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HomeMy WebLinkAboutGeologic Slope and Beach Processes ReportCoastal Solutions, LLC 11027 Manitou Beach Drive NE Bainbridge Island, WA 98110 206.459.7264 April 17, 2017 Michael Edwards 2535 South Point Road Port Ludlow, WA 98366 Re: Geologic Slope and Beach Processes Reconnaissance Draft Edwards Property 2535 South Point Road Port Ludlow, WA 98366 Project No. 16080 Dear Mr. Edwards: vP� C Oct,) This letter -report summarizes Coastal Solutions' observations made during a recent geologic slope and coastal processes reconnaissance of your South Point Road property. The purpose of our study was to observe and evaluate the existing geologic conditions of the beach with regard to the proposed shoreline modification at the project site. Observations Site Conditions and Topography The site is located on the eastern shore of the Toandos Peninsula overlooking the main channel of Hood Canal. The residential property was steeply sloping with an existing wood frame house and attached garage on the upland between the beach and the South Point Road right-of- way. The house was located 20 feet from the inferred ordinary high water line (OHW). A wood deck attached to the east side of the house was located approximately 10 feet at the closest point. The property was surrounded by undeveloped private property to the south, the South Point Homeowners' association property to the north, South Point Road to the west, and the tidelands of Hood Canal to the east. The tideland area consists of the mouth of the South Beach Homeowners marina and sand spit. Beach and Coastal Conditions The shoreline at the site is currently unprotected and is showing signs of erosion. Based on available geotechnical information, the house was constructed in 2002 and included the placement of fill soils adjacent to the shoreline. Excavated material was derived from a mass excavation adjacent to the road. Spread footings were then cast directly on top of the fill. We know a building permit was obtained and is on file at Jefferson County. Both properties to the north and south were unprotected with the exception of scattered beach logs and a rock bulkhead farther north on the club property. Michael Edwards April 17, 2017 Coastal Conditions Project No. 16080 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 drift cell that begins approximately 10 miles south near the tip of the Toandos Peninsula and ends approximately 2 miles to the north within Squamish Harbor. Drift cells contain zones along the shoreline that include erosion areas, transport corridors, and deposition or accretion areas. Littoral sediment transport (direction of sediment movement parallel to the beach), which is predominantly a function of wave direction, is from the south but likely alternates in direction and intensity with seasonal variation. Primary sediment sources for the beach at the project site has been periodic landslide activity and ongoing erosion south of the site. The sand spit that is the current location of the South Beach Association is an accretion shoreform that was created by a prominent westerly turn of the shoreline in the area. The sand spit has been heavily modified by residential development, including the construction of armoring along most if not all of the properties on the spit. These predominantly wood bulkheads retain natural sediment as well as dredge material removed from the channel during initial grading and development of the area. We are aware that the channel is still being periodically dredged. Dredging, unmitigated bulkhead construction, and upland grading has significantly altered that natural sediment movement in the area. Current plans call for installing a bio -engineered erosion control system to protect the fill soils the lie beneath the house. The fills soils are highly susceptible to erosion when exposed to even low energy wave action. The new system will largely be located above the OHW line as it existed before the site was altered by construction. Drainage No groundwater seepage was observed on the slope or beach at the time of our site visit. However, our experience with nearby sites indicates that groundwater seepage from the upland area west of South Point Road can be heavy and may impact the project site during the wet winter months. Groundwater flow is also influenced by other factors such as precipitation, irrigation practices, time of year, upland land uses both on and off-site, and stormwater control. Comprehensive drainage analysis, including testing the downspouts during our field visit, was beyond the scope of services for this phase of the project. Downspouts for the house terminated below ground but we could not determine the location of the outfall. There were no indications of erosion that could be attributed to uncontrolled storm water or surface water flow at the site. Despite relatively high infiltration rates inferred for the sand and gravel sediments exposed at the site, all roof drains and other concentrated stormwater runoff should be located and discharged to a controlled location near the beach. Vegetation Vegetation consisted primarily of scattered shrubs and beach grass seaward of the existing house. The existing vegetation along the shoreline is providing surface erosion control only and cannot be relied on to protect against ongoing wave action in the area. Page 2 Michael Edwards April 17, 2017 Project No. 16080 Geology The South Point spit is an accretion shoreform where littoral sediment moving north encounters a lower energy environment (north east facing shoreline north of the site is somewhat protected from wave action and wave energy in this area is reduced). The location of the point is likely related to pre -historic upland landslide activity that altered the shoreline in the area in a manner that promoted the accretion of sediments. We are aware of ongoing landslide activity above the road. This activity is related to an existing landslide scarp from prehistoric landslide activity in the area. The cause of such landslide activity is unknown but is likely related to deglaciation of the region or more recent seismic activity. Soil borings were not completed at the site for this study but inferred soils below the beach likely consist of disturbed pre -historic landslide debris. These near surface soils have not been compressed by glacial ice and are subject to rapid movement during storm events. As a geologically recent feature, the sand spit is highly sensitive to even small changes in energy such as storm -generated waves and the alteration of terrain during 20`i' century development. Recommendations Soft Bank Protection The existing modified shoreline at the site consists of former natural shoreline, dredge spoils placed during channel modification, and fill placed during construction of the house. Dredge spoils and fill are highly suspect in terms of their engineering qualities including compaction and retention. Exposed at the shore, erosion of the fill poses the biggest threat to the house and deck area. There are a variety of methods to stabilize shoreline erosion ranging from engineered retaining wall and rock bulkhead hard armoring to the use of soft bulkhead techniques. We understand the owners wish to construct a more natural shoreline using anchored logs and imported sand rather than a rock bulkhead. The site is a relatively low energy area that is well suited for soft bank techniques. Soft armoring 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 bank that is threatening the upland fill soils beneath the house. 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. Soft bank protection is best suited for low wave -energy environments where damage to mobile but restrained logs is less likely and long-term sediment replacement can be performed economically and effectively. The site is protected from both the north and south and appears to be well suited for a soft bulkhead application. While soft bank systems provide no guarantee of uninterrupted beach protection, an increased level of protection can be achieved at the site. Page 3 Michael Edwards April 17, 2017 Project No. 16080 The low -angle beach face and relatively flat area landward of the OHW will allow a gradual beach face to be constructed and will provide a greater level of ongoing erosion control. However, the performance of a soft bank system is directly related to the intensity, duration, and frequency of high-energy storm events. Our experience is that the region is experiencing shorter, higher intensity rain and wind storm events than in past decades. This inherently unpredictable weather pattern will have direct impacts on the performance of any bio- engineered solution including the project site. In general bio -engineered systems are dynamic and result in the lowest overall visual and ecological impact to the shoreline environment but provide the least amount of bank protection during storm events. A soft bank bulkhead can reduce the probability of erosion to the upper slope and reduce risk to the fill pad upon which the house is constructed. The system will provide a measure of protection that is greater than what exists there now. However, the system is not storm -proof and will be altered/damaged by future storm events and will require maintenance and repair. See attached Figure 1, Edwards Soft Bank Site Plan, Cross-section and Vicinity Map for details of the proposed project. Flood Zone Habitat and Mitigation The following section addresses geological aspects of the nearshore area that may be identified as a flood zone under current FEMA regulations. No elevations were provided for the site, however inferred elevation 10.2 (mean higher high water) was estimated along the upper beach face waterward of the proposed project. The project as proposed is meant to approximate the natural upper beach and back beach configuration that likely existed at the site prior to residential development. In our opinion, the beach logs and gravel proposed in the areas will neither adversely affect the flood zone in the area nor adversely affect littoral sediment transport along this reach of shoreline as far these coastal geologic conditions may contribute to habitat management in the area. As construction methods and sequencing may relate to flood plain impacts, all equipment and access will be from the upland. No barge landing will be required. Disturbed areas will be replanted immediately following construction. Limitations This letter -report was prepared based on a limited geological .field reconnaissance and no subsurface explorations were performed. A more in-depth characterization of slope conditions and geotechnical analysis 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, in particular bioengineering methods are highly dependent on the frequency and severity of storm events and the skill and care used by the contractor during construction of the bulkhead. Past performance is not necessarily an indication of what to expect in the future based on the inherent unpredictability of the weather that will affect the project site. The opinions in this letter -report are meant to help the property owner manage the natural erosion that is occurring 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 Michael Edwards and his agents with specific application to the project site. Page 4 Michael Edwards April 17, 2017 Project No. 16080 It has been a pleasure to provide these services to you. If you have any questions, please do not hesitate to call Rob Cousins at (206) 459-7264. Sincerely, Coastal Solutions, LLC I Robert F. Cousins I Robert F. Cousins, LEG, LHG Principal Geologist rob@coastalsolns.com EXPIRES: 02/06,/Ib Nina L. Cousins, P.E. Principal Engineer nina@coastalsolns.com Attachments: Edwards Soft Bank Repair Drawing Figure 1. Page 5