Storm Water Best Management Practices: An Overview of Physical Treament Structures

The U.S. Environmental Protection Agency (USEPA) defines Best Management Practices (or BMPs) as schedules of activities, prohibitions of certain practices, maintenance procedures, or physical treatment structures that help reduce pollutants in storm water runoff. While routine maintenance schedules and spill prevention training are parts of an effective pollution reduction strategy, physical structures can also be used to assist in pollution removal and reduce the energy of sheet flow from paved areas prior to discharge to surface waters.

Physical treatment structures and technologies are typically divided into two groups: runoff reduction practices and structural practices. Runoff reduction practices are pervious areas which lessen the volume of storm water runoff that will enter a structural practice. Structural practices remove pollutants through gravitational settling, filtration, or biological/chemical processes. Structural practices also provide storage and discharge management strategies to reduce the volume of storm water entering surface waters. The following provides a brief description of types of physical treatment structures.

Runoff Reduction Practices

Impervious Surface Disconnection

This is a strategy to disconnect paved areas from drainage infrastructure through a receiving grass area, rain garden, or storm water planter. Disconnection strategies include curb cuts at pavement boundaries, installing a gravel verge along parking lots, and disconnecting roof downspouts from impervious surfaces. Such strategies reduce water quantity volume from impervious areas but have limited storage capabilities and provide minor filtration treatment of storm water as it travels across the surface. The maintenance of such areas can be as simple as clearing debris, replacing mulch in gardens, or weeding and reseeding planters.

Sheet Flow to Grass Filter or Conservation Area

Sheet flows to filter strips and conservation areas are the practice of directing uncollected runoff from an area of impervious surface through an adjacent broad, pervious area of healthy vegetation and high soil quality on a mild slope. Filter strips are usually built with turf grass, while conservation areas are natural, mixed vegetation areas. These areas are low maintenance and require periodic reseeding, invasive species management, and mowing.

Grass Swale

Grass swales are wide, dry storm water conveyance channels to provide limited water quality treatment for a short duration flow. Grass swales should not be confused with ditches or other waterways, as surface runoff through grass swales moves slower and below the height of the grass lining. Grass swales dry quickly after storm events without any prolonged or intermittent flow. Mowing and weed control are typical maintenance practices for grass swales.

Green Roof

Typical roof design sheds rainfall as runoff, conveying rain off the surface as quickly as possible. Green roofs reduce storm water runoff quantity and peak flows by retaining a portion of the rainfall in a medium for vegetation or evaporation. Green roofs are divided into two types: intensive rooftop patios with deep planting media that can support shrubs and trees; and lightweight and low-profile gardens for storm water management, water recycling, and/or thermal regulation. Green roofs require considerable construction over the roof, including waterproofing layers, insulation layers, drainage layers, a root barrier, and the growing media. Roof drains should also be constructed on surfaces with easy accessibility for routine and emergency maintenance, including removing obstructions from overflow drains, pruning, reseeding, and weeding.

Rainwater harvesting

Structural Practices

Wet Extended Detention Basin

A permanent pool that provides temporary storage for storm water runoff is a wet extended detention basin. The temporary storage volume is released slowly over at least a 24-hour period to allow for the settling of suspended solids and to protect the receiving stream from erosion. The pool also provides detention time for the water to undergo physical, biological, and chemical processes to remove additional pollutants between storm events. Maintenance includes periodic dredging of sediment, mowing, and maintenance of shoreline vegetation to clear inlets and outfalls.

Dry Extended Detention Basin

Opposed to a wet basin, dry extended detention basins temporarily pond and store runoff to delay peak runoff discharge rates and allow for the settling of suspended solids. Dry basins have an average detention time of 48 hours until captured runoff is fully drained. Periodic basin clean-out of sediments is necessary, in addition to trimming excessive vegetation.

Extended Detention Constructed Wetland

Constructed wetlands are an alternative to extended detention basins. Instead of using a singular pond, constructed wetlands use a variety of shallow pools of variable depth with vegetation. Constructed wetlands are highly effective in removing both sediments and pollutants. Such systems operate long-term with low maintenance, typically pruning vegetation and clearing inlet and outlet blockages.

Bioretention

Bioretention is a shallow depression over an excavation that is backfilled with an engineered media and vegetated. Storm water runoff ponds over these areas and percolates through the vegetation and media to remove sediments and other pollutants. Excess storm water is able to discharge through an underdrain to another sewer system.

Pervious Pavement

Pervious pavement is a permeable pavement surface layer over one or more aggregate layers. These types of pavements are designed to temporarily store storm water and promote infiltration into the underlying soil, reducing runoff volumes. These systems have also been able to filter some solids and other pollutants from the runoff. Pervious pavement is not recommended for high traffic areas or heavily loaded traffic, or in areas with heavy industry to reduce pollution to groundwater. The primary maintenance activities for this system are to keep the pavement clear of pollutants and debris to maximize storm water conveyance.

Infiltration Basin

Infiltration basins are vegetated impoundments with high infiltration capacity. These systems are installed to capture runoff and allow it to infiltrate the underlying soil. Such systems are effective in reducing runoff volume and recharging groundwater. Infiltration basins must be constructed with proper draining soils to promote rapid infiltration through its permeable surface. Maintenance activities will require managing the sediment load to prevent clogging or reduced infiltration.

Infiltration Trench

Similar to the infiltration basin, infiltration trenches allow runoff to infiltrate into the ground and groundwater table. In addition to storage and infiltration, the trenches may direct runoff in excess of the storage capacity to other storm water management structures. Such trenches must be constructed with permeable materials (generally rocks) to prevent erosion. Infiltration trenches are smaller than infiltration basins and are useful for small areas needing storm water practices.

Sand Filter

Sand filters are constructed with a sedimentation chamber and filtration chamber to treat storm water. The first sedimentation chamber filters out large particles by settling, and the second filtration chamber removes finer particles by using a bed of sand and infiltration to a drainage system. Sand filters are typically used near areas with highly impervious surfaces and runoff carrying high particulate loads. These systems are also useful when permanent pools cannot be maintained for a wet pond. Sand filters require frequent maintenance to prevent clogging and may require additional monitoring if groundwater contamination is a concern.

Underground Storage Facility

An underground storage facility is a large subsurface reservoir, usually constructed under impervious areas, athletic fields, or other open space, to manage storm water runoff. Underground storage systems may include infiltration, extended detention, and may be backfilled or use chambers to maximize storage available. Maintenance practices must be regimented with underground storage facilities because they are not easily accessible or inspected like aboveground systems. Additional treatment practices are highly recommended to reduce maintenance requirements and improve the efficacy of the storage system.

When designing storm water control structures, a combination of runoff reduction and structural practices are the most effective in controlling runoff volumes and removing pollution. If you are designing new storm water runoff controls, or are looking for alternate solutions for your facility, PSARA Technologies is available to assist your vision and implementation. “If you’re interested in learning more about storm water management and best management practices, please reach out to us at (513) 791-4418 for comprehensive support and guidance.

Source: Ohio’s Standards for Stormwater Management, Land Development and Urban Stream Protection, https://epa.ohio.gov/divisions-and-offices/surface-water/guides-manuals/rainwater-and-land-development