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HomeMy WebLinkAbout001063018 Geotech AssessmentStratum Group File: 6.14.18 1 STRATUM GROUP P.O. Box 2546 Bellingham, WA 98227 July 6, 2018 Sonya Baumstein 121 A Louisa Street Port Townsend, WA 98368 Re: Geologic Hazard Assessment Jefferson County Parcel 001063018 522 McMinn Road, Port Townsend, WA 98368 Dear Ms. Baumstein, This geologic hazard assessment was conducted to assess the risk of erosion and landslide hazards on the subject property. It is our understanding that you have plans to develop the site and that this report will help inform those plans. The northern portion of the subject property is located along a high coastal bluff on the Strait of Juan de Fuca northeast of Cape George. Based on our geologic hazard assessment, it is our opinion that any development on the site will not be at risk of landslides or erosion for at least 100 years as long as it is setback at least 70 feet from the top edge of the bluff. Furthermore, development of the site will not increase the risk of landslides or erosion on or off the site. This geologic hazard assessment included a field inspection of the subject property and vicinity including the upland area, coastal bluff, and shoreline reach. Our evaluation also included review of historic aerial photographs, available geologic mapping, lidar (light detecting and ranging) imagery, site plans, and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. 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 is underlain by Vashon till (Figure 1). Schasse and Slaughter (2005) describe Vashon till as a very poorly sorted mix of clay, silt, sand, and gravel with scattered cobbles and boulders. The unit is highly compacted to the point of resembling concrete and commonly forms steep, nearly vertical coastal bluffs along Puget Sound and the Strait of Juan de Fuca. Vashon till was deposited directly by overlying glacial ice between 18,000 and 14,000 years ago. The maximum ice thickness in the area has been estimated to be approximately 4,000 feet thick during the last major regional glaciation (Schasse and Slaughter, 2005; Thorsen, 1981). July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 2 The shoreline bluff is mapped as undivided glacial and nonglacial deposits (Figure 1). Geologic units are typically designated as undifferentiated or undivided when it is not possible to distinguish between different units on a map. For this map, it is likely a mapping scale issue in which the authors observed multiple distinct glacial and nonglacial deposits on the bluff but could not represent their interpretations within such a small area of the map. Our geologic observations are mostly consistent with the above described mapping. On the upland portion of the property, hand dug test pits near the road revealed dense silt and clay with lesser sand and gravel and scattered cobbles. This is consistent with the upland portion of the property being underlain by Vashon till. Historic garbage dumping took place at the site and is visible in historic aerial photographs. The remaining garbage obscures the ground surface near the shoreline bluff and has made geologic interpretations difficult in the area proximal to the bluff. However, the observed steepness of the uppermost bluff is suggestive of it being underlain by a compact and cohesive unit such as Vashon till. As part of our investigation, we walked the shoreline area below the subject property to make stratigraphic and geomorphic observations of the bluff. The bluff exposures revealed a complex sequence of glaciogenic, glaciofluvial, glaciomarine, and possible nonglacial sediments, some of the which have undergone post-deposition deformation. Directly below the subject property, the lower bluff is underlain by stratified sand and gravel layers with minor silt layers (Figure 2). In many areas, this unit exhibits folding that has been cut by subdequent normal and conjugate July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 3 faulting (Figure 2). Due to vegetation obscuring the middle portion of the bluff, it is difficult to interpret the units higher on the bluff at the subject property. Figure 2. Stratified and deformed sand and gravel layers on the lower bluff near the subject property. However, bluff exposures elsewhere along this shoreline reach are more extensive. To the southwest of the subject property, the lower bluff is composed of very hard till that ranges from undeformed to highly deformed (Figure 3). The deformed areas are characterized by open to tight folding, layering and foliation, shearing, and load structures (Figure 3). These structures are indicative of ice-induced compression as well as vertical stresses. Layered to massive, compact glaciomarine silt and clay with minor fine sand overlies the deformed till. The glaciomarine drift does not exhibit the same level of deformation as the underlying till although some areas are highly fractured and sheared (Figure 4). The fracturing extends beneath the weathered surface and allows for the compact drift unit to be easily broken into angular pieces. Undeformed till and possible nonglacial deposits and/or outwash overlie the glaciomarine drift. July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 4 Figure 3. Highly deformed and tilted till layers separated by a thin tilted glaciolfluvial layer. July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 5 Figure 4. Highly fractured glaciomarine drift. Although it is not possible to establish a coherent history deposition and deformation of the above described units, it is our opinion that their stratigraphic and structural complexity is a result of the area once being a near marine ice-marginal setting with a fluctuating sea level. This created create complex and dynamic depositional and glaciotectonic conditions involving interaction between the land, sea, and ice. In the Cape George area to the south and southwest of the subject property, there is a series of recessional moraines that are well defined in lidar imagery (Figure 5). These moraines indicate that this area was an ice-margin for a period of time long enough to allow for extended proglacial deposition and the creation of well-defined moraines on the landscape. There is also evidence of a Pleistocene shoreline that truncates the moraines and a meltwater channel that likely emanated from the ice margin (Figure 5). July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 6 Figure 5. Lidar hillshade showing geomorphic features indicative of a near marine ice marginal setting with a fluctuating sea level in the Cape George area. SITE SPECIFIC OBSERVATIONS The 5.11-acre subject property is located along the Strait of Juan de Fuca approximately 1.25 miles northeast of Cape George. The upland portion of the property is gently sloping to the northwest between 4 and 7 degrees (7 to 12 percent). Most of the upland area has been cleared of trees so that it is predominantly covered in grass and low brush (Figure 6). Trees along the edges of the property consist of red alder, western red cedar, Douglas fir, and madrone. July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 7 Figure 6. The gently sloping upland portion of the property. The shoreline bluff is approximately 120 feet high. Along this shoreline stretch, the bluff shape is predominantly planar although there are scallop shaped depressions into the bluff in some areas including two that adjacent to one another on the bluff on the subject property (Figure 7). These features are landslide source areas. The scallop shaped depressions on and near the subject property includes the upper two-thirds of the bluff. The one to the northeast is approximately 90 feet wide and the one to the southwest is approximately 65 feet wide (Figure 7). Both form depressions into the bluff that are between 15 and 18 feet and together form a wider convex depression (Figure 7). There are two lobate slide deposits on the lower bluff beneath the depressions (Figure 7). July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 8 Figure 7. Landslides visible in lidar imagery on and near the subject property. GEOLOGIC HAZARD EVALUATION Based on lidar and field observations, previous landslides on this stretch of bluff were on the order of 15 to 18 feet. The failures were limited to the upper two thirds of the bluff. The slides were predominantly rotational in nature and initiated within a silt and clay rich glaciomarine drift unit. In areas where previous landslides occurred, we observed extensive and pervasive fracturing of the glaciomarine unit (Figure 4). These fractures were formed by compaction from the weight of overriding ice and then subsequent dilation once the load was removed when the ice melted away. The fractures greatly reduce the shear strength of the glaciomarine drift and create potential failure surfaces for landslides. It is our opinion that the slides are caused by incremental undercutting of the lower bluff by wave erosion which exposes the weak fractured drift and causes it to fail. Any material overlying the glaciomarine drift such as outwash or till also fails. There is no evidence of larger failures along this stretch of shoreline. There is one scallop shaped depression approximately 600 feet southwest of the subject property that is 55 feet deep (Figures 7 & 8). However, this deep scallop was formed by at multiple smaller slides rather than one large failure. An oblique aerial photograph from 2000 shows that a slide occurred in this area in the late 1990s that removed nearly all the trees from the slope (Figure 8). Another oblique aerial photograph from 1992 shows the same depression with a similar shape prior to the slide (Figure July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 9 8). This indicates that the depression existed prior to the most recent landslide and is likely the result of multiple failures with periods of stability and tree growth between slide events. Lidar imagery also shows at least two smaller landslide source areas within the larger overall feature (Figure 7). July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 10 Figure 8. Oblique aerial photos comparing a section of shoreline between 1992 and 2000. The depression in the bluff existed prior to the late 1990s landslide. In order to establish a setback distance for develop of the property, we consider both the increment bluff retreat rate as well as the size and frequency of previous landslides. The average long-term bluff retreat rate along this shoreline stretch is on the order of 4 inches per year. This area periodically experiences large waves 2 to 3 feet in height due to the large fetch across the Strait of Juan de Fuca as well as waves occasionally travelling up the strait directly from the Pacific Ocean. Based on a bluff retreat rate of 4 inches per year, 100 years of bluff retreat from shallow sloughing and raveling caused by wave erosion will be approximately 33.33 inches. However, the erosion on the lower bluff may also initiate landslides on the upper bluff that can cause an additional 15 to 18 feet of retreat. However, these slides will be rare and likely spaced apart by many decades. Once a 15- to 18-foot failure occurs on the upper bluff, there needs to be another 15 to 18 feet of retreat on the lower bluff to undercut the same section of bluff and cause a subsequent failure. Using a long-term bluff retreat rate of 4 inches per year, this would take somewhere on the order of 40 to 60 years to occur. Therefore, over the course of 100 years, up to two 15 to 18-foot failures are possible on any given section of the bluff in this area. Adding two 18-foot failures to the 33.33 feet of incremental bluff retreat from wave erosion, we arrive at a maximum bluff retreat of 70 feet over the course of a 100-year life of a home. CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard evaluation, it is our opinion that development on the subject property will not be at risk of landslides or erosion for a minimum of 100 years as long as it is setback at least 70 feet from the top edge of the bluff. Furthermore, development of the property will not increase the risk of landslides or erosion as long as the following recommendations are followed. Rare landslides that are up 18 feet thick may be possible at the top edge of the bluff although most slides will likely be smaller. Stormwater: Stormwater can be fully dispersed consistent with the Washington Department of Ecology’s Stormwater Management Manual for Western Washington. Yard Waste and Debris: No debris or yard waste should be placed on the steep slopes of the property. Yard waste eventually builds up and can form a wet unstable mass that will slide down the slope damaging the slope and increasing the frequency of slope failures. Water Lines: No water lines should be buried on the steep shoreline slope or within 30 feet of the steep bluff slope and shutoff valves should be located at least 30 feet from the top of the steep slope. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding this assessment please contact our office at (360) 714-9409. July 6, 2018 Jefferson County Parcel #001063018 Geologic Hazard Assessment Stratum Group File: 6.14.18 11 Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Geoff Malick, G.I.T., M.Sc. Licensed Engineering Geologist Geologist-in-Training