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<br />Page 1 of 2 <br /> <br /> <br /> <br />Drainage Guidelines for Commercial Developments and <br />Residential Developments with 4 or More Units <br />Stormwater Detention and Treatment <br /> <br />City of Redwood City <br />Engineering Divisionand Transportation Department <br /> <br /> <br />General Requirements <br />A. A Grading & Drainage Plan must be submitted with Building Permit applications for all commercial <br />development projects that include site disturbance . This plan shall be a separate sheet included with the <br />Building Permit submittal and must be prepared by a qualified licensed engineer and be signed by the <br />same. <br />B. The goal of the drainage design is to maintain or reduce the pre-development runoff rate from a site to <br />the City’s storm drain system. <br /> <br />Stormwater Detention Calculations <br />Storm drainage systems shall be designed to control the amount of runoff from the site by using the <br />following steps: <br />a) Use the Rational Formula, Q = CiA <br />b) Determine the rainfall intensity for the site using data from the NOAA Precipitation Frequency <br />Data Server (PFDS). https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=ca <br />i. Select data type “Precipitation Intensity” <br />ii. Search by project address <br />iii. Using the “PF Tabular” data, determine the rainfall intensity for a <br />10-minute storm duration for the 10-year, 25-year, and 100-year average recurrence <br />intervals. <br />iv. If a longer time of concentration is expected, provide calculations to validate the <br />expected time of concentration and adjust the duration accordingly. <br />c) Determine the pre-development discharge rate using a 10-year design storm. <br />d) Determine the post-development discharge rates using a 25-year design storm. <br />e) Determine the post-development discharge rates using a 100-year design storm. <br />f) Determine the difference in discharge rates between c and d. Use this difference to calculate the <br />volume of stormwater required to be detained, based on the City’s Synthetic Unit Hydrograph <br />model (see Attachment “O”). This detention volume must be stored within pipes and/or vaults on <br />the site. <br />g) Restrict the flow from the detention structure(s)/pipe(s) to exit the site at a rate equal to or less <br />than the pre-development discharge rate determined in c. <br />h) Contain the 100-year storm volume on-site with combined use of surface storage and vault/piping <br />system storage. <br />i) Allow for overland release to a City street. <br /> <br />7.A. - Page 45 of 121 <br />51