Agenda - Council - 06/13/2017 - Laserfiche WebLink

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Review File: Covenant Meadows Final Plat Exhibits Engineering Review June 6, 2017 Page 2 of 3 SW1, SW2: 1. A scale and circle denoting 1 mile must be added to the area map. 2. A table listing erosion control measures to be installed and quantities of each item must be added to the sheet. 3. The City must approve the Notice of Termination (NOT) before it is submitted. Erosion control must be inspected weekly and after every 0.5 inch rain event. The weekly reports must be emailed to the City. The complete erosion control inspection report must be submitted before the City will approve the NOT. Ll, Ti, T2: 1. Trees to be removed must be clearly identified on the plan and in the tables. Add a column to the tables identifying trees to be removed. 2. Tree protection fencing must be shown on the plan and installed as noted above. Stormwater Calculations A stormwater summary report was provided. The report states the 100 year storm is retained in the pond. The LRRWMO requires volume reduction (infiltration) on all projects. The LRRWMO rules require that infiltration basins empty completely within 48 hours after a 1" rainfall. The report must be revised to include the 1 inch, 2 year, 10 year, 100 year and 100 year back to back rain events. The report must list the new impervious area and the infiltration volume required. This must be calculated with a formula or the MIDS calculator. Using the volume from a 1 inch rainstorm in HydroCAD is not acceptable, HydroCAD calculations include abstraction which reduces the volume reported. The City has reviewed the stormwater calculations and find that the 100 year event will take at least 56 hours to drain out of the south east pond. This was computed by multiplying the infiltration rate of 0.8 inch/hour times the surface area of the bottom of the pond to determine the cubic feet per hour leaving the pond. The total runoff reaching the pond was divided by this rate to determine the time to drain the pond. The City model also showed that the NE and SE ponds overtop by 0.1 foot. The HydroCAD report indicates 2.7 cfs will leave the site and enter the Germanium Street ditches during the 100 year event. The City is requesting profile and cross section information for the existing ditches so that the impact from this project can be determined and ensure that existing properties will not be adversely impacted by this increased runoff. The report states onsite soils in the mitigation area will be removed and replaced with more permeable soils to promote infiltration. Details for the soil replacement must be included in the plans. The Minnesota Pollution Control Agency has produced the free MIDS calculator for evaluating stormwater practices like infiltration. The pond and infiltration basin must be evaluated with the MIDS calculator. The stormwater report must include the infiltration summary for each pond.