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HomeMy WebLinkAboutGeotechnical ReportCoastal Solutions, LLC 11027 Manitou Beach Drive NE Bainbridge Island, WA 98110 206.459.7264 August 5, 2019 Robin and Bob Westler 621 Ludlow Bay Road Port Ludlow, WA 98365 Re: Geologic and Beach Processes Reconnaissance Westler Property, 621 Ludlow Bay Road Port Ludlow, WA Project No. 19047 Dear Ms. Westler: -a ��---_7 JA N 2 7 2020 This letter -report summarizes Coastal Solutions' observations made during a recent geologic and coastal processes reconnaissance of your property on the southeastern shore of Ludlow Bay in Jefferson County, Washington. The purpose of out study was to observe and evaluate the existing shoreline conditions at the site with regard to the proposed bulkhead replacement project. Observations Site Conditions and Topography The site is located on the southeast shore of Ludlow Bay. The 1.55-acre residential property consists of a relatively flat southeastern upland where the house is located and a steep beach slope to the northwest. The area around the house consisted of landscape shrubs and grass lawn. The house consisted of a wood -frame structure with a daylight basement foundation with a detached garage. The eastern footing for the deck attached to the northwest side of the house was located approximately 30 feet from the top of the slope at the closest point. According to Jefferson County Department of Health records, the septic tanks are located between the house and the top of the slope and the drainfield is located southeast of the house near Ludlow Bay Road. Total vertical relief between the house foundation and the beach was on the order of 27 feet. The area between the top of the slope and the beach consisted of landscape shrubs, a gravel path, and a deck located adjacent to the shoreline. A patio and fire pit area were located in the yard area approximately 4 feet from the top of the bank. A fixed pier, ramp, and float extended over the upper intertidal area near the lower deck. The property was surrounded by similar private properties to the northeast and southwest, Ludlow Bay Road to the southeast, and the tidelands of Ludlow Bay to the northwest. Exisling Shoreline Conditions The existing bulkhead that protected the site was on the order of 80 feet along the northeastern 80 feet of the shoreline at the property. The bulkhead consisted of concrete rubble, wood piling, rocks, and horizontal logs with concrete beach stairs near the southwest property line. The southwest approximately 20 feet was unprotected. The bulkhead was still protecting the Robin and Bob Westler August 5, 2019 Project No. 19047 shoreline but showed signs of settlement, deterioration, and displacement, which is typical of these types of bulkheads. Based on lower slope geometry and voids in the bulkhead face, we estimated 6 to 10 feet (measured horizontally) of fill exists in places landward of the bulkhead. Both adjacent properties were unprotected. The bulkhead at the site was still performing as intended, but is at the end of its service life and in need of replacement. Current plans call for the replacement of the existing bulkhead with a new, irregular -faced rock bulkhead placed 3 to 4 feet landward of the toe of the existing bulkhead. The new rock bulkhead will be approximately 5 feet high with a batter on the order of 6H:1 V (horizontal:vertical) and will extend the entire length of the shoreline at the property. The new bulkhead will be extended approximately 20 feet to the southwest where a landslide caused downslope displacement of a large tree and approximately 25 cubic yards of soil. Placement of the rock bulkhead landward of the original rock bulkhead footprint will result in a net gain of a minimum of 300 square feet of upper intertidal area now buried by the rubble bulkhead. In plan -view, the new rock bulkhead will follow the same approximate layout as the existing bulkhead with the exception of the northwest extension. Both flanks of the bulkhead will end at the property lines. The proposed replacement bulkhead type, location, and configuration will result in a net decrease in adverse impacts to coastal and littoral processes created by the block rubble and creosote treated piling and timbers that exists now. Beach and Coastal Conditions The beach consisted of an approximately 15H:1 V sand, gravel, and mud beach -face. Based on our observations of barnacles and other indications of periodic inundation on the beach, ordinary high water (OHW) was located at the face of the existing bulkhead. The most recent data regarding coastal processes published by the Washington State Department of Ecology, (Ecology) Washington Coastal Atlas indicated that the property lies within a littoral drift cell that begins approximately 1.3 miles northeast at Tala Point and terminates approximately'/z- mile to the southwest near the head of Ludlow Bay. Drift cells contain zones along the shoreline that include erosion areas, transport corridors, and deposition or accretion areas. We noted subtle signs of littoral sediment deposition on the northeast sides and scour/erosion on the southwest sides of rock and other obstructions within the upper intertidal area consistent with the published drift directions. Sediment transport is largely the result of wind -driven waves striking the shoreline at an angle. Erosion on the project site consisted of scour in and around the concrete rubble that is allowing piping of soil and collapse of the log/piling system. Drainage We noted that roof runoff for the house and the garage appeared to be controlled by a series of downspouts that discharged to a subsurface system that discharges via a 4-inch diameter PVC pipe near the southwest property line. There was no evidence of surface erosion within the upland area due to stormwater runoff. We noted no signs of groundwater seepage at the site or on the adjacent sites during our field visits. It should be noted that groundwater seepage levels and flow volumes will vary with precipitation, irrigation practices, time of year, and upland land uses both on and off -site. Page 2 Robin and Bob Westler August 5, 2019 Project No. 19047 Comprehensive drainage analysis, including testing the downspouts during our field visit, was beyond the scope of services for this phase of the project. The proposed replacement bulkhead design should include a suitable stormwater outfall structure where the 4-inch outfall will intersect the face of the bulkhead or discharge via energy dissipation behind the bulkhead. A dispersion/energy dissipater placed behind the bulkhead should be placed in such a manner as not to jeopardize the integrity of the bulkhead or cause erosion at the beach or the toe of the slope. Vegetation Vegetation on the upland portion of the site and around the house consisted primarily of low shrubs and landscaping. The backyard area consisted of grass lawn. Geology Soil exposures on site were limited due to the fill soils that had buried the former natural toe of the slope behind the bulkhead and the presence of the concrete rubble. However, soils exposed on the neighboring property to the southwest consisted of interbedded sand and gravel. This exposure is consistent with mapped soil units of Vashon advance outwash sediments. Vashon advance outwash soils were Vashon Advance Ourwash Advance outwash silt, sand, and gravel was deposited during the last major glaciation of the Puget lowland approximately 15,000 to 18,000 years ago. As the ice sheet advanced south through the area, silt, sand, and gravel were deposited ahead of the advancing ice sheet by pro - glacial streams meandering through a broad outwash plain. The advance outwash consists of interbedded sequences of sand, gravel, and silty sand. Where a prominent silty unit is present underlying more granular sand, the silt layers can act as aquitards that impede the downward percolation of groundwater through the more permeable sand. In places this can result in groundwater seepage at the silty sand/sand contact. The advance outwash deposits have been overridden and compressed by the advancing ice sheet and display low compressibility and high shear strength characteristics. When exposed to waves at the toe of the slope however the advance outwash is highly susceptible to rapid erosion. While not observed during our field visit, groundwater can accumulate at the contact between sand and silt units within the advance. This is the result of the groundwater percolating down through the relatively high permeability sand and encountering the relatively low permeability silt below. This perched groundwater condition can result in localized seepage where the contact is exposed. Groundwater levels and flow volumes should be expected to fluctuate with precipitation onsite and offsite The bulkhead at the site protects fill soils placed landward of the bulkhead and seaward of the former toe of slope. The fill, where exposed consisted of loose, moist, silty sand with gravel, concrete debris, creosote piling, and creosote logs was likely placed with no engineering oversight or control. As a result, the fill and debris is highly suspect in terms of its erosion resistance and strength qualities. Even properly placed and compacted select fill is highly susceptible to shoreline erosion if not protected from wave action. Shoreline retreat rates are dependent on factors such as upland land use, precipitation, geologic conditions, over -water fetch distance and direction, and shoreline protection. However, Page 3 Robin and Bob Westler August 5, 2019 Project No. 19047 adequate protection of the fill soils from rapid erosion is the most important factor that will affect erosion rates and subsequent property damage at the site if the fill is exposed to wave action. Despite its relatively poor condition, the rubble bulkhead is still providing toe protection. A comprehensive natural bank retreat analysis could not be performed due to the long-term presence of the existing bulkhead and the historically placed fill soils. However, available air photos for the neighboring property to the southwest indicated toe erosion on the order of 8 to 12 feet between 1977 and 2019. We noted a well -established colluvial toe -of -slope that supported dense nearshore vegetation in the 1977 image. Colluvium is the natural collection of eroded material and landslide debris from the upper bluff. Colluvium is loose in nature, organic -rich, and provides a growing medium for plants and trees. On beach bluffs colluvium is a sacrificial natural arming system that in an unaltered system is constantly being eroded by wave action but also constantly being replenished by mass wasting from above. During our most recent site visit in June of 2019, the colluvial toe was essentially gone indicating faster erosion rates in the area than deposition and replenishment. Conclusions There is no structural integrity of the rubble portions of the bulkhead and the piling portions show signs of deflection and wood -rot. Based on the existing deteriorated condition and the degree of piping and undercutting observed, the bulkhead is at high risk of being breached and in need of replacement. Continued piping will eventually result in continued settlement and movement of rubble out onto the upper intertidal area and further deflection of the logs and piling. The bulkhead protects the fill soils that in turn provide an erosion buffer for toe of the slope and the upland improvements including the deck and dock approach. Current plans call for replacing the entire rubble and timber pile bulkhead with a new rock bulkhead between 3 to 4 feet landward of the existing bulkhead face with an approximate 20 feet extension to the southwest to protect the existing access path near the southwest property line. Erosion on the unprotected neighboring property has resulted in destabilization of the Westler shoreline in this area. This area is active in terms of erosion and will require future maintenance but the extension will allow the bulkhead to terminate at a point landward of the OHW line that will result in decreased need for future maintenance as erosion continues on the neighboring property. Replacing the existing bulkhead with a properly constructed rock bulkhead, as proposed, will provide a more predictable erosion control structure (in terms of long-term performance), which will protect the existing upland features at the site. Replacement will also result in a net gain of upper intertidal area previously buried by the original bulkhead and backfill and will result in foreign material such as concrete rubble and treated wood being removed from the shoreline environment. Alternatives Shoreline erosion and stability can be addressed by several methods depending on site conditions, location and project goals. Shoreline erosion control methods can range from the use of planted supplemental vegetation to control surface erosion, constructing soft or bio- engineered shoreline protection systems, constructing hard surfaced bulkheads or rock Page 4 Robin and Bob Westler August 5, 2019 Project No. 19047 revetments, or the application of a hybrid system that employs both hard and soft armoring techniques. Replacing an existing erosion control structure however, limits the available alternatives for the property owner to adequately protect existing improvements on the property and on adjacent properties. Su lemental Ve etation In our opinion, the existing upland vegetation at the site does not contribute to protection of the property from wave action. Supplemental vegetation will have no mitigating effect on wave generated toe erosion and is not a viable option as a replacement for protection at the site. Rock Bulkhead Rock bulkheads consist of essentially vertical rock walls constructed at or near the ordinary high water line. 3,000 to 5,000 pound rocks are keyed into the beach approximately 4 feet, stacked to a typical height of approximately 5 to 7 feet above the beach surface and backfilled with free -draining rock ballast. Out of the alternatives discussed in this report, rock bulkheads are the most effective method to control toe erosion over the anticipated service life of the structure. The most significant effect of constructing a rock bulkhead is reduced sediment input to the beach. This is the result of building a structure that is necessarily designed to stop or significantly slow toe erosion. While the effects of reduced sediment input to the beach can be mitigated, rock bulkheads result in an alteration of the shoreline as compared to natural conditions. Where a rock bulkhead replaces an older outdated design such as the bulkhead at the site, erosion control is maintained but adverse effects to the upper intertidal area are reduced. A rock bulkhead represents less of an impact to the upper intertidal zone when constructed waterward of the previous structure. Soft Bank Protection The use of soft bulkhead techniques includes the combined use of beach nourishment and restrained large woody debris (beach logs) at the site. Beach logs are restrained using cables and an earth retention system such as dead -man anchors or tieback style earth -anchors. The logs are arranged in such a way as to impede wave energy acting on the toe of the slope. Imported beach sediments are then placed to embed the logs aiding in energy dissipation. The beach nourishment also provides sacrificial sediment that is actively recruited by the natural coastal processes acting on the shore. In areas of diminished littoral transport volume and areas where sediment recruitment is high, soft bank protection typically results in a net loss of nourished sediment. As the sediment is eroded over time, additional nourishing sand is placed to maintain the erosion control function of the system. The amount of beach nourishment is initially estimated using average bank retreat rates for the project site but is adjusted based on actual long-term performance. Soft bank protection is best suited for low wave -energy environments where a broad, relatively flat back -beach and upland area exists to minimize the angle of the constructed surface and maximize the wave run-up distance. Soft bank structures are also not well suited for sites that are flanked by existing hard surface structures such as the northwest side of the project site or where existing structures such as dock approaches need long-term protection. A breach of the soft bank system during a storm event would not only jeopardize the project site but would affect the integrity of adjacent structures. Page 5 Robin and Bob Westler August 5, 2019 Project No. 19047 The performance of a soft bank system is directly related to the intensity, duration, and frequency of high-energy storm events. As a dynamic system, they result in the lowest overall impact to the shoreline environment but provide the least amount of bank protection during storm events. The topography and aspect of the site is an environment not suited to soft bank protection. Mitigation Construction of the replacement rock bulkhead will result in a net gain of between 300 and 400 square feet of upper intertidal beach area previously buried by the rubble bulkhead. The bulkhead will continue to impede erosion in a similar but less intrusive fashion than the existing bulkhead. Beach nourishment volume estimating for the entire 100 feet of bulkhead was not performed due to the extensive historical modification of the shoreline and because no natural erosion (that could contribute sediment to the beach) is currently occurring behind the existing bulkhead. However for estimating purposes, the 20 feet of bulkhead extension will impede approximately 4 inches of soil per year based on our observations of the neighboring property. This translates to approximately 1.3 cubic yards of soil that will no longer reach the beach each year the bulkhead is in place. The excavated natural silty sand with gravel that will be removed as part of the project however should be placed on the beach as natural sediment input. Based on our calculations, the footprint of the bulkhead will generate approximately 66 cubic yards of soil that may be used for beach nourishment. 66 yards is the equivalent of approximately 50 years of erosion along the extension portion of the bulkhead. Recommendations Based on our field observations, the existing bulkhead is at the end of its service life and is in need of replacement. To reduce the probability of acute and severe erosion of the fill between the slope and bulkhead, and to reduce the impacts of the existing structures to the existing coastal processes at the site, we recommend replacing the existing rubble and piling bulkhead as proposed. The new and extended rock bulkhead should be placed entirely landward of the existing OHW (estimated at the face of the existing bulkhead). Construction of the bulkhead flush to the adjacent bulkhead to the northeast will reduce scour and erosion that may be occurring. Removal of the rubble and treated wood and construction of the new bulkhead above the OHW will reduce the adverse impacts to the existing littoral sediment movement in the area resulting in a no net loss of ecological functions. To reduce the probability of erosion to the fill soils behind the proposed bulkhead we recommend the following: Verify that the existing surface water drainage systems including roof runoff from the house are functioning properly and can still be routed to the beach when the new bulkhead is constructed. This pipe should be located, protected during construction, and replaced to its current location. No concentrated flow should be allowed to discharge within the existing fill soils behind the proposed bulkhead. Place sufficient size rocks landward of the OHW line to create a gravity wall system. The rocks should be placed in a manner to provide a smooth connection to the existing bulkhead to the northeast and to a suitable stopping point landward of the OHW mark at the southwest property line. Page 6 Robin and Bob Westler August 5, 2019 Project No. 19047 Plans Review We have reviewed the proposed plans prepared by Sealevel Bulkhead Builders, titled "Replace Old bulkhead with a Rock Bulkhead for Bob Westler, 621 Ludlow Bay Rd, Port Ludlow, WA 98365" undated provided to us on August 5, 2019, In our opinion. the plans as proposed are consistent with the geotechnical opinions and recommendations in this letter -report. Limitations This letter -report was prepared based on a limited field reconnaissance and no subsurface explorations were performed. A more in-depth characterization of slope conditions and geotechnical analysis, including quantitative slope stability analyses for the subject property or the adjacent properties was beyond the scope of work for this study. There are inherent risks associated with construction in the shoreline environment. The longevity and effectiveness of shoreline armoring is highly dependent on the frequency and severity of wave -generating storm events that are inherently unpredictable. Other factors that impact the effectiveness of shoreline armoring are the frequency and magnitude of upland landslide activity and erosion, and the skill and care used by the contractor during construction of the reverinent. The opinions in this letter report are meant to help the property owner manage issues related to erosion at the site and do not constitute a legal opinion or a warranty, express or implied. This letter -report was prepared for the exclusive use of Robin and Bob Westler and their agents with specific application to the project site. It has been a pleasure to provide these services to you. If you have any questions, please do not hesitate to call. Sincerely, COASTAL SOLUTIONS, LLC I Robert F. Cousins, LEG Principal Geologist rob@coastalsolns.com Nina L. Cousins, PE Principal Engineer n ina@coastalsolns.com 1 Page 7