Agenda - Council - 06/11/2024 - Laserfiche WebLink

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BMP Descriptions Bioretention is a BMP that uses soil and vegetation to treat stormwater runoff from roads, driveways, roof tops, and other impervious surfaces. Differing levels of volume and/or pollutant reductions can be achieved depending on the type of bioretention selected. Bioretention can function as either filtration (biofiltration) or infiltration (bioinfiltration). Biofiltration BMPs are designed with a buried perforated drain the that allows water in the basin to discharge to the stormwater drainage system after having been filtered through the soil. Bioinfiltration BMPs have no underdrain, ensuring that all water that enters the basins will either infiltrate into the soil or be evapotranspired into the air. Bioinfiltration provides 100% retention and treatment of captured stormwater, whereas biofiltration basins provide excellent removal of particulate contaminants but limited removal of dissolved contaminants, such as DP (Table 6). Table 6: Matrix describing curb -cut rain garden efficacy for pollutant removal based on type. Curb -cut TSS .. DPSize o Design Rain Garden Area Type Removal ,Remov'JaJemovJa11 1eduction Treated L Notes A Id A A Optimal sites are low enough in the landscape to capture Bioinfiltration High High High High High most of the watershed but high enough to ensure adequate separation from the water table for treatment purposes. Higher soil Biofiltration High Moderate Low Low High infiltration rates allow for deeper basins and may eliminate the need for underdrains. The treatment efficacy of a particular bioretention project depends on many factors, including but not limited to the pollutant of concern, the quality of water entering the project, the intensity and duration of storm events, project size, position of the project in the landscape, existing downstream treatment, soil and vegetation characteristics, and project type (i.e. bioinfiltration or biofiltration). Optimally, new bioretention will capture water that would otherwise discharge into a priority waterbody untreated. The volume and pollutant removal potential of each bioretention practice was estimated using WinSLAMM. In order to calculate cost -benefit, the cost of each project had to be estimated. To fully estimate the cost of project installation, labor costs for project outreach and promotion, project design, project administration, and project maintenance over the anticipated life of the practice were considered in addition to actual construction costs. If multiple projects were installed, cost savings could be achieved on the administration and promotion costs (and possibly the construction costs for a large and competitive bid). Please note infiltration examples included in this section would require site specific investigations to verify soils are appropriate for infiltration. City of Ramsey Stormwater Retrofit Analysis