Agenda - Council - 04/09/2018 - Laserfiche WebLink

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2. INTRODUCTION 2.1 Authorization The preparation of this report was authorized by the Ramsey City Council on March 27, 2018. This project has been designated as City Improvement Project No. 18-09. This Project was added to the Capital Improvement Program as 16-STM-002 Identify COR Infiltration Banking Area. 2.2 Background The City purchased the Ramsey Town Center properties out of foreclosure in 2009.The City has been marketing the properties for development ever since. The majority of the properties are in the infiltration prohibited zone as shown on Exhibit A and will need offsite infiltration as a condition of receiving a LRRWMO permit. The LRRWMO Third Generation Plan, adopted in 2012, requires infiltration of the first one inch of runoff (volume reduction) from all impervious surfaces on the site. The Drinking Water Supply Management Area (DWSMA).for the municipal water supply wells overlies the COR. State Law discourages or prohibits infiltration in a DWSMA. Final determination of the infiltration exclusion area in the DWSMA is a local decision. The City of Ramsey discussed the options with other jurisdictions and set the 10 year capture zone as the infiltration exclusion zone. Properties that are between the boundaries of adjacent 10 year capture zones are also excluded from infiltrating their stormwater. The Third Generation Plan has a mitigation sequence defined that must be followed when infiltration is not permitted on a new development site. The process includes on site infiltration, off site infiltration, other non-voli nee control treatment on site, or contribution to a stormwater impact fund. The fund will be used to construct volume reduction practices where infiltration is permitted. Infiltration practices must have 3 feet of vertical separation between the bottom of the infiltration area and groundwater. Preliminary designs looked at modifying the outlet of an existing stormwater retention pond to cause the water level to rise to 4 feet and flow into the infiltration area before water flowed out of the pipe. This proved to be impractical, it would have required a large area to achieve the required infiltration, and it would have raised the 100 year elevation of the pond, impacting the floor and parking elevations of adjacent lots. Staff then explored stormwater lift stations and visited with several cities that use stormwater lift stations for level control. It is feasible to use a stormwater lift station to pump the water to the infiltration basin from the existing stormwater retention pond. Staff evaluated available sites including the area between Highway 10 and. the BNSF tracks adjacent to the commuter rail station, which was the former Auto Ranch property, the site east of Ramsey Boulevard formerly known as B & A Cylinder, and the selected site. All of the sites are City owned. Each will be discussed in detail. Feasibility Report COR Infiltration Basin City of Ramsey Project No. 18-09 Page 2