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Drainage Area <br />Pretreatment Device <br />1" Event Peak <br />Runoff Rate (cfs) <br />Design Treatment <br />Capacity (cfs) <br />Drainage Area 1 <br />ecoStorm Model #1 <br />0.42 <br />1.9 <br />Drainage Area 2 <br />Aqua -Swirl #1 AS -3 Model <br />0.88 <br />1.8 <br />Drainage Area 3 <br />Aqua Swirl #2 AS -3 Model <br />1.39 <br />1.8 <br />Drainage Area 4 <br />ecoStorm Model #1.5 <br />1.07 <br />2.6 <br />Event <br />Existing Runoff <br />Rate (cfs) <br />Proposed Runoff <br />Rate (cfs) to <br />Regoinal Ponds <br />2 -yr (2.75 ") event <br />1.56 <br />16.27 <br />10 -yr (4.1") event <br />6.98 <br />26.16 <br />100 -yr (5.9 ") event <br />16.85 <br />39.27 <br />Table 1: Runoff Rates Summary <br />Project No. UWA10002 <br />The Residence at the COR <br />Stormwater Narrative <br />Table 2: Pretreatment Summary <br />The ecoStorm and Aqua -Swirl structures have been sized adequately enough to treat the 1" event and convey <br />the 10 -year rainfall events. A diversion weir was used for each system to contain and treat the 1" event, but <br />also to allow stormwater to by -pass during larger events. <br />ALLEYWAY PONDING ANALYSIS <br />It is our understanding the lower level of the existing parking structure adjacent to the proposed alleyway <br />experiences fairly frequent freezing. It is not our intent to provide a solution to this problem. However, special <br />attention to drainage and ponding in the proposed alleyway between the existing parking structure has been <br />given and analyzed. The maximum ponding depth for each catch basin in the proposed alleyway was <br />modeled assuming the outlet pipe to be 50% clogged during a 100 -yr event. Overland overflow elevations <br />have been designed at one foot below the proposed garage floor and the existing parking structure elevation <br />as shown on the proposed grading plan. Since the ponding depth of each subcatchment will exceed the <br />overland overflow elevation during the 100 -yr event, a theoretical "glass wall" was necessary to model in order <br />to conservatively compute how high each ponding area would bounce during the design event. The results of <br />this analysis indicate the highest subcatchment surface ponding elevation to be 869.63 which is below the <br />proposed garage floor and lower level parking structure elevation of 870.00. This theoretical "glass wall" is a <br />very conservative approach to modeling due to this is not a land- locked basin and the ultimate overland <br />overflow path would travel towards Sunwood Drive. <br />L AN D F OR M December 3, 2010 <br />2 <br />