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Ms. Katie Larsen 03/29/2019 Page 3 Lyngblomsten Preliminary Plat Engineering Review • Stormwater Management The Lyngblomsten development is 20.74 acres in size, with an existing impervious area of approximately 2 acres. Majority of the site runoff is towards the west where there is an 24 inch diameter pipe outlet and to the existing wetland on site. The proposed impervious area is approximately 9.5 acres. There is a proposed pond located in the middle and east side of the site. The City adopts Rice Creek Watershed District’s (RCWD) rules for water quality and quantity requirements. The applicant is proposing a stormwater reuse system that will be further reviewed with the final design. It is strongly recommended that the applicant obtain an approved CAPROC prior to final plat approval. The current submittal now includes rate control via a combination of a dry pond (located in the northwest) and a stormwater treatment pond. The submittal normal water level of the pond is 896, and the 100 year elevation of the pond is at 900.5. 1. The applicant is not showing the existing impervious on Outlot C in the drainage calculations and if it is the intent to redevelop with this area being impervious the applicant would need to revise the proposed drainage map, HydroCAD model and water quality calculations to include this area. Otherwise in the future this area will be required to meet RCWD rule C and City stormwater requirements. Drainage calculations and spreadsheets now account for Outlot C. Genetically, the area has been set to mostly impervious to allow for flexibility in future planning 2. In the Stormwater report, the applicant should include a summary table of the HWLs (2, 10, and 100-year) for the wetland (both existing and proposed) and in the proposed pond. Indicate on the summery the lowest adjacent building opening elevation. a. The applicant shall also verify free board requirements are met. The summary table has been modified. 3. It is not clear on the plans where the Porous Baffle Basin and Floating Head Skimmer are proposed. The applicant shall submit additional information for review. The plans have been modified to show the location of erosion control devices. See sheets C1.50-CI .52. These erosion control devices will be reviewed with the final design submittal. 4. In the plan and modeling, the applicant shall update the NWL of the pond to be at, or above, the elevation of 897.0. This is the conservative estimated seasonal ground water elevation provided by Braun in the Geotechnical Report. a. The applicant shall also verify all HWL and freeboard requirements are still met. The normal water level lies been adjusted The City is comfortable allowing the NWL of the pond to be constructed below the ground water elevation if the pond is lined with a 1.0-foot clay liner. Please provide an updated plan, HydroCAD model/discharge rate calculations, water quality calculations, and irrigation calculations. 5. The City’s standard outlet control structure shall be used without a Fluidic-AMP Hydrobrake. The HydroCAD model shall reflect this. The fluidic amp Hydrobrake has been included in the model to allow for Rice Creek Watershed District’s required 80% reduction inflow rate. If a standard orifice where to be used, it would be small enough where we are concerned about clogging of the actual orifice. The Hydrobrake provides a reduced flow yet allows for the passing of a larger sphere. The model has not been updated. The applicant has communicated to us that this outlet has been changed and modified to be a v notch weir with a 10 inch orifice at outlet. 6. In the HydroCAD models, a. The applicant shall use a MSE3 rainfall distribution instead of the Type II. The MSE3 distribution has been used.