Under the Stormwater Management Program, new development that requires the management of stormwater runoff must use Environmental Site Design (ESD) or construct a Best Management Practice (BMP). These structures include infiltration, flow attenuation, retention, extended detention, detention and underground storage.
Infiltration allows the water to seep into the ground and recharge the groundwater. The infiltration rate, or how quickly the water enters the ground, depends on the type of soil. Sandy soils infiltrate more quickly than gravelly soils. Infiltration can be used as a BMP within a trench or a basin. Not all development can use infiltration as a BMP. The practicability of infiltration depends on groundwater elevations, the depth to bedrock and the infiltration rate.
Flow attenuation delays the flow time of stormwater runoff, which reduces the peak discharge leaving the site. This BMP is typically used when new development causes only small increases in stormwater runoff. This BMP is provided by grading flat swales that prolong the flow path. This process is rarely used because very limited room in new development allows for swells to be designed and constructed.
Retention facilities or wet ponds permanently pond water. Stormwater is managed by providing storage of stormwater runoff above the permanent pool elevation. Wet ponds also provide water quality benefits by allowing pollutants to precipitate from the water before discharging in to receiving streams. Wet pond are not permitted in Class III watersheds because temperatures of the water stored in the pond are increased. These increased temperatures can adversely impact the cool waters required for trout. Wet ponds can be an amenity to a development because of their aesthetically pleasing appearance and the attraction of aquatic creatures.
Extended detention facilities store the first flush (normally the first 1/2" of runoff) of stormwater runoff for 24 to 72 hours. After 72 hours, the bottom of the facility will be dry until the next storm event. Detention of stormwater runoff provides water quality benefits by allowing some pollutants to precipitate from the water before being discharged to receiving streams. This type of facility is the most commonly used BMP in Harford County. New regulations will allow planting within these facilities to create shallow marshes that are more aesthetically pleasing.
Detention facilities are dry ponds which become completely dry with in 24 hours of a storm event. These facilities provide the least amount of water quality benefits. The use of detention facilities as a BMP is discouraged.
Projects may have BMPs completely underground. These underground facilities use the practice of infiltration, extended detention or detention. This BMP is typically used in commercial developments, where tight site constraints do not allow areas to be designed strictly for stormwater management.
Environmental Site Design
Environmental Site Design (ESD) relies on integrating site design, natural hydrology, small scale stormwater management practices and non-structural techniques to mimic natural stormwater runoff characteristics.