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HomeMy WebLinkAbout701224015 Geotech AssessmentMay 9, 2002 To: Mr. David Russ Bath 9345 Fountleroy SW Seattle, WA 98136 For: Soft Shore Protection Design 1423 Camp Discovery Road East shore Dabob Bay Jefferson County, WA Section 22, T 27 N, R I W, WM JE[FERSON COUN]'Y ~ : D£PI'. O~ COMMUNi'f¥ DEVELOPMEN[_.., Background and Purpose Jim Johannessen, Licensed Geologist, visited the above-referenced property on 4/15/02 to assess coastal processes at the site and evaluate the need for shore protection at the site. General approaches for soft shore protection methods to significantly s/ow coastal erosion were discussed at that time. Measurements of the site and the beach and creek area survey were completed dudng the site visit. This included delineating the ordinary high water mark (OHWM), west lot features, beach slope, approximate mean higher high water (MHHW), property boundaries, and creek features. At your request, I have completed a design for soft shore protection of the beach and west yard area. Design drawings are included here and this letter summarizes the design and material needs. Coastal Processes The site is located along the east shore of Dabob Bay. The sediment transport (littoral drift) at the site was mapped by Johannessen (1992). The long-term littoral drift pattern is referred to as net shore-drift in Washington. The site is part of a very long net shore-drift cell termed cell JE-16, which originates at the SW end of the Toandos Peninsula (Johannessen 1992) and continues past the site to Long Spit at the north end of Dabob Bay. Natural shore drift processes seem to occur in this area as the amount of development of the shoreline is not extensive. The beach along this shore has a mixed gravel and sand beach. The sediments that comprised the high-tide beach at the site ranged in size from medium sand to pebble and cobble. Camp Discovery Creek reaches the shore south of the site. The lower reach of the creek channel runs no,lhward across the iow eievation area south of the Bath property. Recent erosion of the high- tide beach has exposed post-glacial peat on the beachface near the northern property line. This suggests that the beachface has experienced significant erosion over the recent decades or centuries and has retreated landward over the old marsh area, exposing the peat. The largest change at the beach has been the movement of the mouth of Camp Discovery Creek. According to Mr. Bath, the creek mouth has usually been located a considerable distance to the nodh of the site. The creek channel was located in the backshore area during these pedods, as net shore-drift of beach sediments forced the creek mouth northward. Mr. Bath stated that there has been approximately 3 creek mouth "blowouts" in the last 40 years, at which time the creek channel would cut directly through the berm immediately south of the site dudng high flow '~x pedods. This occurred in the pedod soon after the summer of 1994, reportedly when a logging road failure upstream triggered a flood event. A pair of oblique aerial photos shows the pre- and post-flood views of the creek mouth and beach (Figures I and 2). Wi~son Ave., Beilins~am, WA 98225-7348 ($60) 647-1845 Fax: 67x-665~ coastalseo~attbi, com ~ OaddRus~ Bath May 9, 2002~ Page 2 of 4 COASTAL GEOLOGIC SERVICES, INC. SITE Figure 1. Oblique aerial photo taken 7/11/94 showing the creek extending landward of the beach berm for more than 1,000 ft north. Photos by WA Dept. of Ecology, photo numbers 176 and 177. Figure :~. Oblique aerial photo taken May 2001 showing the creek channel after it breached the beach berm immediately south of the Bath property. Photo by WA Dept. of Ecology. Comparison of the two images (Figures I and 2) reveals that significant recenl erosion of the upper beach has occurred since 1994. It is estimated that the vegetation line (which is also the ordinary high water mark or OHWM) moved landward at least 50 ft between 1994 and 2001 at the site based on quasi-scaled measurements from the images. Mr. Bath stated the three ecology blocks visible on the 2001 image were not moved but inslead the bank was eroded away beneath them over the winter of 2001-02. The blocks are now on the upper beach (Figure 3). Figure 3. Photo taken 4/15~02 showing the upper beach and bank at the site. Note the ecology blocks that have dropped to beach level as the bank has eroded landward and rock in foreground on adjacent property. ~r. David R~ Bath t~ay ~, 2OO2, ~age $ COA..qTAL ~EOLOGIC SER VICES, INC. Soft Shore Protection Design The proposed soft shore protection design is intended to minimize additional erosion of the Iow yard area waterward of the house. The entire project would be constructed generally 18 ft landward of the mean higher high water (MHHW) line as of 4/12/02. This would allow for quasi- stable installation of coarse gravel and anchored logs. The length of the area to be protected (to the degree possible) would be the width of the property at 65 ft. The cross shore width of the gravel fill area would be 12-18 ft. See the attached site plan and cross section for the extent of the gravel fill and anchored log area. The materials required are as follows: 1.80 tons (60 cy) gravel: I ~ to 2" washed, rounded (drain rock) 2. 80 lineal ff of logs - Cedar or Douglas fir are strongly preferred, grand fir, hemlock, or are acceptable if dimensions are 25% greater. Dimensions may vary, minimum dimensions are: 25 ft x 20 in, preferably with root mass, OR 20 ftx 24 in with root mass, OR 10ft x 30 in with root mass 3. Approximately 4 ecology blocks (1 per pair of log ends; 2x2x4 ft blocks): poured with 2 inch center hole (plastic pipe in form), or drilled on-site 4. 7/16 inch marine grade galvanized lashing chain and shackles (length determined after log procurement) The key elements of the design are both raising the elevation of the uppermost beach and backshore area in the vicinity of OHWM and increasing the sediment size to change it to a more consistent (well-drained) gravel size, making a more resistant backshore area. The addition of anchored logs is also important, as discussed below. Between 0.75 and 1.0 ft of the uppermost beach sand and gravel would first be excavated in the 12-18 ft wide band along the uppermost beach and backshore area (see Site Plan and Cross Section). This would have the lower portion of the imported gravel keyed into the beach, adding to its stability. Drain rock (1 ½ to 2" washed, rounded) will be placed into the excavated area, and above the excavated area. The drain rock would slope from the existing beach surface at a 5.5:1 slope up to the new berm crest at +14.7 ft MLLW. The imported gravel would be generally 1.25 to 1.75 ft thick vertically. The design here would rely on approximately 3 of the largest available, preferably but not necessarily with root mass attached. Drifting logs are often the best size and quality and can be acquired much cheaper than whole logs on land. Logs would be placed in the imported gravel, keyed in approximately one-third of their diameter. 'Root masses would aid in the stability of the logs. The logs would be secured with ecology block anchors (3,600 pound concrete blocks measuring 2x2x4 ft). The top of the ecology blocks would be buried a minimum of 2.5 fl below the existing surface. Madne grade galvanized lashing chain would be used for securing the logs to the anchors, with a complete wrap around each end of each log. Very large galvanized staples (preferably 6 inch length) would secure the chain to the logs. The cut native beach sediment would be placed atop the landward portion of the enhanced storm berm area (see Site Plan and Cross Section). This material would cover the horizontal storm berm top (0.6 ft thick) and extend up to 13 ft landward of the drain rock at a 3.5:1 slope. In effect._., this would move the storm berm crest back approximately 5 ft and the landward edge of the storm*` berm landward 10-12 ff. COASTAL GEOLOGIC SER VICES, INC. Native dunegrass (Elyrnus mollis) would then be planted atop the sand and gravel storm berm to help stabilize this material during storms. Dunegrass would augment the stability of the constructed storm berm through its root strength. Please call if there are any questions at (360) 647-1845. Thank you for allowing Coastal Geologic Services to be of assistance. References Johannessen, J.W., 1992, Net shore-drift in San Juan County and parts of Jef~erson, Island, end Sno/x3mish counties, Washington: final report. Western Washington UnN~,sity, for Shorelands and Coastal Zone IVlanagement Program, Washington Department o~ Ecology, Olympia. 58 pp., 25 maps. Coastal Geologic Services inc. Jim Johannessen, MS Licensed Geologist ATTACHED: Site Plan Cross Section ] I SITE PLAN Bath Property Soft Shore Protection 5i9102: 1423 Camp Discovery Rd E Dabob Bay. Jefferson Co., WA, Section 22, T 27, R 1W, WM Scale 1'=20', Measurements 4/15/02 Vert. Datum: MLLW Whitney Pt, MHHW = +11.55 MLLW Prepared: Jim Johannessen, Licensed Geologist, MS Coastal Geologic Svs. Inc., Bellingham WA (360) 647-1845 I t ,Scd/_E; / '= 2.0' CROSS SECTION Bath Property Soft Shore Protection 519102: 1423 Camp Discovery Rd E Dabob Bay. Jefferson Co., WA Section 22, T 27, R 1W, WM Horiz. scale 1'=10', Vert. scale 1' = 4' (Vert. Exag.= 2.5) Vert. Datum: MLLW Whitney Pt, MFIHW = +11.55' MLLW Prepared: Jim Johannessen, LicenSed Geologist, MS Coastal Geologic Svs. Inc., Bellingham WA (360) 647-1845 12. I0 ! ! , ! I , f I I I t ioo I/O /2.o