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April 21, 2023 Elk River Landfill Page 2 of 2 Reference: South Stormwater Pond u:\227705870\technical\south_stormwater_pond\cup\attachment 4 - drainage calculations\stormwater write-up.docx The southern stormwater basin was designed to provide flexibility depending on mining activities in the area. The maximum bottom elevation of the pond will be at an elevation of 964; the actual elevation may be 964 or lower. Modeling assumed a bottom elevation of 964 with two deeper settling basins at the east and west ends of the pond, as shown on the drainage map. A safety bench has been incorporated into the pond design and perimeter access for cleaning / maintenance activities has been considered. A 5:1 slope on the north side of the pond was implemented to visually change the slope and allow for additional landscaping. Based on site soils, it is anticipated that infiltration will take place within the pond footprint. However, to be conservative, no infiltration was assumed in the stormwater model. The stormwater basin includes an outlet culvert at an elevation of 967.5’. Since it is anticipated that infiltration will occur (onsite soils are HSG A/B soils) the outlet culvert will serve as an overflow mechanism, rather than a typical pond outlet structure. As another conservative design measure, the model assumes that run-off from a 25-year, 24-hr storm event is in the pond at the start of the modeled storm (~2.0 feet of water within the pond). If stormwater does discharge through the outlet culvert, stormwater would flow through the pipe to an existing wooded, low-lying area directly west of the pond footprint. The overflow elevation of this low-lying area is 969.5; the area was included in the modeling to confirm that discharge will remain on Landfill property for the design storm events. Incorporating the shallowest anticipated pond grades into the model, results indicate that the 25-yr and 100-yr storm events will be managed with no off-site discharge. The 100-yr event includes nominal discharge to the low-lying area onsite. Excavation below the grades shown would result in more ponding capacity and the capability to manage even more extreme storm events with no discharge from the Landfill property. A summary of the pond performance for the modeled storm events is included below: Storm Event 25-yr 100-yr Pond Peak Water Elev. 967.88 968.66 Pond Discharge (cfs) 0.00 0.34 Low Area Peak 968.58 969.34 Low Area Discharge (cfs) 0.00 0.00 Stantec Consulting Services Inc. Attachment: Subwatershed Map