Laserfiche WebLink
SURFACE WATER MANAGEMENT <br />(Revised August 2011) <br />Stormwater generated by the landfill modification will be controlled on-site. Runoffwill be <br />controlled and routed by drainage terraces, gabion lined downslope structures, and two <br />sedimentation basins as shown in the drawings. Drainage routing features have all been sized <br />adequately to minimize erosion from the site. <br />Stormwater generated from the site will be discharged via a storm sewer system, under the <br />exterior roadway to the north end of the proposed expansion, then west under the City Trail into <br />a wetland upstream of the Rice Lake wetland complex. The discharge point will be onto <br />property owned by Waste Management, Inc. <br />The new sedimentation basins (P-1 and P-2) are located around the perimeter of the site at the <br />southwest corner (P-1) and southeast corner (P-2) side of the southern development area (SDA). <br />HydroCAD-Stormwater Modeling, using the SCS Curve Number method, was used to calculate <br />run-off hydrographs for the final landfill configuration. To determine surface water run-off for <br />this analysis, the landfill was divided into subwatersheds depending on the contributing drainage <br />areas. Subwatershed areas are shown on Figure 1. Peak flows from the subwatersheds were <br />calculated for 2, 10, 25, 100-year, 24-hour storm events of 2.7, 4.2, 4.7, and 5.9-inches <br />respectively along with a 2.5-inch storm. Run-off curve numbers of 77, 80, and 100 were used <br />for the landfill top, sides and ponded water in the sedimentation basin respectively. Time of <br />concentration was computed using hydraulic distance and flow velocity according to NRCS <br />Hydrology Guide for Minnesota. Calculated time of concentration values were entered into the <br />HydroCAD model as direct entry. <br />These runoff calculations are used as inputs in sizing the various components of the surface <br />water management system discussed below. <br />T:\0742\1406\June] 1 CityApp\Comment responseUtespoffie_08181 tUlttachment I\SurEace Water Management Memo 081011.doc <br />