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MCOB, PHASE II <br /> Calculation: C-4 <br /> Overflow Pipe Design Page 2 of 3 <br /> Design Basis: <br /> The flow splitters and outfalls to Merrill Creek will be reconstructed in their same locations as the existing <br /> conditions.The amount of flow tributary to each flow splitter has changed very slightly since the original <br /> design due to how the tributary areas changed with the removal of the trench drain. The invert elevation at <br /> the flow splitter has changed from the original design. The new outfall pipes must be designed to convey the <br /> 10 year peak flowrate. <br /> Since this is a retrofit project,the conveyance design for this project needs to be designed for the 10 year <br /> storm as calculated using the Rational Method and Manning's Equation. This is consistent with the original <br /> permit conditions for the existing conveyance system. The full flow slope can be calculated using the <br /> Manning's equation. <br /> Methods and Assumptions: <br /> Each flow splitter has two pipes exiting the structure. One continues on to the onsite water quality treatment <br /> system and the other outfalls to Merrill Creek. The flow splitter is designed such that no flows up to the <br /> water quality flow rate will go out the outfall pipe. The 10 year storm flowrates are larger than the water <br /> quality flowrates thus a portion of the flow will go out the overflow pipes and a portion will continue to the <br /> water quality treatment system. To determine the flowrate out each overflow pipe at the 10 year storm,it <br /> was assumed that the pipe continuing onsite to the water quality treatment system would be flowing full. <br /> The full flow capacity of the water quality pipe was then subtracted from the total tributary flow. All of the <br /> remaining flow was assumed to discharge to Merrill Creek through the overflow pipe. <br /> The following software was used for this calculation: <br /> - AutoCAD C3D 2016—project site tributary area information <br /> - Calculation C-3, 10 yr conveyance spreadsheet <br /> Findings and Conclusions: <br /> Manning's equation rearranged for slope is as follows <br /> Q *n <br /> S —_ (0.46 *D2671 <br /> n=0.012 <br /> CB 6: <br /> l0yr flow into Flow Splitter(from calculation C-3)= 10.20 cfs <br /> Full Flow Capacity of pipe continuing to water quality(from calculation C-3)= 1.46 cfs <br /> Required Capacity of Outfall Pipe= 10.20-1.46=8.74 cfs <br /> Pipe Size= 18" <br /> Critical Slope=0.6% <br /> Provided Slope=0.61% <br />