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01 <br /> Table 2 provides a summary of the computed scour results and the recommended total scour depth. This <br /> scour depth is assumed to occur below the existing channel thalweg (low point). Identifying scour depth <br /> along the scour berm would assume that the existing channel thalweg would migrate into it. The thalweg <br /> was typically located approximately 10 feet lower than the existing grades at the toe of the proposed scour <br /> berm. This yielded a total depth of scour at the toe of scour berm of approximately 21 feet below <br /> existing grade. Complete scour analysis calculations are included at Attachment 1. <br /> Table 2 Summary of Flow Rates for Various Design Storms from the Report <br /> Sour Mechanism Scour Depth(ft) <br /> General Scour 7.2 <br /> Long Term Scour 3.3 <br /> Total Scour 10.5 <br /> V. Scour Mitigation <br /> Riprap scour mitigation was sized and designed to account for the 100-year storm event scour and <br /> migration of the St. Vrain thalweg. Riprap was sized using methodology presented in the US Army Corps <br /> of Engineers (USACE) Hydraulic Design of Flood Control Channels EM 1110-2-1601. Riprap sizing was <br /> completed using the USACE ChanlPro Version 2.0 for Windows 7 software. Sizing and design approaches <br /> used hydraulic information from the HEC-RAS 1 D model provided by the Matrix Design Group. <br /> A self launching riprap configuration was selected for the scour berm given the existing grade being <br /> approximately 10 feet above the St.Vrain Creek thalweg. This allows for riprap to be placed on the existing <br /> surface and to self launch into scour as it develops along the toe. Riprap launching was assumed to occur <br /> at a 2 horizontal to 1 vertical slope. <br /> Riprap sizing assumed a rock density of 155 pounds per cubic foot. A bend radius of 850 feet used a 39 <br /> foot active channel top width. USACE gradations were used with a factor of safety of 1.5. Computations <br /> resulted in a USACE gradation Class 11 with a median stone size of 2.33 feet and a thickness of 3.5 <br /> feet. ChanlPro program output is included as Attachment 2. <br /> Riprap installation geometry assumed a 150% increase in rock volume associated with uncertainty in self <br /> launching greater than 15 feet. Total riprap installation dimensions were 29 feet long by 9 feet high <br /> and will extend the full length of the project along St. Wain Creek. Figure 5 illustrates the proposed <br /> scour berm section. Scour berm sizing computations are included as Attachment 3. <br /> VI. Recommendations <br /> HDR completed a scour and scour mitigation analysis for the Lyons Quarry Project to address scour <br /> concerns associated with St. Vrain Creek. Scour depths of approximately 21 feet were computed for the <br /> proposed conditions. Scour mitigation design is based on the 100-year storm event. Scour berm design <br /> resulted in riprap revetment placed along the length of the project fronted by St. Vrain Creek. All riprap <br /> revetment should include an operation and maintenance plan to define processes to identify construction <br /> and performance issues in the field and potential remedies. Analyses and design were based on best <br /> available information. <br /> hdrinc.com 1670 Broadway,Suite 3400,Denver,CO 80202-4824 <br /> (303)764-1520 <br /> 3 <br />