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HomeMy WebLinkAbout989600005 Geotech AssessmentSHORELINE BANK STABILITY ANALYSIS KALA POINT VICINITY On January 30, 1989, i examined two blocks, of undeveloped lots just south of the Kala Point development. Of primary concern was the block of lots south of the end of Prospect Avenue that face northeast, here called the "South Block". The block of lots adjacent to the Kala Point development and east of. Olympic Avenue are here designated the "North Block". The available plat (copy attached) for this area ~ndi~ate~ !c~s generally very narrow by today's standards. However, because of the implied development density, any analysis oi the potential impacts of shoreline bluff development obviously mu~t ~ake into consideration this aspect. The cummulative e~ec~s ~ £uli development need to be conaidered even though ~h~t g~a~e ,~ay be decades away and will undoubtedly be less than a tkird that suggested by the old plat. NATURAL SETTING South Block Access to the South Block is from Elston Avenue, now a one ~&ne unsurfaced road. The upland surface slopes gently to the s~utheast. My examination primarily consisted of traverses down the bank at nba north property boundary, up the bank at the south boundary and along the contour at about midslope across the entire block of !ot~. · The bank is approximately 100 feet high at the south end of the property and 105 feet high at the north end. Judging'from almost continuous exposures along the beach access trail near the ~crth property boundary, it 'consistm of gray unoxidized pebbly sand of gl~clal outwash origin. The upper 5 to 10 feet is predominant~ ~ . ~y g~acial till with scattered lenses of fine gravel This uppermost part of the bank is vertical in places as is typical of such m~terial, whereas the rest of the bank underlain by somewhat cohesive sands generally ranges from- 35 to 47 degrees in slope. Slope aYera~es, from the high tide line to the crest of the bank, range from 41 to 45 degrees. - 1 - DEC. 07 '98 (MON'; 09:59 CONLMUN!CATION N~:33 PAGE. The bank is well forested with Douglas fir and Madrona to about 15 inches in diameter. One core of one of the larger fir indicated an ag~ of more than 45 years. Mid and upper slope fir are generally straight and vertical. Some lower ~lope fir have a gradual curve, commonly terminating in a straight vertical leader of 15 - 20 feet. In places, generally along the lower quarter of :he slope and along the beach, trees were tilted or down. Many were alive but hsd no vertical leader, indicating that they had fallen since the last growing season. . In almost every instance a~sociated with conifers "out of plumb", small slide scarps could be located. These scarps seldom reach as much as 10 feet in height and are discontinuous. Slide moveman~ appears to be confined to the down$1ope-thickening wedge of ca!luviai soil that blankets the slope below the uppermost till layer. No exposures of the underlying glacial sands were visible in scarp ~urfaces. The foregoing all tends ~o confirm that current slide action on the site consists essentially of small, shallow failures that typically move only a few feet in a given episode. They are most common along the toe of the slope and are clearly triggered by storm wave erosion of the bas~ of the bank. In spite of a careful search, no evidence could be found for ground-water-induced instability. No water-perching clay layers could be found in the bank nor was any horizon o~ water-loving vegetation found. The essentially straight bluff line is further evidence that deep-seated f~ilures have not occurred here. In summary, the bluff fronting the South Block is eroding slowly bu~ predictably, with recent ~ctiv.ity conf.tned largely to the ~oe. The relatively uniform slopes and fifty-year-old forest cover suggest that there have been no significant bluff faitur~s for at l~ast that long ~ time. Lacking a perched w~ter table or other planes of inherent weakness, it is highly un~ike!y that even in the event of ~ major storm at high tide, a de,p-seated slope failure cop~ble of damaging a home on 'the uplands could occur. (It should be noted that the storm thor blew ap~r~ the Hood Canal bridge ~bout ten years ago and caused massive ero$isn of many shoreline bluffs in the area apparently had little impmct here.) Thu~, there should be no surprises for a developer or homeowner at this site. - 2 - DEC. ,97 ' 98 ~N~ON) 09:59 CO.VCMUNiCATION No:39 PAGE. ~'-~orth Block Thi~ tract, abutting the Kale Point Development, is fronted by a lagoon. Thus wave erosion at the toe of the slope is not m factor in banE' stability. The glaciated uplands of the block slope northeast, thus the north end has essentially no "bank", and slopem about 15~ towards the lagoon. Th~ south end of the block is also protected by the lagoon and its fronting beach from significant wave action. A shoreline bluff was developed here in the millenia before Kala Point was formed. .Subsequent erosion had reduced this bluff to its natural angle of repose (35~). A dense forest cover mainly of Douglas fir and Madrona, with a thick understand of salal has developed on the slopes of this tract. The foregoing observations and my knowledge of the geology in the general area indicated to me that natural instability of such slopes is not a problem, thus, no more detailed study was felt justified. DEVELOPMENT Storm Drainage. In spite of what appears to be a rather impermeable till blanket over much of the glaciated upland sur.face, no evidence of ~urface runoff such as gullying was found, e~en along roads. Development will change this, as roof~ and paved surfaces partially replace the undisturbed soils. Eventually, as more and more lots are developed and streets are improved, sites of concentrated storm runoff will probably develop. At ~uch sites elsewhere but in similar materials, these concentrations have created gulleys cutting tens of feet back into the upland surface. At one site such a "miniature canyon" developed in a single storm. Thus, runoff concentrations should be minimized, especially in areas such as this, with relatively impermeable uplands and easily erodable bluffs. Where unavoidable they mus't be conducted to beach level in a well engineered 'sYstem ~uch as lined ditches and/or pipes, with appropriate energy dissipators at the outfalls. Clearing-grading The relatively undisturbed soils and small trees and brush on the present upland surface are important elements in the present hydrologic equilibrium. In spite of their clayey nature, the soils apparently allow some infiltration and the ~rees intercept and/or transpire additional moisture. Compaction or - 3 - ' / , T', . DEC. 07 98 ,MON, ~0:O0 COM~.UNiCAT!ON N'~:99 PAGE. 4 r~zo~ai of these soils by heavy equipment will tend to disturb this equilibrium and aggravate potential runoff' problems. Clearing cf vegetation should be minimized and clearings not actually occupie~ by construction should be replaced by d~sireable ~pacies as soon as possible.-Under no circumstances should clearing and grading debris be deposited on the steep bluff face of the South Block. Trees on the upper bluffs of such sites ara commonly too shallow-rooted to carry the extra load. Th~ result is often a shallow landslide, wLth the stripping of alt soils and vegetation fro~ the bluff face. S~ch sites are ex~remely dtff~cut't to reveget~te ~nd much more su~ceptibl, c to erosion tha~n tbs ~atural bluff. Beach Access - Bluff C!earin~ ~'c is my , understanding that the lots under consideration include ~It~!~, if any, of the bluff face. Nevertheless, it seems lik~!y that many homeowners will seek better beach access, either through outright ~urchase of fronting bluffs, easements or unofficial" trails. Others may seek to improve views by cutting trees. The potential impacts of such activities vary considerably between the North and South Blocks. The North Block, especially the northerly lots, has gentle enough slopes so that both beach access trails and selective cutting for view improvement could easily be accomodated. Any rc~s, even on the northerly lots should be carefully engineered. Special attention wo~ld be needed for road drainage. Cuts should be minimized. The The South Block, in view of the steep slopes and lower bank stabiiity, wilt require even more care for any bluff face modifications. Foot traffic on the bluff should be minimized, with community rather than individual beach-access trails carefully laid out to minimize soil disturbance. Roads should not even be considered for beach access in this area. Tree cutting fcr view enhancement should' be very selective, confined to th~ uppermost bank, and create a minimum of soil disturbance. C!earcu~<ing, ~ven o~ ban .run o a e o , = _ a narrow d in f t f singl ! t could start a chain reaction of shallow instability that would be apt to involve the bank nc both sides of the cut. Setbacks Because of relatively gentle slopes and lack of erosion, any setbacks for the North Block should be based more on factors such as aesthetics and protection of nearby wetlands than on the long 5arm safety of a structure. - 4 - DEC. 0v ' 98 ~N, 1O:00 CONt%fUNiCATION N®:33 PAGe' 5 In regard to the South Block current erosion along the bluff appears to be occurring in a manner and at a rate such that safe residential development of she uplands is entirely feasible. The prudent builder here must, however, take into consideration not only the long term effects of construction at his-or-her own sit~, but ~ny re~ulting from careless development by eventual neighborm. Eve~ accidents, such as a plugged drainage ditch or broken water or sewer mmin, could effect bank erosion rates at a particular site. Thus, even though natural ~rocesses may take decades to measurably impact the upland surfaces, the effects of regional development should be considered. Many variables exist in considering the question of safe setbacks at such a site. For example, given "infinite" money, a good engineer could safely cantilever a home over the bluff edge. However, acknowledgin8 that this is not Southern California and that property vu!ues, life styles, and incomes are generally different, such "high tech" solutions seem unjustifiable. Given the geology and current rate of geologic processes, and the height and angle of bluff, a setback of on the order of 30 feet would seem adequate for the long-term protection of a structure. Again, this would be in part to serve as a buffer for potential development impacts as well as for long-term natural processes. It would not serve as ~ substitute for site-specific careless development such as the unengineered disposal of concentrated storm runoff. GERALD W. - 5 - , ,~ ,"~ T TT DEC. 07 98 (MON) ~0:0'! ~Nh~UN.CATION No:33 PAGE. 6 EXISTING K^LA F tq' i)EVF, LOi>HENT ,1,4. L L_i. L kL;l. NORTH BLOCK SO UT }1 Bt,OCK . /.,,,.,-, -~ ..-. ,-, ,¥f_ - °T DEC. 07, 9~ ..... X~ i O: ,_,r'2 ~,~' .MU.'< CA'--- ~ ON' N.:: ~ PAGE. 7