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

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BMP Descriptions Developments prior to enactment of contemporary stormwater rules often included wet detention ponds which were frequently designed purely for flood control based on the land use, impervious cover, soils, and topography of the time. Changes to stormwater rules since the early 1970's have greatly altered the way ponds are designed. Enactment of the National Pollution Discharge Elimination System (NPDES) in 1972 followed by research conducted by the Environmental Protection Agency in the early 1980's as part of the Nationwide Urban Runoff Program (NURP) set standards by which stormwater best management practices should be designed. Municipal Separate Storm Sewer System (MS4) guidelines issued in 1990 (affecting cities with more than 100,000 residents) and 1999 (for cities with less than 100,000 residents) required municipalities to obtain an NPDES permit and develop a plan for managing their stormwater. Listed below are five strategies which exist for retrofitting a stormwater pond to increase pollutant retention (modified from Urban Stormwater Retrofit Practices): • Excavate pond bottom to increase permanent pool storage • Raise the embankment to increase flood pool storage • Widen pond area to increase both permanent and flood pool storage • Modify the riser • Update pool geometry or add pretreatment (e.g. forebay) These strategies can be employed separately or together to improve BMP effectiveness. Each strategy is limited by cost-effectiveness and constraints of space on the current site. Pond retrofits are preferable to most new BMPs as additional land usually does not need to be purchased, stormwater easements already exist, maintenance issues change little following project completion, and construction costs are greatly cheaper. There can also be a positive effect on reducing the rate of overflow from the pond, thereby reducing the risk for erosion (and thus further pollutant generation) downstream. For this analysis, all existing ponds were modeled in the water quality model WinSLAMM to estimate their effectiveness based on best available information for pond characteristics and land use and soils. One proposed modification, excavating the pond bottom to increase storage, often has a very wide range in expected cost due to the nature of the excavated soil. If the soil has been contaminated and requires landfilling, the cost for disposal can quickly lead to a doubling in project cost. For this reason, projects which include the excavation of ponds have been priced based on the following criteria: • Management Level 1: Dredged pond soil is suitable for use or reuse on properties with a residential or recreational use • Management Level 2: Dredged pond soil is suitable for use or reuse on properties with an industrial use • Management Level 3: Dredged pond soil is considered significantly contaminated and must be managed specifically for the contaminants present Costs within each of these levels can even range widely, but were estimated to be $20/cu-yd., $35/cu- yd., and $50/cu-yd. for levels 1, 2, and 3, respectively. Additional costs associated with specific projects are listed in Appendix B. City of Ramsey Stormwater Retrofit Analysis