10025 9TH AVE W 2016-01-01 MF Import

DVE Connyltants, Inc. PRIMARY TREATMENT OF RUNOFF, DETENTION AND RELEASE OF RIINOFF The total site area is 1.93 acres. An underground concrete detention vault was chosen to provide for both detention and primary treahnent, in the form of dead storage, of the generated runoff. A second vault will be open topped and used for secondary treatrnent in a wetland cell. It should be noted ffiat the calculated live storage will be based on the azeas of both vaults in th� foilovring calculations. Analysis for the design wes based on the Hydrog�aph meti7od using the Santa Barbara Urben Hydrog�aph model for the computation of predicted runoff and level pool routing. CalculaGons were aided using Waterworks so&wue. DETENTION AND RELEASE Detention and release were designed to limit peak flows from the developed site during the 2-year, 24 hr storm event to 50 °/a of the exisring condirion 2 yeaz, 24 hr peak runoff and to maintain the existing condition peak flows for the 10 yr. and 100 yr. events as well. As noted above, the soil is of the hydrologic soils group C, The undeveloped azea is covered with tall grass and some brush , from table 1-5.6 use curve numbers of 81 for the undeveloped condition and 86 for the developed condirion. Intensities in total inches in 24 hr event are• 6 MO 1" 2 yr. 1.55>' 10 yr. 2.25" 100 yr. 3.25" (see table 1-5.4) DETENTION AND RELEASE The basins and hydrographs developed by the Watenvorks softwaze are attached to this report. Per City standazds a correction factor for storage volume must be applied to account for errors in the design model due to continuous storm events. For this site, with 60% impervious surfaces, the conection factor is 1.35. This procedure calls for the design to match water sw�face elevarions consistent with the volumetric increase. This can be accomplished in either of two ways; by increasing the hydrographs by 30% and applying them to the desigi pond. Or, by decreasing the design pond area/volume by 30 % and designing the release structure based on ac:ual rainfall events. For this design the latter was chosen. The actual basin is : The underground basin and wedand cell; ( 20' x 30') +(20' x 65') = 1„900 sf 1,900 / 1.35 =1,407 sf Therefor. the modeled basin is onlv 70' X 20' for a total area of 1 400 sf. Drainage Report, Bush Crardens, Job # 98022 C:'�vfy llocuments`,DRN. RE:pT198051.docLast printed 1O101J98 9:32 AM i