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HomeMy WebLinkAboutRS_Channel_Migration_Study_250108_v1 Seattle 9706 4th Ave NE, Ste 300 Seattle, WA 98115 Tel 206.523.0024 Kirkland 750 6th Street Kirkland, WA 98033 Tel 425.822.5242 Mount Vernon 2210 Riverside Dr, Ste 110 Mount Vernon, WA 98273 Tel 360.899.1110 Whidbey 1796 E Main St, Ste 105 Freeland, WA 98249 Tel 360.331.4131 Federal Way 31620 23rd Ave S, Ste 307 Federal Way, WA 98003 Tel 253.237.7770 Spokane 601 Main Ave, Ste 617 Spokane, WA 99201 Tel 509.606.3600 October 18, 2023 Patty Thumann and Jamie Williams 13411 SE 151st Street Renton, WA 98058 Re: Channel Migration Zone Study DCG/Watershed Reference Number: 231005 Dear Patty and Jamie, This report reviews hydrologic and geomorphic indicators of channel migration and possible need for shoreline stabilization on a parcel adjacent to the Cedar River in Renton, WA. Location The parcel is located at 13411 SE 151st Street, Renton, WA 98058. COR parcel #2223059112. GPS Coordinates lat 47.466874. long -122.161703. The parcel includes a single-family residence built in 1961 and several outbuildings on the right bank of the Cedar River, at about River Mile 4. Much of the left bank is in public ownership, the Soos Creek Park and Trail and Cedar River Natural Zone. It is approximately 1,200 feet downstream of a State Route (SR) 169 bridge over the river. The parcel is .87 acres in size with a nominal width of 124 feet and length of 344 feet. The parcel is shown below: Figure 1. COR Parcel 2223059112 on the Cedar River in Renton, WA Channel Migration Zone Study Thumann/Williams October 18, 2023 Page 2 Hydrologic Indicators U.S. Geological Survey flow records are available for the Cedar River near Renton, WA gauge 12-119000, including peak flows from 1948 – 2022. Major floods occurred in 1996 (7,650 cfs), 2006 (6,090 cfs), 2009 (9,390 cfs), 2011 (5,870 cfs), 2015 (5,500 cfs), and 2020 (9,620 cfs). The 100- year flood, based on a 2015 flood frequency analysis, is 12,000 cfs (King County, 2015). The FEMA 100-year flood elevation on the parcel is 76 feet NAVD 88. The FEMA flood insurance rate map shows the flood zone (AE) on the parcel (Figure 2). The upper portion of the parcel, including the buildings, are outside of the 100-year flood boundary. Figure 2. Flood insurance rate map (WA State Coastal Atlas/FEMA). Geomorphic Indicators The parcel is on the right bank of the largest meander on the lower 10 miles of the Cedar River. The meander extends from the SR169 bridge (RM 4.3) downvalley to an area abutting SR 169 (RM 3.2). The parcel is upstream of a small point bar (depositional) at the apex of the meander. The Person Revetment (built in 1972) on the left bank extends upstream of the parcel. Bank armoring on the Erickson Revetment (built in 1963) on the right bank of the meander is upstream of the parcel, as shown in Reach 3 below (Figure 3). Ground elevations on the parcel range from 60+ feet NAVD 88 on the bank of the river to 76+ feet NAVD 88 on the upper 1/3 of the parcel (KC iMAP). The lower 40+ feet of the parcel is a riparian zone adjacent to the river. Channel Migration Zone Study Thumann/Williams October 18, 2023 Page 3 Figure 3. Publicly maintained revetments in Reach 3 of the Cedar River. (King County, 2015) King County Department of Natural Resources and Parks, Water and Land Resources Division conducted a Level 1, Maplewood Area Landslide Hazard Assessment of steep slopes above the left bank on the Cedar River in 2018. The assessment included geotechnical studies and risk analysis relative to potential landslides into the river and possible flooding effects. The assessment recommended additional investigations of subsurface geology and slope stability models, as well as developing alternatives for mitigating risks. The results of additional work have not been published to date. Note: Delineation of landslide hazards, characterizing different types of landslides, and calculation of potential runout distances or potential but unquantified flood hazards are beyond the scope of this review. CZM and Erosion Hazard Mapping The SR 169 Bridge and Erickson revetment are assumed barriers to channel migration (King County, 2015). There are no mapped Avulsion Hazard Zones (AHZs) downstream of RM 6.8 (King County, 2015). There are no mapped Erosion Hazard Areas (EHAs) susceptible to lateral channel migration due to erosion on the right bank downstream of the SR 169 bridge (King County, 2015). The report notes that the CMZ along Reach 3 covers a relatively narrow portion of the valley floor and large wood presently is not believed to be a significant factor regarding channel migration. The Historical Active Channels and Historical Migration Zone Delineation (Appendix Map 3) includes channel configurations prior to construction and improvement of the barriers to channel migration. A comparison of site conditions on the parcel following floods in 1996, 2006, 2009, 2011, 2015, and 2020 shows no significant surface erosion or channel Channel Migration Zone Study Thumann/Williams October 18, 2023 Page 4 migration due to flooding on the upper parcel, and only minor disturbance in the riparian zone (King County, iMAP). Conclusion Based on this review, the upper 1/3 of the parcel is above the regulatory Base Flood Elevation and shows no evidence of surface erosion or channel migration following major floods, including the flood of record (2020). Therefore, new construction within this area is unlikely to require new shoreline stabilization or other structural flood hazard reduction measures. References FEMA. 2020. Flood Insurance Rate Map. FIRM 53033C0983G. https://msc.fema.gov/portal/search. King County. 2015. Cedar River Channel Migration Study. Prepared by Terry Butler and Fred Lott. King County Department of Natural Resources and Parks, Water and Land Resources Division. Seattle, Washington. King County iMAP. https://gismaps.kingcounty.gov/iMap/ King County. 2018. Maplewood Area Landslide Hazard Assessment Report. Prepared by Jeremy Bunn and Judi Radloff, King County Department of Natural Resources, Water and Land Resources Division. Seattle, Washington. Washington State Coastal Atlas. Flood Hazard Areas. https://gis.ecology.wa.gov/portal/apps/webappviewer/index.html?id Disclaimer The information contained in this document is based on the application of technical guidelines currently accepted as the best available science and in conjunction with the manuals and criteria referenced above. All discussions, conclusions, and recommendations reflect the best professional judgment of the author(s) and are based upon information available at the time the study was conducted. All work was completed within the constraints of budget, scope, and timing. The findings of this report are subject to verification and agreement by the appropriate local, state, and federal regulatory authorities. No other warranty, expressed or implied, is made. Channel Migration Zone Study Thumann/Williams October 18, 2023 Page 5 Sincerely, Alan R. Wald, LHg Hydrogeologist