AgdaPkt 2010-11-22 clsd and regular

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7.A REPORT To the Honorable Mayor and City Council From the Ci Mana er November 22, 2010 SUBJECT Study Session — Bayfront Canal Storm Drainage System RECOMMENDATION No action is required at this time. This staff report is an information only item and accompanies a PowerPoint presentation by staff. BACKGROUND Personnel at the Stanford Hospital and Clinics have contacted the city expressing concerns regarding the magnitude and frequency of flooding of the Stanford Medicine Outpatienf Center site and the surrounding 4Q0-500 blocks of Broadway in Redwood City. Historically, this area is prone to flooding. The Stanford Medicine Outpatient Center at 450 Broadway in Redwood City is bordered on the north by Highway 101, and on the south by Broadway, between Douglas Avenue and Second Avenue. The 11.3 acre site is part of the (arger, 48.4-acre Midpoint Technology Park site. The site's storm system drains to a 30" city maintained storm pipe on Broadway, and a 24u city maintained pipe adjacent to Highway 101. Both storm pipes eventually discharge into the city operated Douglas Avenue pumping station. This station serves the Douglas Avenue Drainage Basin, a 655-acre basin roughly bounded by Second Avenue, EI Camino Real, Chestnut Street, and US 101. The station pumps the basin's storm water runoff into a 42-inch diameter force main that crosses the freeway and discharges into the northern terminus of the Bayfront Canal. As part of the approvals for the Stanford Medicine Outpatient Center project, a long term detainment system was built, by which the water detained on site will be stored underground. The detainment system was designed to detain and slowly release storm water so that the total volume of storm runoff experienced post-project will not be more than pre-project. As stated in the Certified EIR, until the plans (that have been in place for several years) to upgrade the Bayfront Canal and Douglas Avenue pumping station are implemented, it is unlikely that the proposed underground vaults will prevent future flooding on the project site. Some of the water wi(I likely end up on the adjoining portion of the Technology Park site, but that would still leave more unconveyed water than could be accommodated by the proposed on-site storage vaults. As a result, flooding should still be anticipated within low-lying portions of the on-site parking lots and landscaped areas during major storm events (Wagstaff and Associates, Environmental Impact Report for the Stanford Medicine Outpafient Center Project, June 2006). The Douglas Avenue pumping station has the capacity to accommodate the runoff from a one-year recurrence interval storm which is a storm of an intensity expected to occur on an annual basis per BKF Engineers, Ufilities Study, Midpoint Technology Park, Redwood City, CA, August 27, 1996. As stated in the EIR for the Stanford Medicine