4.2.b PCSR 05-23-2023 - Laserfiche WebLink

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Memo To: Elk River Landfill From: Megan Ambuehl, PE Ethan Ketelsen, PE File: South Stormwater Pond Date: April 21, 2023 Reference: South Stormwater Pond Re-Design; City of Elk River Application Modifications to the stormwater management system for the southern portion of the MSW disposal area have been made to accommodate gravity drainage for stormwater management. The previous design included two sedimentation basins and would have required a lift station to manage stormwater from the landfill; discharge was under the exterior roadway, then under the City of Elk River trail, and ultimately to an upstream wetland on property owned by Waste Management. Consistent with the previous design, stormwa ter generated from the southern portion of the landfill will still be controlled and routed by drainage terraces, downslope structures, perimeter ditching, and related stormwater management infrastructure. Stormwater will be routed through these features to one large stormwater basin as shown on the attached subwatershed map. The proposed larger stormwater basin will allow for on-site control of stormwater, eliminating the trail crossing. The discharge point will be directly west of the proposed pond to an existing low area, on property owned by by the Landfill, east of the trail. HydroCAD stormwater modeling, using the SCS Curve Number method, was used to calculate run-off hydrographs for the final landfill configuration which assumes all landfill areas are closed with the final capping system. To determine surface water run-off for this analysis, the landfill area draining to the southern pond was divided into subwatersheds depending on the contributing drainage areas. Subwatershed areas are shown on the attached figure. Model inputs were consistent with the previously completed modeling for the site. Run-off curve numbers of 77, 80, and 98 were used for the landfill top, sides and ponded water in the sedimentation basin, respectively. Perimeter areas were assigned curve numbers of 55 (woods), 69 (fair grass cover), and 81 (developed urban industrial area) depending on existing or expected condition at the time of final closure of the current MSW disposal area. Time of concentration was computed within the HydroCAD model based on the characteristics of each subwatershed (i.e. hydraulic distance and slope). Peak flows from the subwatersheds were calculated for the 25-year and 100-yr 24-hr storm events based on NOAA Atlas 14 precipitation frequency estimates for the site. • 25-year, 24-hr = 5.19 inches • 100-year, 24-hr = 6.86 inches Runoff-calculations were used to size the various components of the surface water management system routing drainage to the south stormwater basin. Landfill development is in progress and currently Cell 20 Phase 3 is planned to be constructed in 2023. The drainage area contributing runoff to the pond will depend on waste filling activities and final cover construction; with the ultimate drainage area realized at the time of final closure of the currently permitted MSW disposal area. Prior modeling completed for the site included open channel flow within the landfill footprint. The re-design project modeling has incorporated a piping network. MPCA solid waste rules require management of the 25-year, 24-hr storm as the minimum design storm event. For the purposes of the southern pond re -design project, piping has been sized based on the 100-year, 24-hr storm event. Actual materials used for downslope structure or other infrastructure construction may vary; open channel flow or piping networks may be utilized for drainage. At minimum, the 25-year 24-hour storm event will be considered during construction level design/implementation.