05-11-2020 Council Packet - Laserfiche WebLink

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9 100. Pond 100 will pick up surface water from the northwest portion of the site and discharge to the west to an existing private ditch. Pond 400 will receive the remaining surface water and discharge to an existing ditch to the north. o The design includes a standard outlet control structure with pipe outlet. An outlet control structure is a device that holds water back in the proposed pond and then discharges it slowly into the ditch so that the flow to the downstream ditch is less than the existing condition flow into the ditch. o The original design included an outlet pipe to the ditch. The applicant was unable to obtain an easement for the outlet pipe to the ditch as required by the City and RCWD. The applicant revised the design and enlarged the pond to provide additional storage and added a plunge pool dissipater at the discharge location. • Adjacent property owners expressed concerns over the s stormwater runoff from the site and the downstream private ditches. Staff had multiple conversations and Zoom meetings with the property owners to better understand their concerns and explain the existing and proposed conditions. This included an onsite evaluation, reviewing topographic data, record plans, survey, and incorporation of the NorthPointe development drainage plan. The downstream systems appear to be working correctly and the NorthPointe development has reduced the surface water flow to the subwatershed in question. We will continue to work the property owners and provide information and findings. o The adjacent private ditch owners are responsible to perpetuate existing runoff rates and are to maintain the ditch flow through their property. Staff reviewed private ditch requirements with the City Attorney and RCWD staff. o 2019 was a record year for rainwater in Minneapolis-St. Paul area according to the Minnesota DNR Year to Date Precipitation Chart. Soil saturation in the fall of 2019 was similar to a spring snow melt condition where typically the fall is a dryer condition. These conditions have led to standing water due to soil saturation and higher flows in conveyance systems. o We are working with one of the downstream property owner to complete culvert maintenance on their property. • In addition, we are having the developer verify the downstream conditions to confirm their assumptions.” Per the RCWD CAPROC dated April 8, 2020: Soil conditions vary over the sloping site, with HSG D sandy lean clays (CL) at the higher elevations, and HSG A/B poorly graded sands and sands with silt (SP/SP-SM) with sandy lean clays below. The HSG D soils in addition to the high water table below the HSG A/B soils make infiltration infeasible for the site. Thus, stormwater ponds are acceptable to meet the water quality requirement. The pond sizing, and outlets and overflows are consistent with design criteria. 100% of the project area either flows to the ponds or meets design criteria. Additional TSS removal is not required. The applicant has met all the Water Quality requirements and the design criteria of RCWD Rule C: Stormwater Management Plans.