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Home My WebLink AboutRS_ScourAnalysis_200710 _V1.pdf 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com TECHNICAL REPORT Date: March 30, 2020 Revised July 10, 2020 To: Matt Herrera From: Ed McCarthy, Ph.D., P.E. and Logan McClish, EIT Project Name: Cedar River Apartments Project Number: 170314 Subject: Cedar River Apartments Scour Analysis Technical Report This report has been prepared to document a scour analysis that we performed for the proposed Cedar River Apartments project at 1915 SE Maple Valley Highway, Renton, WA 98057 (King County Parcel #1723059026). The project proposes to construct an apartment complex, pedestrian trail, park amenities (railings, stairs, boardwalk, signage and benches), shoreline buffer restoration, and increased flood storage capacity. This analysis focuses on the potential for scour to occur at the base of an existing retaining wall, on the subject site, along the Lower Cedar River (LCR). While the wall was installed in 1961 – 1962 and currently shows no signs of serious degradation, there is the concern that excessive river scour could jeopardize the structural integrity of the wall. Methods Historical Flow & Scour Data Flow and scour data were collected from public resources and recent studies. Table 1 lists flood flow rates for given return periods for the reach of the river adjacent to the subject site (FEMA, 2005). Cedar River USGS gage data (Figure 1) were compared to these selected return period flood flow rates to determine the size and frequency of various floods that have occurred over the period of gage record. The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 2 of 8 Table 1. HEC-RAS model peak flow rates Return Period (years) Flow Rate (cfs) 2-Year 3,000 10-Year 5,940 50-Year 9,860 100-Year 12,000 Source: FEMA Preliminary FIS (September 15, 2017) Figure 1. Peak annual flow events in Renton, WA at USGS stream gage 1211900 from 1906 to 2018 50-YR 10-YR 2-YR The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 3 of 8 Significant peak flow rates have been summarized in Table 2 below for data from 1985 to 2020. For much of this time frame, streambed elevation data has also been collected (see Figure 2). The range in flood flow return period is provided for each peak flow rate for each given time period. Table 2. Summary of peak flow events from 1985 to 2018 and the associated occurrence interval 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 2017 - 2018 2,330 <2 2018 - 2019 2,370 <2 2019 - 2020 9,620 10 – 50 Northwest Hydraulic Engineers (NHC) performed a scour study of the LCR spanning several years (NHC 2018). The project studied dredging needs at and downstream of the I-405 crossing. From 1985 to 2018, changes in bed elevations were monitored at different cross section locations The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 4 of 8 along the LCR (Figure 2 and Scour Map A). The sharp drops in bed elevation that occurred in 1998 and 2016, at some of the downstream cross sections, are associated with dredging events. The magnitude of channel degradation associated with the dredging reduces as the distance from the dredging site increases. Figure 2. River bed elevations from 1985 to 2018 (NHC 2019) Scour Calculations Depth of scour was calculated for the sharp bend in the LCR immediately upstream of the project site, identified as Location A (Appendix A, Scour Map A). We applied commonly used bend scour equations to calculate potential scour including the Maynord Equation (Maynord 1996), Thorne Equation (Thorne 1997), and the USACE Gravel Bed Channel Chart (USACE 1994). Each of these is presented in analytical terms below. Detailed calculations for each approach are provided in Appendix B with a summary of calculated results provided in Table 3 below. The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 5 of 8 Maynord Equation 𝐷𝐷𝑚𝑚𝑚𝑚𝐷𝐷𝑢𝑢=1.8 −0.051 �𝑅𝑅𝑅𝑅𝑊𝑊�+0.0084(𝑊𝑊𝐷𝐷𝑢𝑢) Dmb = maximum water depth in bend Du = mean channel depth at upstream crossing W = width of flow at upstream end of bend Rc = radius of curvature of bend Thorne Equation 𝑑𝑑𝑦𝑦1 =1.07 −log (𝑅𝑅𝑐𝑐𝑊𝑊−2) d = maximum depth of scour y1 = average flow depth directly upstream of the bend W = width of flow Rc = radius of curvature of bend USACE 1994 – Gravel Bed Channel Chart The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 6 of 8 Table 3. Summary of Scour Calculation Results Method Scour Depth (ft) USACE 1994 - Gravel Bed Channel Chart 10.8 Thorne Equation 4.4 Maynord Equation 5.4 Average 6.9 Note: Scour depth is measured downward from the average bed elevation. Field Measurements & Observations of Scour Pool Field observations confirm that there is a scour pool at the base of the wall located on an outside bend of the river (Location A on Scour Maps A and B). River hydraulic forces interacting with the armored streambank in this area creates significant turbulence that has formed the scour pool. It is likely that this location is where the greatest potential for scour would occur in the river along the subject site. No other significant scour or areas of bank erosion were identified during our field observations along the subject site. A measuring tape with a weight was dropped into the pool from the adjacent wall and an approximate depth from the bottom of the pool to the water surface of 11-ft was measured. An average water depth of 2.5’ was measured in a run section of the river. Subtracting the average depth from the field-measured pool depth provides an estimated scour depth of 8.5’. The geotechnical report for the proposed project (Terracon 2019) noted multiple layers of concrete were used as fill behind and beneath the wall to provide structural support. Given the observed supporting concrete mass beneath the wall at Location A (Photo 1) and its current integrity, it is assumed the streambank is hardened all the way to the bottom of this scour pool, although further field investigation would be required to verify this condition. Recommended Design Scour Depth The observed scour depth of 8.5 feet at Location A has formed in the presence of floods exceeding the 50-year return period but not quite reaching the 100-year return period. To predict a scour depth that could result from a 100-year flood flow we used the USACE 1994 Gravel Bed Channel Chart to adjust the observed scour depth at Location A (Appendix C). This adjustment calculation associates the observed scour depth of 8.5 feet with a 50-year return period. HEC-RAS modeling results predict a one-foot difference in flood depth at Location A for the 50-year and 100-year floods. The USACE chart predicts an incremental depth of 0.55 foot The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 7 of 8 for scour calculations using flood depths associated with the 50-year and 100-year floods at Location A. A reasonable estimate of the 100-year scour depth at Location A would therefore be 9.1 feet (observed depth of 8.5 feet plus the calculated incremental scour depth of 0.55 foot). This recommended design scour depth is most relevant for Location A. Based on river geometry and observed conditions in the field, the 100-year scour depth will likely be smaller at other locations along the site’s river front. Photo 1. Concrete wall footing at Location A on the right bank of the Cedar River at the upstream end of the subject site The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 8 of 8 D iscussion Bend Scour The measured scour depth of 8.5-ft at Location A (Appendix A, Scour Map B) lies within the range of the calculated scour depths of 4.4 to 10.8-ft using commonly applied equations (Appendix B). The most conservative calculation of the potential scour to occur at Location A resulted with the Army Corps of Engineers chart. The chart is based on a regression line produced from measurements in gravel river beds (USACE 1994), consistent with what has been observed at the bend. The scour predictions from the empirical equations do not suggest that scour at the river bend would significantly exceed the depth of scour that has been observed in the field. The historical channel bed data in Figure 2 shows that the section of the LCR at the project site has varied less than one foot since 1985, indicating that the reach is stable. Extreme scouring events and significant changes in average stream bed elevations that could jeopardize the structural integrity of the wall on the subject site have not been observed. Summary Based on the data and analysis provided in this report, the retaining wall on the LCR at Location A has remained structurally sound and stable during its 59-year life. The wall and associated river bank have withstood effects of river scour and erosion for a wide range in river flow conditions with floods exceeding a 50-year return period. Long-term channel stability monitoring of the river bed at River Mile 1.93 (Scour Map A) near the project site has demonstrated with conclusive evidence that the channel has been stable, even in response to downstream dredging. While a scour pool in the river has formed at the upstream end of the subject site, in a location where scour would be anticipated, no other notable erosional features have been noted along the bank. If you have questions regarding my assessment or need additional information, please call me (425) 271-5734. The Watershed Company Scour Analysis March 30, 2020, revised July 10, 2020 Page 9 of 8 Sincerely, Edward McCarthy, Ph.D., P.E. 750 Sixth Street South | Kirkland, WA 98033 | P 425.822.5242 | f 425.827.8136 | watershedco.com References FEMA, September 15, 2017. Preliminary Flood Insurance Study – King County, Washington and Incorporated Areas, Vol. 2 of 4. FEMA FIS Study Number 53033CV002B. Washington D..C. King County. Flood Insurance Study 53033CV001B. Prepared for the Federal Emergency Management Agency (FEMA). Updated April 19, 2005. Maynord, S.T. Toe-Scour Estimation in Stabilized Bendways. Journal of Hydraulic Engineering, American Society of Civil Engineers, 122(8):460-464. 1996. Northwest Hydraulic Consultants (NHC). 2017-2018 Post-Dredge Cedar River Scour Monitoring Report. Northwest March 11, 2019. Northwest Hydraulic Consultants (NHC). 2018 Annual Sediment Report. December 3, 2018. Terracon Consultants, Inc. Bulkhead Wall Stability Addendum. Report prepared for SRM Renton, LLC. June 10, 2019. Thorne, C.R., R.D. Hey and M.D. Newson. Applied Fluvial Geomorphology for River Engineering and Management. 1997. United States Army Corp of Engineers. Channel Stability Assessment for Flood Control Projects. Manual No. 1110-2-1418. October 31, 1994. United States Geological Society. National Water Information System: Web Interface. https://nwis.waterdata.usgs.gov/wa/nwis. Accessed March 27, 2020. Washington Department of Transportation. M 46-03.12 Geotechnical Design Manual, Chapter 5. July 2019. Web Soil Survey (WSS). United States Department of Agriculture. Natural Resources Conservation Service. Available online at http://websoilsurvey.nrcs.usda.gov/ APPENDICES Appendix A – Scour Maps A & B Appendix B – Scour Equation Calculations Appendix C – Estimate of 100-Year Scour Depth APPENDIX A– Scour Maps A & B LOCATION B SCOUR MAP A CEDAR RIVER APARTMENTS Renton, Washington THE WATERSHED COMPANY Science & Design 750 Sixth Street South Kirkland WA 98033 p 425.822.5242 www.watershedco.com ARMORED SECTION OF RIVER BANK LOCATION A UPSTREAM END OF BULKHEAD WALL DOWNSTREAM END OF BULKHEAD WALL BULKHEAD WALL MAPLE VALLEY H WY CEDAR RIVER03-27-2020050100200 RM 1.93 (APPROX.) RM 2.1 (APPROX.) SCOUR DEPTH = ~11-FT SCOUR MAP B Renton, Washington THE WATERSHED COMPANY Science & Design 750 Sixth Street South Kirkland WA 98033 p 425.822.5242 www.watershedco.com 03-27-2020 SECTION OF UNARMORED BANKWALL TIES INTO EMBANKMENT OBSERVED THALWEG LOCATION A BACKWATER CONDITIONS CEDAR RIVER APARTMENTS BULKHEAD WALLMAPLE VALLEY HWYCED A R RI V E R 0 20 40 80 APPENDI X B – Scour Equation Calculations Project: SRM Cedar River Date: 2-17-2020 Bend Scour Calculations HEC-RAS hydraulic model outputs (bend located at RS 204.7) Rc = 600 ft (source GIS) W = 103 ft (source HEC-RAS) Ave Depth = 9 ft (source HEC-RAS) Rc/W = 5.83 (source HEC-RAS) Maynord Equation 𝐷𝑚𝑏 𝐷𝑢 =1.8 −0.051 (𝑅𝑐 𝑊)+0.0084(𝑊 𝐷𝑢 ) Dmb = max water depth in bend Du = mean channel depth at upstream crossing W = width of flow at upstream end of bend Rc = radius of curvature Dmb = Du * (1.8 - 0.051*(Rc/W) + 0.0084 * (W/Du) = 14.39 ft Scour Depth = Dmb – Ave Depth = 5.39 ft Thorne Equation 𝑐 𝑦1 =1.07 −log⁡(𝑅𝑏 𝑊−2) d = max depth of scour y1 = average flow depth directly upstream of the bend W = width of flow at upstream end of bend Rc = radius of curvature d = y1 * (1.07 - log(Rc/W - 2)) = 4.39 ft USACE Gravel Channel Bed (1994) Max Depth at Bed = 2.2*Mean Water Depth Max Depth at Bend = 19.8 ft Scour Depth = Max Depth - Ave Depth = 10.8 ft APPENDIX C– Estimate of 100-Year Scour Depth Use USACE 1994 Gravel Bed Chart to Estimate 100-Year Scour Depth at Location A: Location A - Bend at RS 204.7 100-Year Scour Depth Source Bend Radius - Rc 600 ft GIS Water Surface Width - W 103 ft HEC-RAS Rc/W 5.83 Hydraulic Depth 9.0 ft HEC-RAS USACE Predicted Scour Depth 10.80 ft USACE Chart (see Chart in Appendix B) 50-Year Scour Depth Source Bend Radius - Rc 600 ft GIS Water Surface Width - W 101 ft HEC-RAS Rc/W 5.94 Hydraulic Depth 8.2 ft HEC-RAS USACE Predicted Scour Depth 10.25 ft USACE Chart Use USACE Chart results to adjust observed scour depth: Observed Scour Depth 8.50 Ft Difference between 100-year depth and 50-year depth: 0.55 ft Recommend 100-year scour depth (Observed + Delta from USACE): 9.1 ft Observed Scour Depth 8.5 ft Field Measurement