What is a BMP (Best Management Practice)?

A BMP is a structural or non-structural stormwater practice that minimizes the impacts of land use changes on surface or groundwater systems. Structural BMPs are basins or facilities such as wet ponds, dry ponds, infiltration basins and bioretention areas designed to reduce pollutant load in stormwater runoff. Non-structural BMPs are land use or development practices such as preservation of open space, vegetative buffers and filter strips which minimize the impact on receiving stream systems. BMPs can also serve a dual purpose by providing for runoff (quantity) control and stream channel protection.

Are There Different Types of Structural BMPs?

Structural BMPs are categorized into the following groups: ponds, wetland systems, infiltration systems, filtering systems, open channel systems and manufactured BMP systems. Although the groups are wide-ranging, it is easy to tell the difference between them.

Wet and dry ponds are the most common structural BMP types found in residential neighborhoods. Wet ponds hold a permanent pool of water and dry ponds are mainly dry, except during and after storm events. All ponds usually have an earthen dam, or similar structure such as timber crib or concrete wall, which impound water. A wet or dry pond almost always has a principal flow control structure such as a riser (standpipe), a discharge pipe or culvert and an emergency overflow spillway.

Wetland systems are similar to wet ponds, except they have distinct high and low marsh areas with diverse wetland plant communities.

Infiltration systems consist of infiltration basins or infiltration trenches. Infiltration basins are larger facilities which sometimes resemble dry ponds; however, water which temporarily stores in the pond infiltrates through the bottom of the basin into natural (permeable) soils beneath the facility. Infiltration trenches are usually underground stone-filled chambers which allow runoff to infiltrate back into the ground.

Filtering systems mainly consist of bioretention basins, which are highly specialized BMPs, that have underdrain, planting soil and mulch layers and a wide variety of native trees, shrubs and ground cover.

Open channel systems are linear BMPs usually found along roadways or in yard areas around buildings. Open channel BMPs consist of wet or dry swales. These systems are very similar to most stormwater conveyance channels; however, a wet swale has timber or rock check dams in the bottom of the channel and a dry swale has an underdrain and a permeable soil layer beneath the channel.

Manufactured BMP systems are specialty devices used solely for water quality purposes in intensely developed urban or redevelopment areas where surface BMPs are not feasible. Manufactured BMPs usually are small size chamber or filtering units that are located in underground manholes or vaults on storm drain pipe systems. Manufactured BMPs are sometimes used as pretreatment devices in combination with other BMP types.