03-04-2019 Council Packet - Laserfiche WebLink

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Ms. Katie Larsen 02/07/2019 Page 2 Lyngblomsten Preliminary Plat Engineering Review 5. The applicant should provide proposed centerline street grades (either grading sheets, or in profile on street plan sheet). 6. The SWPPP is being reviewed by the City’s Environmental Coordinator; below are our preliminary comments: a. The applicant should provide silt fence around the existing wetland. b. The applicant should provide silt fence along the project perimeter adjacent to Hodgson Road. 7. The applicant should provide and label a maintenance access route to the outlet control structure. The access should be a minimum of 10-feet wide, 8.0% longitudinal slope, and 2.0% cross slope. 8. The applicant should include riprap at storm sewer outfalls. The City’s standard detail for Rip Rap at outlets should be included in the plans. 9. The applicant should review the rear yard drainage of Lots 6-8. The current plan shows drainage onto private property. Proposed surface drainage needs to be directed westerly towards Wetland 1. 10. The applicant shall review the grades between Hodgson Road and the berms at the end of the private cul de sac. It appears that drainage is trapped in that area. 11. The minimum allowable grade across turf areas and drainage swales is 2%. 12. The grading plan needs to be adjusted at the southwest existing retaining wall. • 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. Below are the comments relating to stormwater management: 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. 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. 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. 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. 5. The City’s standard outlet control structure shall be used without a Fluidic-AMP Hydrobrake. The HydroCAD model shall reflect this. 6. In the HydroCAD models, a. The applicant shall use a MSE3 rainfall distribution instead of the Type II. b. In the HydroCAD model the applicant shall not include infiltration. The HydroCAD model is for flood modeling verses water quality. Water quality is calculated separately. c. The applicant shall update all HydroCAD results and HWL elevations on the plan. We anticipate that the modifications required to meet City standards for to the HydroCAD model and site design will increase the flood storage required for the site.