"wet" stormwater pond at New Bristow Village in Prince William County
When it rains in Virginia's urban areas, the raindrops often hit impervious surfaces such as roofs and roads, patios and parking lots. That water can not seep into the ground, unlike raindrops that fall on lawns and gardens and can soak into the ground. Stormwater that races off the impervious surfaces will flow downhill, and eventually drain into a creek or lake.
In densely urbanized central cities in Virginia, stormwater drains into a dedicate set of underground pipes - the municipal separate storm sewer system (MS4) network - before reaching the creeks or rivers. In three Virginia cities (Alexandria, Lynchburg, and Richmond), however, the stormwater pipes are connected with the sanitary sewer pipes.1
Normally all the sewage and stormwater flows to the wastewater treatment plant, and is treated as required by the Clean Water Act before being released. However, a surge of stormwater from an August thunderstorm can flood into the combined pipes, exceeding the capacity and forcing the water/sewage mixture through outlets that drain directly into creeks and rivers without any wastewater treatment.
In the suburbs, stormwater typically runs into specially-built ponds that delay the speed at which the runoff drains into the creeks. That delay reduces the speed of the runoff, and slowing down artificially-accelerated runoff with artificial ponds reduces erosion. Excessive sediment is one of the three major pollutants that have damaged the Chesapeake Bay, along with excessive nitrogen and excessive phosphorous.
Erosion and sediment control ordinances now require developers to reduce the impact of impermeable surfaces by building flood control structures or stormwater management ponds. When the water rushes off the parking lot, it fills the pond rather than races into the stream (and water in a pond rises rapidly during a storm). The outflow from the pond is a relatively small pipe which limits the amount of water flowing downstream, so water level in a stream does not rise rapidly if the pond performs its intended function. The speed at which the pond drains should mimic the speed at which the now-paved area naturally drained excess rainwater. This reduces the "flashiness" of the stream, so water levels rise slowly downstream of a stormwater management pond.
"Dry" stormwater management ponds are empty between storms, while "wet" ponds always keep some water in them. Ponds are fenced to keep children from drowning when . That reduces the chance that children playing on the water's edge could get trapped during a storm, when the water level in the pond rises rapidly. However, such ponds are "attractive nuisances" that insurance agents view as hazards, instead of being an attractive water accent to a neighborhood already impacted by ugly parking lots. New "rain garden" alternatives to the stormwater management pond are now coming into favor.
One possible alternative is to reduce runoff by growing grass right on the roof of large buildings. An aerial pasture can retard some of the runoff, since the grass and soil would retain the rainfall. The additional costs for the roof might be offset in part by reduced costs of air conditioning, since evaporating moisture in the soil would cool the roof.
Replacing paved drainage pathways with grassy swales would also reduce the spead at which water reached a stream. The swales could even develop into wetlands in some areas, mitigating the impacts of development on wildlife values. Since parking lot and roof runoff is generally warmer than natural drainage, a rain garden also can reduce thermal stress to fish and other stream life. Runoff from a hot, black asphalt parking lot can be warm enough to kill trout, as occurred on Four Mile Run in Arlington.2