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HomeMy WebLinkAbout101323002 Geotech Assessment (2021)PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 November 29, 2021 Brian Gregory brian@allworthdesign.com Re: Geology Hazard Assessment Jefferson County Parcel 10132002 345 McCurdy Point Road Port Townsend, Washington It is my understanding that a pool and garage are being proposed to be added at the existing residence on the property. The Jefferson County Public Land Record shoreline slope stability map indicates that the shoreline bluff on the property is within an area the map indicates as unstable old slides and unstable and the Jefferson County Public Land Record landslide hazard amp indicates the steep bluff is a high hazard area and the slopes above are moderate hazard areas. I had previously completed a geology evaluation (Stratum Group, 2004) of the site prior to the existing home being built. The home was built at a location consistent with my recommendations. The proposed garage and pool will also be located in a manner that is consistent with my recommendations for set back from the top edge of the shoreline bluff. Based on my assessment of the subject property and vicinity, I conclude that the existing home, the proposed pool and garage will be outside of any geologic hazardous areas and will not be at risk from landslides or shoreline bluff retreat for at least 100 years. Furthermore, the proposed pool and garage will not increase the risk of landslides or erosion on or off the property. The purpose of this geology evaluation was to evaluate the potentially unstable slopes on the subject property and vicinity and the risk of slope failures impacting the site. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the steep slopes and other slopes in the area, an inspection of the shoreline conditions at the site and vicinity, review of available geologic mapping in the area, review of lidar (light distance and ranging) imagery of the vicinity, and my own notes and observations I have made in the vicinity of the site and at locations with similar geologic conditions. 2 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 GENERAL GEOLOGY The Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) indicates that the upland portion of the subject property above the steep shoreline bluff is underlain by glacial marine drift and glacial advance outwash. The bluff is mapped as being underlain by undifferentiated glacial and non glacial deposits. Figure 1. Portion of geologic map (Schasse and Slaughter, 2005). Qb = beach, Qguc = glacial and non glacial deposits undivided, Qgdme = glacial marine drift of Everson Stade, Qga = glacial advance outwash, Qgt = glacial till, Qgta = glacial ablation till. Observations on the uplands, bluff and shoreline on and in the vicinity of the subject property are partially consistent with the mapping described above. The uppermost part of the bluff all along the shoreline of the subject property and at least portions of the upland portions of the property are underlain by glacial till consisting of silty 3 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 sand and gravel in a very compact condition. The till is on the order of a few feet to 15 feet thick. The thickest portion of the till is near the northern tip of the property at McCurdy Point itself. The till thins to the southeast and southwest of the point. Glacial marine drift, if present is of limited extent. The marine drift and till will essentially appear the same as both were deposited by glacial ice. The difference is that marine drift was derived from floating ice on sea water during the late stages of the last glacial period when the land surface was briefly submerged. The mass of glacial ice had suppressed the land surface downwards hundreds of feet and as the ice thinned the area became inundated with sea water with ice floating on the water. The melting ice then ‘rained’ sediment onto the sea floor. After the land rebounded, the glacial marine drift was compacted by seasonal wetting and drying which makes the unit very hard like the glacial till; however, the self compaction forms fractures in the glacial marine drift. The result is a subtle but important difference in stability as the fractures in the marine drift can result is slightly different stability with somewhat larger failures in the marine drift than in the glacial till. Glacial marine drift is present in the vicinity, but does not appear to be present on the subject property. Most of the upland on the property appears to be underlain by slightly reworked glacial till and the till appears to be mostly ice wasting ablation till. The reworking is fro wave action that has left very subtle wave cut terrace strands on the upland area that are more apparent in lidar bare earth imagery (Figure 2) than visually on the ground. Figure 2. Lidar bare earth via Jefferson County GIS. Note the very subtle wave strands across the uplands 4 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 The steep bluff is underlain by dense sand, silt and gravel beneath the till all along the bluff face. The advance outwash varies in composition along the bluff face and much of appears to be consistent with under ice flow as blocks of till and very high energy flows are embedded within the unit. Near the northern point there is very little sand. Towards the southeast the sand unit immediately beneath the till increases in thickness. This dense sand unit contains very little silt or clay and therefore has very little cohesion. Towards the southwest there is very little cohesionless sand. Below the sand the bluff is underlain by silt, clay, sand and gravel units with silt and clay being the predominant units. A glacial drift layer consisting of clayey silt with pebbles, cobbles and boulders embedded into the fine grained matrix is present along the base of the bluff at the point. This is the only area along the shore that this unit is present, but it can also be seen as a platform shore area towards the east. A result of this unit is that the beach at Point McCurdy consists of cobbles and boulders whereas elsewhere along this stretch of shoreline the beach consists primarily of sand with a few gravel sections. This also is the likely the reason for the existence of the point, as the very compact drift unit at the base of the point and the cobbles and boulders within the drift are much more resistant to erosion that the bluff materials southeast and southwest of the point. I interpret the glacial drift at the base of the bluff as Possession Drift, a glacial drift unit deposited during an older ice age that predates that last glacial period. The Map Showing Coastal Erosion, Sediment Supply, and Longshore Transport in the Port Townsend, 30- By 60-Minute Quadrangle, Puget Sound Region, Washington (Kueler,1988) notes that this shore reach is an area of relatively low erosion rate with infrequent small slides. Historic aerial photographs of the shoreline dating back to the 1940s are consistent with this reach being an area of relatively slow bluff retreat over the historic record. However, the shortness of the historic record should be considered as bluff retreat can be episodic as beach conditions change or the bluff becomes very undermined. I did not observed any obvious bluff changes since my last site visit in 2004 and noted that the old gun emplacement appeared to be in the same position relative to the bluff as it did in 2004. Some ravel of loose material and small scale shallow slides has taken place on the bluff face. 5 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 SPECIFIC SITE OBSERVATIONS The property is approximately 33 acres in size and consists of an upland area bounded on the northwest and northeast by a very steep northwest and northeast facing shoreline bluff. Except for the shoreline bluffs on the outer edges of the property, the upland portion slopes gently towards the northwest. The shoreline bluffs on the property range in height from approximately 60 feet on the northwest bluff to 70 feet at the point on the northern end of the property to approximately 100 feet on the northeast bluff. The northwest facing bluff is very steep with much of the bluff being nearly vertical. The northeast facing bluff is vertical on the upper 10 to 15 feet and the slopes at approximately 40 degrees for 10 to 40 feet in height and then steepens down to the shore at an angle typically on the order of 65 degrees or steeper. The existing home on the property is located approximately 140 feet back from the top edge of the shoreline bluff slope to the northwest. The home is accessed via a gravel driveway from McCurdy Point Road. Outside of the home, driveway and landscaped areas immediately around the home, the upland portion of the property is covered primarily with second growth Douglas fir with an understory of salal and evergreen huckleberry and other brush. The northwest facing bluff is mostly devoid of vegetation with the exception of a few areas of grass and wild rose. The northeast facing bluff has stands of trees on portions of the slope consisting of Douglas fir, red alder and western red cedar as well as areas of brush and no vegetation. None of the trees on the northeast-facing slope are mature. Past slope failures are apparent all along the northwest facing bluff. This entire section of the bluff has very little bluff vegetation and the bluff is generally very steep, but can hold steep slopes for long periods of time. A wedge of loose soil with vegetation is located along most of the lower slope and wave erosion of this lower slope appears to be infrequent based on driftwood and vegetation at the base of the slope and areas of the uppermost beach. The southeast facing bluff is more vegetated and, although still very steep, the bluff is not as steep as the northwest facing bluff. Vegetation is better established on this bluff slope, but there is evidence of relatively recent slides on portions of the slope on the subject property and on slopes to the southeast of the subject property. Remnants of a very old car is present within the beach area at this location. The bluff slopes on the subject property are eroded primarily by wave action that undermines the 6 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 base of the bluff. The shoreline at the site is exposed to open waters of the Straight of Juan de Fuca. Large waves are common during storms and hence periodically the shore and the base of the bluff are exposed to very large waves. The northwest facing bluff receives more frequent large wave action than the northeast facing bluff. However, even the northeast facing bluff will receive large waves because of the open nature of the shore in this area. While the northwest facing bluff is subject to more frequent wave action, the area near the point is more resistant to erosion than other areas on this section of shoreline because the base of the northwest facing bluff near the point is composed of very compact silt and clay drift with boulders. Because of the very compact nature of the bluff soils, the slope failures appear to consist of relatively narrow slab type failures. Ravelling of material off the exposed soils on the unvegetated portions of the bluff and periodic shallow topsoil failures also play a minor role. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the east by wave action. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. The lack of large mature trees along the bluff on the subject property and adjoining property bluff slopes is indicative of erosion rates that do not allow enough time to for trees to be become established for long periods. The beach at the base of the bluff is a sand, gravel and cobble beach and at the point area is a platform shore underlain by very hard glacial till with boulders. Overall the beach on this reach of shoreline is built up such that there is no large undermining of slope as the upper beach abuts the base of the slope. This may periodically change for short periods of time during large wave events with high tides and storm surge. Long shore drift moves sediment from the southwest to the northeast along this section of coast. With very little shoreline armoring and active erosion of the high bluffs to the southwest, the sediment supply to the beach remains very near natural conditions. Overall the beach appears to be in very good condition with no lowering of the beach that could lead to an increased rate of shoreline erosion. Large volumes of sediment are provided by the very high eroding bluffs to the southwest of the subject property. This sediment supply maintains the beach such that erosion at the toe of the bluff slope is limited to very high tides and storm wave events. 7 November 29, 2019 345 McCurdy Point Road, Port Townsend, WA Geology Hazard Assessment Stratum Group File: 11.1.21 CONCLUSION AND RECOMMENDATIONS Based on my assessment of the site, it is my opinion that the pool and garage as well as the existing home on the site will not be at risk from slope failures or bluff retreat for well over 100 years. Furthermore, it is my opinion that the proposed pool and garage will not increase the risk of landslides or erosion on or off the site. For this shoreline bluff, the frequency of slope failures is highly dependent upon the erosion rate along the shoreline. For purposes of this evaluation I selected an overall long term average bluff recession rate of 4 inches per year for the bluff at this site. This recession rate is likely a conservatively high rate based on the historic record as well as my own observations along this shore reach. The top of the bluff will not systematically retreat 4 inches per year, but the bluff will progressively get steeper and slope failures will take place similar in scale to the past bluff failures that took place at this location and in the vicinity. Top of bluff failures should be expected to be on the order of 5 feet. Stormwater: Stormwater from the new garage should be infiltrated consistent with Western Washington Stormwater Manual and consistent with the current infiltration of stormwater. Soil Bearing: For foundation loads, a soil bearing of 3,000 psf may be used for foundations. Foundation excavations should be 18 inches deep or down to unweathered glacial drift soils, which ever is deeper. A friction angle of 35 degrees should be used if retaining walls are required. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist