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<br /> <br /> <br /> <br />Frattalone’s Mondello <br /> <br />SUMMARY OF STORMWATER RUNOFF <br /> <br />Introduction: <br />This project will consist of constructing a residential development which will include the <br />construction of new roadways, houses, driveways, and associated stormwater and utilities <br />throughout the entire 23-acre development. <br /> <br />There is an existing wetland on the west side of the project, during the project grading will be <br />done alongside the wetland to increase the flood storage capacity. The wetland boundary will <br />be undisturbed during the project. <br /> <br />Existing Conditions: <br />The existing site consist of a few small houses, wood areas, farm fields, and a wetland. The <br />majority of the property is undeveloped. <br /> <br />Grassed areas were modeled as >75% Grass Cover, Type D Soils, with a Curve Number of 80. <br />Existing crop areas were modeled as Row Crops, Type D Soils, with a Curve Number of 89. <br />Wooded areas were modeled as Woods, with a Type D Soils, with a Curve Number of 77. The <br />impervious surfaces were modeled with a Curve Number of 98. <br /> <br />Proposed Conditions: <br />The majority of runoff from the new proposed conditions will be routed to a filtration basin on <br />the North side of the property, which is now designated Outlot B. The runoff will discharge into <br />a forebay for pre-treatment prior to entering the filtration basin. <br /> <br />The filtration basin will consist of plantings, 12” of a soil mixture (80% clean free draining sand <br />and 20% organic matter) with 4” perforated draintile below the sand section. The filtration <br />basin was modeled with an infiltration rate of 1.6 in/hr. The basin will outlet through an outlet <br />control structure that is controlled by a weir wall, into the wetland. This basin will be used to <br />control runoff rates and volumes leaving the site. <br /> <br />The proposed conditions were modeled as >75% Grass Cover, Type D soils, with a Curve <br />Number of 80. The impervious surfaces were modeled with a Curve Number of 98. <br /> <br />Analysis: <br />The project area was analyzed using HydroCAD Version 10.0 Stormwater modeling software. <br />SCS TR-20 modeling method, along with the MSE 24-hour storm event, were utilized in the <br />modeling of the existing and proposed runoff conditions. The 2-year, 10-year, and 100-year <br />storm events were used to model runoff conditions. Output from NOAA Atlas 14 precipitation <br />frequency data server was used for storm event depths. <br /> <br />Runoff Rate: <br />Per Ramsey-Washington Metro Watershed District standards, the proposed peak runoff rate <br />must not exceed the existing. The existing and proposed peak runoff rates leaving the site are <br />listed in the table below. <br /> <br />Existing peak runoff rates (in cubic feet per second): <br /> 2-year 10-year 100-year <br />Off Site to the East 1.47 2.44 4.70 <br />Wetland 22.70 43.93 96.09 <br />Total Site 22.83 44.14 96.55