2005-08-23_PWETC_AgendaPacket - Laserfiche WebLink

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The disadvantages of captnring the rnnofffrom the 0.8-inch precipitation event are: ~ Conld result in low water quality volume (and treatment) for sites having less than 15% . . ImpervIOUS cover. . If the rest of the pelmeable areas contain tight soils (SCS Type C or D), tbe infiltration facility may not fully captnre runoff and nutrients generated from tnrf, lawn and open space areas. EXAMPLE IMPLEMENTATION The example road reeonstruction projects discussed below were evaluated based on captnring all stonns less than or equal to 0.8-inches and utilizing the following methodology for BMP sizing. For the majority of small storms impervious areas generate the bulk of the rnnoff. This allows us to simplify the calculations and focus on the impervious areas. A rnnoff coefficient (C) of 0.9 was used for all impervious areas. For the examples, the following equation was utilized to determine the required BMP storage volumes. Vrequired = (0.8 inches) x (0.9) x (lfoot/12 inches) x (Impervious Area) Calculation of impervious area could be preformed utilizing one of two methods. The preferred method would be to measnre the impervious areas directly using snrvey or GIS information. The second option would be a simplified approach utilizing an average lot size for a subwatershed and then using percent impervious fraetions based on available literatnre. For example if you had a 10 acre subwatershed with 20 homes located within that subwatershed the average lot size would be Yz acre. Based on the McGraw-Hills Series in Water Resources and Environmental En"ineerin" (1988) a single-family home development with Y2 acre lots has 25% impervious area. Therefore 2.5 acres of impervious area would need to be utilized in the volume required calculation for the 10 acre subwatershed example. Example 1 - White Bear Lake. TIlls neighborhood street reconstruction was completed in 2004 and incorporated underground infiltration and a structnral, sediment removal device (V2B 1) prior to discharge to White Bem Lake. This project was reassessed in light of the proposed 0.8-inch rainfall standard with an attempt locate all stormwater featnres within road right-of-way. The neighborhood is relatively flat with several matnre trees. The area directly tributary to the 0.5 miles ofreconstructed roadway is approximately 10 acres. Soils in this area are sand, with good separation from the ground water table. Locations for BMPs included areas that were utilized for BMPs in the 2004 construction and additional open spaces shown on drawings provided with the permit. In order to meet the 0.8-inch rainfall standard, ten raingardens with a total length of 470 feet and an average width of 15-fcet were incorporated. Underground featnres included a 75-foot french drain (perforated pipe sunounded by washed rock) and a 100- foot infiltration trench. Sump catch basins were incorporated to provide pretreatment prior to the infiltration trench. Additional opportunity is available for incorporation of an increased amount of underground infiltration Emmons & Olivier Resow.ces, Inc. Page 5 of7