1010 N BROADWAY EVCC 2018-01-01 MF Import

mg On It Index Hall Basin Area % Impervious 25-yr, 24-hr Storm 2001 Basin 6.10 acres 90% 3.52 cfs 2010 Basin 7.36 acres 92%* 4.34 cfs Difference 1.26 acres NA 0.82 cfs *the additional 1.26 acres was assumed 100%impervious for a conservative approach As indicated in the 2001 Stormwater Hydraulic Report the total 25-year, 24-hour flow rate for the entire basin tributary to the downstream storm system was calculated to be 7.9 cfs. Considering the additional area for the Index Hall Replacement site, the 2010 total flow rate for the entire basin tributary to the downstream storm system is 8.72 cfs (7.9+0.82=8.72). Pipe Capacity The 2001 Stormwater Hydraulic Report shows that the most restrictive downstream pipe is an 18-inch diameter pipe with a design slope of 0.99 percent located within Waverly Avenue. The capacity of this pipe was calculated to be 70%full for the 25-year event. Considering the 2010 calculated flow rate with the increased area, the same pipe was analyzed and determined to perform at roughly 74%full for the 25-year event. Conclusion The downstream conveyance system continues within Waverly Street until it combines with the City combined storm/sewer system approximately 460 feet north of Tower Street. Based on the 2001 Stormwater Hydraulic Report and the updated information as described above, drainage from the proposed project, the existing storm system within Waverly Avenue has sufficient capacity to accept drainage from the Index Hall Replacement site. Q:\2009\209459\10_CIV\NON_CAD\Project_Memo Downstream Conveyance.docx Project Memo Page 2 of 2 ® - Everett Community College Index Hall Replacement 209459.10 11/16/2010