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HomeMy WebLinkAboutBLD2021-00082-09A-Geotech ReportStratum Group File: 10.29.19B 1 PO Box 2546, Bellingham, Washington 98227 May 3, 2021 Lisa Volkert Re: Geologic Hazard Assessment 142 North Maple Street, Irondale, WA Jefferson County Parcel 961804001 Dear Ms. Volkert: This geologic hazard assessment was conducted to assess the risk of landslides and erosion on the subject property. It is our understanding that you are planning to construct a new home, driveway, and septic drainfield on the site. The property includes a shoreline bluff on the west shore of Port Townsend Bay in Irondale. Based on our geologic hazard assessment, it is our opinion that the proposed development on the site will not be at risk of landslides or erosion for at least 100 years as long as it is located at least 35 feet from the top edge of the bluff. This setback distance will provide adequate protection from potential future incremental bluff retreat and will also provide space behind the home to prevent concentration of stormwater near the shoreline bluff. The site plan provided for the development of the site indicates the home and septic will be located much more than 35 feet from the top edge of the slope. This geologic hazard assessment included a field inspection of the subject property and vicinity including the upland area, shoreline bluff, and shoreline reach. Our evaluation also included review of historic aerial photographs and maps, available geologic mapping, lidar (light detecting and ranging) imagery, the proposed 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 property is underlain by Vashon recessional delta deposits (Figure 1). The shoreline bluff is mapped as mass wasting deposits (Figure 1). A level area at the base of the bluff in the northeast corner of property is mapped as modified land. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 2 Figure 1. Clipped and annotated geologic map showing the subject property location. Qgod = Vashon recessional delta deposits, Qmw = mass wasting deposits, Qml = modified land, Qga = Vashon advance outwash, Qgome = Everson glaciomarine drift, Qf = fill material. Our observations are generally consistent with the above described mapping. Machine dug test pits on the upland portion of the property reveal moderately dense low cohesion medium sand with gravel with scattered small cobbles consistent with Vashon recessional outwash (Figure 2). Hand dug test pits on the slope revealed loose sand and gravel overlying moderately dense sand and gravel of similar composition. Based on these observations, it is our opinion that the bluff is underlain by a thin mantle of colluvium overlying Vashon recessional outwash deposits. This is consistent with observations made along a steeper bluff face to the south of the subject property where similar recessional deposits are found overlying Vashon till and Vashon advance glaciolacustrine or glaciomarine deposits. The till and glaciolacustrine units are not indicated on the map, but this is likely primarily due to the scale of the map. Figure 2. Vashon recessional deposits on the subject property. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 3 Lidar imagery of the site (Figure 3) and vicinity shows a level area at the base of the bluff in the northeast corner of the property. This level area is at least in part the result of filling and grading that took place for historical iron works that included numerous iron kilns along waterfront. The waterfront in this area was also modified by adding dredged material from the bay just to the north as part of fill used for a log yard associated with a saw mill that was formerly located further to the north. The historical use of this area is consistent with the mapping of the level area at the base of the bluff as modified land. While the area designated as mass wasting on the map on the steep shoreline bluff slopes is consistent with our observations, we note that the wider mass wasting area designated to the north of the subject property is actually a large area of modified land associated with the former steel mill. Figure 3. Lidar hillshade image showing modified land at the northeast corner of the property from historical iron works and saw mill along the shoreline to the north. SITE SPECIFIC OBSERVATIONS The 1.26-acre subject property is located on a shoreline bluff on the western shore of Port Townsend Bay in Irondale. An easement across the adjacent property to the southwest (parcel 961804003) will be used to access the site from North Maple Street. Modified land area May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 4 The property is currently undeveloped. The upland portion of the property slopes gently to the northeast (Figure 4). Measured slope angles in this area range from 0 to 11 degrees. The upland portion of the property is predominantly covered in brush including Scotch broom, Nootka rose, ocean spray and Himalayan blackberry. Trees on the perimeter of the previously cleared upland area include western red cedar, red alder, and big leaf maple. Figure 4. The gently sloping upland portion of the property. The shoreline bluff on the property (Figure 5) is between 65 and 80 feet high. Measured slope angles on the bluff ranged from 34 to 38 degrees. The bluff is essentially a planar angle of repose slope. Vegetation on the bluff consists of mixed forest stands that include big leaf maple, western red cedar, Douglas fir, and red alder with an understory including sword fern, snowberry, ocean spray, thimbleberry, stinging nettle, and other brush. During our site investigation, we observed evidence of minor soil creep and raveling on the bluff. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 5 Figure 5. The shoreline bluff on the subject property. The northeast corner of the property below the bluff there is an elevated platform between the base of the bluff and the shoreline (Figure 3). This platform is associated with the former iron works and later saw mill that were formerly located at the base of the bluff along the shoreline at and to the north of the property. Visible remnants of the ironworks on and near the subject property include remnants of brick walls and scattered loose bricks from iron kilns along the beach (Figure 6). The shoreline reach fronting the subject property is characterized by a sand and gravel beach with scattered cobbles (Figure 6). The lower beach face includes an abundance of small rocks likely associated with the former iron works. There is also some slag material present. The beach also contains abundant shell fragments. The beach has been altered significantly by past fill. Much of the elevated platform consists of sand and shells from dredged material placed on the shoreline to create a broad area for log storage to the north (Shipman, Gravel Beach Blog). Some of this sand and shell material has eroded and makes up a significant portion of the current beach (Figure 6). The beach to the north was again modified immediately to the north of the subject property during a cleanup of the former iron works site and included the addition of additional beach material, placement of logs and site grading post cleanup work (GeoEngineers, 2014). May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 6 Figure 6. The shoreline reach at the subject property. View is to the north. Note kiln bricks and rocks as well as significant shell material derived from the dredge fill material to the north. Maximum fetch at the site is approximately 9 miles northeast across Port Townsend Bay and across Admiralty Inlet to Whidbey Island. Despite this relatively large fetch in one direction, the site is relatively protected in all other directions due to its position in Port Townsend Bay. SHORELINE PROCESSES Shoreline landform and drift mapping completed by Coastal Geologic Services (2013) for the Washington State Department of Ecology mapped the subject property shoreline as a transport zone (Figure 7). Transport zones are areas where sediment is transported by wave and current movement along the beach, but little to no sediment is being added to the beach by erosion of the bluff area above the beach. The shoreline just south of the property is mapped as a feeder bluff. The shoreline drift or sediment transport direction along the shoreline has been mapped as towards the north at the subject property. The transport direction shifts to the south a few hundred feet south of the property. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 7 Figure 7. Annotated Washington Coastal Atlas coastal landforms map showing the subject property location. Our site observations and interpretations are consistent with the above described shoreline mapping. We did not observe any evidence of wave erosion at the base of the bluff on the subject property. However, we did observe evidence of wave erosion at the base of the bluff and shallow landsliding approximately 300 to 600 feet south of the subject property. We have observed some minor erosion of the elevated area of the beach fill material to the north, but no significant erosion was noted at the subject property itself. It appears that there is a small area of accretion at the south end of the dredge platform at the subject property as the dredge platform protrudes out from the base of the bluff and has modified the shoreline transport at the subject property creating an area where the beach has moderately been filled in and widened south of the dredge fill area Figure 8. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 8 Figure 8. Shoreline at the subject property. GEOLOGIC HAZARD ASSESSMENT During our site investigation, we did not observe any evidence of previous, ongoing, or incipient landsliding on the subject property. We also did not observe any groundwater perching, seepage, or springs. There are no conditions on the bluff slope that portend future landslides. Observed shallow landsliding on the steep bluff approximately 300 to 600 feet south of the subject property is caused by groundwater perching within recessional sand and gravel overlying compact glaciolacustrine or glaciomarine silts and clays as well as till. However, these perching layers are not present on the subject property. The bluff on the site is essentially an angle of repose plainer slope. Therefore, top of bluff retreat will be minor as long as no erosion takes place at the base of the base of the bluff. The shoreline waterfront at much of the property and to the north has been significantly altered dating back to the waterfront kilns associated with the iron works, the later dredge material fill placed on the shore associated with the saw mill that used the fill area as a log storage yard and the most recent addition of beach material and fill associated with the environmental remediation at the former iron and steel works. The cleanup as built drawings (GeoEngineers, 2014) indicates that the kiln foundations were left in place (we observed some of the kiln foundations) and the shoreline beach was built up including gravel beach nourishment to the north of the site. The result of the fill material and kiln foundation locations is such that we do not anticipate base of bluff erosion will take place for many decades into the future. The fill platform, kiln May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 9 foundations and beach nourishment are effectively precluding erosion of the base of the bluff at the subject property. Keuler (1988) mapped the shoreline at the subject property as an approximately neutral coastal segment of little erosion or deposition. We concur with Kueler (1988) that this shore reach is subject to little erosion. The lack of erosion is primarily due to the placement of fill immediately to the north that has pushed the beach outward well away for the base of the bluff. For determining the appropriate home location setback from the top edge of the bluff, we used an average rate of bluff retreat of 2 inches per year at the subject property. Given the lack of erosion and shoreline processes described above, 2 inches per year is a very conservatively high rate. Using this conservatively high bluff retreat rate, the bluff should be expected to retreat approximately 17 feet over the next 100 years. We do note that the underling soils on the bluff slope consist of sand that may be susceptible to erosion. We recommend an additional 15 feet be added in order to create ample space to allow for infiltration of stormwater. Yhe site plan for the home and septic have the home set approximately 80 feet back from the top edge of the bluff and the septic drainfield located approximately 65 feet back from the top edge of the bluff at its closest point. May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 10 CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard assessment, it is our opinion that a proposed home on the site will not be at risk of landslides or erosion for at least 100 years as long as it is located at least 35 feet from the top edge of the bluff. This distance will provide adequate protection from potential incremental bluff retreat and will also provide space behind the home to for stormwater infiltration without impacts to the bluff slope. Based on site plans the home will be located approximately 80 feet back from the top edge of the bluff slope and the septic drainfield will be approximately 65 feet back from the top edge of the bluff at its closest location. Stormwater management: Stormwater from the home should be dispersed or infiltrated on site in the vicinity of the home consistent with the Stormwater Management Manual for Western Washington. Stormwater should not be concentrated within 15 feet of the top of the bluff. The site plan indicates stormwater will be dispersed approximately 70 feet back from the top edge of the slope. Vegetation management: Tree removal on the bluff should be limited to hazard tree removal. Clearing on the bluff will lead to an increase in raveling and bluff retreat due to reduced soil root strength. Limbing of trees for view corridors is acceptable and topping of big leaf maple trees is acceptable. Yard Waste and Debris: No yard waste or dead vegetation should be disposed of on or near the top of the bluff. Yard waste and debris may become a wet unstable mat that can slide down and damage the slope and increase the frequency of slope failure and raveling. The waste also kills vegetation and damages root structures that support slopes. 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. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Geoff Malick, L.G., M.Sc. Licensed Engineering Geologist Licensed Geologist May 04 2021 May 3, 2021 Jefferson County Parcel 961804001 Geologic Hazard Assessment Stratum Group File: 10.29.19C 11 References: GeoEngineers, 2014. As-Built Report Shoreline Habitat Restoration Irondale Iron and Steel Plant. Keuler, R.F., 1988. Map showing coatsal erosion, sediment supply, and longshore transport in the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington. USGS Map I-1198-E. MacLennan, A.J., J.W. Johannessen, S.A. Williams, W. J. Gerstel, J.F. Waggoner, and A. Bailey, 2013. Feeder Bluff Mapping of Puget Sound. Prepared by Coastal Geologic Services, for the Washington Department of Ecology and the Washington Department of Fish and Wildlife. Bellingham, Washington. 118p. 42 Maps. Schasse, H.W. and Slaughter, S.L., 2005. Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington. Washington Division of Geology and Earth Resources, Geologic Map GM-7. Shipman, H, Gravel Beach Blog multiple posts under Irondale. May 04 2021 May 04 2021