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Home My WebLink AboutRS_CMZRiskAssessment_200408_V1.pdf750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com TECHNICAL REPORT Date: April 8, 2020 To: Matt Herrera From: Alan Wald, LHg Project Name: Cedar River Apartments Project Number: 170314 Subject: C hannel Migration Risk Assessment Memo The following report includes review of hydrologic indicators, geomorphic indicators, and historical records relevant to potential CMZ hazards on a lower Cedar River reach from River Mile (RM) 1.75 to 2.5. L ocation The location of the proposed Cedar River Apartments (Project Site, parcel #1723059026), is located on the right bank upstream of I-405, as shown in Figure 1 (King County iMap, 2020a). Figure 1. Project site on the Cedar River near Renton, WA The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 2 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com H ydrologic Indicators The Cedar River drains an area of 184 square miles from the foothills of the Cascade Mountains to Lake Washington and Puget Sound. River discharge (flow) is regulated by Seattle City Light Masonry Dam built in 1914 for hydropower, water supply, and flood control. Frequency analysis of peak flow on the Cedar River near Renton (NHC, 2018b) is reported to be: • 2-year flow of 3000 cubic feet per second (cfs), • 5-year flow of 4600 cfs • 10-year flow of 5940 cfs, • 50-year flow of 9860 cfs • 100-year flow of 12,000 cfs Annual peak flow of record varies from less than 2,000 cfs to 10,600 cfs, as shown in Table 1 and Figure 2. (USGS, 2020). Period Peak Flow Rate (cfs) Return Period (years) 1985 - 1991 10,600 50 – 100 1991 - 1997 7,650 10 – 50 1997 - 1998 1,920 <2 1998 - 1999 2,840 <2 1999 - 2000 2,890 <2 2000 - 2001 1,100 <2 2001 - 2002 2,620 <2 2002 - 2003 2,060 <2 2003 - 2004 2,510 <2 2004 - 2005 2,410 <2 2005 - 2006 4,380 2 – 10 2006 - 2007 6,090 10 – 50 2007 - 2008 2,980 <2 2008 - 2009 9,470 10 – 50 2009 - 2010 2,120 <2 2010 - 2011 5,870 2 – 10 2011 - 2012 2,790 <2 2012 - 2013 3,860 2 – 10 2013 - 2014 2,800 <2 2014 - 2015 5,500 2 – 10 2015 - 2016 2,500 <2 The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 3 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com 2017 - 2018 2,330 <2 2018 - 2019 2,370 <2 2019 - 2020 9,620 10 – 50 Table 1. Annual peak flow. Cedar River at Renton, WA Figure 2. Graph of annual peak streamflow. Cedar River at Renton WA. Peak flows in excess of 6,000 cfs (10-year return interval flow) at this site are considered channel forming flow of sufficient depth, velocity, and stream power to initiate channel migration by rapid meandering and channel avulsion (WDFW, 2009). There have been seven of these channel forming flows on the lower Cedar River since Water Year (WY) 1940, not including a major dam break flood in 1911, as follows:  WY1966 7,650 cfs  WY1972 6,201 cfs  WY1976 8,800 cfs  WY1991 10,600 cfs  WY2007 6,090 cfs  WY2009 9,390 cfs  WY2020 9,620 cfs The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 4 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com G eomorphic Indicators The Cedar River downstream of I-405 is a constructed, reinforced channel with no meanders. Aggradation of bed sediments is controlled by regular dredging and the channel is fixed in place by the I-405 abutments and bank revetments (NHC, 2019). Effects of channel constriction by the I-405 abutments extends upstream and potential channel meanders are constrained by outlet control through the bridge. The Cedar River upstream of I-405, for about 2 miles to Maplewood Subdivision and Soos Creek Park, is an incised reach with narrow, irregular meanders, hardened banks, and several industrial developments on the shoreline. Almost every outside bend is armored by levees and revetments or bounded by an erosion-resistant feature (KC, 2015). Erosion control projects for the Milwaukee Railroad mainline (now Cedar River Trail), SR169 (Maple Valley Highway), and King County levees and revetments at Riverview Park limit the width of the channel and disconnect the river from its floodplain. Woody debris loading is very low and sediment transport exceeds sediment supply in the reach (NHC, 2018; KC, 2015). Average meander width is about half the unconstrained (historical) meander width and average channel bed elevation varied by less than 1.0 ft from 1985 to 2020 (NHC, 2019). The riverbank at the project site is protected from erosion by a massive concrete revetment built sometime around 1961, as shown in Figure 3. The revetment was not affected by six floods, ranging from 6,090 cfs to 10,600 cfs, from WY1996 to WY2009. Figure 3. Concrete revetment at the project site RM 1.75 - 2.5 upstream of I-405 The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 5 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com H istorical Record Aerial photographs show no side channel development, channel avulsion, or significant bank erosion on the project site for 82 years (since 1938). LiDAR imagery (KC, 2020) show no side channels or meander scars on the project site, including after the WY1991 flood of 10,600 cfs, as shown in Figure 4 (KC, 2020b). Figure 4. LiDAR image of project site on right bank of Cedar River upstream of I-405 S ummary Department of Ecology guidelines for delineation of channel migration zones (WDOE, 2003) include assessment of the Historical Migration Area (HMA), Avulsion Hazard Area (AHA), Erosion Hazard Area (EHA), and Disconnected Migration Areas (DMA). There are no geomorphic or photogrammatic indicators of an HMA on the project site for more than 100 years since the dam was built. There is no AHA or EHA mapped on the Cedar River downstream of RM 5.0 (KC, 2015. Table 6 and Map 5), as shown in Figure 5. The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 6 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com Figure 5. Map of potential CMZ features on the lower Cedar River Published estimates of channel widening by bank erosion (KC, 2015, Appendix B) are for unarmored areas and would not apply to the project site. There is no evidence of a DMA on the project site. Although the project site is mapped as a severe hazard area (KC, 2015. Map7), shown in Figure 6 below, the delineation appears to be a projection of a mapping algorithm unverified by the necessary field investigation and site surveys. The Watershed Company Channel Migration Risk Assessment April 8, 2020 Page 7 of 7 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com Figure 6. Map of potential CMZ classifications on the lower Cedar River Based on hydrologic, geomorphic, and photogrammatic information contained in this report, there is no quantitative information to suggest there is a channel migration hazard on the project site today. Sincerely, Alan R. Wald, LHg 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com R eferences (KC, 2020a). King County. King County Parcel Viewer. https://kingcounty.gov/services/gis/Maps/parcel-viewer.aspx (KC 2020b). King County. LiDAR Swipe. https://www5.kingcounty.gov/lidar/ (KC, 2015). King County. 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. 2015 (NHC, 2018a). Northwest Hydraulic Consultants, Inc. Cedar River 2018 Annual Sediment Report. Final Report. Prepared for City of Renton, WA. Seattle, WA. December, 2018 (NHC, 2018b). Northwest Hydraulic Consultants, Inc. Cedar River 2017 Annual Sediment Report. Final Report. Prepared for City of Renton, WA. Seattle, WA. January, 2018 (NHC, 2019). Northwest Hydraulic Consultants, Inc. Post-Dredge Cedar River Scour Monitoring Report. Memo Report. Prepared for City of Renton, WA. Surface Water Utility. Seattle, WA. March, 2019 (USGS,2020). US Geological Survey. Surface Water Data for Washington State. Station #12119000 https://waterdata.usgs.gov/wa/nwis/rt (WDOE, 2003). Washington Department of Ecology. A Framework for Delineating Channel Migration Zones. Prepared by Cygnia Rapp and Timothy Abbe. In cooperation with Washington Department of Transportation. Ecology Publication #03-06-027. Olympia, WA. November, 2003. (WDFW, 2009). Washington Department of Fish and Wildlife. High Flows for Fish and Wildlife in Washington. Report of Investigations in Instream Flows. Prepared by Alan Wald, Habitat Program. Olympia, WA. 2009