Agenda - Public Works Committee - 02/21/2023

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Name: Ramsey Parkway Center Median Analysis Date: February 19, 2018 Page: 2 of 3 are restricted from infiltration. Therefore, the water quality medians must be filtration systems (rather than infiltration systems), with a perforated underdrain and impermeable liner. Three liner types were considered for this analysis: • Woven geotextile with very low permeability, • Clay liner, and • Rubber liner (ethylene propylene diene monomer, EPDM, rubber). Figure 2 depicts a typical section commonly used for filtration applications. For depressed median alternatives, we assumed tree, shrub and perennial plantings would be used in the intersection and high visibility areas. Native seeding (prairie grasses with flowering plants) would be used in other areas. This type of vegetation provides for nutrient uptake, volume reduction, offers aesthetic benefits, and provide a traffic calming effect. Native, salt -tolerant plant species are typically utilized to offer the greatest potential for long-term sustainability. Trees are recommended throughout the median area per The COR Design Framework (November 27, 2012). For comparative purposes, a planting plan similar to that used in the median east of Center Street was used for the raised median option. Benefits and Disadvantages of Liner Options Given the infiltration restrictions on site, an impermeable liner is recommended to ensure that that critical groundwater resources are protected from unexpected contamination. The advantages and disadvantages of each liner option considered are presented below: 1) Woven Geotextile: Woven geotextile fabric is a cost effective option for separating the bioretention medium from the underlying sandy soils. While the geotextile has a very low permeability, it is not 100% impermeable. The underlying coarse filter aggregate and perforated drain tile will naturally force water along the path of least resistance into the tile. Little water is expected to pass through the geotextile layer. 2) Clay Liner: Clay liners have a long history of use and are a popular choice in bioretention basins that have difficulty holding water. The clay liner must be thick (up to 12") to be truly impermeable. These liners have a tendency to fail over time and must be continually monitored for effectiveness. If the liner were to fail, a portion of the bioretention basin would have to be removed for repair of the clay liner. 3) EPDM Rubber Liner: The EPDM liner is the safest choice for groundwater protection because it is 100% impermeable and has a relatively long service life. EPDM liners are the most expensive option and can be difficult to install. Estimated Costs Costs are based on standard cross sections and median widths, as shown in Figure 2. The filtration medium, planting medium, and under drain depths are based on the Minnesota Pollution Control Agencies recommended design for a bioretention facility with an underdrain. Estimated costs are summarized in the following table. Costs are shown per linear foot, with project costs reflective of 1,565 feet of total depressed or raised median length. Costs do not include curb and gutter,