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HomeMy WebLinkAbout998200118 Geotech AssessmentIhLR o 5- 3 ~ S/ X1.17 o S- U a3 "7j u~.1 4 qg - aoo - ~~g STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 June 13, 2005 Thank N~uyen 265 243 Avenue SE Samamish, WA 98074 Re: Geology Assessment of Beach Access Trail 20 Ta1a Shore Drive Hadlock, WA Deaz Mr. Nguyen: JUN 1 52005 ., t _ Stratum Group is pleased to present the results of our geologic evaluation of a beach access trail. The purpose of this geology evaluation was to evaluate the existing trail route and evaluate the feasibility of reconstructing. portions of the route to the beach as the wooden stairs have partially collapsed at several locations such that they are impassable. The evaluation was done in regards to geologic slope stability and potential impacts to beach processes as well as cost impacts. This evaluation was based on a visual inspection of the shoreline bluff slope, properties in the vicinity of the route, a review of available geologic mapping in the area and conditions on the shoreline. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was deeply eroded. The Geologic Map of the Lincas- Port Ludlow Area Jefferson County, Washington (Hanson, 1976) and the Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard, and Dembroff, 1993) indicate that the subject property is underlain by Vashon glacial till. The till consists of very compact, poorly sorted clay, silt, sand and gravel with occasional boulders deposited directly by glacial ice approximately 16,000 years ago. Observations of the upland area on the property and on the upper slope of the shoreline bluff are consistent with the mapping described above. However, consolidated preglacial deposits consisting of very compact silt are present on the bluff slope of the subject property. The shoreline bluff at the subject property and along the coast at this location is an erosional bluff and provides sand and gravel for beaches along the bluff and beaches to the south of the bluil=. The bluff is exposed to periodic lazge wave action from the open waters towards the northeast of the bluff. Hence north to northeast winds will generate large waves that will erode the base of the bluff. This erosion undermines the base of the bluff causing slope failures at the base that create an over steep slope. Subsequent failures then take place further up the slope as June 13, 2(Ids JUN i 5 ~ 20 Talc Shore Drive, Port Ludlow, WA Geology As~sment of Trail _ the slope reestablishes itself to the angle of repose. _ _:_:____~___- ~ SPECIFIC SITE OBSERVATIONS The slope of the shoreline bluff traversed by the trail is underlain by very compact glacial till and very compact preglacial silt deposits. The overall slope of the bluff is approximately 45 degrees. However, there are steeper sections of the bluff and a few areas that are less steep. Steeper areas were limited to areas neaz the base of the bluff and areas neaz the mid portion of the bluff where past landslide scarps were present. The overall height of the bluff at the subject site traversed by the trail is approximately 50 feet. The steep shoreline slope is covered primarily with blackberry with a few alder trees and willow trees neaz the base of the slope. The existing trail traverses the upper slope of the bluff along a cut slope .trail that angles down the bluff slope from the south towards the northeast. The trail switches back neaz the north properly line and ends at the top of the existing wood stairs approximately mid way between the north and south property lines. The top of the wood stairs is located slightly higher than the mid- point down the bluff slope. The existing trail is in good condition and except for slightly uneven surface areas does not require any improvements. The wood stairs leads directly down the slope approximately 15 feet to a wood platform landing. The support beams for the wood stairs span the entire distance to the platform. Because of wood rot at the platform, the support beams have partially lost support and hence the stairs are leaning are very difficult and dangerous to negotiate. From this wood platform the stairs angle down the slope in similar fashion to another landing. Like the landing area above, this landing has partially collapsed and has been completely overgrown with blackberries. The combination of tutted supports and blackberries has rendered this section of stairs impassable. A final set of steps leads to the shoreline. The shoreline of the property consists of a rock riprap approximately 6 feet above the beach. The beach is a cobble beach with sand and gravel. The shoreline along this stretch of coastline is an eroding bluff. Erosion from the bluff provides sediment to the beach along the base of the bluff: Essentially wave action at the toe of the bluff erodes the lower slope and oversteepens the bluff slope. Overtime the slopes above will then collapse onto the shoreline below and the process will repeat itself. The overall net drift at the shoreline is from north to south. Sediment on the beach is transported ficom the north to the south in the vicinity of the subject property due to the angle at which the prevalent waves strike the shoreline. Rock riprap similar to the subject property is located at the base of the slope along numerous properties to the north of the subject property including the property immediately north of the subject property. A rock riprap bulkhead protrudes out over the beach three lots north of the Strum Group File: 6.8.OSB 2 June 13, 2005 20 Tala Shore Drive, Port Ludlow, WA Geobgy Assessment of Trail JUN 1 52005 -~ s _._J subject property. This bulkhead protrudes faz enough out into the tide zone to effect shoreline drift. The beach north of this bulkhead is broader than the beach south of the bulkhead and the bulkhead is deflecting sediment into deeper water such that the shoreline south of the bulkhead will over time be impacted. Besides the impact of shoreline armoring along this stretch of coastline, the shoreline bluff slopes along most of the shore in this area has been stripped of trees. The tree removal has two effects: the loss of root strength will cause bluff slopes to be more likely to fail, and there will be a lack of lazge, erosion resistant woody debris in the landslide material if the slopes do fail. CONCLUSIONS AND RECOMMENDATIONS Based on beach morphology and the slope conditions, the existing route to the shoreline is presently stable and would be a good location for accessing the beach with minimal disturbance to the slope above the beach. The reconstruction of the damaged sections of wood stairs will not cause slope stability problems that will impact the slope or beach as long as our recommendations are followed. I recommend that the existing ladder stairs spanning the steep lower portions of the bluff slope be replaced with similaz ladder stairs. Any type of structure that spans the steep slopes is acceptable. The supports for the ladder step spans should be supported by posts embedded into the ground surface of the slope at least three feet. The use of concrete piers for the supports will last longer than using wood. The stair landing should continue to be located at the top of the rock riprap bulkhead. The trail landing on top of the rock riprap bulkhead is stable. Some minor scouring of material from the top of the bulkhead is evident. The scour is the result of periodic lazge waves that flow up onto the bulkhead. Replacement of gravel on top of the bulkhead can be done to level the bulkhead. However, gravel and cobbles used should be of similar size to the sediment on the beach and no beach material should be used at the bulkhead site. Cutting back blackberry brush in order to access the trail is acceptable in order to be able to install the trail and walk on the stairs without getting cut. The cutting back of blackberry brush will provide the opportunity to plant native vegetation along the trail route that has better root strength than blackberry, will be easier to maintain and esthetically more pleasing. There are a variety of plants that would improve the route to the shoreline. Willows are already present neaz the base of the slope. Hooker Willow has good rooting and is tolerant of salt spray. Pacific Willow (on wet areas) and Scouler Willow (on drier areas}are also usefiil and do not grow to heights that will block views and are tolerant of trimming. The Washington State Department of Ecology suggests a variety of plants for slope stability. Stratum Group Fik: 6.8.058 3 r June 13, 2005 20 Tala Shore Drive, Port Ludlow, WA Geology Assessment of Trail ~~~ t ,t _ i CLOSURE Please note that there aze inherent risks associated with lots neaz or adjacent to steep slopes. These are risks that the building owner (in this case the trail owner) should recognize and be willing to accept. If conditions appeaz different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regazding please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Shawn Group File: 6.8.056 4 r• Figure 1. 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