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HomeMy WebLinkAboutGeological Slope and Beach Processes Reconnaissance for BLD2013-00671 - BLD Engineering / Geo-tech Reports - 7/17/2013 COASTAL SOM71ONS, LLC. 11027 Manitou Beach Drive NE.Bainbridge Island,Washington 98110 206-459-7264 July .17,2013 Project No. 13003 Dwayne Griffith P.O.Box 1710 Allyn, WA 98524 Re: Geologic Slope and Beach Processes Reconnaissance Griffith Property 6570 East Grapeview Loop, Allyn, Washington Dear Ms.Griffith: This letter-report summarizes Coastal Solutions' observations made during a recent geologic slope and coastal processes reconnaissance of the subject property. The purpose of our study was to observe and evaluate the existing shoreline conditions at the site with regard to the proposed concrete bulkhead replacement and repair project. Observations Site Conditions and Topography The site is located on the west shore of the North Bay area of Case Inlet approximately 1.5 miles south of Allyn, Washington. The residential property consists of a relatively flat upland on the west side adjacent to the East Grapeview Loop right-of-way where the house and driveway were located and a steeply sloping eastern portion adjacent to the beach. Three structures were located on the property, two wood-frame houses and a storage cabin. The main house consisted of a wood-frame structure with a conventional footinglcrawlspace foundation with an attached wood deck on the east side adjacent to the beach slope. The deck was located approximately 3 feet from the top of the slope and the hour foundation was located approximately 17.5 feet from the top of the slope. The toe of the slope was protected by a concrete seawall that extended along the entire 155 feet of the shoreline at the site. A narrow road/path excavated through the bluff allowed pedestrian access to the beach area and a narrow concrete boat ramp through the bulkhead. We understand that the septic infiltration system is located in the wooded upland area west of the houses. The flat area landward of the seawall was approximately 10 feet wide and appeared to be fill soils.placed at the toe of the slope when the wall was built. The top of the northern concrete bulkhead was on the order of 3.5 feet above the beach face. The southern approximately 55 linear feet was on the order of 4.5 above the beach face. The wall showed signs of undermining and piping of the fill material, likely as a result of periodic overtopping by wave action. Placing fill over the upper intertidal area was common from the I950's to the 1980's. The Property was surrounded by similar private properties to the north and south,the existing East Grapeview Loop right-of-way to the west, and the tidelands of Case Inlet to the east. The existing concrete seawall that protects the site was essentially vertical,but showed signs of cracking, deterioration,and undermining. The wall protected the fill area directly behind the wall,and in turn,the upper exposed bluff adjacent to the foundation area for the deck and Dwayne Griffith July 17, 2013 Project No. 13003 house. The seawall was still performing as intended but is at high risk- of collapse resulting a subsequent breach. in Higher concrete seawalls protected both adjacent properties to the north and south. Both adjacent shoreline structures were in good condition and performing as intended. Existing Shoreline Conditions Current plans call for the replacement of the northern compromised seawall with a new irregular-faced rock bulkhead on the order of 5 feet tall placed approximately I foot landward of the existing wall face. The rock wall will be angled in such a manner as to blend with the existing seawall to the north and with the seawall that will rcmain on the southern portion of the subject property. The boat ramp is slated for removal and will be replaced with pre-cast concrete steps. The southern approximately 55 linear feet of concrete wall will be raised by I foot, The proposed northern replacement wall type,location, and configuration will result in a net decreased impact to coastal.and littoral processes created by the existing flat surface wall. Raising the southern wall by one foot will have no detrimental impacts to existing shoreline conditions. A second rock wall approximately 6 feet high will be constructed adjacent to the exposed bluff near the house foundation. Depending on field grades during construction,sloping backfill will be placed in the area between the upper rock wall and bluff. The upper rock wall will be keyed into undisturbed natural glacial sediments inferred to underlie the fill soils that currently exist behind the concrete seawall. Beach and Coastal Conditions The beach consisted of an approximately I OH:I.V sand,gravel, and cobble 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 seawall. 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 at a divergence zone approximately 1/2 mile south of the site and terminates approximately I mile north of the site at an accretion sand spit. 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 north sides and scour/erosion at the boat ramp and at the seawall north of the site consistent with the published drift directions. However,due to the north-south alignment of the beach at the site, littoral drift in the area likely varies between the summer and winter months. Low pressure storm systems from the south during winter will result in a northern drift direction and summer high pressure resulting in northerly winds will result in southerly sediment drift directions. Erosion on the project site consisted of severe scour at the toe of the seawall that had exposed and undermined the footing of the wall. Drainage Downspout drains for the southern house were routed to a temporary overland piping system that ran down the access path toward the beach. Downspouts for the main house and outbuilding adjacent to the upper bluff appeared to discharge to the ground adjacent to the foundation. The upland was largely vegetated and we saw no signs of standing water. There Page 2 Dwayne Griffith July 17,2013 Project No. 13003 were also no signs of groundwater seepage at the site during our field visits. It should be noted however 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. Comprehensive drainage analysis was beyond the scope of services for this phase of the project. The proposed replacement bulkhead design should include a suitable storm water outfall structure that the existing downspout drains can be routed toward. The outfall structure such as a pipe penetration to the beach or 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 consisted primarily of undisturbed, second growth forest west(landward)of the houses. The back yard area consisted of low ground cover vegetation.The flat fill area between the bulkhead and the upper bluff was sparsely vegetated with grass and weeds. Geology Soil exposures were generally good based on the vertical nature of the bluff and the lack of vegetation on the bluff face. Sediments exposed along the bluff consisted of very dense,moist gray silty sand with gravel interpreted as Vashon Lodgment till. These sediments were deposited during the last major glaciation of the Puget lowland, between approximately 15,000 and 18,000 years ago. As the ice sheet advanced south through the area, a mixture of gravel, sand, silt,and clay entrained in the lower portion o:fthe glacier was deposited directly onto the pre-existing landscape. Unweathered till exhibits high shear strength and low compressibility characteristics and is fairly resistant to wave erosion where exposed. The unweathered till was mantled by a 2-foot thick weathered layer where root-wedging, bioturbation,oxidation and cycles of freeze-thaw have reduced the density and strength of this layer. Depending on the percentage(by weight)of sand, both the unweathered and weathered till is still susceptible to wave erosion. Groundwater can accumulate at the contact between the weathered and unweathered till. This is the result of the groundwater percolating down through the higher permeability weathered till and encountering the lower permeability unweathered till below. This"perched" groundwater condition can result on localized seepage where the contact is exposed on the slope. Evidence of seasonal perched groundwater seepage was observed north of the main house along the vertical till bluff approximately 20 feet north of the house. The flat area behind the seawall is consistent with typical historical fill placement during construction of this type of wall. The fill has buried the former toe of slope and has essentially stopped toe erosion in the area. Topography of adjacent lots is similar and fill placed over the former upper intertidal area was common in the 1950's through the 1980's. This time frame is consistent with the construction of this type of wall. The seawall at the site protects fill soils placed landward of the seawall and seaward of the former toe of slope near the houses. The fill consisted of loose silty sand with gravel and cobble likely placed with no engineering oversight or control. As a result this material 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.. Seawalls not designed to Page 3 Dwayne Griffith July 17, 2013 Project No. 13003 resist hydrostatic pressure or lateral earth pressures associated with uncontrolled fill often result in deterioration and undermining as observed at the project site. 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, 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 generally poor condition,the wall is still providing a degree of toe protection and has resulted in its intended purpose of stopping toe erosion. A comprehensive"natural"bank retreat analysis could not be performed due to the long-term presence of the existing wall and the historically placed fill soils_ Conclusions The structural integrity of the seawall is unknown. However based on its existing deteriorated condition, and the degree of deflection observed, the wall is at risk of collapsing and is need of replacement. Rapid increases in hydrostatic pressure associated with overtopping wave action and limited drainage behind the wall to alleviate the buildup of groundwater will eventually result in the seawall toppling over and exposing the fill soils behind the wall. The wall protects the fill that in turn provides an erosion buffer for the toe of the slope and the upland improvements,most importantly the deck and the house footings adjacent to the top of the slope. Current plans call for replacing the northern 80 feet of wall with a rock bulkhead on the order of 5 feet high located landward of the existing seawall face. The existing seawall footing and boat ramp will be removed as part of the project. Replacing the existing seawall with a properly constructed rock bulkhead,as proposed, will improve drainage, provide a more predictable erosion control structure(in terms of long-term performance), that will protect the existing upland features including the house. The southern 55 feet of wall is in better condition and will remain.The height of this wall will be increased by approximately 1 foot. The existing deck is at imminent risk of Ding damaged by landslide activity. The upper bluff is exhibiting signs of periodic landslide activity that is consistent with the soil type exposed. The upper weathered lodgment till is highly susceptible to erosion due to cycles of freeze/thaw, root wedging,and preferential groundwater flow paths. The lower unweathered till is relatively erosion resistant but is susceptible to slab-type Iandslides. These types of landslides occur less frequently but can result in the loss of 1 to 5 feet of the bluff during a single event. The proposed upper rock will serve as erosion control and partial buttress of the slope that will reduce upland landside activity and will increase slope stability as it relates to the existing deck and house. 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 revetments,or the application of a hybrid system that employs both hard and soft armoring Page 4 Dwayne Griffith July 17,2013 Project No. 13003 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 etatiorz In our opinion, the existing upland vegetation at the site(grass) is providing surface erosion protection only and does not contribute to protection of the property from wave action. Supplemental vegetation can slow surface erosion from rainfall and upland surface water flow but has little to no mitigating effect on wave generated toe erosion. As a result supplemental vegetation is not a viable option as a replacement for protection at the site. Rock Bul.knead 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 `P y into the beach approximately 4 feet, stacked to a typical height of approximately 5 feet above the beach surface and backftlled with free-draining rock ballast. Out of the alternatives discussed in this report, rock bulkheads p t, kneads are the most effective method to control toe erosion over the anticipated service life P 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 seawall at the site,erosion control is maintained but adverse affects to the upper intertidal area are reduced. A rock bulkhead represents less of an impact to the upper intertidal zone due to reduced wave reflection that results from the energy dissipating irregular face of the structure as compared to the flat surfaced existing seawall. Rock Revetments The second most effective erosion control method is the construction of a rock revetment. Rock revetments are similar to rock bulkheads in that they consist of"hard"armoring of the shoreline by constructing a sloping rock surface at the toe of the beach slope. The benefit of revetment is a more gradual wave run-up area at the shoreline that can reduce wave reflection and potential scour in front of the bulkhead. The negative aspects of a revetment are that their larger footprint results in a larger area of the upper intertidal that is buried and depending on their height may not provide the needed protection for the site during storm events or extreme high tides. The lower the angle of the face,the more beach area is covered. To minimize footprint and maximize the erosion control aspect of the revetment they are typically built at steep angles to take advantage of the surcharging affect of the upper rocks similar to the structural characteristics of a vertical rock bulkhead. Absent the surcharge,the rocks at the toe of a revetment are more susceptible to movement and redistribution than a rock bulkhead. A revetment in lieu of a rock bulkhead will result in more intense and more frequent maintenance due to the angled face and would result in an unpredictable erosional environment where the revetment connects to the adjacent structures located to the north and south. More maintenance at the project site would require additional barge landings and heavy equipment on the upper intertidal area. Most importantly, the revetment would occupy a larger footprint Page 5 Dwayne Griffith July 17,2013 Project No. 13003 within the upper intertidal area due to the limited space to construct the revetment behind the existing face of seawall. Soft Bank Protection The use of"soft"bulkhead techniques includes the combined use of beach nourishment and restrained Iarge 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 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 project site(rock bulkhead to the north and concrete seawall to the south). A breach of the soft bank system during a storm event would not only jeopardize the project site but would affect the integrity of both adjacent structures. 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. Hybrid Soft-bank/Revetment Hybrid systems employ elements of both a hard surface stopgap component and the dynamic component of a soft-bank system. A rock revetment or bulkhead is constructed at or above the ordinary high water line and is partially buried and further protected by the addition of anchored beach logs,sand,gravel and cobbles placed as beach nourishment. The sand and logs mimic natural beach conditions on a day-to-day basis and the rocks serve as a structural stopgap measure during large storm events. The benefits of a hybrid system are that they partially mimic natural coastal processes while providing a high level of erosion control due to the rock structure. The drawbacks to a hybrid system is that similar to the soft bank system, the dynamic portion of the hybrid system (restrained logs and imported sand)will require long- term maintenance demands that cannot be fully determined until the system has been in place for a period of years. The hybrid system also leaves adjacent hard-surface structures at risk of being breached or damaged. Page 6 Dwayne Griffith July 17, 2013 Project No. 13003 Mitigation Construction Of replacement rock bulkhead will result in a net gain of upper intertidal beach area and a net reduction in wave reflection and scour that is currently occurring on the beach due to the existing flat-surfaced seawall. Beach nourishment volume estimating was not performed due to the extensive historical modification of the shoreline and no natural erosion (that could contribute sediment to the beach) is currently occurring at the site. Recommendations Based on our field observations,the existing concrete seawall north of the boat ramp is at risk of collapsing and is in need of replacement. To reduce the probability of acute and severe erosion of the fill between the slope and seawall and to reduce the impacts of the existing flat- surface wall to the existing coastal processes at the site we recommend replacing the existing seawall as proposed. The new rock bulkhead should be placed entirely landward of the existing OHW(estimated at the face of the existing seawall). Construction of the bulkhead flush to the adjacent seawalls to the north and south will reduce scour and erosion that may be occurring. Construction of the bulkhead landward of the OHW and angled as proposed will reduce the adverse impacts to the existing littoral sediment movement in the area. To reduce the probability of erosion of 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 are effectively routed toward the beach when the new bulkhead is constructed. This pipe should be connected to an appropriate outfall neat the beach that will not jeopardize the bulkhead or cause erosion on the beach. No concentrated flow should be allowed to discharge within the existing fill soils behind the proposed bulkhead. Energy dissipaters,rock revetments and other means may be needed to reduce the likelihood of concentrated seepage at or behind the 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 both the adjacent bulkhead to the east and the existing seawall to the west. • Place sufficient size rocks adjacent to the upper bluff to serve as erosion control. Base rocks should be embedded a minimum 4 feet and must bear directly on undisturbed lodgment till inferred to underlie the area. For back slopes greater than 2HA V,we recommend the use ofMirafi BX 1200 geotextile fabric placed every 12 inches within the proposed quarry spall backfill. Topsoil may be placed on the back slope for revegetation purposes. Plans Review We have reviewed the proposed plans prepared by Sealevel Bulkhead Builders, titled"Replace Concrete Bulkhead with Rock, Raise Concrete Bulkhead, Replace Concrete Ramp with steps, Rock Retaining Wall for Duane Griffith, P.O. Box 710 Allyn WA 88524"dated 4/2/13. In our opinion,the plans as proposed are consistent with the geotechnical opinions and recommendations in this letter-report. Page 7 Dwayne Griffith July 1.7,2013 Project No. 13003 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 bullhead. 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 Duane Griffith and his 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 U ` o ' P was U�,r � o { b 32152 ed Gea �1 � �Ia1VAL <1 i Robert F. Cousins ; 'EXPHRES 021051 ' Robert F. Cousins,LEG Nita L.Cousins,lP.E. Principal Geologist Principal Engineer rob@coastalsolns.com nina@coastalsolns.com Attachments:Construction Plans Prepared by Sealevel Bulkhead Builders(2 Sheets) cc: Jenny Rotsten,Sealevel Bulkhead Builders Macintosh IID;Coasta3 solutions 1.1.C:projccts:13003-Griffitb:Grif WinRpt.doc Page 8 _ 4 nrui�ry jlu ,�,`_ ro._y.,. a1NlMdit4Y870tIgUO{'ONNDQB1Nidd s � •• .., 1.. ti8:un T h a Mw G. i 00 ..V. " )PPV Nit a Naarr.�a-,UVDsryd f IJM 4l1Y:7#•r`rjv OWIVSW5107hoo 9vidad a�tf►88 td�aj Nartu� dnvYY� ��'r� iij `V vt�d�7��t�►�d�r�t�J•3�59 � - r ••--.��- � yet';�� ` 6Ct I IV—f�im:,4' s C 12ECA I�fIxX �,7r uPA R ! , U tITr--R , vv ! 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