2005-08-23_PWETC_AgendaPacket - Laserfiche WebLink

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capture rule is based on a regional analysis of the rainfall frequency, and is intended to capture and treat runoff produced from the 90th percentile of rainfall events. The MPCA analysis was based on the last 30-years of precipitation records at the Mim1eapolis/St. Paul International Airport. For this period, the design storm for the 90tl1 percentile was the 1.05 inch event. Also, as part of the technical analysis done for the 2005 MPCA Stormwater Manual, a comparison was done on the removal efficiency of ponds based on various runoff volumes generated by the 90% capture rule. Although the analysis was completed for water quality ponds and not infiltration, these results still are useful in demonstrating the effectiveness of capturing runoff from the 0.8-inch storm event. For comparison purposes, the current RCWD standard was also modeled and both results are sho\\m in Table I. Table 1. Comparison of estimated Wet Volume Treatment utilizing two different water quality models PondNET Results P8 Results Percent Reduction Phosphorous Loads Percent Reduction Phosphorous Loads Development Scenario Develonrnent Scenario -- I , 100-acre I 50-acre I 10 I 2 1 aD-acre 50~acre , ~acre -acre Detached Detached IO-acre 2-acre Detached Detached i .. I . . Institutional Commercial Criteria R 'ct . 1 R"ct . 1 I InstItutIOnal I CommelcIal Residential i Residential eSl entIa i eSl enha 90% Capture I , I , Rule* (1.05" 57.9 55.0 54.8 53.1 47.7 46.8 47.4 43.8 rainfall) District Standard 65.1 62.1 59.9 60.4 54.9 55.0 54.0 53.8 (2.5" rainfall) I , i , ! I * Based on the analyses perfonned for the 2005 lVlPCA Stormwater Manual based on the last 30-years of precipitation records at the MSP Airport. The effectiveness of the 0.8-inch stonn event standard would actually be increased on average to near 60% if infiltration BMPs are incorporated. The MPCA's 90% capture rule efficiencies above are more indicative of efficiencies for standard water quality ponding. Based on this analysis, we reconunend that the 0.8-inch rainfall event be used as the design event for sizing road reconstruction infiltration practices. Also, due to the constraints typically associated with roadway projects and the difficulty of enforcing the standard under tight soil conditions, we recommend the 0.8-inch rainfall design event be applied only over the impervious areas of the subwatershed served by the facility. If applied to highly impervious land cover, the advantages of capturing the runoff from the 0.8- inch precipitation event are: . Approximately 90% of stonns are captured on an average year thereby establishing a reliable threshold for runoff capture and treatment. . Pmiial treatment is provided for lmger stonns above the 90% threshold. . Cunent District standards for Total Phosphorus are met. . Relatively simple to compute, administer and review for most sites. ETIllTIons & Olivier Resources, Inc. Page 4 of7