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9 Center for Marine Coastal Studies - Nor COAST University* Z/ INFO, ATIG-41 CENTER A PRELIMINARY STUDY OF STORM-INDUCED BEACH EROSION FOR NORTH CAROLINA BY C. E. KNOWLES and JAY LANGFELDER and RICHARD McDONALD "REPORT NO. 73-5 OCTOBER, 1973 GB 458.8 X56 1993 th Ca linAState rAT CENTER FOR MARINE AND COASTAL STUDIES NORTH CAROLINA STATE UNIVERSITY RALEIGH, NORTH CAROLINA A PRELIMINARY STUDY OF STORM-INDUCED BEACH EROSION FOR NORTH CAROLINA by C. E. Knowles and Jay Langfelder and Richard McDonald Report No. 73-5 October 16, 1973 U.S. DEPARTMENT OF COMMERCE NOAA COASTAL SERVICES CENTER 2234 SOUTH HOBSON AVENUE CHARLESTON, SC 29405-2413 Property of CSC Library A Preliminary Study of Sto@m-lnduced Beach Erosi.on for North Carolina Introduction Storm erosion of beaches and dunes of the North Carolina coast has always occurred, but it.has not been a serious economic problem until recently when increased development of beach front property has.taken place. In some locations, structures have been built seaward of the beach storm recession line and have sustained consid- erable damage. This report presents the results of a study to deter- mine -the.expected storm induced beach erosion which can provide 'preliminary information needed for coastal land management.. Preliminary beach development limits recommended in this report are based orf calculated recessions of beaches for storm.occurrences of one-in twenty five, one in fifty, and one in a hundred year-fre- quencies. Necessary field data were collected from fishing piers along the entire North Carolina coast except for Currituck and Hyde Counties where no piers exist. Beach recession values were.calcu- lated usi,ng essentially,a semi-empirical procedure that is a modifi- cation of .,a technique adapted by Vallianos of the Wi-l-mington District of,the U. S. Corps of Engineers, from work done earlier by..Edelman (1). Procedure The general, method employed in this study to predict storm in- duced beach recession involves the balancing of the areas of upland 2 erosion and offshore deposition due to a storm. It is assumed, there- fore, that all sand eroded by the storm is deposited directly offshore or that it is moved along the shore at a steady rate; as much is coming into the area up-current as is leaving down-current. Based on field observations and model study tests (2.3) it has been determined that the. offshore profile flattens by a factor of about two from its prestorm slope. The sand required for this flattening comes from the beach-and dunes. Beach- profiles needed for this study were obtained, as noted earl.ier, from fishing piers along almost the entire North Carolina coast. Soundings (the distance from the beach and offshore,bottom to the top of the pier) were made at closely spaced intervals,.but away from the pier pillings to.avoid making measurements in scour holes. Most piers-were near enough to a U. S. Coast and Geodetic Survey bench-ma.rk so that these soundings could be converted to beach pro- files based on mean sea level (MSQ. In those locations where es-_ tablished bench marks were not available, assumed elevation relative to the waterl-ine was used. Using the time that the waterline soundings were made, elevations were converted to an estimated MSL using N.O.A.A. tide tables (4) applicable to the pier location. T-he height, width and shape of the dunes at each pier location were recorded and in those locations where man-made structures changed the shore profiles, measurements of dunes characteristic of the area were used. 3 Storm surge levels above MSL as a function of storm return fre- quency were obtained (5,6,7,-8,9) for the five areas of North Carolina and,are shown in Figure,l and listedin Table 1. The storm surge level is defined as the still water height that can be expected to occur once duri-ng the indicated return frequency. For the purposes, of this study surge levels for storms with a once.in twenty7five, fifty and one-hundred year probability of reoccurrence were used to obtain a range of beach recession values., These surge levels .(S) were used to.obtalin the height (H) and breaking depth (Hb) of the waves. associated with the storm. They were then-used to calculate the ass.oc-iated beach recession using th.e modified technique of Edelman discussed in detail in the remainder of this section. The techn-ique (see Figure 2 for exa,mple cal-culati,on) involves plotting the before-storm beach profile and A line representing the storm surge level (S). For each of the three storm reoccurrences. wave height-.,(H) is ca,lculated by H = 1 .5 (S) and the breaking depth of the wave (Hb) above the bottom is given by Hb = :1.3(H). (2) To find the breaking depth relative to MSL (Hm), the storm surge is subtracted from Hb, i.e., Hm = Hb - S. (3) Values.of HM for th.is study are shown in Table 1. The outer limit@of sand,deposition is assumed to occur at a depth approximately equal, to thebrea.king depth ofthe wave. The foreshore limit. of off- 4 shore.bottom slope or-beach breakpoint in th,e-before-storm profile is then.found. An examination, of..all th,e profiles.obtained by this study where the-fo'reshore was clearly,discernible,,indicated that the,breakpoint occurred at.a. mean depth.of approximately -1.5.ft. bel-ow MSL,.and-a horizontal line at this depth was then drawn and acted as the pivot line.along which thestorm-beach profile slope changed. Except-at the two extreme ends of the storm profile, the beach and offshore slopes were-flattened by a ratio of 2.1:1 compared@to the original profile. As discussed earlier.this flattening was deter- mined-from model study tests for storm waves.. In constructing the storm-beach profile from a pre7Storm multi-sloped beach.,it was necessary to..establish a procedure for selecti-ng the locations at which the storm profile would change., Landward of and-including the breakpoint,pivot line,, all storm-beach.slopes change at a 2.1 to. 1 ratio on a. line drawn vertically from the,point.where the,pre- storm profile changes slope, as shown, in Ff.gure 2. The dune characteristics that were measured during this,study may be classified in th.ree,ways. First, the-dune will be,sufficiently. high-and wide to remain above.the-storm surge level during the erosion process.. Secondly, the top of the dune. will be,above the storm surge initially, but will be bel.ow the storm surge as erosion of the dune progresses., and thirdly, the dune will be below the storm surge level initially so that, the dune,will be overtopped through- out the entire erosion process. 5 For the first classification the 2.1:1. storm profile line will inters-ect storm surge level below the.top, and, a 10(V):l(H) slope is drawn from this intersection to the top of the dune. Under the. second and third classifications the storm profile will not inter- sect storm surge level,so the storm profi.le retains its 2.1:1 ratio of-the originaT until it,intersects-the back side of the dune. Finally, to determinethe storm-beach profile it was necessary to match,the area of erosion,with the area of deposition as shown in Figure 2. A p1animeter wa.s used to measure areas.and the storm pro- file lines were adjusted until the.erosion and depositionareas were equal. For-the second and third conditions, it is apparent that with. overtopping of the dune not,,all sand eroded by the storm will be. deposited offshore, as assumed by this procedure and more erosion will. actually occur than is predicted by this technique. This addi- tional erosion at overwash areas has'not been considered in the cal- culati,ons. Finally, a measure of the rec.ession of the beach due to the storm from a relatively'fixed and identifiabl:e point was made.- The measurement of the extent@_of storm damage was taken as the distance from the seawardItoe of the dune to the.landward point at which the. storm profile@crossed the.original profile. The recession values for each pier,location are-shown in Table 2. The measurements from the, toe of the.dune.were.-chosen to express the range of recession for the'reoccurring storms., because this point was judged to be the most stable and most easily identified. 6 Results and Discussion The calculated storm recessions for each of the 29 piers that were-analysed are presented in Table 2. This table not only indi.- cates the..amount-of,.anticipated recession but presents information on-the dune characteris.tics:in the area. The calculated values of storm erosion depehd-on several factors; ffrst, the storm surge level. that can,be expetted for thelvarious reoccurrence frequencies,. secondly,the height and massiveness of the'dune at each, location and thirdly, the,distance-of the dune from the mean-water line. The affects.,of these factors can readily be noted in the.data, presented in Tabl;e@-2. From the results1of Table 2 recommended ranges of recession lines along the North Carolina coast are,presented in Table-3 and in Figures 3 and 4 for reoccurrance frequencies of-one in twenty fi,ve years and one in a hundred years. These recommended, ranges were obtai-.ned by@wetghting the various data.according to the degree. of reliability that could be assumed for each of the.individual pier locations. Thus,- for,pier.locations where the elevations were obtaIned from estimates of the water level obtained from N.O.A.A. tide tables, less weight was given to.the.results than for those which were.ob- tained from,U.S.G.S. bench-marks. At several of the pier.locations there were sea walls constructed and in those areas no adjustment wa-s made for the difference in erosion affect at the sea.wall. Also, the top of the dune at three locations was, unidentifiable or was found to be below ormery near.the mean high water level. 7 Although the,ranges presented in Table 3 are based on a.semi- empirical method and are somewhat preliminary, they-represent a rea- sonable estimate of storm induced beach erosion limits. They-are recommended for use in coastal zone.management as preliminary cri- teria-for establishing a dynamic zone. The,above resuItsmst be considered preliminary since the present profiles were only taken where:piers exist and then only at a single time,during the summer. [email protected] could obviously be obta,,ined by securing additional profiles, during other times in theyear. In add.ition, in areas where piers are-not lo- cated., profiles,should be obtained by soundings from a boat. The storm induced. recession prediction presented in this report is considered to be useful for determining the distance,from the toeof the primary dune.in which any structures,mig ht be considered to be in d.anger'. However, if a building set.back,line.is to.be established, additional factors,such as long time erosion, contin-. uity_of@the dunes, size and shape of the.dunes, potential for over- wash and other existing@features should be cons-idered. This type- of information would require a more detailed study than was under- taken in the.present report. However, such information can be ob-. tained,,.and by combining all, of this information a reasonable setback.line could be establ,ished that would provide some!gui.dance for development-of our beach front areas. Acknowl edgment This study was supported in part,.by the Department of Natural and Economic Resources of the State of North.Carolina. The field 8 work was able performed by Bruce Harvey, Cid Carrilho and John Holli- day.of the Department of Geosciences of North Carolina State,Unive.rsity. References 1. Edelman, T. - Dune Erosion During Storm Conditions, Proceedings Of Deventh Conference On'Coastal Engineering, London, England, September 1968. 2. Rector, Ralph L. - Laboratory Study Of@Equilibrium Profiles Of Beaches, Beach Erosion Board Technical Memorandum No..41, August, 1954. 3. Noda, Edward K.'- Coastal Movable-Bed Scale-Mo-d 'el Relationship, Final Report Prepared For U. S. Army Coastal Engineering Re-, search Center, Tetra Tech., Inc., March 1971. 4. Tide Tables - High And Low Water'Predictions-East Coast.Of North And South America U. S. Department of Commerce,.National Oceanic And Atmospheric Administration, National Ocean Survey, 103. 5. "Outer Banks Between Virginia StateLine And-Hatteras..Inlet, N. C." Interim Survey Reoort%Of@Hurrican Protection, U. S. Army Corps Of@,Engineers, Wilmington, N. C.- District Aprili 1965. 6. "Ocracoke Inlet To Beaufort Inlet, North Carolina" Interim Sur- vey Report Of Hurricane Protection, U.-S. Army Corps Of-,Engineers, Wilmington, N. C. District, December, 1964. 7. Tide-Level - Frequency Analysis For Ocean Shore OfBogue Banks@ North Carolina. National. Oceanic And Atmospheric Administration.National Weather Se.rv1cb. July 18, 1972, (An Anal,ysis, Made At The Request Of U.S. Army Corps,Of Engineers, Wilmington, North.Carolina District). 8. Survey Report.Of Hurricanes, Carolina Beach, N.C.,,And Vicinity, And Cooperative Beach Erosion Control Study, Carolina Beach, North Carolina,Damag,e Benefit Analysis, Data. U. S. Army Corps Of Engineers Wilmington, North Carolina District September 1960. 9. Brunswick County Beaches, North Carolina General Design Memorandum, Phase I, Yaupon And Long Beaches July 19, 1973. 9 Table 1. Storm stillwater surge levels and breaking depth of waves for one in a twenty-five, fifty, and one hundred years storm return frequency, respectfully. SURGE LEVEL(S) (+Ft.MSL) BREAKING DEPTH(Hm) (-Ft.MSL) Virginia To 1/25 .1/50 1/100 1/25 1/50 1/100 Cape Hatteras 7.43 8.20 8.80 7.06 7.79 8.36 Cape Hatteras 7.10 7.63 8.00 6.75 7.25 7.60 To Cape Lookout Cape Lookout To 7@63 9.33 10.95 7.25 8.86 10.40 New River Inlet New River Inlet 8.80 10.55 12.05 8.36 10.02 11.45 To Cape Fear Cape Fear To 9.67 11.23 12.45 9.19 10.67 11.83 South Carolina Table 2. Results of beach recession study for North Carolina Coastline County. and Dune Height Toe of Dune Recession from toe of dune (ft) Pier'Name (ft. above Height(ft), Dist. from for'.three storms with.specific MSL) from MSL MHW(ft) return frequencies inlyears Dare-Coun@y 1/25 1/50 1/100 Kitty Hawk 19.6 12.1 215 11 34 54 Avalon 18.5 9.3 150 40 70 94 Nags Head 22.9 7.6 112 94 - 107 126 Outer Banks 30.0 4.1 75 69 73 78 Hatteras IsTand 21.7 2.2 102 99 104 108 Cape Hatteras 24.0 8.8 61 57 66 74 Carteret County Wall Triple Ess, 22.0 4.3-9.6 24 91 114 161 Oceana 21.7 5.9 83 101 132 168 Iron Steamer. 15.7 6.4 136 80 95 106 *Elevation's assumed and corrected to estimated MWL using N.O.A.A. TideTables Table 2 continued, County and Dune Height Toe of Dune Recession from toe of dune (ft) Pier Name (ft. above, Height(ft) Dist., from for three storms with specific MSL) from MSL MHW(ft) return frequencies,in years 1/25 1/50 1/100 Carteret County ,Emeral.d Isle 24.0 6.6 83 66 74- 98, Bogue Island 16.0 8.9 158, 21 162 220 Onslow-Coun@y McKWs 11.0 4.7' 78, 95 108 134 Paradise 14.1 10.5 173 143 189 223 Ocean City 12.3 7.0 113 161 206 229 Pender County Scotch Bonnet 23.0 9.4 154 133 144 178 *Elevation's assumed and corrected to estimated MWL using N.O.A.A. Tide.Tables C\j Table 2 continued County and- Dune Height@ Toe of Dune Recession from toe of..dune (ft) Pier Name (ft. above Height(ft) Dist. from for three storms with specific MSQ from MSL MHW(ft) return frequenc-ies--in years- 1/25 1/50 1/100 Pend6;r County Surf City 25.0 5.6 98 113 186 227 Dolphin 28.3 7.7 114 103 120 188 Ocean 17.9 8.2 103 99 178 234 New Hanover-County. Johnny Mercer 14.0 5.7, 112 165 175 180 Crystal 15.4 6.4 112- 123 145 194 Carolina Beach 10.4 Wall (Rock) 23 264 347 363 4.1-11.3 Center 12.9 153 206 270 Kure 12.8 Wall 109 74 124 1144 12.5-18.5 *Elevation's assumed and-corrected to estimated MWL using N.O.A.A. Tide Tables Table 2 continued County and Dune Height Toe of Dune, Recession from top ofdune .(ft) Pier Name (ft. above Height(ft) Dist. from for three storms with specific MSL) from.MSL MHW(f-t-) return frequencies in years- 1/25 1/50 1/100 Brunswick County Ya,upon 10.7 2.1 2 175 227 237 Ocean Crest 12.9 8.0 87 83 101 116 Long Beach 16.7 8.5 105 142 175 192 Holden Beach 14.9 9.9 15,7 199 220 265 Ocean Isle 14.9 None 120 134 150 *Elevation's, assumed and corrected to estimated MWL using N.O.A.A.1 Tide Tables 14 Table 3. Recommended range of recession lines from the dune toe for one in twenty five and. one in a hundred years storm return frequency Location Range of recession. lines (ft) 1/25 frequency 1/100 frequenc I Virginia to Cape Hatteras 40-100 80-1.20 II Cape,Hatteras to Cape Lookout (one value) 50-100 70-120 III East-West Portion to Carteret County 70-100 100-170 IV Onslow County 100-160 130-230 V Pender County 100-140 180-'230 VI New Hanover County 120-170 180-270 VII Brunswick County 120-190 150-260 12 J U! 10 2 w 0 w 9- w LL z w 10 > w 8 @A 7 25 30 40 50 60 70 80 90 100 ESTIMATED AVERAGE RETURN PERIOD IN YEARS Figure 1. Storm surge levels related to return frequencies for five subdivisions of &-,010 0000@ the North Carolina Coast. ELEVATION IN FEET FROM MS.L ril co r- :0 C:) cp Cil mo ol rr 0 0 (A. 0 CD 0 0 el% e- C: 0 C+ 0) Fn 0 qr+ m rn =in cn(j) > ju D-H 11 -f CO) IrWm 0 -0 :C;K ZA 0 X m 0 X:E 11;E ocn 0 > tT N Wrn PD< CD C)m G) m (A CD -4 M C G) om (.nr- --A m \/<NA m rn r-m > r > X 0 m 0 uld 0 Z < 11 < CA rTl k<@4j I rn m- N CA 0 ;D rT, m 3> 0 K\ H b BERTIE MARTIN WASHINGTON PITT TYRRELL BEAUFORT DARE HYDE CRAVEN JOMES 'PAMLICO ONSLOW PENDER COLUMBUS CARTERET N EW ANOVER. CP 0 BRUNSWICK too- j(0 O@, 10 \AID 420- /90 Figure 8. Recommended range of recession lines from the dune, toe for.on,e in twenty five years storm return frequency TIP BERTIE ARTIN WASHINGTON PITT TY R RELL BEAUFORT DARE HYDE CRAVEN JOME.S PAMLICO ONSLOW PENDER CARTERET COLUMBUS 1-n:0 NEW HANOVER BRUNSWICK 01/ /so- Figure 4. Recommended range of recession,lines from the dune toe for one in a hundred years storm return frequency Lr, 3 6668 00001 8442