiles, at the left stake shows some distortion caused by logs. -During the boating season, the left profile experienced a slight episode of, bank erosion that is evident on the August 9, 1979-profile. The next monthly profile at the left stake was collected after the passage of Tropical Storm David.and more change was observed4 By October 31, 1979, the sediments which had accumulated at the base of the bank were mostly removed. In summary, there were only minor changes in.the shoreline profiles at this site.. A small amount'of bank erosion was measured during the boating seaon, and.some additional erosion was observed after the passage of Tropical Storm David. Site FF: A beach and sandy marsh on Beards Point in the upper South River. This:1site is the community beachin a subdivision known 5-29 as Glen Isle (Figure 5.17). The shoreline segment chosen for-monitoring consists partially of a lawn bank landward of a beach, and an adiacent marsh on Beards Point. The profile layout for Site FF contains three transects spaced 30 feet apart. Figure 5.16'illustrates the exact location of these profiles. The vegetation for Site FF at the right and center profile locations consists of ordinary lawn grass which is maintained by cutting. This lawn grass extends up to a 2 foot scarp at the waters' edge. Beyond the scarp is a beach compos ed of brown sand. At the left profile,stake, the vegetation consists of a dense cattail marsh which extends up to the shoreline scarp. Sediments exposed in the scarp contain very compact root masses. Sediment samples were collected from the scarp, beach, c 'ind nearshore in the upper 1-2 inches of the shoreline profiles (Table 5.1). The sediments in the nearshore zone at Site FF are considered to be primarily derived from.the erosion of the grassy beach. This is shown from samples collected from this scarp which were composed of 97% sand containing pebbles 1-10 mm., in size. The beach.samples collected in front of the scarp, and the nearshore samples Next pages: Figure 5.17.(left) Location map showing Site FF. Figure 5.18 (upper right) Aerial view.of Site FF. Figure 5.19 (lower right) Typical profile of Site FF in October 1978. 5-30 P.1, ""A SDU vo Goose 'Cap@�Wohn Island, WeP@,nt 4p Porter Point Addi@son ((j top. - . ptf N., ne. 21 @I.Lv n 'L Boyd @Point Cedar 7 Pt SCALE, 1:240W 0 1 MILE 1001) 0 1001D 2000 3DOD 4000 5000 6m _7TO FEET r 0 1 KILOMETER F i gure .5.17 5-31 FF C.19 mw Ui@ ...... ... . gt, WIN "fr@ .......... 7,p moll Olt Figure 5.18 15 10 VEGETATION 14 IISITE _F7F 13 APPROX. MEAN 12 7 TIDE LEVEL 10 --LEFT STAKE 9 8 7 -CENTER STAKE LJJ 6 _j 5 3 3 0'< 4 3 -RIGHT STAKE 2 0 -16 0 10 20 30 40 50 60 70 PROFILE LENGTH (ft.) Figure 5.19' 5-32 SITE- FF KEPT 9TAKO INORT 9TAK tation approx. wder NOV. 3.1976 47 level DEC 1. 1978 DEC. 28.1978 FE&28.1979&--,,Ce Al@ ice MARCH 30.1979 MAY 2. 1979 JUNE 4.1979 JUNE 29.1979 AUG. 2.1979 SEPT. 13.1979 OCT. 17. 1979 OCT 31,1979 10 zo FEET Figure 5.20 5-33 contained 97% sand and pebbles rang ing from 1-5 mm. in size. Sediment samples taken from the scarp on the left profile contained less sand and slightly more silt and clay, (Table 5.1). The mean tidal ranqe at Site FF is approximately 1.0 foot. The only shoreline structure present in the area is a community boat pier and mooring area which includes a wooden bulkhead that extends 10 feet out and is approximately 150 feet downstream from the study site. This pier is not considered to have an.eff.ect on shore erosio n at the study site. The shorefaceof Site FF receives boat-wake energy from boats travelling up and down the South River. There is no speed limit restriction and most of the traffic is travelling at 6igh speeds.. The location of Beards Point results in relatively close passages of boats within 75-100. feet of the shoreline. Wakes from boats travelling upstream probably impact the downstream side of Beards Point in the vicinity of site FF more than wakes travelling downstream. Wakes were not measured at this site, but waves were observed to be undergoing refraction around Beards Point during the time profiles were taken. The shoreface at Site FF also receives wind-wave energy with the longest.fetches from the east and southeast. Regional winds from these directions c an generate opposite: Figure 5.20 Comparison of monthly prof iles collected at Site FF. 5-34 tly on Beards appreciable wave energy which would focus direc Point. Wind waves from the northwest will create the same type of refraction previously mentioned, and could have some effect on this site. The fastland boundary for all 3 profiles was defined as the edqe of vegetation which is also-the first pronounced change of slope. Profiles at Site FF were collected'monthly and a comparison between successive months is illustrated in Figure 5.20. The comparison shows that the small scarp at all three profile stakes'experienced slight changes during the year of study. The left profile stake in the cattail marsh experienced the greatest change during the passage of Tropical Storm David on September 8-9, 1979. The profile for September 13 at this stake shows a change in the.profile near the scarp, and erosion of sediments, when compared to the August 2 profile. There was also noticeable modulation of the sediments on the beach from month to month before the start of the boating season. But the changes in the location of the fastland boundary were minor at all of the stakes at this site.. 5-35 VI BOATING FREQUENCIES AND CHARACTERISTICS Robert J. Byrne, John D. Soon III Rhonda Waller, and Deborah Blades A. Introduction This chapter describes the f requencies of boat passes and other boating characteristics which were observed in front of those sites described in Chapter IV. At the beginning of the study these sites were known to be located in areas of Anne Arundel County which were popular for. boating, but there was no information available on the exact levels or patterns of boating which might be expected at each site. Since this inforynation is usef"ul in interpreting the behavior of the shoreline profiles during the boating season and at other times of the year, each of the five sites wasoccupied on a.daily rotating basis for several weeks during the summer of 1979 (Table 6.1), to inventory the boating characteristics and to measure the boat-wake energies which ar e discussed in the next chapter. Theresults presented in this chapter show there were markedly different frequencies of boat'passes at each of the five study sites, together wi.th different patterns of boat speeds, hull configurations, and distances of boat passes from the shoreline. The experiments with controlled boat passes discussed in Chapter VIII show specifically how these different characteristics can affect the wave heights (and Next pages: Table 6.1 Boating inventories at the study sites. 6-1 Table 6.1 Dates and Sites of Boating Inventory Total Averaqe Date Day Site Boats Boats/Hr. Weather 25 May @Fri A 98 14 26 Sat B 72 9 27 Sun C 208 26 28 Mon D 522 Navy Day 75 2� Tue E 1 0 Rain 30 Wed A 221 28 31 Thr C 5 1 Rain/Haze 1 June Fri B 18 3 2. Sat D 231 29 Rain 3- Sun E 9 4 Rain 4. Mon A 64 13 5 Tue B 206 26 6:` Wed C 107 13 1-'July Sun A 400 100 2. Mon B 67 10 3 Tue C 203 .25 4 Wed D 281 70 5 Thr E 35 5 6 Fri A 357 45 7 Sat B 511 64 8 Sun C 647 81 9 Mon D 174 22 10 Tue E 39 6 il Wed A 188 24 12 Thr B 149 19 13 Fri C@ 88 15 Haze/Rain 14 Sat D 397 50 1@ Sun E 120 15 16 Mon A 234 29 11 Tue B .106' 13 1-8 Wed C 149 19 19 Thr D 148 18 20 Fri E 62 12 Haze/Rain 21 Sat A 302 76 22 Sun B 537 67 2 J 90 13 Haze/Rain Mon C .24 Tue D 118 15 Rain/Clear 25 Wed E 38 6 26 Thr A 170 21: Haze/Rain 27 Fri B 15 28 Sat C 337 42 29 Sun D 438 63 30 Mon E 30 4 31 Tue A 171 21 6-2 Total Average Date Day Site Boats Boats/Hr. Weather 1 Aug Wed B 159 23 2, Thr c 118 20 3 Fri D 105 21 4 Sat E 106 13 5' Sun A 1505 188 6'. Mon B 107 13 7. Tue c 147 18 8 Wed D 127 16 9 Thr E 74 12 10 Fri A 187 23 11 Sat B 257 51 Haze/Rain 12 Sun No Data 13 Mon No Data 14 Tue E 46 6 15 Wed No Data Thr B 30 6 17 Fri c 75 9 18 Sat D 228 28 Clear/Rdin 19 Sun E 84 10 20 Mon E 49 6 21 Tue No.Data 22 Wed' c 85 11 23 Thr D 86 11 24 Fri No Data 25 Sat No Data 43 26 Sun B 215 27 Mon c 43 6 28 Tue D 44 6 29 Wed No Data 30 Thr E 28 7 31 Fri B 26. 4 1 Sept Sat c 28 -2' Sun No Data 3 Mon B 60 8 4 Tue A 15 2 5. Wed No Data 6- Thr No Data 7 Fri B 11 8' Sat E 30 .4 9 Sun D 38 5 10 Mon B 28 4 ii Tue c 32 5 12 Wed D 40 7 13 Thr E 0 0 14 Fri No Data 15 Sat No Data 6-3 wind qualifier cloud cover visibility H - C, q, S - steady 0 clear hate R - rain G - gusts I scattered F - fog C - calm 2 broken U - unlimited 3 overcast V - variable time I)oat boat speed (mph) hull length (feet) ype direction distance (feet x 10) 1 5 10 15 20 25 30 :12 14 16 18 22 26 30 P0S, YY:US DS UT 6 8 10 12 14 16 IS 20 22 24 28 32 36 40 44 48 2 S 10 IS 20 2S 30 :12 14 16 18 22 26 30 :pDS:YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 3 :.S 10 IS 20 2S 30 :12 14 16 18 22 26 30 :PDS:VY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 4 S 10 IS 20 2S 30 :12 14 16 18 22 26 30 :PDS:Yy:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 S S 10 is 20 25 30 :12 14 16 18 22 26 30 PDSYV @US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 6 S 10 15 20 25 30 :12 14 16 18 22 26 30 :PDS:YV:US bS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 7 S 10 IS 20 2S 30 :12 14 16 18 22 26 30 :PDS:YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 8 S 10 IS 20 2S 30 :12 14 16 18 22 26 30 :PDS- YY:US DS UT 6 8 10 12 14 16 IS 20 22 24 28 32 36 40 44 48 9 S 10 is 20 2S 30 :12 14 16 19 22 @6 30 :PDS:YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 10 5 10 15 20 25 30 :12 14 16 18 22 26 30 :PDS: YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 01 1 1 1 11 5 10 IS 20 2S 30 :12 14 16 18 22 26 30 :PDS:Yy:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 12 S 10 15 20 25 30 :12 14 16 IS 22 26 30 :PDS:YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 4 I 1 8 13 S 10 15 20 2S 30 :12 14 16 18 22 26 30 :PDS.'YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 14 S 10 15 20 25 30 @12 14 26 16 22 .26 30 PDS', YY;US DS U-T 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 15 5 10 IS 20 25 30 :12 14 16 18 22 26 30 :1 PDS:1YV:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 16 S 10 15 20 2S 30 :12 14 16 18 22 26 30 :P0S:YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 17 5 10 IS 20 2S 30 :12 14 16 18.. .2.2 26 30 :PDS.'YY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 18 5 10 15 20 25 30 :12 14 16 18 22 26 30 :P ()SYY:US DS UT 6 8 10 12 14 16 18 20 22 2-4 28 32 36 40 44 48 19 S 10 15 20 25 30 :12 14 16 18 22 26 30 :P0SYY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48, 20 5 10 15 20 @S 30 :12 14 16 1 @ 22 26 30 :P0SYY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 4 16 1: 22 26 30 'PDS:.YY:US D S UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 21 5 10 IS 20 2S 30 :12 1 22 S .10 IS 20 25 30 :12 14 16 18 22 26 30 :PDSYY:US DS UT 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 -P IS 20 22 24 28 32 36 40 44 48 23 5 10 IS 20 2S 30 :12 14 16 18 22 26 30 DS:YY :US D S UT 6 8 10 12 14 16 Figure 6.1 thus the wave energy) in wakes which break along the shore- line. The experimental data helps to explain why there is not a clear increase in boat-wake energy (discussed in the next chapter) at the sites.with the highest boating f requencies. B. Methods An inventory of boating activity was conducted at the study sites during an initial 13 day period (25 May - 6 June, 1979) and a following 77 day period (I July - 15 September, 1979)., The initial sampling design c alled for each s ite to beinventoried on each day of the week twice (i.e. 2 Sunda*ys', 2 Mondays, etc.).- This level of sampling strategy was determined by the fiscal constraints on the study; these clons.traints precluded anything other than a simple rotation of a single observer from site to site in a sequential series. An.observer categorized all boats passing at each site between 1000,hrs. and 1800 hrs. EDST. Figure 6.1 shows an ..example of the log sheet which was used on each day that observations were made. Each boat passing the study site was logged, in sequence, noting: o Time of day.. o Boat speed (estimated). o Rull' length (estimated). o Hull.type; displacement or planning. o Sailboat. 0220site.: Figure 6.1 The log sheet which was used to inventorT-boating frequencies and other characteristics at the profile sites described in Chapter IV. 6-5 o Presence of a skier in tow. o Direction: up stream, downstream or turning. o Distance: estimated or determined with range finder. On each of the days when boats were inventoried at a site,@the incoming boat-wake waves were also measured using a surface electronic wave gauge with a strip-chart recorder output (described in Appendix C). The wave heights were used to construct the wake-energy budgets, which are described for each'study site in Chapter VII. When boating activity and boat wakes were measured at a certain study-site, observations were also-made each hour of: o Wind speed and direction at the study site, using a wind-speed gauge and compass. o Cloud cover noted as clear, scattered, broken, or overcast. o Visibility: haze, rain,'foq? unlimited. o Position of*still water on the shoreline profile. o Position of the breaker zone and the upper and lower limits of the swash zone on the shoreline profile. o Hourly recording'of the wind-wave field using the wave gauge. C. Results A comparison of the boating sta tistics for all the sites is given in Table 6.2. Daily summaries of all the opposite: Table 6.2 Comparison of the boatinq statistics for all the study sites. 6-6 TABLE 6.2 Summary of Boating Characteristics and Boating Activity: Site Averages Expressed as a Percent Site Avg. Boats/Day Speed (mph) Hull Length (ft) Type Skier Sail Distance (ft) <10 20 30 >30 <16 22 30 >30 P D <100 200 '360 480 >480 A WD 170.5 29 47 23 1 32 32 24 12 75 25 14 7 12 34 8 0 46 WE 735.7 41 31 27 1 33 34 23 10 54 46 0 15 1 8 26 0 65 B WD 91.9 14 60 26 0 49 35 11 5 82 18 46 0 3 12 5 4 76 WE 344.2 9 78 12 1 45 45 9 1 91 9 45 0 3 7 4 6 so C WD 95.2 9 75 15 1 53 40 7 0 90 10 63 0 28 56 16 0 0 WE 326.2 11 76 13 0 49 39 12 0 87 12 57 0 36 47 17 0 0 WD 155.8 22 37 35 6 27 21 28 24 73 27 2 8 0 1- 2 6 91 WE/H 268.8 30 43 26 1 26 35 25 14 68 32 1 17 0 1 1 2 96 E WD 44.6 23 51 26 0 59 35 5 1 76 24 53 4 18 68 14 - WE 69'.8 97 2 1 0 63 20, 11 6 3 97 1 10 3 55 42 - WD = Weekday Averages; WE/H Weekend, Holiday Averages WE - Weekend Averageol P Planing; D Displacing boating characteristics at each site are presented in Tables 6.3 thru 6.7. SITE DESCRIPTIONS Site A is located on the vegetated spit along the lower South River near the entrance to Harness Creek. There are two distinct patterns of boating in the vicinity of this s@ite. One group of boats operates in a popular sailing area well away from the shoreline, out near the main'channel of thelower South River. Another qroup.of boats enters and,leaves a popular anchoraqe area in Harness Creek, and passes the study site at a much closer distance. Table.6.3 shows that the total frequency.of boat passes, and the portion of boats passing close to shore at Site.A can vary from day to day, and from weekday to weekend. The summary of data from all sites in Table 6.2 shows that, on the average, more than two-thirds of the weekend boat Passes at Site A take place well away from the shoreline out in the South River and a larger percentage of th.e'weekday boat passes at Site A take place nearer to the mouth of Harness Creek. 012posite: Table 6.3 Daily inventory of boating activity ror site A. .6-8 Table @.3 Inventory of Boat Activity, Daily Summaries Site A Date Day Total Boat Speed Hull Length Type Skier Sail Distance (1979) Boat (mph) (ft) (ft x 10) Passes <10 20 30 >30 <16 22 30 >30 P D <10 20 36 48 >48 25 May Fri 98 30 50 18@ 18 43 19 18 48 50 2 - 18 is 62 30 I-lay Wed 221 33 54 117 17 14 53 59 95 178 43 32 - 56 57 108, 4 Jun Mon 64 43 21 6 21 21 16 52 12 - 19 7 38 I Jul Sun 400 162 126 ll@ 43 118 78 61 200 104 2 - 21 124 1 247 6 Jul Fri 357 152 204 1 115 109 83 42 283 14 57 26 50 126 26 1 178 11 Jul Wed 188 26 126 30 2 87 58 27 10 171 2 38 12 17 86 34 46 16 Jul Mon 234 61 103 70 120 61 41 13 170 40 43 24 3 46 56 127 21 Jul Sat 302 130 123 45 4 75 126 96 6 165 97 2 43 is 108 2 152 26 Jul Thr 170 46 93 29 2 62 69 32 7 107 46 52 17 25 85 11 43 31 Jul Tue 171 50 58 ' 70 2 72 55 44 6 90 74 8 17 17 68 5 92 3 Aug Sun 1,505 616 436 431 22 573 479 318 153 653 572 4 285 1 37 454 1 1,021 10 Aug Fri 187 54 94 49 1 46 70 71 2 73 98 10 19 6 83 98 4 Sep Tue 15 4 9 2 4 6 3 3 15 15 7% of the weekday boat passes at Site A consisted of sailboats compared to 15% of the weekend boat passes. This observation partially explains the lower percentage of planing boats which were present in the weekend boating patterns. The average frequency of boat passes on weekends at Site A was 4 times as high as on weekdays. The average hull length and average speed were approximately the same on both weekdays and weekends. Most important, Table 6.2 shows Site A experienced more boat passes on both weekdays and weekends, on the average, than any of the other 4 study sites whose profiles. are described in Chapter IV. site B is located at the steep bank on the upper South River near Goose Island. This site is located along a relatively straight reach of shoreline, and in a relatively, sheltered portion of the upper South River. The site is not @near any popular anchorage or docking.facility, and is a popular running ground.for high-speed power boats and ski boats. This popularity is reflected in several of the stati stics.of boat use on Tables 6.2 and 6.4. Virtually no boats under sail were observed at this site, and almost 50% Opposite: Table 6.4 Daily inventory of boating activity at Site B. 67-10 Table 6.4 Inventory of Boat Activity, Daily Summaries Site B Date Day Total Boat Speed Hull Length Type Skier Sail Distance (1979) Boat (mph) (ft) (ft x 10) Passes <10 20 30 >30 <16 94 JU --30 P D <10 20 36 48 >48 26 May Sat 72 7 55 10 39 28 5 60 12 23 2 29 21 15 15 1 Jun Fri 18 11 7 2 12 4 12 6 1 1 1 1 14 5 Jun Tue 206 62 144 1 4 90 101 11 195 11 80 9 11 14 4 168 2 Jul Mon 67 13 53 1 .13 39 13 2 58 10 32 67 7 Jul Sat 511 60 447. 8 63 379 92 10 508 5 191 3 22 30 12 31 419 12 Jul Thr 149 12 127 10 132 83 4 4 147 1 91 5 16 3 .4 121 17 Jul Tue 106 4 101 1 61 39 6 105 71 3 1 1 4 97 22 Jul sun 537 40 420 71 6 359 167 25 1 493 60 287 1 3 31 10 21 490 27 Jul Fri 104 4 88 12 61 39 4 96 7 55 4 9 5 5 82 I Aug Wed 159 3 62 94 110 31 2 37 127 19 87 2 20 8 10 107 6 Aug Mon 107 17 73 26 1 67 48 84 34 53 3 12 2 6 94 11 AuL3 Sat 257 14 162 79 2 181 77 200 46 121 10 14 8 20 209 16 Aug Thr 30 3 23 4 16 14 22 8 12 1 3 1 25 26 Aug Sun 215 8 42 162 3 184 30 6 5 176 50 97' 1 26 12 16 184 31 Aug Fri 26 3 5 16 2 8 15 1 2 10 16 4 1 7 2 4 13 3 Sep Mon 60 36 24 13 26 6 14 61 4 2 45 7 7 Sep Fri 11 2 1 8 1 9 1 8 .3 5 10 1 10 Sep Mon 28 4 21 2 13 11 4 19 9 8 3 2 3 2 18 1001, of the total number-of boats inventoried were pulling skiers both-on weekdays and weekends. About 90% of all boats inventoried had speeds which were estimated at 10 mph or moret and lengths of 22 feet or less. Between 75 and 80% of the boats remained mo re than 480'feet from the shore, probably because of shallow depths and obstructions (tree trunks) near the shore at Site B. @Table 6.4 shows that Site B had, on the average, the second-hiqhest frequency of boat passes (next to Site A) on the,weekends. The averaqe number of boats on the weekends was.slightly less than 4 times the average number of boats on weekdays.. .Site C is located at the small promontory on Broad Broad,Creek is Creek near the entrance.to the South River. comparatively deep along both shores and relatively straight. Like Sites A and B, considerably mote boating activity occurred on weekends as compared to weekdays. Table 6.5 shows Site C experienced the second largest. boating frequency on weekdays of the five sites, and some 63% of its average 95 boats per day were pullinq skiers. Table 642 shows about 93% of the weekday boats were 22-feet or less in length and ab out 90% had speeds exceeding 10 mph. more than 80% of all boats observed passed within 200 feet, Opposite: Table 6.5 Daily inventory of boating activity at Site C 6-12 Table 6.5 Inventory of Boat Activity,- Daily Summaries Site C Date Da y Total Boat Speed Hull Length Type Skier Sail Distance (1979) Boat (mph) (ft) (ft x 10) Passes <10 20 30 >30 <16 22 30 >30 P D <10 20 36 48 >48 .27 flay Sun 208 27 147 34 63 92 51 2 172 36 73 131 77 30 May Thr 5 6 Jun Wed 107 29 74 2 2 @30 63 14 87 20 so 45 56 6 3 Jul Tue 203 13 189 3 23 119 52 201 3 145 90 71 17 4 a Jul Sun 647 69 542 33 3 366 174 93 3 638 9 400 147 228 191 1 13 Jul Fri 88 6 75 8 69 22 91 70 5- 71 22 18 Jul Wed 149 7 133 7 2 8 5 63 145 4 97 55 64 31 .13 Jul Mon 90 7 74 4 5 64 25 1 55 34 51 20 61 6 28 Jul Sat 337 34 249 54 143 184 8 241 90 204 121 202 9 2 Aug Thr 18 6 88 23 53 65 94 24 89 25 78 5 7 Aug Tue 147 13 121 12 106 35 6 97 50 98 48 95 4- 17 Aug Fri 75 6 35 34 69 4 2 63 12 47 4 35 48 22 Aug Wed 85 9 18 58 73 12 74 11 48 5 47 31 27 Aug Mon 43 3 20 18 2 15 25 2 1 33 10 10 30 6 I'Sep Sat 113 9 57 44 3 54 .52 7 89 24 65 37 64 11 11 Sep Tue 32 1 23 6 2 6 18 6 22 10 15 7 27 of the shore and roughly 30% came within 100 feet of shore. Thus Site C is clearly a site in which a high level of activity was concentrated very near the shoreline being. monitored. Site D is located at the bluff near Severnside on the northern.shore of the Severn,River. This site is the most exposed of the five sites towind-wave activity. Compared to the sites discussed for the South River, weekend boating activity was not greatly in excess of that observed during weekdayst even though the Fourth of July holiday was included among the former. Table 6.2 and 6.6 shows that very little skiing was observed at this site and boating charact- eristics were rather mixed with a broader distribution of boat sp eeds and lengths than at other sites. This was not unexpected.in view of the close proximity of Site D to the por t-of Annapolis,,a major center for yachts of all types. It is also apparent in Table 6.6 that most of the boating activity occurs well out in the Severn. Most of the traffic appears to betransiting to and from the Bay. Site E is located at.the pocket marsh inside a cove that opens to the Severn River. The site is well protected, Opposite: Table 6.6 Daily inventory of boating activity at Site D. 6-14 Table 6.6 Inventory of Boating Activity, Daily Summaries Site D Date Day Total Boat Speed Hull Length Type Skier Sail Distance (1979) Boat (mph) (ft) (ft k 10) Passes <10 20 30 >30 <16 22 30 >30 P D <10 20 36 48 >46 28 May Mon 522 34 181 243 69 16 74 177 249 374 138 5 522 2 Jun Sat 231 99 127 5 24 58 68 91 163 68 1 6 225 4 Jul tied 281 90 129 62 67 184 58 32 175 46 2 65 1 5 1 1 273 9 Jul Mon 174 47 86 40 77 45 41 13 144 5 17 1 173 14 Jul Sat 397 92 171 130 4 178 121 46 46 244 72, 1 25 7 4 385 19 Jul Thr 148 69 61 18 73 30 42 1 86 21 1 @41 2 59 29 Jul Sun 438 96 161 155 15 102 152 170 14 209 146 3 88 1 2 3 444 3 Aug Fri 105 29 36 39 1 55 15 15 19 48 39 6 17 5 7 1 90 8 Aug Wed 127 36 47 41 45 37 41 4 72 41 2 19 3 1 3 119 18 Aug Sat 228 104 85 36 1 53 55 65 49 72 70 10 86 2 2 224 23 Aug Thr 86 .29 26 29 2 35 25 15 11 48 20 18 1 85 28 Aug Tue 44 9 8 19 7 12 24 4 5 24 1 3 13 5 22 q Sep Sun 38 6 9 21 1 4 25 5 3 16 18 4 3 2 10 16 10 12 Sep Wed 40 15 12 1-1 2 14 12 14 21 12 7 3 37 the nearshore zone is muddy and extremely shallow at low tide in the vicinity of the monitoring.site. In contrast to the other sites, Site E experienced minimal boating activity during both weekday and weekend observation periods. Roughly 60% of all boats inventoried had lengths of 16 feet or less. On weekends, 97% of all boats travelled at speeds of 10 mph.or less. Skiing activity accompanied about 53% of weekday boating, but dropped to about 1% during weekends. This is an-indication that the weekend skiing restriction is being respected by the boaters.. Opj2osite: Table 6%7 Daily inventory of boating activity at Site E. 6-16 M*W0 mom an Table 6.7 Inventory of Boating Activity, Daily Summaries Site E Date Day Total- Boat Speed Hull.Length Type Skier Sail Distance (1979) Boat (mph) (ft) (ft x 10) Passes <10 20 30 >30 <16 22 30 >30 P D <10 20 36 48 >48 29 Mav Tue I 3 Jun Sun 9 8 1 3 3 2 2 7 2 6 1 5 Jul Thr 35 18 17 28 6 1 26 4 6 6 3 11 22 10 Jul Tue 35 6 32 1 15 21 3 38 1 29 - 1 35 3 15 Jul Sun 120 116 3 1 82 23 8 7 4 111 4 5 5 23 91 20 Jul Fri 62 6 39 15 37 20 59 4 36 2 26 34 1 25 Jul Wed 38 7 25 6 34 1 4 29 9 28 - 34 4 30 Jul Mon 30 3 1 25 6 2 17 13 13 3 1 30 1 4 Au-, Sat 106 104 1 1 56 28 16 7 2 65 1 21 3 85 is 9 AuT, Thr 74 9 42 23 15 55 4 49 23 41 2 4 66 4 14 Aug Tue 46 11 22 13 31 13 2 33 11 29 2 1 38 7 19 Aug Sun 84 80 3 1 61 13 5 6 2 77 5 1 57 27 20,Aug Mon 49 13 10 26 42 4 2 1 30 17 26 2 33 16 30 Aug Thr 28 4 '6 18 8 17 2 2 16 11 6 1 5 20 3 8 Sep Sat 30 30 16 6 8 27 3 19 9 13 Sep Thr 0 VII COMPARISON OF BOAT-WAKE AND WIND-WAVE ENERGY BUDGETS Robert J. Byrne, John D. Boon III, Rhonda Waller and Deborah Bl,ides A.. Introduction This chapter presents a comparison between the wind-wave energy at each of the study sites described in Chapter IV for the year of observations (October 1978 thru October 1979), and the wave energy in boat wakes .during the summer of 1979. This information was produced as part of the study.in order to interpret the seasonal P11- appearance of the shoreline proftles at eazh of the study sites. A discussion of the association.between the wave energy budgets and the fastland response is contained in Chapter IX. A relatively easy Way to.compare the-potential for shore erosion from boat wakes and.win.d.waves is to. compare the wave energies from each source. Wave-energy is simply proportional to the square of the..wave heights. .However, a singlevalue of the magni,tude.of..wave.energy within a qiven hour does not explain how that energy.may have been distributed within that hour. For example, a few large waves-in an othe,rwise.calm hour,would..co.ntain the same energy as a greater number of smaller waves. during the hour..Even a very.small wave of 0.1 foot is capable of moving sand when it breaks.on the beach, but its zoneof influence on.the shoreline profile is smalli 7-1 A "Larger wave, say 0.5 foot, has the capacity to move more sand per unit area over a larger area. For this study, the field measurements were used to construct models of the total enerqy contained in boat wakes and wind waves.. In spite of the fact that information on the individual waves is lost when the boat-wake and wind-wave energy budgets are.drAwn., the e,<pression of energy provides an index for the capacity of boats and the wind to do work oft the shoreline profile. Methods It is important to realize.,that.thevalues presented for total wind- and boat-wake energies-are estimates. A complete portrayal of the wave energy at each site would have required continuous measurement of the waves at each site which was well beyond the scope of the,present study. The principal steps involved in the calculation of the boat- wake energy, budget were; 1.) Develop for each site the regression relationship between hourly boating frequency and total boat-wAke energy per hour. This relationship allows.the simple estimation of hourly wake energy from the hourly boating frequency. 2.) Establish the duration of the boatinq season. This was assumed to extend from 15 May throuqh 15 September. The data obtained in the boating inventory (Chapter VI) indicated a dramatic decrease in boating afterabout 20 Auqust. Thus two levels of boating activity were assumed to apply:. a high level between 10 June and 20 August, and a lower "transition" level between 15 May 9 June and between 21 August 15 September., 7-2 3.) Establish the average hourly boating frequency for both- weekdays and weekends at each site. This was achieved by separately averaging, at each site, the weekday and weekend hourly boating frequencies observed during the inventory of boating.activity. In order to describe the higher levels of activity, the values and the averaqing was restricted to those observations between 10 July and 20 August. The transition periods (15 May 9 June and 21 August -15 September) were assumed to contain one-half thehourly boating frequency described during July and August. 4.) For the purposes of computation, the period of boating activity each day was taken as 8 hours. This is reasonably consistent with the observations that most boating occurred between mid-morning and very late afternoon or early evening. 5.) Following steps (1) through (4), the wave energy due to boat wakes was then calculated on.a monthly.basis.and also for the periods between surveys of the shoreline. i., Boat Wake Energy Calculations Analyses from the initial 13-day observation period indicated that the hourly boat wake energy was linearly correlated with hourly boat frequency at each of the five sites (Figures 7.1 to 7.5 When boat traffic was light, the signatures of individual boat passes could be discriminated in the record. In these cases, the hourly boat-wake energy was simply the sum of the energies in individual boat passes during the hour. When the boat traffic was.so heavy that it was not possible to discriminate individual boat passes, the wave recorder was turned on for 15 minutes-each one-half hour so that battery energy would be conserved, thereby insuring the capability of the instrument to measure waves throughout.the day.. In these situations, the hourly wave energy was, calculated as a 7-3 multiple of the wave enerqy contained wi thin the 15 minute segements. But the frequency of boat passes and other characteristics of each passi nq boat were still continuously recorded on the log sheets. The actual enerqy of a wave is directly proportional to the square of the wave heiqht. The total energy contained An each boat wake reaching the shore was calculated as: EB fqT4 li 2rms 7 8 where N Number.of boat waves recorded Hrms Mean square wave height H2/Ni) 1/2.&1 i=.lr2 F N /Pq = Specific gravity of water = 62.5 lbs/ft3 The enerqy given by equation 7.1 requires an adjustment for background enerqy due to wind waves if any are present. Wind-wave enerqy contributions were determined from samples of wind waves taken during the absence of boats approximately at the beginning of each hour. Using equation 7.1 together with the number of wind waves present in the sample, an energy IICwI' is calculated as an'estimate of the wind-wave energy contribution during subsequent boat-wake events- CW = Ew,&t,3/ Atw 7.2 where tb = Time duration of recorded boat wave event tw Time 'duration of wind wave sample 7-4 The adjusted individua.l.boat-wa ve energy is therefore EBI ='EB - Cw. Tf two or more boat-wave trains were encountered at one time, the resultant waves are treated as a single event. ii. Regression Analysis between Wave Energy and Boating Frequenc For the levels of boating activity-and boat-wake energies-which were collected at the study sites, a line of best fit to the data collected at each site was calculated using linear regression through the origin (Table 7.1, Fiqures.7.1 thru 7i.5).. Regression through the origin is required since boat-wake energy must approach zero as the number of boat passes approaches zero. The model for this regression is:. EH f H+ F- 7.3 where EH Total boat energy for a given hour fH Frequency of boat passes during the hour Regression coefficient Deviation from regression opposite: Table 7.1 (top) Results of regression analysis: hourly boat-wake energy as a function of hourly boating frequency. Figure 7.1 (bottom) Regression curve for boating and wake-enerqy at Site A. next pages: Figures 7..2 to 7.5 Regression curves for boating and wake energy at Sites B thru E. 7-5 Results of Regression Analysis: Hourly Boat Wake Energy as_& Function of Hourly Boating Frequency, E. - b f H' Site Regression Equation Confidence Interval sample Staimlard Estimate on Regression Deviation from Coefficient Regression, S E-f A 'H - 4.89 fH 0.73 3.84 iH ' 3.24 fH + 0.71 7.47 13H . 2.24 f 0.46 3.69 H B EH - 2.92 fH + 0.47 5.98 C iH - 15.78 f H + 1.47 22.95 D iH - 10.64 f H 1.14 18.86 E iH - 2.27 f H 0.34 2.95' RELATION BETWEEN HOURLY BOAT PASSES AND WAKE ENERGY AT or SITE A FOR SUMMER OF 1979 Cr C3 z w w Uj cr to CO V.-w- ib.w ab.oc ab.w a vbC1 *.Go 85aT ArREGLirlic7IN g.u r-e. 7-6 coo, RELATION BETWEEN HOURLY BOAT PASSES AND WAKE ENERGY AT SITE B FOR SUMMER OF 1979 cr C3 0 ts ir Lo dr cc at Figure. 7.2 I RELATION BETWEEN HOURLY BOAT a PASSES AND WAKE ENERGY AT 4 SITE C FOR SUMMER OF 1979 cr a: C3 z C) U% UA 9c L2 Uj w L3 'Ia... own Figure 7.3 7-7 9 RELATION BETWEEN HOURLY BOAT PASSES AND WAKE ENERGY AT SITE D FOR SUMMER OF 1979 tr .9 C3 x R tn cc C.3 Ln lo.m wdo lb.so tb -01 Ual A.= Wn 06.0 so'k T11ble, PIC Gouk"'. c Y Figure 7.4 RELATION BETWEEN HOURLY BOAT PASSES AND WAKE ENERGY AT cr cr SITE E FOR SUMMER OF 1979 ca z V w cc Uj uh it... ME. Fi gure 7.5 7.8 For the purposes of this study, an extended form of this model was constructed where the variance of EL is assumed to be directly proportional to the value of N. Using the assumption, 19 is,then-estimated by the sample regression coefficient "b", which is' computed as "b" E E H/ fH H/fH- obviously, the use of the model for obtaining daily boat-wake energy assumes that the mixtures of boating characteristics remains consistent throughout the boating season, and that the s.ample'data are unbiased. The primary benefit of the model is that it enables the prediction of boat-wake energy to extend to all days during which the fundamental variable of boating frequency had.been measured. Reqression,ana.lyses were performed for each site using samples from.the data acquired throughout the boating season. The results of the regression analyses are, presented in Table..7.1 and shown in Figures 7.1. through 7.5. Table 7.1 conta ins the indiviudal regression estimates for each.site, the sample standard deviation from regression @(S and a confidence.interval estimate on the E'f Tegression coefficient (Sb t.05) using Student's 'It" at an alpha level of 0.05. Multiple regression equations were developed for Site A since this site was,exposed to wave -energies arising from boat traffic using both Harness Creek and South River. Table 6.2 indicates that on weekends approximately 65% of the passes were associated with South River traffic, but the 7-9 level dropped to 46% on weekdays. The three regression lines described in Figure 7.1 (and Table 7.1) represent conditions reflecting the different proportions o f.'traffic on South River. The upper curve (July '79) was derived from data samples on the 6th, 11th and 31st of July when only about 25% of the passes were in South'River, and thus reflects the energies.derived from boats relatively close to shore. Thelowest regression line (may 179) represents sample from two day s in late May when the majority of boat passes were in the South River and the resulting wave energies reaching the site were relatively small. In the calculation of total boating energy during the boating season (which will be discussed shortly) the regression relation EH = 2.24,fH was usedfor weekend days,and the combined" regression (Figure 7.6) was used.to approximate the relationship for weekdays. iii.I.Average Hourly 8oating Frequency and Wave Energy Due to Boats The regression analyses discussed above enable the hourly boat-wave energy to be estimated from the hourly boating frequency At each site, a number of weekdays and weekend days were inventoried during the boating season (Table 6.1). For each of these days an average hourly boating frequency was calculated (Table 6.1 and Figure 7.6). Examination of Fiqure 7.6 shows that during the latter part of August and during September the average hourly frequency 7-10 had diminished relative to the mid-summer period. Althouqh there are fewer observation days in late May and early June, there is also a suggestion that the average hourly frequencies were also less in this early part of the boating season. These periods of diminished activity can be predicted since the local schools recess for summer in early June and -return in late August. For the purpose of estimating the average hourly timid-summer" boating frequency at each site, the period between 10 June and 20 August was used. The weekday and weekend hourly boating frequencies.were separately averaged. It was then assumed that the "transition" periods were characterized by one-half of the respective "mid-summer" levels. The boatinq season was assumed to start on 15 May and to end 15 September. Thus the transition periods were 15 May-to-9 June and 21 Auqust-to-15 September. The average .hourly boatinq frequencies so derived are listed in Table 7.2. The total wave enerqy in any monthly period (or profile period) isestimated by the hourly wave energy at each site opposite: Figure.7.6 Graph of average hourly boating frequencies at the five study sites. 7-11 AVERAGE HOURLY BOATING FREQUENCIES 00 6 -*&188 Lij 4 *,67 51 U) Li U) 11SITE:EJ 0 5 &0* 3 co 1 ISITE LLJ < LIJ 5 0 JISITE 40- <> 30- 20- 0 MAY SEPT. *WEEKENDS AND/OR HOLIDAYS 'Figure .7..6 7-12 TABLE 7.2 Comparison.of Wave Energies for Year and Boating Season YEARLY SUMMARY BOATING SEASON Rank Wind Wave Boat Wake Percent Rank *Wind Wave Boat Wave Rank Percent % Wind Wave Wind Boat Wave Wind 2) Boat Boat Wave in Boating Site Wave Energy ft-lbs/ft2) Energx Wave Energy (f -lbs/f t Wave Energy Season A 2 5,450,816 118,100 2.2(l) 2 2,651,.077 118,100 3 4.4 (2) 49 (3) B 3 4,133,173 70,680 1.7 3 1,938,797 70,680 4 3.6 47 C 4 3,823,991 376,040 9.6 4 1,847,511 376,040 1 20.4 48 D@ 1 6,969,310, 247,660 3.6 1 2,948,847 247,660 Z 8.4 4Z E 5 3,181,249 15,650 0.51 5 1,75 6,115 15,650 5 0.9 55 summed for entire months of May through September, 1979 (1)Percent Boat Wave*Energy = (Boat Wake Energy year total wind wave energy) x 100 (2 Percent boat wave energy - (boat wake energy boat season wind wave energy) x 100 (3) Percent wind wave in = (wind wave in boating year total wind wave energy) x 100 boating season season Multiplied by the number of days (weekdays and weekend days) and by the number of boating hours per day (which was assumed to be 8 hours). C. Results The values for wave energy from wind waves and boat wakes for the year and for the boating season are shown in Table 7.2, together with the relat ive magnitudes. With respect to wind-wave activity, the sites rank (in decreasing order) D, A, 8, C, E for both the total year, and the' 19 79 "boating season". The sites range C, D, A, B, E with respect to bo.at-wake energy. Site C exhibited the hiqhest (20.4%) percentage'of boat wake e-nerqy during the boating season. Note that Figure 7.7 shows boats were not the principal source of wave energy at any of the shoreline.sites during the summer.. months. Nearly one-half (42-55%) of the total, annual wind wave energy occurred during the boating season (May-September). Monthly summaries.of the wind- and boat-wave energies' are given in Table 7.3 and Figure 7.7. A summary between opposite: Table 7.2 Comparison 'of Wave Energies for the. Year and Boating Season. next pages Table 7.3 Wind-wave and boat-wake energy budgets. at each site between prof ile.periods". Table 7.4 (left) Wind-wave and bo.at-wake energy budgets at each site by month. 7-14 Table 7.3 Vlind--Wave and Boat--- lake Ener 2 ;7,,r - - erioF@ ft-lb /ft Profile P Period S I T E A B c D E 10/29/78- 357,845 361,120 241,211 390,062 227,638 11/25/78 11/25/78- 397,659 292,156 .223,281 409,392 172,665 12/20/78 12/20/78- 501,347 .388,154 350,278 1,0_69,687 219,930 2/3/79 2/3/79- 36,737 25,469 244,837 512,135 28,905 3/10/79 3/10/79- 637,492 526,448 383,118. 69.0,178 334,576 4/15/79 4/15/79- 571,142 460,157 435,388 651,301 381,474 5/25/79 5/25/79- .529,469 297,318 348,538 556,232 297,539 6/23/79 Boat 43,080 14,160 83,850 56,240 3,560 % 8.1 .4.8 24.1. @10.1 .1.1 .. 1 6/23/7�- 538,453 400,853 374,921 610,303 347,469 7/28/79 Boat 56,260 23,240 137,080 90,750 5,720 % 10.4 5.8 36.6 14.9 1.6 7/28/79- 369,195 310,968 236,258 408,779 220,484 8/18/79 Boat 32,030 13,3.00 78F880 53,280 .3,370 % 8.7 4.3 33.4 13.0 1.5 8/18/79- 578,233 426,361 411,982 670,357 437,187 9/15/79 Boat 17,820 9,400 65,400 36,500 2,300 % 3.1 2.2 15.9 5.4 0.5 9/15/79- 721,058 518,193 451,789 770,309 422,181 L 10/20/79 % Boat Energy Boat Ener gy Wind Wave Energy x 100 7-15 Table 7. 4 Wind-Wave and Boat-Wak,-2 Energy ft-lbs/ft4; bv Month Month S I T E A B c D E Nov,'78 368,876 382,030 243,274 401,653 231,533 Dec,'78 568,327 397,942 302,422 580,851 232,657 Jan,'79 286,499 2201384 203,256 729,965 13i,259 Feb,'79 0 0 241,940 459,637 0 Mar,179 426,045 290,674 284,571 582,364 240,740 Apr,119 449,694 436,867 282,900 50l'OP979 244,569 May,179 494,260 373,589 550,673 327,697 *Boat 9,200 4,750 28,450 19,740 1,250 % Boat 1.9 1.4 1.6 3.6 .04 June,179 525,098 297,350 367,197 572,675 320,8 64 Boat 31,200 25,170 96,980 65,120 4,110 % Boat 5.9 8.5 26.4 11.4 1.3 July,'79 453,541 377,932 303,306 512,831 286,220 Boat 37,700. 19 1 790 117,250 78,940 4,990 % Boat 5.2 38.7 15.4 1.7 Aug,'79 591,475 443,806 407,363 663,395 30,749 Boat 30,300 15,840 94,180 64,130 4,060 % Boat 5.1 3.6 23.1 9.7 .1.0 Sep,179 586,703 486,024 3961,056 649,273 4.23,585 "Boat 9,700 5,130 30,180 19 730 1,240 % Boat 1.6 14 7.6 3.0 0.3 Oct,179 697JP298 466,529 418,117 764,014 344,376 Boating Energy Based on 15 May .31 May Boating Energy Based on 1 Sept. 15 Sept. VBoat Energy Boat Energy Wind Wave Energy x 100 7-16 profile period's is given in Table 7.2. The zero entries for wave energy at Sites A, 3, and E during February, 1979 represent ice-bound conditions. A relatively strong contribution from boat-wake energy at Site C is shown-in Figure 7.7. In July'1979, boat-wake energy was 38.7% of the wind-wave enerqy (27.9% of the total wave energy (Figure 7.7). It is of interest to compare Sites B and C. Both sites were subject to essentially the same levels of wi nd energies with the same percentage of that activity occurring during the boating season (47-48%).0 Inspe ction of Table 6.2 indicates-the two sites have very similar levels.of boating activity and about the same ratios of planning versus displacement hulls. Site C had a somewhat higher percentaqe.of water-skiing activity (45% versus 60%). The major difference in the boating activity at the two sites was the distance of the boat passes relative to the shore. At Site B about 80% of the boat passes were at distances next pa2e;. Figure 7.7 Histograms of monthly wave energy.* Open boxes represent wind-wave energy and blocked boxes represent boa t-wake energy. The values entered above the boat-wake energy represent the fraction of boat-wake energy relative to the total (wind plus boa t) energy for the month. 7-17 mmmm Mm Mao m M M MIM 5 MONTHLY WAVE ENERGY (Xio FT-LBS/SQ FT to 4c cr OD 3 7 2Cb co ch to Cd IND to 03 to greater than 500 feet whereas at Site C over .80% of the boat passes were at distances less than 200 foet. it thus appears that the close proximity of passage at Site C is the principal cause of the relatively hiqh boat-wake enerqies. in addition, the steeper nearshore botto@n qradient at Site- C results in less frictional influence on the incoming waves. 7-19 VIII WAVES CENERATED RY PASSAGE OF A RoAT Robert J. Byrne, John D. Boon III, Rhonda Waller,-and Deborah Blades A. Introduction This chapter presents the results of a modest experiment conducted at one of.the study sites to understand the behavior of wakes produced by boats cruisinq at different speeds and distances from the shoreline. As a boat passes over the water's surface, part of the energy transmitted by the craft's propulsion unit is taken up by the water in the form of surface waves..Thus the wakes are a manifestation of the resistance offered by the still water to the deformation caused by the boat's hull. The earliest studies of the waves caused by ships were conducted from the viewpoint of how waves effect the resistance of a ship (Froude, 1.881; Kelvin,'1887). More recently, attention has-been devoted to the relation between ship waves and.the stability of banks on thewaterways. 'through which the boats pass.(Johnson, 1957; Das, 1969; Sorenson, 1967). The pattern of waves in a boat wake depends partially on the value of the Froude.number "F"* (which- is the ratio *The Froude Number is not directly measurable, but represents the ratio of two variables which often become "lumped together" in theoretical wave-energy equations. The Froude Number "F" is the ratio of the boat speed 'Vs" and the speed "C" of a wave in shallow water. The wave speed is in turn a function of the basin depth "d", since C = 19 ("g" is the acceleration due to qravity; 32 feet/ sec.2). So... Froude Number "F"= Vs/ 8-1 between the boat speed "Vs", and the speed "C" of a wave in.shallow water). Both the Froude number and the configu- ration of a boat hull influence the maximum wav e height which will be experienced at a given distance from the sailing line of the boat. A displacement hull will generate a series of waves at the bow and stern (Figure 8.1). At values of "F" below 11 the wave pattern in the vicinity of the boat, together with, the maximum wave height, can change fairly dramatically as ,the wake travels away ftom the boat.' Each set of waves produced at thebow and stern include a series of' waves diverging from the sailing line and a series of transverse waves which move in the direction of boat passage. The intersections of the transverse and diverging waves are points of higher wave heights where breaking waves are most likely to occur in the wake. iThese "cusp" locations may be connected to form a locus, of cusps which define an angle."W" which the wave front makes with the sailing line (Figure 8.1). The theoretical development of Kelvin (1887) predicts a value of jO 19928' for Froude number values less than 0.7 and for values greater than about I. However, for intermediate "F" values, the angle ",V approaches a maximum of 90* when-"F"=I. At this point thetransverse and diverging waves combine.to opposite: Figure 8.1 (top) Schematic drawing of waves genera by moving boat.. ,Figure 8@2 (bottom) Definition sketch of boat- wake packet. 8-2 DIVERGING WAVES TRANSVERSE WAVES SAIL NG LINE WAKE WAVES11 Fi gure 8.1 av 1 2 + 3. 4 t (elapsed time) Figure 8.2 8-3 form a sinqle wave with its crest normal to the sailinq l1ne. Besides the angle 11, the maximu m wave height (and thus,tota.1 en erqy) in the wake wave "packet" varies with the Froude nUmber.. A typical boat-wake wave packet is shown schematically in Figure..8.2. Within the packet there is a single wave with maximum:N@ight. Results'of some experiments with boat models in a towing tank (Johnson, 1957) are shown in Fiqure 8.3 to illustrate the n,onlinear behavior of IIH max " with Froude number "F". After passing the criticalvalue of "F"? the values of "Hmax" tend.to approach a constant value. B. Field Measurements of Controlled.Boat Passes The experiment was conducted at Site C (Broad Creek), using boats operated by the Maryland Department.of Natural Resources Marine Police. Two boats were usea: a 26 ft. Uniflite cruiser (Marine Policeboat "Somerset#'), and a 16 ft. Boston whaler. The Uniflite is a deep-V pl aning hull,while the Boston Whaler is a 3- point plan-ing hull. Replicate passes were made at distances of 206, 150, and 100 ft. (also 50 ft. in the case'of the Whaler) from the shoreline for a range of speeds between 6 and 30 oj2posite: Figure 8.3 (top) Maximum wave height as a functionof Froude Number for typical ship model (Johnson, 1957). Fi5ure 8A (bottom) Typical record of boat wake passing the wave gage in shallow water. 8-4 .16 0 8 A%h E-04 oir 0 40 0 vs 467 FF Figure 8.3 WAVE GAUGE RECORD (26ft. UNIFLITE CRUISER) 3 WAVE HEIGHT (f t.) 0 9b 6b 3o b, TIME (sec) Fig ure 8A 8' knots. Boat speed was determined by measuring the time for the boat to travel between two buoys anchored 100 ft. apart. The surface wave gauge (described in Appendix 4) was located approximately 24 feet from the shoreline in a water depth of about 2.2 feet. With very rare.exception.none of the waves in the generated trains broke seaward of the wave gauge.' position. A typical wave record produced during the trial runs is shown in Figure 8.4. The results of the experiment are shown in Tables 8.1 and 8.2. In these calculations several different parameters are of interest. These are: Rmax" = the highest wave of the group (measured in feet). V = the average wave period (defined as the number of waves divided into the duration of the wave packet). "E" average enirgy per unit surface area (ft-lbs/f,t ?gH2ms, where "Hrms" H2/N.i)1/2 8 r ET total ener2y in wave train (ft-lbs/ft 1 2 - rms 8 11F11 1.667 Vs/ /gd for boat speed in knots. The results of the trial runs are graphed in Fiqures 8.5:thru 8.12. In the plot of "Fg' versu.s"'Hmax" (Figures 8.5. and 8.6), thereis an apparent peak in "Hmaxl@ at values of "F" betwe-en 0.8 to 1.0. A definite relation of 11H max" to the distance of the boat from the shore is. also 8-6 apparent, and this relationship is stronqer'for the deep-V hull. Figures 8.7 and 8.8, which are of boat speed versus "Hmax"., offer a 'simpler- il lustration of how 11H maxog varies with different boat speeds. As expected, the deep-V hull of the 26 ft. cruiser qe nerated the larger, waves. The largest "Hmax" occurred for speeds between 8 and 10 knots when the cruiser was in the displacement mode', and the "Hmaxll.values ranged'between 1.25 and 1.75 ft. for the distances tested. These values far exceed those which. were.expected for wind-generated waves. It is of interest-to note that the dependence of, 11H (or energy, Table 8.1) upon.boat speed is highly max nonlinear. For the circumstances tested, "H max" varies in a nonlinear fashion.with.the inverse of speed. The "Hmax" for the Uniflite cruiser decreased as the-boat speed was increased beyond a "critical" value of"8 to 10 knots. In the case of the 16 ft. planing hull of the Boston Whaler, the "critical" speed occurred between 6 and 8 knots when the, Whaler was in the displacement mode. For distances of 100 to,200 ft. from the shoreline, the' data in Figure 8.5 show there is little dependence,for either the Whaler or the Uniflite Cruiser. between ."H max. and thedistance of the boat from shore. However, at the. closer boat passes of 50 ft.,.the range of "Hmax" is dramatically increased. Again the nonlinear dependence of next pages.: Tables 8.1.and 8.2 Summary of observations of controlled boat passes. 8 7, Table 8. 1 Summaryrof Obser .vations: 26 ft. Uniflight Cruiser Run Speed Distance Number of H m ET Duration :F (ft.) Waves 2 2 MPH Knots (ft.) (ft-lb/ft (ft-lb/ft --t (sec) (sec) (V in knotl 11 28.4 24.9 200* 15 0.58 1.09 16.35 34.2 2.3 2.04 12 31.0 27.3 200 15 0.58 1.08 16.20 33.0 2.2 2.23 13 23.5 20.1 200 17 0.74 1.36 23.12 39.0 2.3 14 20.0 17.6 200 0.85 J.41 26.79 41.-1 2.2 1.44 15 8.6 7.6 200 15 1.30 2.75 41.25 38.1 2.5 0.62 16. 10.2 9.0 200 15 1.8.1 4.71 70.65 45.0 3.0 0.74 17 6.3 5.5 200 17 0.56. 0.48 8.16 28.2 1.7 0.45 18 6.5 5.7 200 17 0.71 0.53 9.01 @24.3 1.4 0.47 19 32.5 28.6 -150 17 0.67 1.06 18.02 37.8 2.2 @2.43 20 11.0 27.3 150 17 0.69 1.09 18.53 33.9 2.0 2.32 21** 20.7 18.2 150 9 1.12 4.08 -36.72 21.6 2.4 1.55 22** 20.7 18.2 150 11 0.96 2.99 32.89 25.2 2.3 1.55 CO 23** 9.6 8.4 150 11 1.58 4.64 51.04 30.0 2.7 0.71 1 24 13.6 12.0 150 9 0.98 4.25 38.25 22.2 2.5 1.02 CO 25 11.8 10.4 150 9 1.23 6.04 54.36 26.4 2.9 0.88 26 6.4 5.6 150 6 0.60 1.13 6.78 9.0 1.5 0.48 27 6.8 6.0 150 6 0.71 1.37 8.22 9.3 1.6 0.51 28 29.6 .26.0 100 6 1.03 3.76 22.56 12.9 2.2 2.42 29 29.6 26.0 100 6 0.96 3.53 21.18 14.1 2.4 2.42 30 22.0 19.4 100 9 1.14 3.81 34.29 19.8 2.2 81 31 20.0 17.6 100 9 1P07 3.18 28.62 18.0 2.0, 32 11.0 9.7 100 6 1.36 7.50 45.00 3.1 0.90 33 11.8 10.4 100 6 1.34 7.08 42.48 19.8 3.3 0.97 34 6.6 5.8 ido 9 0.71 1.05 9.45 14.7 1.6 0.54 35 6.9 6.1 100 9 0.74 1.26 11.34 18.6 2.1 0.57 Largest wave broke just seaward of wave gage Water depth at 200 ft - 13 ft. 150 ft = 12 ft. 100 ft = 10 ft. 50 ft = 3 ft. Mff P MM Table 8. 2 Summary of Observations: 16 ft. Boston Whaler Run Speed Distance Number of H E E Duration y F MPH (ft.) Waves m 2 ft.T 2 t.(see) (sec) Knots (ft.) (ft-lb/ft -lb/ft.@ (V in knots) 1 28.4 25.0 200 28 .179 0.11 3.08 46.8 1.7 2.04 2 32.5 28.6 200 29 .201 0.11 3.19 48.0 .1.7 2.34 3 20.7 18.2 200 21 .290 0.25 5.25 36.3 1.7 1.49 4 26.2 23.0 200 25 .290 0.23 5.75 42.6 1.7 1.88 5 9.6 8.4 200 22 .335 0.38 8.36 40.2 -1.8 0.69 6 11.8 10.4 200 22 .446 0.49 10.78 45.6 2.1 0.85 7 6.5 5.7 200 15 .312 0.30 4.50 24.3 1.6 0.47 8. 6.9 6.1 200 19 .290 0.26 4.94 30.0 1.6 0.50 9 31.0 .27.3 150 22 .246 0.15 3.30 33.9 1.5 2.32 10 31..0 27.3 150 22 .223 0.16 3.52 34.5 1.7 2.32 11 20.7 18.2 150 16 .335 0.34 5.44 27.6 1.7 1.55 12, 22.0 19.4 150 17 .312 0.29 4.93 31.2 1.8 1.65 13 13.4 11.8 150 15 .468 0.55 8.25 30.9 2.1 1.00 14 9.6 8.4 150 14 .402 0.66 9.24 27.9 2.0 0.71 15 6.2 5.4 150 10 .357 0.37 3.70 17.4 1.7 0.46 16 6.5 5.7 150 10 .469 0.56 5.60 14.1 1.4 0.48 77- 34.1 30.0 100 12 .312 0.31 3.72 19.8 1.7 2.79 18 32.5 28.6 100 16 .268 0.20 3.20 21.9 1.4 2.66 19 20.7 18.2 100 10 .402 0.54 5.40 17.7 1.8 1.70 20. 23.5 20.7 100 11 .357 0.40 4.40 .18.3 1.7 1.93 21 10.2 9.0 100 10 .513 1.00 19.5 2.0 0.84 22 11.4 10.0 100 10 .491 0.87 8.70 16.2 1. 6 -0.93 23 6.2 5.5 100 11 .469 0.41 4.51 20.5 1.9 0.51 24 7.9 7.0 100 10' .670 .1.10 @11.00 17.7 1.8 0.65 25 32.5 28.6 50 4 .491 0.80 3.20 6.0 1.5 4.86 26 17.9 15.8 50 .4 .759 1.71 6.84 6.3 1.6 2.69 27 9.7 8.5 50 5 .781 1.45 7.25 9.9 2.2 1.44 28 .27.3 24.0 50 5 .446 0.73 3.65 8.4 1.7 4..08 29 17.0 15.0 50 4 .670 1.30 5.20 @6. 0 1.5 2.55 30 11.4 10.0 50 4 .737 1.44 5-.76 7.2 1.8 1.70 31 7.6 6.7 50 13 .893 0.93 12.09 31.5 2.4 1.13 32 7.2 6.3 50 14 .759 0.90 12.60 30.6 2.2 1.07 T3 _* 23.5 20.7 200 19 ..312 0.26 4.94 29.7. 1.6 1.69 34* 27.2 23.9 150 16 .357 0.34 5.44 30.6 L 9 2.03 35* 27.2 23.9 50 15 .402 0.52 7.80 23.4 1.6 4.06 -with water skier T, max'11 and distance is shown, althouqh "Timaxle for the Whaler approaches a constant value beyond the "critical" speed more quickly than in the case of the deep-V hull. This,is no doubt due to the fact that the planing mode is achieved at lower speeds in the Boston Whaler than in the Uniflite Cruiser. Since the principal concern of this-study is the magnitude of enerqy reaching the shoreline with each boat pass, plots of total wave energy "E" in the respective wave packets is shown as a function of the Froude Number '!F" (with distance as a parameter) in Figures 8.9 and 8.10. The results presented above show there is a strong nonlinear relationship.between "F" and "14max fit so it is not surprisinq that a similar nonlinear relationship exists between "F" and total wave packet energy at the shoreline. In the case of the deep"V hull (26 ft. Uniflite cruiser) there is only a sliqht suqgestion that wake energy is dependent upon the distance from the shorefor any given speed. In the case ot the Whaler,'only those boat passes at the,50 ft. distance show'clear separation in their wake enerqles. Part of the reason for a reduction in wave energy from boats passinq at greater distances from the shore is that the number of wavesin a packet-depends upon distance'. For example, Tables 8.1 and 8.2 show that wave's generated at close distance for any given speed.contain higher waves but fewer in number. 8-10 It is important to note that the peak values of "ETIO and "Hmax" in Figures 8.9 and 8.10 lie in vicinity of 'IF" 0.8 rather than the theoretical value of 'IF" = 1. This observation is consistent with the results of similar experiments with the wakes of larger-hulled craft reported by Sorenson (1967). Three runs were made by the Boston Whaler with a water skier in tow. This condition was tested to see in a preliminary way whether the effects of the skier's weight would cause the planing hull to "squat" and thereby generate larger "ETO' in the wake. The plot of "ET" versus 'IF" (Figure 8.9) does not clearly distinguish a difference. However, a plot of "ET" versus boat speed (Figure 8.11) suggest there may be an effect, since two of the three runs do show values fo r "ET" which are higher than the general trend.. While thesefew runs cannot be considered to display a truly significant difference, the results do suggest that the effect of water.skiers on boat.wakes should be examined further in future-tests. The most important observation to be drawn from these experimental boat runs is that maximum values of next pages Figures 8.5 (upper left) and 8.6 (lower left) I Variation in maximum wave. heiqht "Hmaxe. as a function of Proude Number with distance of passage as-a parameter. Figures 8.7 (upper right) and 8.8 (lower right) Variation in maximum wave hei4ht wHmax" as a function of boat speed with distance of passsage as a parameter. 67Ft. Boston Wh 2.5. F, 2.0 DISTANCE FEET 4) 0=0 X 150 L5 U- 100 A 50 1-0 A- x x 0 0 0 1.0. 2.0 3.0 4.0 5.0 6.0 7.0 Y%rg-d Figure 8.5 26 ft. Unif lite Cruiser 2.0 DISTANCE -E-F..-ET 0 200 x 150 LL too was* 1.0 X vw 0 0 .1. .0 2.0 3.0 IF 3N9T F i g.u r e8.6 OKI 8-12 r IA- 2-0 16 Ft. Boston Whole IF rd DISTANCE 15 - FEET LL 6=0 0 X 150 10 - b * 100 A 50 0 0 SK IER il w 5 - 00 0 0 0 5 10 15 20 25 30 35 Boat Speecl- (Knots) Figure 8.7 U- Unif lite Cruiser 100 126 ft. DISTA NCE 80 FEET) 0200 x 150- 100 4) 40 AL- x- X* w 20 0 0 0 5 10 15 20 25 30 35 Boat Speed (,Knots) Figure 8 .8 i-A 0 wave-heights (and wake ene6y) are generated for'Froude numbers in thd range between 0.7 and 1.0. Since the Froude number is dependent upon water depth as well as boat speed, those boat speeds which generate maximum wakes will vary with different water depths in different waterways. Table 8.3, which lists the various Froude numbers arisinq from different combinations of boat speed and water depth, provides a simple illustration of what might be @xpected for a variety of "typical" conditions. For example, suppose a boat was travelling at a steady speed of 6 knots while. running up a creek in which the depth decreased from 18 ft. at the mouth to,4 ft. near the head. During the run the Froude number would be small near the mouth F11 = 0.42 to 0.56, respectively, for water depths of 18 ft. and 10 feet) and relatively small wave heights would be generated in the wake. But, when the boats reached depths less than 6 feet, the Table shows that maximum wave heights would be generated. Table 8.3.can also be used to show the effects of another kind of boating pattern. Consider a creek where the water depth varies from 10 feet at the centerline to 2 feet opposite: Table 8.3 Froude number for different combinations of.water depth and boat speed. next pages: Figure 185.9 (upper left) and 8.10 (lower left) T r- ota energy in the wave packet as a function of Froude Number with distance of passage as a parameter. Figure 8.11 (upper right) and 8.12 (lower right) Total energy in the wave packet as a function of boat speed with distance of passage as a parameter. 8-14 nomm:moom moons go-womml TABLE 8.3 FROUDE NUMBER for COMBINATIONS Of WATER DEPTH and BOAT SPEED DEPTH. SPEED (Knots) (ft) 2 4 6 8 10 12 14 16 18 co 2 0.42 0.83 1.25 1.66 2.08 2.49 2.91 3.32 3.74 Ln 4 .0.29 0.59 0.88 1.17 1.47 1.76 2.06 2.35 2.64 6 0.24 0.48 0.72 0.96 1.20 1.44 1.68 1.92 2.16 8 0.21 0.42 -0.62 0.83 1.04 1.25 1.45 1.66 1.87 10, 0.18 0.37 0.56 0. 74, 0.93 1.11 1.30 1.49 1.67 12 0.17 0.34 0.51 0.68 0.85 1.02 1.19 1.36 1.52 14 0.16 0.31 0.47 0.63 0.78 0.94 1.10 1.26 1.41 16 0.15 0 .'2 90-44 0.59 0.73 0.88 1.03 1.17 1.32 18 0.14 0.28 0.42 0.55 0.69 0.83 0.97 1.11 1.25 LL. -F N1,1 20 6 Ft. Boston W .d -j DISTANCE 15 - LA- @1%0 150 100 cl, 10 b- A 50 e 0 S K I @.Rj r- A 0 w 5 0 7.0 0 1.0 2.0 3.0 4.0 5.0 6.0 v - I Y%rg d Figure 8.9 r 100 [2 6 ft. Unif lite Cruise DISTANCE FEET 80' 0 200 X 150 100 CP 60 kx w 40 20 0 F- 0 0 1.0 2.0 3.0. VS/ Fi gure 8.10 to jer wo 8-16 2.5 116 Ft. Boston Whaledl DISTANCE FEET 0@=b E5- X 150 * 100 A 50 1.0, SKIER A 5- x 0 0 0L 2 0 5 .10 15 20 2 5' 30 35 Boat Speed Knots) F.igure 8.1 1 ly6ft- Uniflite Cruiser DISTANCE P .0 (FE ET) 0 200 LAL X 15 .0. %MOO 1.5 I IL 100 1.0 x 0.5 0 5 10 20 @5 30 35 Boat Speed K,nots)' Figure 8..12 8-17 near the bank. A boat travelling the centerline at a steady soeed of 6 knots would have'a low Froude number (0.56) The same boat travelling at the same speed and small wake. closer to the shore in water depths of 6 feet or less would be in the Froude number ranqe between 0.7 and 1.25 and would be generating a-maximum wake. Table 8.3-does not,. unfortunately, allow for the. prediction of the magnitude of the wave energy reaching the shore. The absolute magnitude of wave energy in any wake would depend upon-hull characteristics and the slope of the nearshore bottom, together with the boat speed and water depth.where.the boat passes a,ny.particular.shorelin-e site. But, Table 8.3 shows that distance from shore is important in producing the wake in any specific boat pass. C. Suspended Sediments Resulting from Boat Wakes Besides measuring,wake characteristics in sometrial runs, other data were collected'at Site.C,to.give a very. preliminary idea of the increase in suspended sediment associated with breaking waves in boat wakes alonq the shoreline. For this experiment, theUniflite cruiser travelling at a speed of approximately 21 knots made repeated passes 200 ft. offshore, and samples were taken after the breaking of the lst, 5th, and 10th wake packets. The water in the nearshore was also sampled prior to the passage of the boat and again at-,the end of all of the passe s of the Uniflite cruiser. The samples were collected by immersinq one-quart jars about 5 cm. under the water surface immediately after the last wave in each packet broke on the shoreline profiles. The water samples were filtered through pr e-weighed 0.6'.,pm Nuclepore filters. After desiccation, the filters were reweigheA to determine total, weiqht of suspended sediments. Then. the samples were completely combusted to obtain the percent of organic material. The results are shown in Table 8.4. Table 8.4. Suspended Sediment Concentrations Total Percent Run Time Concentrations organic 1.) Ambient 1045 EDT 0.0053 grams/liter 35.7 2.) 1st Packet 1125 0.440 20.3. 3.) 5th Packet 1127 0'120 4.) 10th Packet. J130 0:330 23.2. 5.) End of Pa sses 1407 0.081 14.3 Due to a rel atively high stage of. the tide when the Uniflite runs were conducted, breaking waves extended across the entire foreshore, and the swash impinged against the bank scarp at Site C (Figure.4.17). Table 4.4 shows the foreshore sediments at this site are composed pri-ncipally of sand with only a few percent of s-ilt and clay. Yeot, the data in Table 8.4 show that the breaking.waves.resulted in an enhance,m e.nt of the short-term load of suspended material by more than two orders of magnitude over the ambient level. 8-19. Inspection of the filters showed most Of the material supsended by the boat wakes was clay and silt,but some of the organic material was observed to be buoyant detritus. This increased' amount of suspended sediment could come from either the bottom sediments in the nearshore, or from 'the bank scarp which was within the range of the wake swash. it is interesting to note that once the first boat pass was made, the data show no tendency towards higher or lower concentrations of suspended sediments with an increasing number of boat passes. So this one set of trial runs with. the Uniflite'.cruiser principally' demonstrates that boat wakes breaking alonq the shoreline can increase,the short. term concentrations of suspended sediments in the nearshore zone at Site C. 8-20 Ix DISCUSSION, CONCLUSIONS. AND THOUGHTS-FOR MANAGERS Rober t J. Byrne, John D. Boon, III, Rhonda Waller, and.Deborah Blades A. Discussion This study presents fourlines of evidence which when considered together, provide the basis for inference as to the role of boatinq activity as a cause of fastland erosion along the tidal shorelines of small coves.and creeks. These are: 1.) D11rect observation of the fastland and beach changes at five sites in Anne Arundel County over a one-year period.. 2.) Estimates of the wind-wave energy throughout the year and that due to boat wakes during the boating season at the five sites. 3.) An inventory.of the boating characteristics. at five,sites reported as having heavy boating traffic. 4.) Field observations at one site of the wave. characteristics generated by controlled boat passes at various speeds and distances from the shore. The purpose of this chapter is to integrate these findings and thereby offer an interpretation of the role boating activity plays in fastland erosion at the tidal. shores. Point 1. Discounting the effects of Tropical Storm David, the direct observation of fas t land changes at the five sites (Chapter IV) indicated that only at Site'C, in a narrow waterway, was there siqnificant-fastland retreat dd rinq the boating season. The question naturally arises 9-1 as to whether comparable behavior of the fastland at the -five sites would havelbeen observed in other one-year periods. T o address this we.must bear in mind that the total erosi,o,n response is . a combination of that induced by wind waves plus that induced by boat-wake waves. The magnitude of the wind-wave. energy wi 11 vary somewhat from year to year as a function of gross weather patterns and storm activitv. On the other hand there is no reason.to assume that the boating activity during the 1979-boating season was atypical of.averaqe conditions overrecent years. Thus, between years we expect the total wave energy to be a combination of a constant contribution due to boats and a variable contribution due' to wind. waves. More directly, the fastland response is dependent upon the frequency.of storm activity which may fluctuate considerably from.year to Year. Observations for a' sev6ral'-yiear period which'includes this variability in storm activity would be required.to estimate the "average" erosion response due to the total wave energy. A"hypothetical case will illustrate the point. Suppose at a given site boat-wave energy was [email protected] for a fastland recession of 0.25 ft. every year but because of variation'in storm activity the total yearly recession, ove@r a four year period, was 4 f t. , 3 f t. 2 f t. and 1 ft.! respectively. The yearly percentage of recession due to boat wakes would'then be 6, 8, 12, and.25% -respectively. Over the four-year period the total 9-2 recession would- be 10 ft. with 10% due to boat-wake. energy. Thus in any given year there could be appre ciable error in estimating the level of erosion attributable to boat wakes. In spite of the fact that the observations were conducted for only one year,@certain inferences can be drawn about the four sites which showed either no erosion,, or where the response during the boating season was very slight. Storm activity during the observation year was relatively slight. No major northeast storms with a strong storm surge occurred (the effects of Tropical Storm'David which occurred near the end of the period will be discussed separately). This being the case, the contribution of erosion fromboat-wakes would be amplified relative to a year with high storm-frequency Thus the results showing negligible impacts due to boats at fourof the sites indicates thati in general, boat.wakes play a relatively minor role in the total erosion process at those sites. The same.conclusion would apply for sites with similar physiographyj bank composition, fetch., and boating activity. The two sites with bluffs, Sites B and D, warrant special discussion. The principal fastland..modification which occurred was slumping in winter and.early.spring and reduction of that material by wave action. The cause of the slumping action was likely percolation of groundwater,, and surface.runoff during freeze thaw cycles. By late May.much of the material in the slumps 9-3 had been suiected(to wave action and was displaced. There is no reason to.assum6 that all.slumping activity is confined to the winter and spring. Had slumping occurred in early summer then we must assume that the combined wind-wave and boat-wake action would have displaced some of @these materials.. In such circumstances it would be reasonable to attribute a fraction of the erosion to boat wakes. However, as Table 7.2 indicates, the boat-wake energy appears to be a relatively small percentage of the wind-wave.energy (3.6% at Site B and 8.4% at Site D). Such being' the case', attribution of erosion to boat.wakes woul& be relatively small. Point'2. It was previously indicated that only at Site C was there significant fastland retreat during the boating season. Site C, on.Broad Creek, ison a narrow.channel (600, ft. width) -with a relatimely steep nearshore gradient. Two ofthe three profiles showed fastland retreats. of 6.8 feet and 5.2 feet (Figure 4.19). Site C received the highest amount of boat-wake energy of the five-sites. As well? boat-wake' energy accounted.for a substantially higher fraction of the total wave energy (Figure 7.7) at Site C than at the other sites. it is of particular interest.to compare Site C and Site Bwhich has 'a similar nearshore profile, but a wider and shallower channel. The current nautical charts show a MLW dePth of 12 feet near Site C and 8 feet near Site B. The two sites received about the same wind-wave energy throughout the year. and.during theboating season. Site C 9-4 was exposed to about 5 times more boat-wake energy than Site 9. Inspection of the boating characteristics (Table 6.2) shows that the two sites were very similar with respect to averaqe.boating frequency, speeds, boat lenqths, and hull types. The strikinj difference between the boating characteristics at the sites is the distance of passage from shore. At Site B, 80% of theboat passes occurred at distances greater than 500 feet, while at-Site C 80% occurred at distances less than 200 feet from the shore. These results. illustrate the importance of distance of passage in.controlling the level of boat-wake. energy at the shore. The physical setting at Site C, the nature of its. fastland, and the low sand supply from adjacent fastland are all conditi.ons conducive to erosion in the presence of. wave action. The site is a low terrace composed of unconsolidated sand.and qravel-capped with a very thin marsh. There is evidence-that-the site is-at least partially composed.of fill material.. More important however, the site represents a'transition point where Broad Creek widens, and very little sand is supplied to Site C from the fastland along the.shoreline., Thus the erosion of the beach is not inhibited by the addition of sand.. Point 3.. The-fastland response at Sites B and D.to the passage of Tropical Storm David illustrates the relative importance of extreme events in the erosion process of bluffs along tidal shorelines. At Site D,the combined 9-5 effects of the storm surge (estimated 2.5 ft.), and*wave action generated by the southeast wind, resulted in fastland retreat throughout the year including.recession of the bluff-face itself. However, at Site B, which is-more protected from wave action from the southeast, the steep bank showed no response to the storm passage. R. Conclusions This study Indicates that a significant contribution to the total wave energy (and potential erosion) from boat wakes is likely only when there is a high frequency of boat passages close to shore. While there may be several circumstances wherein boats pass close to shore, the. greatest relative impact is likely to occur in narrow creeks where the channel width:forces passage within two or three hundred feet.from the shore. Since, wind-wave activity is likely to be suppressed in narrow creeks, it is under these circumstances that a high frequency of boat passages would generate a large portion of the.total wave energy. But it is not likely that further studies at othersites in Anne Arundel County would show boat wakes contribute more enerqy for Prosion@than wind waves. The level of fastland erosion response depends upon the nearshore depth gradient, the.composition of the.fastland.? [email protected] supply of -littoral sands.from the adjacent shoreline. The condition&most susceptible to erosion would be the combin,ation of an,exposed pointof land composed of hiqhly-erodible material such as'sand and gravel.with a steep nearshore gradient. The site which had the greatest 9-6 change in the shoreline profiles (Site C) possessed all these factors. Experiments with controlled boat'passes at Site C indicate that for a g iven water depth the amount of wave energy generated depends principally upon the boat speeds. At low boat speeds the wake energy is quite small. At intermediate speeds (7 to 10' knots) the. wave energy was maximum. At higher speeds the wave energy again decreases. The magnitude of the wave energy as a function of d istance was of secondary importance for the conditions tested (50 to 200.ft.). The role of this parameter would be more important at larger distances. The results of the observations at Site can be generalized in terms of the Froude Number (proportional to the ratio of boat speed to the square.root of water depth). Maximum wave energy occurs in the Froudenumber range of 0.7 to 1.0 with enhanced wave energy in th e ranqe.of Froude number values of 1.25 to 1.5 (Figures 8.5, 8.6,, 8.9,. and 8.10). Inspection of various combinations of boat speeds and water depths (Table 8.3) indicates that a boat speed of 6 knots would generate near-maximum wakes when the water depth is less than 6 feet. A boat speed of 8 knots in water depth ranging between.10 and 4 feet would generate,maximum or near-maximum wake. Boats travelling.at 4 knots, on the other hand, would not generate their highest wakes except when in water depths of 2 feet or less. For the range of depths frequently found in narrow creeks fringing the shores of Chesapeake Bay, three 9-7 particular conclusions may be drawn: 1.) Boats reducing speed to conform to the speed limit pass through the speed range which generates maximum wake. 2.) If the approach to the speed control.arga it within a narrow creek the shores adjacent to the approach zone will be exposed to the higher wake energies noted in 1. 3.) Boat operators underestimating their speed by only a few knots while in a speed control area could generate a'near-maximum wake while transiting the waterway. C. Thoughts for Managers Three points which would mitigate the potential erosion impacts due to boats are offered for consideration: 1.) The study shows that depth conditions exist in some creeks wherein maximum boat-wake energies are generated close to the standard 6 knot speed limit. The results.can be used to estimate the speeds at which maximum wake is generated for various water dept hs. In some cases a reduction of the speed limit would decrease the unintentional generation of maximum wake. 2.) Since boats approaching a speed-control zone will pass through the speed which genera.te,s.maximum wake as they slow from high speed, the speed-limit signs should be placed, when possible, at'locat.ions,where the creek is so wide that the wake energy can dissipate before reaching the shore. 3.) The study indicates that thegreatest potential for erosion-impacts due to boat wakes is to be expected 9-8 when high frequency boat passages Occur within a few hundred feet from the shore. Restrictions in such areas-would-@ reduce the potential for shore erosion. D. Recommended Further.Studies ,The present study indicates that it is in narrow creeks and other circumstances wherein boats pass close to shore that the highest potential for boat-wake erosion exists. The question then naturally arises, "How close to the shore can boats pass without causing the significant wake energy at the shoreline?" The com parison between two sites, one of which.showed dramatic erosion during the boating season and the other very little, provides a partialanswer. The two sites.had similar boating.characteristics with respect to frequency, hull sizes, and speed. The only major difference was the distance from shore at which passage occurred. At the Broad Creek site(Site C), where erosion occurred, about 80% of the traffic occurred within 200 feet or less from the shore. In contrast, at the Goose Island site (Site B) about 7.5%,of the boat-passes occurred at distances greater than 500 feet. -Consequently, the wave energy at Site B was only about 20% of that experienced at the,Site C. Thus itappears that passage distances of at least 500 feet are required to appreciably reduce the level of wake energy at the shoreline. Further observations.of controlled boat passes over a wider range.of distance from shore would permit a more 9:-9 accurate determination of the creek-width necessary for negliqible.wake enerqy at the shoreline. The controlled boat passes conducted in the present study covered the range of distances from 50 feet to 200 feet at a single site. This ranqe should be extended to at least 500 feet. In addition other sites with constrasting depth gradients should be added to the data set.. As well, the range of hull lengths and types, could be extended. 9 -10 x REFERENCES CITED Anderson, F. E., 1976, Rapid settling rates observed in sediment resuspended by boat waves over a tidal -flat: Netherlands Journal of Sea Research, vol. 10, p. 44-58. Boon, John D., 111, 1978, A Storm Surge model Stu Vol. 1. "Storm surge height-frequency analysis and model prediction for Chesapeake Bay", Gloucester Point,, Va.: Special Report No. 189 in Applied Marine Science and Ocean Engineering, Virginia Institute of Marine Science, 155 pp. Brebner, Authur, P-. C. Helwig, and J. Carruthers, 1966, Waves produced'by ocean-going vessels: a labo- ratory and field study: Proceedings, 10th Confer- ence on Coastal Engineering, Tokyo, p. 455-459. Collins,, J. Ian, and Edward K. Noda,.1971, Causes of levee d-amage in the Sacramento-San.Joaquln d;-ltal Pasadena, Ca., 91IU7: Tetra Tech Inc., Report No. TC-218, 55.pp. Corps ofEngineers, 1973, Shore Protection Manual, Ft. @Belvoir, Va. Coastal Engine-e-FIL-ng Research Center,'3 vols. Das, M. M., 1969, Relative Effect of Waves Generated by Large_ShiEs and Small Boats in Restricted Water- waxs, Berkeley, Ca.: Report No. HEL-12-9,. Hydraulic Engineering Laboratory, University of California, 112 pp. Das, M. M., and J. W. I Johnson, 1910, Waves generated by large ships and small.boats: _Proceedin@s,, 12th. Conference on Coastal Engineering, Washington, D. C., p. 2281-2286. Froude, R. E., 1881, "On the leading phenomena of the wavemaking resistance of ships": Transactions, Institute of Naval*Architecture, London_,_`V`=. @2. Glaser, John D., 1976, "Geologic Map of Anne Arundel County, Maryland", Baltimore, Md.:- Maryland Geological Survey. 10-1 Harris, D. Lee, 1972, Wave estimates for coastal regions; Lin, Swift, D.J.P., David Duane, and Orrin H. Pilkey, eds., Shelf Sediment Transport: Process and Pattern, Stroudsburg, Pa.: Dowden, Hutchinson and Ross, Inc.1 99-125. Hay, Duncan, 1968, Ship waves in navigable waterways: Proceedings, llth Conference on Coastal Engineer- ing, London, p. 1472-1487. Hicks, Steacy D., 197 2, on the classificatio n and trend's of long-period sea level series: Shore and Beach, vol.. 40, No. 1, p. 20-23. Jackivicz, Thomas P., Jr., and Lawrence N. Kuzminski, 1973, A review of outboard motor effects on the aquatic environment: Journal of the Water Pollution Control Fede @atio@n, vol. 45, No. 8, p..1759-1770. Johnson, J. W., 1,948, The characteristics' of wind waves on lakes and protected,bays: Transactions, American Geophysical Union, vol. 29, No. 5, p. 671-681. Johnson, J. Wo, 1950, Relationships between wind and waves,'Abbott's Lagoon, Ca.: 'Transactions, American Geophysical Union, vol. 31, No,.3, p. 386-392. Johnson,,J. W.., 1957, "Ship waves in navigational' channels": Proceedings, 6th Conference on Coastal-Engrg.., Gainesville, Fla., p. 666-690. Johnson, J. Wo, 1968, Ship waves in shoaling waters: Proceedings, Ilth Conference on.CoastAI.Engrg, .London, p. 1488-1498. Johnson, J. W., 1969, Ship waves at recreational beaches: Shore and Beach, vol. 37, No. 11. p. 11-15. Lord Kelvin (Sir William Thomson), 1887, On ship waves, Proceedings., Inst. of Mechanical Engineers, London. Kinsman, Blair, 1960, Surface waves at short fetches and low wind speeds -- a field study, BaMmore, Mdo: Chesapeake Bay.Institute, The Johns Hopkins University, Technical Report 19, 3 vols, 10-2 Kinsman, Blair, 1965, Wind Waves, Englewood Cliffs, N.J.: Prentice-Hall Inc., 676 pp. Liou, Y. C., and J. B. Herbich, 1971, Velocity distribution and sediment motion induced by ship's propeller in ship channels: in, Hydraulics in the Coastal Zone,.Proceedings, 25th Annual Hydraulics Division, Speciality Conference, ASCE, Texas A&M University, College Station, August 10-12, 1977, p@. 228-235. Roy Mann Associates, Inc., 1974, Recreational boating impacts: Chesapeake.and Chincoteaque Bays_* Part 1. Boatinq capacitx Rlannin2 system, Annapolis, Md.: Maryland Department of Natural Resources, 160 pp and Appendices. McGoldrick, L. F., 1969,.A system for the generation arid.measurement of capillary - gravity waves, Chicago, Ill.: Technical Report No. 3, Dept. of Geophysical Sciences, University of Chicago, 27 pp. Moss, Brian, 1977, Conservation problems in the Norfolk Broads and Rivers of East Anglia, England-phyto- plankton, boats, and the causes of turbidity: Biological Conservationist, vol. 12, p. 95-113. Munk, W. H., 1944,@Proposed uniform procedure for observing waves and interpreting instrument. records, Lajolla, Ca.z Wave Project, Scripps Institution of.0ceanography. Palmer, Harold D., 1973,'Shoreline erosion in upper Chesapeake Bay: the role of groundwater: Shore and Beach, vol. 41, N.6. 2, p. 1-5. Plate, E. J., P..C. Chang, and G..M. H idy, 1969, Experiments on the generation of small water waves by wind: Journal of Fluid Mechanics,-Vol. 35, part 4, p. 625-656. Seymour, Richard J., 1977, Estimating wave qeneration on.restricted fetches: Journal of the Waterway, Port, Coastal, and Ocean Division, ASCE, vol. 103, No. WW2 Proc. Paper 12924, May, 1977, p. 251-264. Sorenson, R. M., 1967a, Investigation of.ship-generated waves: Journal of the Waterways and Harbors Division, ASCE, vol. 93., No.-WW1, Proc. Paper 5102, February, 1967, P. 85-89. 10-3 Sorenson, R. M., 1967b, Waves generated by a moving ship: Shore and Beach, vol. 35, No. 1, p. 21-25. Sorenson, R. M., 1973, Waterwaves produced by ships: Journal of the Waterways, Harbors, and Coastal Engineeeing Division, ASCE, vol. 99, No. WW2, Proc. Paper 9754, May, 1973, pe 245-256. Sverdrup, H. U., and W. H. Munk, 1947, Wind, sea and swell; theory of relations for forecasting: U.S. Navy Hydrographic Office Pub. No. 601, 44-pp. Thompson, Edward F., 19,80, Energy spectra in shallow coastal waters, Ft. Belvoir, Va.: U.S. Army Corps of Engineers,.Coastal Engineering Research Center, Technical Paper.No. 80-2, 149 pp. Williams, Jerome, and"Fred Skove, 1980, The effects of boating on turbidity in relation to. submerged -gq'uatic vegetationr Annapolis, Md..: EPA Chesa- peake Bay Program Report, (In Press). Wu, Jim, 1972, Physical and dynamical scales for gener- ation of wind waves: Journal of the Waterways, Harbors, and Coastal Engineering Division, ASCE, voT-.98, No. WW2, Proc. paper 8879, p. 19'3-175. Yousef, Yousef A., 1974, Assessing effects on water quality-by boating activity: U.S. Environmental Protection Agency, Environmental Protection Technology Series Report No.. EPA-670/2-74-072, 58 pp. Yousef, Yousef A., Waldron M. McLellon, Robert H. Fagan, Herbert H. Zebuth, and Carl R. Larrabee, .1978, Mixing effects due to boating activities in shall6w lakes: Final Report to-the U.S., Dept. -5T InteriFr-,Office of Water Research and Technology, Orlando, Flai: Florida Technological University, College of Engineeringr.Environmental Systems Engineering Institute, 352,pp. .10 .4. APPENDIX A HOUSE JOINT RESOLUTION No. 40 A Hou.se.Joint Resolution concerning Anne Arundel County Small Creeks and Coves FOR the purpose of requesting the De partment of Natural Resources to design and undertake a study to determine whether continuous high-speed,boat traffic is in fact detrimental to small coves and creeks Along the Anne Arundel County coastline. WHEREAS, The Anne Arundel County coastline is highly indented,, and the tidal water. indentations form shallow, narrow creeks with highly erodible shorelines and fragile biological ecosystems; and WHEREAS, Continuous high-speed-boat traffic may have an iInjurious effect on the smallcoves and.creeks; now, therefore, be it RESOLVED BY THE GENERAL ASSEMBLY OF MARYLAND, That starting this year,:the Department of Natural Resources is hereby requested to design and undertake a study to determine:whether.continuous high-speed boat traffic is A-1 in fact detrimental to small coves and creeks; and be it. further RESOLVED, That consideration shall be given to closing at'least one cove or.creek in South, Severn, and Magothy Rivers at all times to vessels operated at a speed in,excess of six-(6) knots, for such a period as required to.facilitate the,scientific study; and be it further. RESOLVED, That the Department of Natural Resources shall submit an interim progress report to each member of the House Environmental Matters Committee.and the Senate Economic Affairs Committee annually starting 1977,.a.nd the report shall' be made available to the public. A final report summarizing the results of the study.shall be submitted to the General Assembly not later than the 1981 Session, and shall.be made available to the public; and be it further RESOLVED, That copies of this Resolution be sent to The H6n. James B. Coulter, Secretary, Department of Natural Resources,@ Tawes State Office Building, Annapolis Maryland 21401. A-2 Approved: Governor. Speaker of the House of Delegates. President of the Senate. A-3 APP ONDIX B WIND-GENERATEb WAVES Deborah -Blades, Rhonda Waller, Thomas Burnett, Michael Perry, Tristina Deitz, Mark Alderson A. Introduction This appendix presents a.summary of the wind- generated wave heights which were observed at the study sites. These measured wave heights were used to produce site-specific estimates of the wind-wave energy budget during the year-lonq period of observations. There have been several previous studies of wave generation by winds in shallow coastal waters, (Johnson, 1948, 1950; Kinsman, 1960,; Harris, 1572; Seymour, 1977; and Thompson, 1980), and several mathematical models already exist to predict the charateristics of,waves (height and period) if the wind speed, duration, and fetch are known. Two examples' of these are shown in Figure B.l.. These models are helpful for forecasting general wave conditions in many areas. But, physical oceanog- raphers and.mathematicians,continue to discuss which Opposite: Figure B.1 (top) Growth of wave height with time and distance from the upwind edge.of a fetch (after Sverdrup and Munk,. 1947). Figure B.1 (bottom) Forecasting curves for, shallo%# water waves in a basin with constant depth equal to 5 feet (from the U.S. Army Corps Shore Protection manual, 1973). B-1 GROWTH OF VAVE HEIGHT WITH TIME FROM SVERDRUP AND MUW (1947) 36 AFTER .24 HOURS 30 24 w 18 -12 w ro 0 0 500 1000 FETCH (km) FORECASTING CURVES FOR SHALLOW-WATER WAVES CONSTANT DEPTH- 5 FEET goo 0 so 60 40.0 M 20 ft. 15 ^2 @87@c. I10 1,000 5.600 10.'000 50.boo FETCH (feet) H- WAVE HEIGHT T- WAVE PERIOD Figure B. 1. B-2 'jag, 0 4% ftb r f t 0so @o theoretical approach should be useful.to produce the "best description of how waves are generated by the wind blowinq across the sea surface (Kinsman, 1965; Plate, et al, 1969; Wu, 1972;). As Figure B.1 suggests, none of the existing information is very useful for predicting the wave heights which could be expected at the study sites described in Chapter IV, since none is particularly sensitive to either the range of basin depth or the range of fetch.which are present at.the study sites. In this absence of adequate theoretical models, empirical site-rspecific wind-wAve energy models were constructed by making wave observations at the study sites under different wind conditions. Since wind duration is a factor in wave height, three such models were constructed for each site corresponding to short- medium- and lonq-duration winds. Month.ly budgets of wind-wave energy were then developed for each site from these wind-wave measurements.' B. Methods Throughout the year of study (October 1978-October 1979), measurements of wave characteristics were made at each of the study sites. These observations included: o Wave Height - an observer visually measured wave heights at the points where the waves broke in opposite: Figure B.2 Portions of the continuous meteorological record collected at the United States Naval Academy gauging station in 'Annapolis. B-3 "E S @s w Pr -TF-r-rIrv-T"rmIr ir N .41 E s S 9-Ifflopow" * T 20 -400 7r z 40. Go. WIND RECORD .40 US. NAVAL ACADEMY ANNAPOLIS. MARYLAND -410 40 8 DECEMBER. 1979 .400 too. i2o lao N N E s 11 S w w N N E E s s 0 -4oo 01 40 0-11to-4 -^-,Pl Jt-OVLIW-UA# A I*Iw@ qo 0 z 40 60 INNO RECOM 60 US NAVAL ACADEMY ie so ANNAPOLIS. MARYLAND so ub 22 DECEMKR, M76 0035 ift N E S IF lo N N0 E 0400 0 0 t*0 0100 .0;W0 40 .44 IIIIIND RECORD US NAVIAL ACADEMY ANNAPOLis, MAim-AND so 21 JANUAW. 1979 1002 1201 1 1 Fi I Ilia B .2 ..ff-4 nearshore or on the beach using a graduated staff. Munk (1944) has found that the average height of waves so estimated by an observer is about equal to the average height of the 1/3 highest waves. This has been defined as significant wave height. o Wave Period - An observer timed ll,successive wave crests with a stop watch. This was repeated three times and the average wave period was calculated. o Time of Da - measured with a watch. o Wind Speed and Direction - an observer placed a Simmshand-held annemometer (model ss) one meter above the water surface and noted the approximate du'ration.of,qusts as well as the dominant wind speed. Wind direction was measured by a compass. The local wind record that was selected for use was taken from the meteorological station at the U.S..Naval Academy at Annapolis (Figure B.2) which is located Tjithin 3 miles, 5.5 miles,.4.6 miles'@1.7 miles, and 6.8 mileg of study sites A-E respectively. When the wind velocity at the Naval Ac'ademy Gauging Station was compared to the wind velocity at each of the study sites (Table B.1), there were minor differences which are attributable to terrain effects, station separation, and,measurement correlation between the opposite: Table B.1 Comparison of winds at,the Naval Academy Gauging Station and at the field sites described in Chapter IV. B-5 Table B.1 SOME COMPARISONS OF WIND MEASUREMENTS On-Site Wind Naval Academy Description At Gauging Station 1 Meter Above Hourly Average Date Site The Water Surface Wind Speed March 5, 1980 D 4-5 m/sec 6 knots gusts to 9 m/sec (3 m/sec)- March 5, 1980 A 2-6 m/sec 6 knots gusts to 9 m/sec (3 m/sec) March 5, 1980 E 5-7 m/sec 8 knots (4 m/sec) March 5, 1980 D 5-7 m/sec 9 knots usts to 10 m/aec (4.5 m/sec) March 1.0 1980 D 3-5 m/s'gc 7 knots gusts 7-10 m/sec (3.5 m/sec) March 10, 1980 A 3-5 m/sec .7 knots qusts 5-7 m/sec (3.5 m/sec) March 11, 1980 E 1-2 m/sec 10 knots gusts to 7 m/sec (5 m/sec) March 11F 1980 D 10 m/sec 10 knots gusts to 13 m/sec (5 m/sec) March 11,' 1980 P 10 m/sec 8 knots gusts to 14 m/sec (4 m/sec) March 11, 1980 E 0 8 knots - gusts to 4 m/sec (4 m/sec) March 11, 1980 B 4-5 m/sec 11 knots --gusts to 7 m/sec (5.5 m/sec) March 11, 1980 C 4-6 m/sec 11 knots gusts to 12 m/sec (5.5 m./sec) March 18, 1980 D 7 m/sec 12 knots gusts to 14 m/sec (6 m/sec) March 18, 1980 E 0-2 m/sec 12 knots gusts to 5 m/sec (6 m/sec) March 18, 1980 B 3-5 m/si@c 12 knotl gusts to 11 m/sec (6 m/sec) March 18, 1980. C 6-7 m/sec 12 knots@ '6c gusts to 14 m/s (6 m/sec) March 18, 1980 A m/sec 12 knots gusts to 10 m/sec (6 m/sec) March 18, 1980 D 7-10 m/-sec 12 knots. gusts to 13 m/sec (6 m/sec) March 18, 1980 A 2-4 m/sec 12 knots gusts to 6 m/sec (6 m/sec) B-6 wave heiqht at a particular site and the average hourly wind velocity at the Naval Academy. The hourly averages of wind speed and direction were visually determined from continuously recording strip charts (Fiqure B.2). These were compiled to produce the monthly wind roses shown in Figure B.3. This diagram also contains monthly wind roses documenting wind patterns at the Annapolis Naval Academy over a previous 15 year period. The comparison of the two sets of wind roses contains no evidence to suggest the winds in the study year were .substantially different from normal, considering that the present study uses hourly averages, and that the 15 vear record used two daily instantaneous measurements (probably infrequently collected at night). The wind rose data for the year of observation shown in BA is presented in another form in Figure B.4. This Fiqure figure indicates the distribution of winds which were-used to construct the models of wind-wave energy. opposite: Figure*B.3 Two sets of monthly wind data 'collected t Annapolis (obtained from the U.S. Dept. of Commerce, National Climatic Center, Asheville, N.C.). next paqes: Figure BA (left) Wind distribution at Annapolis, Rd from November 1978 through October, 1979. Figure B.5 (right) Plots of wave measurements at each of the study sites, presented according to the wind speed and direction measured at the Naval Academy gauging station. T3- 7 FREQUENCY OF WINDS FREQUENCY OF WINDS U.S. NAVAL FACILITY- US NAVAL FACILITY ANNAPOLIS, MD ANNAPOLIS. MD CUMULATI VE. THIS SIUUY CUMULATIVE THIS STUDY 1945-1960 NOVEMBER 1978-1979 1945-1960 MAY 1978-1979 HOURLY READINGS 1970 HOURLY AVERA13ES HOURLY READINGS 1979 HOUFLY AVERAGES 12% 30 MO. CALM-941L __E Ip 744 Nft DECEMBER JUNE 19" 1974 C". Be % COA. 2 m W 10.375"W 11. rzo HAS W%RL N @ 'L .(i@ *`57 JULY JANUARY 19" t!)CALIII-21% 744"m FREQUENCY OF WINDS FREQUENCY OF WINDS U.S. NAVAL FACILITY U.S. NAVAL FACILITY ANNAPOLIS, M_PTHIS -97`9" ANNAPOLIS, MD CUMULATIVr CUMI)LATIVE . . THIS TUDY 1945-1960 FEBRUARY 1978-197.9 1945 1960 AUGUST .1978-1979 HOLIRLY READINMI 1979 HOLIRLY AVERAGES Notomy READINGS 1979 HOURLY AVERAGES CALE - 14 % CAL1111-21% W10.957 HIM Ill-avalm MARCH SEPTEMBER 1979 CALM-12% CALM-211% -IS w4% 720 No APRIL OCTOBER Bell" 1979 .9% CALM -U% c 111-K01116WIL W It 1.71 @*F- 7*.::. TZIN111111. 3 'Figure B.3.. 13 8 WINDS AT ANNAPOLIS, MD. .NOVEMBER, 1978-OCTOBERs 1979 51 :]-.6-10. 11-20 )20 A lhours .30 30 u @Ep [h:'o hours [hours cf) 20- 20 z CD (D QD - LLJ iL -.10 c/) z CALM -CALM N NE SE S sw NW Figure B .4 1-5 .F[h:o All SITE A 30 3w& SITE SrTE. C to to w w w w G. 1 0 10 mir " a 1 0 @ 1 0 A, a I I - - z I ; I f MM PALM N NE E SE S SW W NW N NE E Sk s SW IN NW N NE E SE S sw IN mw WIND DIRECTION AT ANNAPOLIS WIND DIRECTION AT ANNAPOLIS WIND (MAECTICk AT ANNAPOLIS GD 30 - WE D SITF E SrM F A 20 - to 69 Me A w 0 -io 16 040 of 4, 04 . " 2 1 ac As 0 w 0 0 0 Me . " I " S a T- . 1 11 . , -W ; v T 'ALM ALM I I CALM M F N NE E SE S SW IN NW NE E sE S SW IN NW N ME E sl S sw w KW WIND DIRECTION AT ANNAPOLIS WIND DIRECTION AT ANNAPOLIS WIND D(RECTION AT ANNAPOLIS Figure . B .5. It Figure B.5 indicates the rangeof wind speeds for which on-site measurement of wave heights were collected. This. figure shows that few observations were made at the higher wind speeds. As a result, the contours of wave heights at the higher wind speeds in Figure 8.6 a-f are shown by dotted lines. These diagrams show the ranges of -measured. significant wave heights plotted according to the wind conditions recorded at the Annapolis Naval Academy meteorological station. The diagrams also show the fetches at each study site in shaded areas. The site-specific models of wave height are particularly reliable within the range of the most frequent hourly average speeds (0-10 knots). Three models were prepared'for each site for three different velocity durations.. The 0-1 hour models were. compiled, from wave observations collect.ed.at times when there was a change in wind velocity greater than 2 knots at the Annapolis Naval Academy gauging station within the hour. The 1-2 hour models were com. iled from wave observati p ions collected at times when no change in wind velocity greater than 2 knots occured within the previous two.hours. The >2 hour models were compiled from wave observations collected at times when no change in wind velocity greater than 2 knots occurred for more than 2 hours. B-11 C Results i Site-specific Models The larqest significant wave heights at each site generally coincide with winds blowinq from the directions of greatest fetch. However, at Site B (near Goose Island), the local topography and wave refraction (bending of the wave fronts around irregularities in the seem to have influenced the waves so that the largest wave heights were measured when the wind at Annapolis was blowing from a direction with very little fetch at the study site. Site FF located near Site B shows similar behavior in the wind-wave distribution. only ripples (wave heights less than 2 cm. were measured'at each study site when the winds.at Annapolis.were blowing from directions with.no fetch. But the diagrams in Figure B.6 a-f show that some wave activity is inferred.to be present at the study sites.under_@strong winds greater than 15 knots from these directions,of no fetch It is rtant to note that Figure BA shows there were very few. impo hours of wind speeds higher than 10-15 knots during the year of observations, and manv of these hours of higher wind speed were at times when the shoreline sites were covered next pages: Figures B.6 a-f Site-specific models of wind- generated waves at each of the five study sites described in Chapter IV. The shaded areas show .the distribution of fetch. The wave measure- are plotted according to the wind speed and direction measured at the Annapolis Naval Academy. B-12 WIND-GENERATED wIND-GENERATED WW* GENERATED WAVE HEIGHTS WAVE HEIGHTS WNE HEIGHTS 40 2 00 0 3000 I "a \, 6 , /I - .\ , j 540 so 000 \ / 9 000 to is to to N. 00 10 Soo 10 000 1 Soo Soo 0 tz- Z_ I N ME E S go W No N FETCH CALM 0 . @ I . . . E' (motors) N "It E U S 11, W Ow N FETCH NME E U S Sw IN ON IN FETCH WIND DIRECTION IN ANNAPOLIS WIND DIIRECTION IN ANNAPOLIS (motors) VAND DIRECTION IN ANNAPOLIS (Motors) OMNI I- =h- 40 oe t 3000 40 00 3= Z CD 30 1 k (5) 1, is so so \ 1 000 to w / / / 000 go to 0 00 Zk\1 k b\-. 0 00c) Soo 0 0 500 500 N 49 E 99 S SW W No N FETCH CALM ro@_V_ 11 ; ... -,T-L@, __jj 1. N at E So S $9 W NO N FETCH CALM 0 (meters) N 09 E 99 S 80 IN M N FETCH WIND DIRECTION IN ANNAPOLIS (motors) (motors) WIND DIRECTION IN ANNAPOLIS *nNO'DIRECTION IN ANNAPOLIS ho,^ 40 X a - - - - - - - 00 loo, 0 c 2000 000 on to 00 93 Soo ow it g, 500 10 500 0 Irv: 0 N No E 61 S to W 01111 N FETCH N 09 E 99 -S 1w W fig N FETCH 0 (molars) (meters) N K E K S NO IN NO N FETCH WIND DIRECTION IN ANNAPOLIS WIND DIRECTION IN ANNAPOLIS fmalars) WIND DIRECTION IN ANNAIPOLIS Figure B.6 '0 WIND-GENERATED WOO-GENERATED WAVE HEIGHTS WAVE HEIGHTS WIND GENERATED Fm WAVE HEIGHTS A000 40 Q9 2000 i 10 n,' 40 00 0 0 so 30 Uj :a 000 to .11.,1 N 0 S to go 00 CALM CALM 500 500 N 0 CALM 0 #49 E N S 80 IN No N FETCH N lot E 119 S sw W Not N FETCH N HE E st S S* W No N FVCH WIND DIRECTION IN ANNAPOLIS (ritaters) WIND DIRECTION IN ANNAPOLIS (motors) WIND DIRECTION IN ANNAPOLIS 0"tWO 3000 40 40 ___v_ 000 CD 0 2000 15 Uj to to mr- W It 20 10 1000 10 500 Soo Soo CAL .0 0 CAI'mo 0 N Of E a S Sw W Mw N FETCH N III S for W low N FETCH N ml E BE S Sw W POW N FITCH WIND DIRECTION IN ANNAPOLIS (motors) WIND DIRECTION IN ANNAPOLIS (motors) WIND DIRECTION IN ANNAPOLIS (mvias) 0 40 3000 70 30W 10 .00' k I, to Vvv 2000 0 so 15 [TV 2000 to 1\ 15 Q. to to It 1000 10 It, 10 5 500 N5 10 Soo CAL . . . . . AL" . . . . . . 0 N ke E 39 S sw W No N FETCH N Oil E 01 S So! W Mw N FETCH N at E 61 S sw W 110 N FETCH WIND DIRECTION IN ANNAPOLIS (motors WIND DIRECTION IN ANNAPOLIS imolerO WIND DIRECTION IN ANNAPOLIS Figure B.6 C "C' 44- / @14 0 "I al'I @5. with ice. So, the inferred distribution of wave heights at these large wind speeds does not have any important effect on the computation of the wind-wave energy budget for this study. ii. Computation of Wave-Energy Budget In order to be able to transform wave height into wave enerqy, the following experiment was conducted. Both the electrical resistance continuous wave height recorder and a graduated staff were used simultaneously to measure wave heights over a range of wave conditions. From the wave recorder strip chart, the RMS wave height was determined for each parcel of waves measured. This in turn was converted into a measure of energy by the equation.- EW @09NH2 rms B.1 8 .where: EW average energy per unit surface area (ft-lbs/ft); Hrms Root mean square wave height; (EH2/N)1/2, i i ?g =Specific gravity of water =62.5 lbs/.ft3 Figure B.7 shows the relationship between observed breaking wave height as measured by the graduated staff, and total energy in the corresponding,individual wave packets-as measured by the wave recorder. The dotted line in Figure B.7 is the least square polonomial regression line which models the relationship between these two quantities. The equation for this model is.: B 15 Ew = -2.877 + 3.867 h - .068 h2 B-2 where: Ew = wave energy (ft-lbs/ft/min) h = observed wave height in centimeters The presence of a negative leading term on the right hand side of this equation suggests there is negative wave energy at zero wave heiqht. This spurious result shows the model is approximate, and is a consequence of sampling error*and measurement error. In practice, this is of no consequence as all wave heights leading to negative energies were ass igned zero energy. On the basis of the above formula, wave heights at I cm. intervals were transformed to wave energies and summed within months. In this-manner, monthly wind-wave energy budget s for each of the sites were*developed, and are shown in Tables 7.2. 7.3, and Figure 7.7. iii. Precision of Wave-Energy Estimates One important question about the wave energy budget is: What is the precision with which the monthly total wind-wave energy is estimated by the-above method? The following discussion presents a rough estimate of this precision. Total Energy "EhIl is the sum over the hours in the month "M." of the energy-per-hour resulting from waves of a given height "h" which were generated by a wind'of velocity. 11V" at Site "S". This can be symbolically represented by: B-16 Total Energym,s Energy (h(V,S)) hours in month The relationship h(V,S) is given by the models displayed in Figure B.6 a-f. A relationship between wave energy and wave heiqht is given by the graph in Fiqure B.7 The variability associated with-each hour of estimated wave energy is an accumulation of: o the errors in estimating the average hourly wind velocity; 6 t'he variability in observed wave height for a given wind velocity; o the variability in ener4y per hour as a function of observed wave height. In the analysis of the'study data, the average hourly wind speed on the,strip charts was estimated to within + I knot, and the average wind direction was estimated to within +.22.50. These magnitudes-of error in measuring wind speed and direction typically translate into a wave height error of + 1 cm. on the wave height models of Figure B.6 a-f. The data from which these wave" height models were developed also had typical variabilities which were estimated as follows: - +-l.cm. for wave heights measured at wind velocity <5 knots - + 2 cm. for wave heights measured at winds between 5 knots and 10 knots - + 4 cm. for wave heights measured at winds greater than 10 knots opposite: Figure B.7*.Observed breaking wave heights plotted against the energy in the waves. B-17 RECORDED WAVE ENERGIES 100- AT DIFFERENT OBSERVED WAVE HEIGHTS 90- so-- 70- so- 50- Fo 0 40- LU 00 0 LU 30- A '20- 0 10 0 o 2 4 6 8 io 12 .14 16 18 2o 22 24 26 213 3o OBSERVED BREAKING WAVE HEIGHT (cm) Fi gure B. 7 The errors in measuring wave heights and in correlating. wave height to wind speed and direction together result in an error in wave height of + 2 cm. associated with waves of 5 cm.; an error in wave height of + 3 cm. associa ted with 5-10 cm. waves, and an error in wave- height of + 5 cm. associated with >1.0 cm. waves. This variability in wave height translates into a variability in wave energy which is shown in Figure B.7. -For example, waves of 5 +2 cm. have an estimated energy within +6 ft-lb/,ft/min; and waves of 8 +3 cm. have an estimated'enerqy within .+8 ft-lb/.ft/min. For a single value of +8 ft-lb/ft/min. (equivalent to, +480 ft-lb/ft/hr), there is a standard deviation of 2.40 ft-lb/ft/hour, assum ing + 480 represents + 2d . Summing this variability over 72,0 independent hourly energy estimates for the month qives a .total variance of; 720 (240)2 41,472,000 ft-lb/ft/month2 or a standard deviation of 6440 ft-lb/ft/month. since tota 1 wave energy for any month is typically.on the order of 400-000 ft-l.b/ft/month (Table 7.3), the error +26 in the calculation of total energy "Eh" by the method described in this chapter yields a.precision of 2(6440/400,000). This i,s equivalent to an errorof + 3.2%. This estimate is rough, but it is very unlikelv to be off by any factor greater than 2.. Even in such a case, the precision of monthly wave-enerqy estimates are judged to be quite good. T3_19 APPENDIX C GAUGE SHALLOW WATER WAVE A shallow water wave gauge was constructed by CEA based on a design by McGoldrick (1969). The sensing element of the device is a capacitance probe featuring a loop of Teflon-coated wire (No. 20) mounted on a supporting rod. The Teflon insulation forms the dielectric and the central conductor and conducting fluid surrounding the wire form electrical,plates. If the insulation is uniform and end effects are negligible, then the capacitance varies linearly with the proportion of the wire length immersed in the conducting fluid (sea water). A transistorized detector (Figure C.1) converts changes in capacitance into a variable D.C. voltage which is routed to a strip chart recorder (linear model 142). Teflon must be used as the insulating material because of its high resistance to "wetting" by films of water that would otherwise delay the- response of the gauge in sensing the rapid fall in water level following the passage of a wave. The CEA wave gauge is designed primarily for shallow- water applications in small estuaries and creeks. The sensing unit containing the detector and wire loop is a Next pages: Figure C.l. (left) Transistor Wave Detector (after McGoldrick, 1969). Figure C.2 (right) Wave gauge calibration data. C-1 Vic I IOK I 2.?K Of (4) IN 191 2111004 01 ice .1 2N 1305 C4 .009 R* RS Re 04 .001 1OK IOX IOK 2N 1304 C, C loop T, Tj ... ... 03 -------- vh, ... ... ... I "C' CS Ce T Ce 1. 101 Do$ OS 5 005 BNC Rt 3 "TTKI OUTPU T 1509 2N 130 .10 PROBE SENSITIVITY fOft PUAtLY CAPACITIVK PROD[ VS. Re. WITH Vcc 15.00 V.D.C. Tj 20000 - 500 A IF TRANSFORMER 1455 KC) 40 Tt: moo-soon IF TP &%sromtt t 455 Kc I 36 ALL CAPACITORS IN #0. UNLESS OTHERWISE NOTED 20 24 Transistor wave detector 16 1 . . . (after McGoldrick, 1969) 1 10 it 14 16 16 to 22 24 Pg. Kn Figure C I 2N U I C loop t 03 C. ?N . @R 5011 5 4 lu w @L Z 3 w 0 2 W I 0, 0 2 3 4 5 TRUE HEIGHT IN FEET WAVE GAUGE CALIBRATION OATA Figure C-2 I-inch diameter PVC rod installed by thrusting its sharpened end into the bottom.' A circular footplate mounted 18 inches above the bottom of the rod aids in the installation and provides added.stability to the probe in maintaining a vertical position. A 100-foot conductor cable attached just above the foot plate carries the D.C. voltage output of the detector back to the recording unit on shore. The sensing unit can be installed in depths varying between 1 and 3 feet and-will sense changes in water level over a vertical range of -4 feet. Markings on the rod at half-foot intervals are provided to allow field calibration checks to be obtained as necessary. Calibration checks should be performed in calm water by holding the probe at 2 or more depths for several seconds and noting the indicated depth intervals on the recorder. Calibration adjustments-are made by adjusting the signal attenuation control until the intervals agree. The detector circuitry ishoused in a water-resistant casing at the top of the probe. The unit is activa :ted by means of a switch exposed when the housing cap is removed. Power is supplied by a 9-volt transistor battery located inside the casing. This battery should be replaced after each 50 hours of use.' The circuit diagram of the detector unit is presented in Figure C.l. Laboratory tank calibration tests show excellent linearity in gauge response over the full 4-fobt depth range (Figure C.2). IC-4 I I I I I I I I I I I I I I I 'I I I i ! @ 11 @ I I- 3666800000891A I