Agenda - Planning Commission - 01/04/2018

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Review File: Ramsey Elementary School Site Plan Application Engineering Review December 21, 2017 Page 5 of 6 pond if the pond will be dry. The local soils are easily eroded and soils down slope of flared end sections have eroded in the past, requiring extensive repairs. The flow areas are identified as West, East and North. Our observations on each area will be presented separately. East area This area includes two (2) existing wetlands near the eastern edge of the property. The wetlands will not be impacted by the project. The outlet for Wetland 3 is 887.9. City analysis of the topography east of the property indicates the grade rises to near 890 before dropping into a small confined basin. The adjacent plat, Shady Lawn Estates was recorded in 1972. Easements were taken along the right-of-way only, not along side lot lines or back lot lines. There is no route for discharging stormwater to the East. This area will need further analysis and the proposed 100 year water level of pond W3 will need to be revised. North area This area includes one (1) existing wetland near the northern edge of the property. The wetlands will not impacted by the project. The outlet for Wetland 1 is listed as 890.5. City analysis of the topography North of the property indicates the grade rises to near 891 before dropping into a the Trott Brook valley. The area to the north is not platted. There is no route for discharging stormwater to the North. This area will need further analysis and the proposed 100 year water level of pond W 1 will need to be revised. West area This area includes the new building and most of the new impervious surfaces. The LRRWMO requirement is to infiltrate the first 1" of runoff from all new impervious surfaces. It appears that all of the runoff is directed to the new pond. Ponding areas can serve as the infiltration basin as long as the pond is completely dry within 48 hours after the 100 year event. The pond was modeled as a wet pond, thus it cannot be used for infiltration credit if it retains water all the time. This ponding area is also using the emergency overflow to pass part of the 100 year event. The emergency overflow may not be used to pass part of the 100 year event. All of the 100 year event must pass through the outlet piping.