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I II .1 l4 CHARACTERISTICS AND PROCESSES OF THE COASTAL ZONE IN SKAGIT COUNTY, WASHINGTON U.S. DEPARTMENT OF COMMERCE NOAA COASTAL SERVICES CENTER 2234 SOUTH HOBSON AVENUE CHARLESTON1 SC 29405-2413 BY RALPH F. KEULER DEPARTMENT OF GEOLOGY. % - 11%- ,I cm IA . ,-c S- WESTERN WASHINGTON UNIVERSITY "The preparation of this report was financially aided through a grant from the Washington State Department of Ecology with funds obtained from the National Oceanic and Atmospheric Administration, and appropriated for Section 306 of the Coastal Zone Management Act of 1972." Property of CSC Library LII I [I~ i LJ I [II I 1l I I 4 ci 1. El I j: j j j j ICNTENTS Page 1 .2 .7 11 & 17 20 * 22 29 * 36 * 37 * 38 *. 39 a 40 0 41-48 Introduction Physical Setting Physical Description and Natural Processes Within Drift Sectors Prograded Beaches Shoreline Erosion Shore Defense Structures Slope Stability Discusaion of Selected Individual Drift Sectors Summary References Index to Maps Xey.to Ma-oSymbols Explanatory Notes for-Map Symbols Maps * i 4, .. r it -.. Ell .LI l.. A. 1- El ' .i II [I -I [I 11 I I INTRODUCTION This report summarizes the results of an investigation of coastal zone processes and characteristics in Skagit County, Washington. The study was incorporated as part of the author's research for graduate study in geology at Western Washington University. The author wishes -to thank the Skagit County Plan- ning Department, especially Mr. Robert Schofield, Director, and Mr. Stephen Harvey. Both of those gentlemen expressed an inter- est in the project in it's initial stages, and also made logis- tical and partial financial support possible. The investigation covers a number of aspects of coastal zone processes and morphology. Among those subjects are: 1) delineation of drift sectors within the county, 2) directions of net long term sediment transport on beaches, 3) identifica- tion of prograded beaches, 4) study of shoreline erosion rates, and 5) delineation of hazardous zones with respect of landslides in coastal bluffs. This report is designed to be used in conjunction with and to supplement the Coastal Zone Atlas presently in preparation by the Washington Department of Ecology. That atlas, by it's very nature, is a generalized reconnaissance'of the coastal zone. This study is more detailed in a number of respects, and the emphasis is on beach and erosional processes. 1 I v LI ~~~~~~~PHYSICAL SETTINTG LI ~~~Little study of windsa and wind generated waves has been done in western Skagit County. Until 1973 no wind data was col- LI - lected on a regular basis. Table I Is a summary of winds near Anacortea collected at the Shell Oil Company refinery. Since Anacortes Is located approximately.in, the center of the coastal zone area the winds there are thouLzht to be fairly representa- tive of the area as a whole. [I ~~Based on that wind data the larges waves that would common- ly occur would be about 6 feet. That estimate is produced by utilizing wave forecast graphs used by the U.S. Army'Corps of Ii ~Engineers (1973). Further confirmation comes. from an unpublish- ed study done in the southern Strait of Georeia in British Col- umbia (B.C. Research., 1974). There' the fetch of open water Is - ~much longer than those found in Skcagit County; at that site ex-' Li ~treme winds produced waves uD to 8feet high. [U ~~Of equal significance in Skagit County Is the larEe amount of time during the year that wind velocities are quite low. LI ~As noted in Table I the winds are under 12 mph for 80% of the year. Those mild winds produce waves that have no importance with regard to coastal erosion. The generally low wave heights, relative"ly high tidal r'anLzet and the poorly sorted sediments Inherited from glacial deposits [I ~combine to produce a beach morpbology in this area that is not LI ~particularly common worldwide. The upper part of local beaches Li ~(the high tide beach) is-typically comp'osed of sand, gravel, 2 _41 rII; Lli f- -: tL I I i, L r-- LA Table 1. Wind Velocity and Direction Shell Refinery, Anacortes, 1975-1977* Velocity (mi/hr) Velocity % 0-2.99 4.4 3-6.99 7-11.99 12-17.99 18-23.99 >24 .9 38.4 14.2 37.2 5.0 Direction Direction % 5.8 7.2 N NNE 2.5 2.7 NE ENE 6.1 7.0 ESE 13.7 SSE 13.2 6.3 3.3 SW 3.1 5.7 WS W W 7.2 WN W 5.2 ' NW NNW 5.7 5.4 *Based on 25,168 hourly observations over the 3 year period. Courtesy of Northwest Air Pollution Authority, Mt. Vernon, Wn. 3 ~~ Icobbles., and boulders Intermixed In varying proportions. The f ~~highi tide beach is moderately steep witb slopes typically being 4 to 11 degrees. The lower portion off tbe beach (the low tide Li terrace) normally is composed primarily off sands and ffiner mat- erials and is nearly horizontal. The Junction point between Li the high. tide beach and low tide terrace most often occurs LI ~at about the mean lower low water mark (zero feet) and the Li ~change in slope at that point is often'quite abrupt (ffigure1) LI two 'In areas where the sboreline is composed of rocky cliffs tomorphological settings are common. In the first, where LI ~almoBt no erosion has occurred, the cliffs plunge directly into LI ~deep water. The second, where the rock is less resistant to erosion, has an abrasion platform developed due to the removal Li. of rock. These are -planar features that slope seaward at 2 to 4 deLzrees and can be in excess of 150 feet wide (ffigure 2). Li The effect of precipitation in Skagit County.plays an LI ~occasionally signif'icant role In the coastal zone, but gener- ally Is only a minor factor. Average ann.ual precipitation in LU ~the Anaoortes area Is abo ut 26 inches; however, only about 25% of that amount appears as stream flow (Phillipsy 1966). The balance.is released back to the atmosphere as evaporation and transpiration by plants. As a result, most local streams are small with sediment contributions to the beaches being negli- I g able. Furthermore., the size off sedime-nts delivered to be'aches Is generally significantly smaller than the particles useable Li ~in building the beaches. In very localized areas precipitation 4 F I Li r- Figure 1. High tide beach and low tide terrace exposed during a very low tide. Note the abrupt change in slope between the horizontal low tide terrace and the steep high tide beach. I Abrasion platform eroded into bedrock; approximate width is 100 feet. Figure 2. 5 LI LI I can be of importance in that infrequent, heavy rainfall triEEers -landslIdes which can be a substantial contribution to the sedi- ment bud6et of a drift sector. I I 6 LI [I [IA I ri I 1 [I LI [I LI [I AI PHYSICAL DESCRIPTION AND NATURAL PROCESSES WITHIN DRIFT SECTORS A drift sector, as commonly understood in the earth sciences, spans a shoreline segment from an area that is contributing sedi- ment (the beginning) to an area that is receiving and accumula- ting the transported sediment (the, terminus). As noted previously, there are' few streams in Skagit County that contribute any significant amount of sediment in sizes suit- able for beaches. Locally the contributions of sediment are de- rived mainly from the erosion of shoreline bluffs. Therefore, within a drift sector the beginiing is an entirely erosional setting, whereas further along the length of the sector the more abundant transported beach materials afford greater protection to the bluffs. All other factors being equal then, erosion will proceed at a faster rate at the beginning of a sector than at a point down drift from there. The results of the above erosion/ transport processes are usually clearly manifested in the appear- ance of the beach, the beach sediments, and the bluffs. The beginning of nearly every drift sector within the county is typified by very low angle beaches (slope of 2-4 degrees), with the profile being essentially planar. The sediment on the beach is usually very coarse cobbles and boulders, often with the bare, eroded sub-beach surface visible beneath the cobbles. At the foot of the cliff there is little or no accumulation of finer sediment in the form of a berm or backshore deposit. Often a high tide will lap at the foot of the cliff. The bluff itself usually is at a very steep angle (greater than 50-60 degrees) 7 and in many cases, nearly devoid of vegetation. All of the abov'e features can be seen in Figures 3 and 4. The physical setting Just described is in sharp contrast to the appearance of the shore zone further along the length of the drift sector. Figure 5 is typical of a beach and backshore located about 3/4 of the way along.a drift sector. Here the much steeper beach angle (6-7 degrees) is immediately obvious, as are the smaller particle sizes of the beach sediment, typi- cally mixed sand, gravel, and a few cobbles. Also noticeable in the-photo is the beach ridge or berm (partially vegetated) which helps protect the bluff. The face of the bluff is well vegetated, with a low slope indicating that wave erosion is not proceeding at a rapid rate. LI LI Li Li C - 4.- 4t' *1 -_ tO * -. ... -r,. -- - - t'. s-* AK, - A, g_j AP_.w_ "S _ Fisure 3. Beginning of drift sector to the east of Dewey Beach. The coarse beach sediment, bare patches of eroded beach surface, and steep cliff are evident. t_tz y_#2_t U -- -_ A tr_- 4 '-_1' -c \_ Fl c--St--- ,r' '4 C ,C�t? *.t It r. yti_ _. _ Figure 4. Near the sector. no berm, beginning of the East Sinclair Island drift Here also characterized by coarse sediment, and steep cliff f&ce. 9 I. A~~ - -= .~ ,- ~:' �4-.. '-i ..~ ~, ~-.- :.> ---- -..�:" "~....� *"'-':' '< ,- ' . '- -''-~ ".~- . �-".'.-:'-:,f 9 e-iW4 _ ~~~~~~~~~~~~~~~~~~~~~~- h I Photo taken about 3/4 of the way along the length of a drift sector. Note the steeper beach slope, the finer beach sediment (mixed gravel and sand), a partially vegetated berm, and well vegetated bluff. Figure 5. 10 PROGRADED BEACHES - Throughout this report the term "prograded beach" is used as opposed to other terms, such as "accretion beach", that have i ~ been used by some investigators to decribe beaches that have accumulated sediment and built seaward. The difference is not simply one of individual preference; rather, the word "prograded" directly implies a beach that has built seaward due to additions of sediment supplied by beach drift (American Geological Insti- * tute, 1972). The term accretion does not necessarily carry the same implication. Furthermore, the word prograded (past tense) does not imply that the building process is continuing at pres- ent. Indeed, there is evidence that some beaches in the county g ~ that have prograded in the past are presently being cut back. In still other cases continuing progradation cannot be estab- lished without long term monitoring. As noted earlier, prograded beaches are typically located LI| at the terminal ends of drift sectors where they receive accum- ulations of transported sediment. In my inTestigation I have found that there are many more prograded beaches in Skagit Coun- |I ty than have been previously recognized or reported. Table 2 is a listing of prograded beaches associated with the terminal 1 ~ends of drift sectors. In addition to the prograded beaches covered in the previous paragraph and Table 2 there are also prograded beaches occasion- I ~ally located within drift sectors. These are formed where there is a reentrant or embayment in an otherwise relatively straight . .- 11~3 .I I I (i Table 2. Listing of Prograded Beaches Associated With the Terminal Ends of Drift Sectors . Geographic Name/ - Location Appears on Map Comments Strawberry Bay/ SW Cypress Is. Tide Point/ V. Cypress Is. none/ NE Cypress Is. Eagle Harbor/ NE Cypress Is. Cypress Head/ E. Cypress Is. Secret Harbor/ SE Cypress Is. none/ S. Cypress Is. none/ SW Sinclair Is. none/ N. Sinclair Is. none/ S. Guemes Is. A A A A A A A A A B -The active beach fronts an ex-' tensive system of older, vege- tated beach ridges and back- shore marsh Nearly all progradation has oc- curred on the north facing side of the point; southerly waves drive sediment around the point. A small beach (about 500 feet long). Sediment is derived from short drift sector to the north A bayhead beach, most sediment provided by the slow erosion of a rocky shoreline to the south. A tombolo connecting Cypress to Cypress Head; sediment supply similar to Eagle Harbor Description and sediment supply very similar to Eagle Harbor above. Most sediment supplied from the west. Slight erosion of prograded sedi- ment along the northwestern part. One of the larger accumulations of transported beach sediments in Skagit County Probably the finest example of a prograded beach in Skagit County. Sediment is supplied from both the east and west. 12 .'j 7 Fe- _i - *i. i. ... Table 2 continued none/ -W. Guemes Is. B Begins approximately where Edens Road meets the shoreline and continues north another 2200 feet Indian Village/ W. Guemes Is. B -none/ NW Guemes Is. Begins about 1000 feet south of Clark Point and continues south for about 1200 feet none/ NE Guemes Is. none/ E. Guemes Is. Sleepy Hollow/ E. Guemes Is. Kirby Spit/ SW Samish Is. Samish Beach/ Blue Herron Beach/ Fish Point/ N. Samish Is. Scotts Point/ E. Samish Is. Ship Harbor/ NW Fidalgo Is. Weaverling SDit/ W. Fidalgo Bay Some slight erosion near the southern boundary of the pub- lic park Begins where Guemes Is. Road meets the east shore; continues north for approx. 2.000 feet. Sediment almost. all derived from bluffs to the north. B B It appears that most of the sedi- ment comprising the spit was supplied in the past; the present rate of beach drift along the south shore is not plentiful. A very large prograded beach that is being presently eroded; see text for complete discussion. 0 ' D D Even though the shoreline to the north has been extensively indus- trialized and filled a moderate amount of sediment is still being supplied to the spit Crandall Spit/ NW March Point The spit is now mostly inactive; surrounded mostly by mudflats. Very little sediment is being supplied because of rip-rap on the bluffs and because of docks associated with oil refineries. 13 l I F! 4[I . I in F! ' 1. I! il I' I I Table 2 continued I., _ none/ - nE March Point Dewey Beach/ N. Skagit Bay Similk Beach/ N. Similk Bay Turners Bay/ N. Similk Bay Kiket Island/ same Kiket Island/ E. G A small prograded point; upper parts of beach are rip-rapped. Small prograded area on the east end of Dewey Beach. G G G G Spit now mostly inactive due tb sedimentation on mudflats Tombolo connecting Eiket Is. to Fidalgo Is. Small prograded beach at west tip of Kiket Is., sediment de- rived from erosion on S. shore of Kiket Is. same Lone Tree Point/ E. Skagit Bay Hope Island/ SBE Hope Is. Snee-oosh Beach S. Fidalgo Is. Flounder Bay/ Nl Fidalgo Is1 Small beach. G H F Eroding spit that has been rip-rapped; see text for full discussion 14 I I rI._ 4' I 1 ii I I I I Ii I ii p shoreline. These embayments, over the course of time, have had ..a barrier accumulate across the mouth. The barrier beach ini- tially was'a spit that progressively lengthened until it be- came connected to the opposite side of the bay mouth. Present- ly these beaches pass transported sediment along their length which then continues onward to the. ultimate terminus of the sector. Some of the barriers are still accumulating sediment, others are being cut back as the bluffs on either side of what was the former bay mouth are eroded. Table 3 lists the loca- tions of the mid-sector prograded beaches. 15 Beaches Located WIithin Drift Sectors Table 3. 1rograded . . *' A - -, Geographic Name/ Location Comments Appears on map t - none/ NW Guemes Is. O Presently undergoing erosion; owners have installed shore defense structures Anaco Beach/ NW Fidalgo Is. Alexander Beach/ ni Fidalgo Is. D D Gibraltar/ G N. Skagit Bay Three prograded beaches within E this drift sector which spans the shoreline from Dewey Beach to northern Similk Bay. MIost have a moderate amount of shore defense structures installed. 16 LI SHORELINE EROSION LI Erosion of Skagit County shorelines, particularly bluffs composed of glacial materials, is a highly intermittant pro- cess. The process is very much dominated by extreme climatic events. That is, beaches protect cliffs from wave attack exeefj during large, infrequent storms when berms are overtopped and Li beach sediment is temporarily removed, -leaving little effective protection. The exact timing of those erosion episodes is ran- dom in the same sense that river flooding is random. They can only be predicted in a statistical manner; that is, expressed as a probability such as the 50 year event or the 100 year Li event. Since very little study has been done on the frequency of occurrence of extreme winds and waves there is, at present, no way of even attempting a statistical prediction. Therefore, to be able to estimate what erosion rates might be in the future one can only assess what erosion has been in the past and assume ED that it will continue at least at the same rate into the future. While analysis of past erosion is useful in determining an aver- age rate of shoreline retreat, that average rate itself obscures Li the great year to year variability mentioned above. Because of that variability it should be emnphasized at the outset that mean an erosion rate of 0.1 feet/year does notAan owner can look at his shorefront and expect to see a tenth of a foot eroded every year; nor does it mean that after 10 years a foot will have LI been removed. It is entirely possible to lose 3 feet in a sin- gle storm and then not experience an easily noticeable loss for 17 kFu ,171, I TI l 1 . I al - I ! I D [I [I I LI LI I,1 a long period thereafter. The average rate does mean that over a longer period of time (Ereater than 20 years or more) the ex- treme events will probably occur often enough to maintain an overall rate of erosion somewhere near the average. Methods Used to Evaluate Erosion Rates The rates of erosion shown on the accompaning maps were derived in a variety of ways. The most reliable method makes use of bench marks installed in shoreline areas by the U.S. Coast and Geodetic Survey. By comparing the distance from the marker to the bluff when it was installed, to the present distance,a net change can be ascertained. Unfortunately, those markers are not numerous and only some have had the necessary original measurements recorded for comparison with the present measurement. If benchmarks were not available, then more in- direct methods were used. Those include: 1) the amount of undercutting of a structure whose age an4position relative to the original shoreline is fairly well known, and 2) the amount of root system exposed on bluff-top trees. Under certain con- ditions sea stacks found on rock abrasion platforms and lag deposits of boulders can also be used to yield an indirect assessment of past erosion. The irdirect methods described often give only minimum rates of erosion that may be only 50% of the true average rate. However, on some shoreline segments nothing else is available so knowing at least the minimum rate is helpful. 18 Erosiont Rates - As displayed on maps A through H the rates of erosion for glacial materials ranges from about I foot per 10 year period *~ to 4 feet per 10 years. The most reliable of those values cluster at 1.5 to 3 feet per 10 years. Some of the values shown.are derived from the indirect methods described above so tend to be lower, mostly in the area of I foot per 10 years. But since those methods underestimate what erosion has been, I the higher, more reliable figures remain as the best guide. For general planning purposes the,county will be most nearly correct if it assumes an average rate of shoreline retreat of 2 to 3 feet per 10 year period in areas where the shoreline is composed of glacial drift materials. This is especially il true near the beginning of drift sectors where erosion tends to proceed more rapidly as described earlier. The most rapid rates of removal in rock occur where the rock is highly fractured. But even in those areas the erosion rate is very much slower than in glacial materials, typically 0.4 feet (5 inches) per 10 year period. In less fractured Fl- rocks the rates are only one half of that cited above, and in some areas there is little or no evidence of removal in the Il ~~~lasI00yas !u I !I*1 [II I lu I -I II- II II LI LI i . SHORE DEFENSE STRUCTURES A wide variety of defense structures are visible along the county shorelines. They range from massive rip-rap used by Burlington Northern along their right-of-way to imaginative attempts by individual owners using many different materials. In looking at these schemes one fact seems to be repeated over and over. Where defense structures are set too far out into the beach, that is, where they protrude to the mean higher high water mark or beyond, they tend to disrupt the normal protective nature of the beach to a significant degree. That is particu- larly true where the structure is an impervious concrete wall or bulkhead. Under normal conditions (no wall) the- swash of the just broken wave moves up the beach and it's energy is dissipa- ted. Also the volume is reduced by percolation into the under- lying sediment so the.backwash is significantly reduced. With an impervious wall in place, whose foot is at the water's edge or below, the normal swash dissipation cannot operate. The result is wave reflection from the wall, with a much stronger backwash, which scours the beach and removes the protective beach sediment. In these cases the owner is in essence trading no erosion of the bank for an eroded beach. That unconcious choice is ironic in that the presence of the beach is one of the most desireable features owners look for in acquiring shore- front property. Figure 6 shows one example of beach scour due to a concrete wall being placed too far out into the beach. It would, seem that owners would be well advised to locate defense structures as far back of the high tide mark as possible. 20 [I 171 LI---- LI LI_ [1 , ,N .^t'... .4 ' : i' X ' -_ . ~' ~.~�, ~ X-o~-~7 :r ~ ~ x , ~,:~ -~�i�f~ :,_ ,~---~'-~" ...... '' Lo I L i L i An example of beach scour due to an impervious con- crete wall. Campare the steep, unmodified beach in the foreground having good sediment cover with the very low angle beach with many cobbles in front of the bulkhead in the background. Also note the pro- nounced drop from the unmodified beach down to the scoured beach. Location: west side of Similk Bay. Figure: 6. 21 F' I [I [I -I LI LI LI LI Li LI LI LI LI SLOPE STABILITY Mass movements in bluffs are quite common along all Skagit County shorelines. However, it is Important to distinguish be- tween small, non-hazardous slope readjustments that go on more or less continuously, as opposed to large mass movements that have the potential for economic los-s or loss of life. In the former category are small slumps, soil falls, and rock falls that are ubiquitous in coastal bluffs and are basically adjustments to wave cutting at the toe of the slope. I have found that those type of slope movements present no particular hazard unless a structure were located at the very edge of a bluff. I found no such examples of poor building practice in SkaEit County. Several zones of large hazardous mass movements are found on Fidalgo, Guemes, and Samish Islands. Those are shown on Maps B, C, F, and G; with the area between Biz Point and Edith Point (western FidalEo Island, Map F) being the largest and potential- ly most dangerous. In that area there are 7 large bowl or amphitheater-shaped scars representing sites of long continued landaliding (see figures 7 and 8). Active sliding into these amphitheaters is continuing to occur. At the time of slope failure a wedge of material is released from the upper rim of the bowl, falls or slides to the bottom of the bowl, and then travels to the beach through a narrow chute as a debris elide or flow. In some cases the broken up, failed material does not evacuate all the way to the beach; 22 I FE 'I LI LI LI [I LI LI LI LI LI LI Li Li Figure 7. Diagrammatic map view of amphitheater-type landslide site (not to scale) I / /'~~~~~~~~~~Lfalure ':"~? ~ lab - 300 feet � //;' ,sandy. .... ' o.twa ' s, - ---- silty, Impermeable sediment- beach. q ': ___-- -- --- :---"- ... *... .: * :'.:. --. Figure 8. Diagrammatic cross section of amphitheater-type landslide site (not to scale) 23 -l I i. [I El.* LI I -r I I LI I LI LI LI 1 - rather, it comes to rest in vegetation in the bottom of the amphitheater or forms a plug inside the chute so that the evi- dence of failure is not easily visible. The most recent failure (winter of 1975/76) occurred in the amphitheater labeled number 4 on Figure 9. A conservative estimate of the volume of that failure is 1OOO cubic yards of material. At least 3 of the 7 amphitheaters have had major failures along their upper rims within the last 20 years, and probably all 7 bave been active durinS that time. The amphitheaters are old features, surely pre-European settlement, and quite possibly greater than 1000 years old. The most landward portion of the bowl rims have now retreated inland an average of 750 feet from the beach implying their continued activity has little if anything to do with wave erosion at the base of the slope. They will continue to regress inland because the headward portions have steep slopes (all at least 45 degrees) and because the failed material does not usually come to rest at the foot of the steep headscarps which might begin to stabalize them. One of the primary factors controlling landsliding in this zone is the particular combination of sediments that make up the bluffs. As shown in Figure 8, the base of the slope is composed of relatively impermeable silt which is overlain by a large thickness of glacial outwash sand. That particular com- bination has been found repeatedly in western Washington to be susceptible to landsliding (for example Tubbs, 1974; and Heller, .1978). The time of failure is nearly always during wetter than ~.o .~~24 24 - . LI Figure 9. Sketch map of the Biz Point/Edith Point area. Amphitheater-type landslide sites are arbitrarly numbered from north to south. 25 I~~~~~~~~~~~~~~~~~~~~~~~~ normal winters followed by heavy individual rainfalls. Since - excess water seems to be the triggering factor, large additions of water to these slopes should be avoided. In that regard, LI! ' the amount of water added by septic systems is not commonly realized. Using U.S. Government figures on average per-person water usage, and assuming a housinig density of 3 per acre, com- putations show a net input of an extra 12 inches of water per acre per year. Since the average runoff in this area is only about 6 inches per year as explained earlier, the 12 inches of added water would be a 200% increase which could exacerbate an already difficult situation. The same combination of materials are present in the bluffs that continue north for more than 0.6 miles from Edith Point. With the exception of one large amphitheater immediately adja- cent to the north side of Edith Point, no other amphitheater- Li type failures have yet developed in that northerly part. How- . ever, there are many places along the bluff top where wedges of material have failed and traveled to the beach. Those failure Li - - wedges produce a scalloped bluff edge where the failures have cut back into the bluff 20 feet or more. Furthermore, the top sur- face of the bluff has a number of small semicircular depressions and cracks ranging up to at least 15 feet from the edge indicating incipient failures that will occur in the not too distant future. Li Nearly identical situations exist in each of the mass move- ment zones listed on Table 4. That is, all have the unfavorable combination of materials where flne grained, relatively imperm- eable deposits, are overlain by'a sandy/gravelly deposit. No 26 I E m [I .' 1' I I I 1' I I-[ 1, I . I A .o I 1. LI Table 4. Listing of major landslide zones Geographic Name Location Edith Point Western Fidalgo Island, from Biz Point north to 0.6 miles beyond Edith Point Similk Bay Western Shore of Similk Bay Miller Bay East of Deception Pass Yellow Bluff Southwestern Guemes Island none Northwestern Guemes Island, 2000 feet south of Indian Village ' Clark Point North tip of Guemes Island none. Northeastern Guemes Island, 1500 feet south of the public park none Northern Samish Island, 2000 feet west of public picnic area Iep F,D e F B B B B a 27 [1- Fri- I Li [I Li I [I very large amphitheater-type failures have yet developed in those other zones, rather the scalloped cliff edge type failures predominate. It should be noted however, that each of those zones has at least one failure that has cut back into the bluff tbp further than the adjacent failures. In doing so those lar- ger failures developed a small bowl-shaped scar and chute on the' bluff face. Whether or not those failures are in the process of developing into large amphitheater type failures is somewhat speculative, but the possibility cannot be ignored. The hazard in all these zones, particularly in the Edith Point area, is that a failure might include a dwelling that is close to the edge of the bluff. Based on the depth of the scars left by failed wedges that cut back into the bluff an average of 20 feet (more than 30 feet in some cases) it is obvious that any structure closer than about 30 feet could be severely dam- aged or tumbled down the cliff. The 30 foot value given should be considered as an absolute minimum for safety because it might only be a reasonable setback for one episode of failure. Since houses are considered to have a useful life well in excess of 50 years, doubling the setback to 60 feet or more would be quite reasonable. Those values are only approximate and might serve as a rule-of-thumb guide; in no case should they be considered as a substitute for site evaluation by qualified personnel. At present, in the Edith Point area, there are a number of houses that are less than 30-50 feet from the bluff edge. In addition, there are several houses built on the narrow finger of land that separates two adjacent amphitheaters. 28 ri Fl' [i Li -E I I Irn II I [I I LI I ' DISCUSSION OF SELECTED INDIVIDUAL DRIFT SECTORS NORTH SAMISH ISLAND (Map C). Owners of shQreline proper- ty within this drift sector have experienced problems with erosion for a number of years. In the past most of the problems seem to have been near the center of the island where it is narrowest. A couple of shore defense structures in that area date back over 20-30 years. In talking with residents there is an indication that more recently the problem is also being experienced further to the east, in the vicinity of Samish Beach and Blue Herron Beach. Comparison of an old photo,to a mecent one (Figures 10 and 11) confirms, on the basis of beach morphology; that condi- tions have indeed changed within this sector. The visible change in the amount and size of beach sediments is the result of an overall decrease in the amount of sediment in transport along the leneth of the sector, rather than an isolated occurrence at Samish Beach. A' number of pieces of evidence indicate that the deficiency of sediment is more closely related to a very long term deple- tion of the original source of supply instead of construction of shore defense structures. As shown on Yap C, the entire northeastern portion of Samish Island is a prograded beach (200-500 acres). That huge volume of sediment implies a pro- lific supply source in the past, far larger than anything now visible in the shoreline bluffs. A second feature which pro- vides a similar indication is a very wide, sandy low tide ter- race that becomes visible at low' tides. In other parts of the - 1 29 _ I I : n~~~~~� k~~~~~~~~~d D . W gaa iF~~MB- - ~ i- Figure 10. Photo taken about 1930 at Samish Beach. Note the large accumulation of fine gravel that forms the slightly convex upper beach. _PTIM 9""''7""""".r--7� =zfF * -.. r lf ,- s;--- *,11 1.74' I %,,T � ii I i I i i f I I I f I I I 1~~~ 1~8' `1��� ' ' ' %y ,C, ~. :< a~i~~ 6: ~ ~ ~ ~ ~ ~ ~ ~ ' - � or Y / -" v , > 5 F _,', l ~N S . .' ' !A9. _- , --,, , � - . .1; a , I ' 5 Figure 11. Photo taken-in 1977 in almost the same spot as figure 10 above. Easily noticeable are: the beach surface is now concave, the sediment is coarser, and the high tide drift line is almost to the edge of the lawn. 30 county similar low tide terraces preferentially occur where - .... erod-ing bluffs contain abundant sand. At present, only a rel- LU atively short stretch of shoreline (300-400 yards), located to the west, at the beginning of the drift sector, has glacial outwash sand and gravel exposed in the bluff. While that de- iposit does supply a moderate amount of useable material to the beach system by erosion and landslides, it appears to be far too small to have supplied the large volume of sediment present LI in the prograded beach and on the low tide terrace. All of the above evidence indicates that a large supply of sand/gravel material has been removed by erosion and as a result, the drift sector is presently in a net sediment deficit due primarily to natural causes. There is no doubt that the shore defense struc- tures already built slow the erosion and deprive the beaches of still more sediment, which exacerbates the situation. However, LI when the volume of sediment involved is considered it becomes obvi6us that man's activities play a relatively small part in the problem. The erosion rate of 4 feet per 10 years shown on Map C -is derived from one measurement and the period of time assessed was only 15 years. Because of that short time period the Indi- cated rate may be somewhat faster than -if a longer time period had been available on which to base the computation. Since erosion will continue along this sector, possibly even faster than it has, the question of a solution arises. Unfortunately, in a sector with a naturally low sediment supply rate, none'of the choices are particularly appealing. The LII~~~~~~~~~~~~~~~ I alternatives are: (l) NO er i ti i LI..- me bu ba (2) Aa TIR~~~ ~ab be Th : al vi l (3) SH s t --U~~~ ~th pr [1 I',' El. [I' . .. . . ACTION. By taking no action and letting mod- ate erosion continue the beaches would be par- ally supplied with needed sediment. Adjust- nts might be able to be made in locating new ildings and/or relocating existing buildings ck from the shore. TIFICIAL BEACH NOURISHMENT. Sediment of suit- le size could be brought in and placed on the ach near the beginning of the drift sector. Lis would allow the sediment to move naturally ong the length of the drift sector and pro- de shoreline protection. lORE DEFENSE STRUCTURES. Additional defense ructures could be built. In the longer term lis alternative could conceivably involve every ,operty owner. 32 Zn El Li NORTHWESTERN FIDALGO ISLAND (BIZ POINT TO FLOUINDER BAY, M Maps. F and D). Upon initial evaluation the southern portions of this sector appear to have no influence on the northern part. That is, it appears as if the rocky headlands interspersed be- tween embayments form large pocket beaches that have no trans- �fer of material between them. Upon closer inspection, I find that there is indeed movement of sand past the headlands. The headlands do stop an appreciable amount of coarser material (gravel) which normally resides higher on the beach. The sand on the low tide terrace is easily transported around the head- lands as can be seen in Figure 12.' The longshore transport of only sand results in both Alexander Beach and Anaco Beach being totally sandy beaches, which is in sharp contrast to the situa- tion of other beaches in the county. Virtually every other beach has at least some gravel and coarser material mixed with the sand; most, in fact, are predominately gravel and coarser material, as explained earlier. Most of the length of this drift sector is essentially un- modified by man. Near the terminus however (Flounder Bay and Anaco Beach), a number of shore defense structures have been put in. Older navigational charts show the mouth of Flounder Bay; mostly blocked by a westward growing spit that had it's base where the mouth of the marina is now. As a consequence of cut- ting through the base of the spit, the supply of sediment was cut off and the seaward face then required rip-rap to protect it. Some small amount of sediment may presently be transported across the,marina entrance. I 33 TI~~~, .1 i'l I~~~~~~~~~~~~~... ~~, t.~~~~~~..�. ~ ~~ II~~~~~~~~~~~~~~~~~~~~~~~~~~~~~--' "I. , I-~~~~~~~~~~~~~~~~~~~~~~ LI ,I. ;;-t I ~~~At Anaco Beach seawall-s'and bulkheads have been installed 1o most of the 8horeline. One property owner claims that I ~~the strilotures became necessary due to 30 to 60 feet of erosion that occurred during the last 13 years. I have not been able to find any direct evidence to dispute that claim. However, FM ~since that rate of erosion would be 6 to 12 times faster than anything encountered elsewhere In the county it is probably Fl g~reatly exaggerated. In fact, such a rate would be 4 to 8 times faster than the highest rates reported from the western part of Whidbey Island (Island County) where the wave energy is significantly higher than in'Skagit County. The continued movement of sand past the headlands in this I ~~sector Is obviously vital to the continued health of the beaches. I ~~Therefore, I would suggest that thelcounty evaluate carefully any proposal that might Interfere with sediment transport In this sector. As covered in the slope Btability section of this report, Li ~this drift sector derives a significant portion of it's sediment [I ~from landslides that are 'occurring oni the bluffs. While it has been impossible to estimate the percentage contribution from [U .~that source, the fact that very few sandy bluff materials are .exposed at the base of the slopes is significant. Since the Fl ~primary material transported alongsbore is sand, that would [U ~suggest that the landslide contribution from the sandy, upper portiona of the slopes Is quite Important In the sediment budget [U ~of this sector. Li Li 35 I II I I [I LI 3I I [I 1 I I I TI I I lii LI SUMMARY The north Samish Island and northwestern Fidalzo Island drift sectors were chosen for an expanded discussion because they illustrate a number of important points that are applicable to all drift sectors in Skagit County. First and foremost is the quedtion of sediment supply and transport. It becomes immediately obvious that beach sediment can be supplied in a variety of ways other than direct wave erosion. Additionally, the sediment one sees on the beach to- day, or lack of it, may be intimately connected to events that happened in the distant past. EquallY signi'ficantis the fact that one must be aware that sediment transported alongshore is not necessarily stopped by imposing rocky headlands. The traditional view that a prograding beach is being supplied it's sediment by the nearest large bluff is overly simplistic, at best. A third point that should be made is that interfering with the natural transport of sediment can often cost more in the long run in shore defense measures and property lost than any -short term benefit gained. Finally, it is worth noting that the alternatives for remedying erosion problems in sectors with a low sediment supply are not easy choices. There are other sectors in Skagit County that also are not well supplied. The difference in one major respect, is that those other sectors are not as heavily developed as Samish Island is. Therefore, the problem does not affect as many people. 36 I REFERENCES CITED ..American Geological Institute, 1972, Glossary of Geology: Gary, M.; Mcaffee, R.; and Wolf, C. (Editors): American Geological Institute, Washington, D.C., 805 p. - B.C. Research, 1974, untitled, unDublished report on wind and wave conditions at Halibut Bank, southern Strait of Georgia, British Columbia. Heller, P., 1978, Occurrence of landslides in the Lower Skagit and Baker River Valleys, Skagit County, Washington: Unpub. report to the Skagit CountyyPlanning Department, 32 p. Phillips, E., 1966, Washington Climate-: Snohomish, Clallam, *Jefferson, Island, Skagit, San Juan, and Whatcom County: Wash. State Univ., Agricultural Ext. Svc. Pub. EM 2626 Tubbs, D.W., 1974, Landslides in Seattle: Wash. Dept. of Nat. Resources Info Circ. 52, 15 p. U.S. Army Corps of Engineers, 1973, Shore Protection Manual: Washington, D.C., U.S. Government Printing Office, 3 vols. L *'I '1 - 1. r" A 37 I I .i r o - :_ Lu i I LI0 ! F' I I l L! I. .I 'U LI r[ LI. C. C N Figure 13. Index to the maps accompaning this report 38 I' F' Fi_ I F' I LI F' [I I Li LI LI Li I LI Li Li KEY TO IMP SY.BO0LS USED oooooo Bluffs composed of glacial and interglacial materials, less than 10 meters (30 feet) high � --- ~Bluffs composed of glacial and interglacial materials, more than 10 meters (30 feet) high flIIIIIillI Rock shoreline with abrasion platform ------ Pl1unging rock cliffs, no abrasion platform �-----... Bluffs composed of mixed or alternating glacial materials and rock **** ~Prograded beaches 06R Mudflats ~'-' - Direction of net long term sediment transport - ..'Lightly modified shoreline, small shore defense structures __ BSignificantly modified shoreline, large shore defense structures, original shoreline now isolated. Heavily modified shorelines, industrialized or filled, original shoreline now nonexistant 44 ~ Major landslide zones 30(1) C-entimeters (feet), mean minimum erosion per 10 year period o 39 EXPLAUATORY NOTES FOR MAP SYMBOLS A) Where the round symbols (low-and high bank glacial mater- ials) are used it may be assumed that a typical beach of mixed gravel/sand is also present. If the mudflat symbol is combined with the above, then the mudflat forms what normally would be the low tide terrace. B) If used alone the mudflat symbol means that the shoreline itself is a low lying area with no beach, typically found adjacent to the Samish delta( e.g. Bow and Edison vicinity) V) The size of the beach drift arrows are a qualitative measure of the supply of sediment in a sector and the rate at which the sediment is transported. D) A" diamond (prograded beach) used without sediment transport arrows means a pocket beach. E) For ease of display, the symbols indicating modified shore- lines are shown on the se-award side of the symbol which indicates the nature of the bluff and beach. In all cases defense structures are associated with the bank and not Li' out in the middle of the beach as might be implied by their placement on the maps. :;II LI LI~~~~~~~~~4 - . I r .' I I I '. I I. I I- I .. I MAP A. SINCLAIR AND CYPRESS ISLAND 41 i I I I .i I I, I I ! I.I LI I I ! I I - - MAP B. VENDOVI AND GUEMES ISLAND 42 [71~ ~ ~ ~ ~~~~r- rC C i--, '" �-o ~~~ U)~ ~ ~~~~~~~~~~~~~~L;. v,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~C r~~~~~0 cO Ir '-4~~~~~,~ I-- sr -I S Amli IS H m~~ 1 P1 --I m o z1 -, U),I~j 'a,oo, 'TI~ ~ ~ ~ ~~~~~~~~~~. * r ~~~~~~~~~~~~~~~~~~~RIVER CD -I~~~~~~~~~~~At m ~ ~ ~~~~~~~~Q C~~~~~~~~~~~~~~~ I-~~~~~~~~~~~~~~~ I ' I rl 11 I I I ' U I I I .U U U L I L 1 I l I I ] i II ;' I b . ?. f Pa di Ila Bay March Point Swl no ml h S1 MAP E. MARCH POINT AND EASTERN PADILLA BAY 45 FE I LI LI El m Li I Li I [I I, I 1 1 1 I I I MAP F. WESTERN AND SOUTESTERN FIDALGO ISLAND, DECEPTION PASS, AND ALLAN ISLAND F. WESTERN AND SOUTHWESTERN F~~~~~~~~~~~~~~IDLOILN,DCPONAS,NDLANSAD 46 rri~~~~~~~~~~~~~~~~~~~~~i 26(.9 tz,~~~~~~~~~~~~~~~~f ISLAND cn1' U ~~~lI adjoining [ - -~winmis -.~~~~~~~~il map . i 128(4r Lb :: �~~~~~~~~N* Sn.e-ooah I4 Swinomich Slough V Go~~ [I, Fl~~~~~~ S k a [I A IKA I S LA N N. Fai iiRi El LI I Li MAP H. SOUTHEASTERN FIDALGO ISLAND AND SKAGIT RIVER DELTA 48 I 11 1