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p,u�c� �irs <br /> AfEMORANDUM <br /> TO: Kirk Hrooks <br /> FROM: Dan Mathias <br /> DATE: Jan. 15, 1999 <br /> SUBJECT: On-site Detention Requirements fc r Multi-Modai Transit Center <br /> Based upon information provided by Tod Fukutomi (attached), I have calculated the on-site <br /> detention requirements for the Multi-Modal Transit Center. The detention requirement based <br /> upon a IO year pre-and post development design storm is 21,780 cubic feet. The existing peak 10 <br /> year discharge is approximately 6.9 CFS. <br /> This site is just upstream of a combined sewer overtlow on 36'"St.The City is required by state <br /> law to reduce the CSO frequency at al! CSOs to an average of one event per year. To reduce the <br /> potential for an inc�eased CSO frequency, and since there are no basements in the vicinity, the <br /> ' on-site detention system s;►ould be designed to minimize discharge ap to and including the 2 year <br /> storm. Therefore,the orifice fortn the on-site detention system should be designed to discharge <br /> 3.3 CFS at the overtlow. This flow rate is approximately two-thirds of the existing 2 year peak <br /> flow. With an orifice designed for 3.3 CFS and 2�,780 cubic feet of storage,the overflow <br /> frequency should be approximately once every lwo years,as intended. <br /> C.C.Jane Zimmerman. <br />