7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

MCOB, PHASE II Calculation: C-3 10 yr Conveyance Calculations Page 2 of 3 Design Basis: The project proposes to remove several of the existing trench drains and replace with a surface v-gutter which drains into a series of catch basins. The existing trench drains act as stormwater collection and conveyance. The new proposed system will consist of conveyance pipes between catch basins. Since this is a retrofit of an existing system,and there are no changes to the amount or runoff entering the conveyance system,the modified conveyance system is being designed to convey the 10-year storm as originally designed and permitted. The conveyance system was designed to convey the 10 year storm as calculated using the Rational Method and Manning's Equation. Methods and Assumptions: The following software was used for this calculation: - AutoCAD C3D 2016—project site tributary area information - Excel worksheet for conveyance calcs The Rational Method was used to calculate the runoff at the 10yr storm event. The equation used is as follows: Q=CiA Where Q =runoff in cubic feet per second C=runoff coefficient(assumed 100%impervious at MCOB so C=0.9 i=average rainfall intensity, inches/hour A=area in acres i=m/T2° Where Tc=time of concentration(6.3 minimum) m=6.31 (from Table 4-3.1(1)-1 COE Design and Construction Standards) n=0.575 (from Table 4-3.1(1)-1 COE Design and Construction Standards) The conveyance capacity spreadsheet was modified to account for the flows overtopping the weirs and exiting to Merrill Creek rather than continuing in the onsite conveyance system. The amount of flow exiting to Merrill Creek at the 10 year event was calculated by assuming that the pipes which continue to the water quality treatment system would be flowing full. The remainder would bypass out the overflow pipe. See calculation C-4 Overflow Pipes,for more information.