08-10-2020 Council Packet - Laserfiche WebLink

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8 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. Discharge Rate Comparison 2-year (cfs) 10-year (cfs) 100-year (cfs) Existing Proposed Existing Proposed Existing Proposed North to Private Ditch 3.5 2.3 5.1 3.9 6.9 5.7 West to Private Ditch 11.6 9.2 6.8 34.1 28.7 25.7 82.8 74.7 73.9 South through Culvert under Birch Street 4.4 3.5 8.0 6.5 14.6 11.9 Totals* 19.2 12.5 9.8 46.7 35.6 32.2 103.3 87.8 87.0 *Difference due to time of concentration Post-development stormwater discharge rates are less than the pre-development rates. The City’s stormwater management requirements for rate control and infiltration have also been met. The City’s requirements for rate control and infiltration are the same as RCWD. The applicant has submitted the revised plans to RCWD for final review and approval. RCWD approved and issued Permit 20-002 on July 20, 2020. During the Public Hearing for the project, there were concerns raised by a few of the adjacent property owners. Staff, the developer, and developer’s engineer have worked to address these questions. As a result, the downstream ditch conditions were surveyed to obtain baseline data of the existing conditions. A culvert on the Rehbein parcel was reviewed by the property owner for blockage. The proposed outlet and associated drainage and utility easement to the Vickers’s parcel is being processed. The original design for Pond 100 outlet was a conventional outlet structure and discharge pipe. This design required a drainage and utility easement. Note there is an existing Great River Energy easement that already encumbers this same area. The Vickers were concerned about the proposed outlet discharge and thus granting an easement was declined originally. The developer then re-designed the outlet without an easement. This revision included a plunge pool dissipator and enlargement of the pond. However there were still concerns regarding erosion and overland flow. Staff met with the Vickers on Thursday, May 7, 2020 to review options for a stormwater outlet. The owners explained their concerns over possible ditch erosion and ongoing maintenance concerns. Staff explained that if an easement is obtained, the City will be responsible for ongoing maintenance of the improvements including the outlet area. The owner informed staff of their preference for a piped outlet with a dissipater still