Structural Stormwater Controls

Structural stormwater management (SWM) controls are engineered BMPs that receive stormwater runoff from developed areas. By using a variety of mechanisms, SWM controls reduce pollutants and slow runoff velocities to minimize impacts when discharged to local waterways. SHA is locating roadways that currently do not have these controls and is implementing new BMPs to maximize water quality of roadway runoff.
 
The SWM Act of 2007 that was fully implemented in 2010 relies on the concept of environmental site design (ESD) to the maximum extent practicable (MEP). ESD emphasizes small-scale SWM practices, non-structural techniques, and better site planning to mimic natural hydrologic runoff characteristics and minimize the impact of land development on water resources.

Grass Swale

Grass swales are grass-lined channels that convey stormwater runoff provide water quality treatment and decrease and slow flow. They help remove pollutants removal through vegetative filtering, sedimentation, biological uptake, and infiltration into the underlying soil. The linear nature of grass swales make them suited for placement along roadway median and side channels.
 
Grass Swale
Grass Swale at I-70 median

Bio-swales

Bio-swales are linear constructed filters that carry stormwater runoff and provide water quality treatment and decrease flow. They collect runoff from roadways and allow water to slowly filter through layers of soil, sand, and stone and discharge through an under-drain that is usually connected to a storm drain inlet. Pollutants are removed by filtering runoff through a 2 to 4 ft. depth engineered soil media, designed to maximize pollutant removal. Check dams are often used to enhance surface water storage which promotes better filtering. The linear nature of bio-swales makes them suited for placement along roadway medians and side channels.
 
Bio-swale Section
Bio-swale Section

Bio-swale Construction along MD 119 in Montgomery County Bio-swale Construction along MD 119 in Montgomery County
Bio-swale Construction along MD 119 in Montgomery County
 

Wet Pond

Wet ponds are large-scale practices that have a permanent pool, or a combination of an extended detention or shallow wetland with a permanent pool. A wet pond uses both a forebay and permanent pool to treat stormwater runoff. Stormwater runoff is filtered and treated through settling and nutrient uptake by plants and other aquatic organisms.
 
Wet Pond at MD 147 and I-695 in Baltimore County
Wet Pond at MD 147 and I-695 in Baltimore County


Wet Pond along MD 291 in Talbot County
Wet Pond along MD 291 in Talbot County

Infiltration Trench

Infiltration trenches are excavated trenches filled with stone that capture and temporarily store stormwater runoff. Runoff captured by the trench percolates into the surrounding soil where pollutants are removed by means of natural filtration before recharging the groundwater. The linear nature of infiltration trenches make them suited for placement along roadside channels. 
 
Infiltration Trench
Infiltration Trench

Submerged Gravel Wetland

Submerged gravel wetlands are small-scale filters using wetland plants in a rock media to provide water quality treatment. Pollutant removal is obtained through biological uptake from algae and bacteria growing within the filter media. Wetland plants also provide additional biological uptake. Submerged gravel wetlands are best suited in areas where a high water table or poorly drained soils are present.

Wet Swale

Wet swales are channels with permanent pools that provide conveyance, water quality treatment, and flow attenuation of stormwater runoff. They collect runoff from roadways and enable the water to pond within the swale. Check dams are often used to enhance surface water storage. Pollutants are removed through vegetative filtering, sedimentation, and biological uptake. Wet swales are installed in areas with a high groundwater table or poorly drained soils. The linear nature of wet swales makes them best suited for placement along roadway median and side channels.
 
Wet Swale along US 113 in Worcester County
Wet Swale along US 113 in Worcester County

Sand Filter

Sand filters are practices that capture and temporarily store runoff before passing it through a filter bed of sand. Sand filters are typically designed with a forebay and a filter bed filled with soil, sand and stone. As stormwater flows into the forebay, large particles settle and then finer particles and other pollutants are removed as stormwater flows through the filtering media. Pollutants are removed through filtering, sedimentation, biological uptake, and infiltration into the underlying soil media. Filtered runoff may be collected and returned to the conveyance system or allowed to partially infiltrate into the soil.
 
Sand Filter
Sand Filter

Bioretention

Bioretention facilities capture and temporarily store stormwater runoff before passing it through a filter bed mixture of soil, organic matter, sand and stone. Pollutants are removed through filtering, sedimentation, biological uptake, and infiltration into the underlying soil media. Filtered stormwater is either returned to the conveyance system or partially infiltrated into the soil. Bioretention facilities provide water quality treatment, aesthetic value, and can be integrated into landscaped areas.
 
Newly Constructed Bioretention Facility receiving Stormwater Runoff
Newly Constructed Bioretention Facility receiving Stormwater Runoff

Stormwater Wetland

Stormwater wetlands are large-scale practices that create shallow wetland areas to treat stormwater and often incorporate small permanent pools and/or extended detention storage. Stormwater wetlands are structural practices similar to wet ponds that incorporate wetland plantings into the design. As stormwater runoff flows through the wetland, pollutant removal is obtained through sedimentation and biological uptake within the practice. An added benefit of stormwater wetlands is the aesthetic and habitat value.
 
Stormwater Wetland along US 50 in Anne Arundel County
Stormwater Wetland along US 50 in Anne Arundel County

Stormwater Control Retrofit

Stormwater control retrofits involve modifying, enhancing, or replacing existing structural SWM facilities to improve water quality treatment of stormwater runoff. Older SWM facilities built in the 1980s and 1990s typically do not remove as much pollution as newer facilities designed to modern water quality design standards. Existing dry detention ponds are a primary target for retrofit since they remove only a small amount of pollution and normally can be converted to another type of facility such as a wet pond, wetland, or infiltration basin where pollutant removal is greatly increased.
 

Stormwater Retrofit MD 4 and Plummer Road from a Dry Pond to a Shallow Wetland.

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