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Introduction to the Minnesota Stormwater Manual

Stormwater concepts and stormwater management

Stormwater issues

Stormwater control practices (Best Management Practices)

Infiltration basins, infiltration trenches, dry wells, and underground infiltration systems capture and temporarily store stormwater before allowing it to infiltrate into the soil. As the stormwater penetrates the underlying soil, chemical, biological and physical processes remove pollutants and delay peak stormwater flows.Bioretention (rain garden) is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation.Tree trenches and tree boxes (collectively called tree BMP(s)), the most commonly implemented tree BMPs, can be incorporated anywhere in the stormwater treatment train but are most often located in upland areas of the treatment train. The strategic distribution of tree BMPs help control runoff close to the source where it is generated. Tree BMPs can mimic certain physical, chemical, and biological processes that occur in the natural environment.Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir for temporary storage and/or infiltration. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). Permeable pavements have been used for areas with light traffic at commercial and residential sites to replace traditional impervious surfaces in low-speed roads, alleys, parking lots, driveways, sidewalks, plazas, and patios.A stormwater harvesting and use system is a constructed system that captures and retains stormwater for beneficial use at a different time or place than when or where the stormwater was generated. A stormwater harvesting and use system potentially has four components: collection system (which could include the catchment area and stormwater infrastructure such as curb, gutters, and stormsewers), storage unit (such as a cistern or pond) treatment system: pre and post (that removes solids, pollutants and microorganisms, including any necessary control systems), if needed, and the distribution system (such as pumps, pipes, and control systems).Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention. Green roofs occur at the beginning of stormwater treatment trains. Green roofs provide filtering of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream Best Management Practices (BMPs).Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. They typically have vegetative cover such as turf or native perennial grasses. Dry swales may be constructed as filtration or infiltration practices, depending on soils. If soils are highly permeable (A or B soils), runoff infiltrates into underlying soils. In less permeable soils, runoff is treated by engineered soil media and flows into an underdrain, which conveys treated runoff back to the conveyance system further downstream. Check dams incorporated into the swale design allow water to pool up and infiltrate into the underlying soil or engineered media, thus increasing the volume of water treated.Wet swales occur when the water table is located very close to the surface or water does not readily drain out of the swale. A wet swale acts as a very long and linear shallow biofiltration or linear wetland treatment system. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams. Wet swales planted with emergent wetland plant species provide improved pollutant removal. Wet swales may be used as pretreatment practices. Wet swales are commonly used for drainage areas less than 5 acres in size.Stormwater step pools address higher energy flows due to more dramatic slopes than dry or wet swales. Using a series of pools, riffle grade control, native vegetation and a sand seepage filter bed, flow velocities are reduced, treated, and, where applicable, infiltrated. The physical characteristics of the stormwater step pools are similar to Rosgen A or B stream classification types, where “bedform occurs as a step/pool, cascading channel which often stores large amounts of sediment in the pools associated with debris dams”. Stormwater step pools are designed with a wide variety of native plant species depending on the hydraulic conditions and expected post-flow soil moisture at any given point within the stormwater step pool.Vegetated filter strips are designed to remove solids from stormwater runoff. The vegetation can consist of natural and established vegetation communities and can range from turf grass to woody species with native grasses and shrubs. Because of the range of suitable vegetation communities, vegetated filter strips can be easily incorporated into landscaping plans; in doing so, they can accent adjacent natural areas or provide visual buffers within developed areas. They are best suited for treating runoff from roads, parking lots and roof downspouts. Their primary function is to slow runoff velocities and allow sediment in the runoff to settle or be filtered by the vegetation. By slowing runoff velocities, they help to attenuate flow and create a longer time of concentration. Filter strips do not significantly reduce runoff volume, but there are minor losses due to infiltration and depression storage. Filter strips are most effective if they receive sheet flow and the flow remains uniformly distributed across the filter strip.Iron-enhanced sand filters are filtration Best Management Practices (BMPs) that incorporate filtration media mixed with iron. The iron removes several dissolved constituents, including phosphate, from stormwater. Iron-enhanced sand filters may be particularly useful for achieving low phosphorus levels needed to improve nutrient impaired waters. Iron-enhanced sand filters could potentially include a wide range of filtration BMPs with the addition of iron; however, iron is not appropriate for all filtration practices due to the potential for iron loss or plugging in low oxygen or persistently inundated filtration practices.Sand (media) filters have widespread applicability and are suitable for all land uses, as long as the contributing drainage areas are limited (e.g., typically less than 5 acres). Sand filters are not as aesthetically appealing as bioretention, which makes them more appropriate for commercial or light industrial land uses or in locations that will not receive significant public exposure. Sand filters are particularly well suited for sites with high percentages of impervious cover (e.g., greater than 50 percent). Sand filters can be installed underground to prevent the consumption of valuable land space (often an important retrofit or redevelopment consideration).Stormwater ponds are typically installed as an end-of-pipe BMP at the downstream end of the treatment train. Stormwater pond size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, due to their size and versatility, stormwater ponds are often the only management practice employed at a site and therefore must be designed to provide adequate water quality and water quantity treatment for all regulated storms.Stormwater wetlands are similar in design to stormwater ponds and mainly differ by their variety of water depths and associated vegetative complex. They require slightly more surface area than stormwater ponds for the same contributing drainage area. Stormwater wetlands are constructed stormwater management practices, not natural wetlands. Like ponds, they can contain a permanent pool and temporary storage for water quality control and runoff quantity control. Wetlands are widely applicable stormwater treatment practices that provide both water quality treatment and water quantity control. Stormwater wetlands are best suited for drainage areas of at least 10 acres. When designed and maintained properly, stormwater wetlands can be an important aesthetic feature of a site.Pretreatment practices are installed immediately preceding one or more structural stormwater BMPs. Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas.Sediment control practices are designed to prevent or minimize loss of eroded soil at a site. Typical sediment control practices focus on 1) physical filtration of sediment by trapping soil particles as water passes through a silt fence, drop inlet screen, fiber roll, etc., 2)settling processes, that allow sediment to fall out of flows that are slowed and temporarily impounded in ponds, traps, or in small pools created by berms, silt fencing, inlet protection dikes, check dams, etc.Erosion prevention practices include 1) planning approaches that minimize the size of the bare soil area and the length of time disturbed areas are exposed to the elements – especially for long, steep slopes and easily erodible soils, 2) diverting or otherwise controlling the location and volume of run-on flows to the site from adjacent areas, 3)keeping concentrated flows in ditches stabilized with vegetation, rock, or other material, and 4)covering bare soil with vegetation, mulch, erosion control blankets, turf reinforcement mats, gravel, rock, plastic sheeting, soil binder chemicals, etc.Pollution prevention (P2) is a “front-end” method to decrease costs, risks, and environmental concerns. In contrast to managing pollution after it is created, P2 reduces or eliminates waste and pollution at its source. P2 includes a variety of residential, municipal, and industrial practices.imagemap for stormwater BMPs
Stormwater Best Management Practices. Mouse hover over an i box to read a description of the practice, or click on an i box to go to a page on the practice.

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