Combined Sewer System (CSS). In some older sections of the District, just one pipe handles both runoff and sewage. In dry weather, this pipe carries sewage to the Blue Plains Wastewater Treatment Plant. In wet weather – such as a gentle spring shower – rainwater runs into storm drains into the same pipe. This pipe – the combined sewer – carries both runoff and sewage to Blue Plains for treatment.
During heavy rains – imagine a typical summer thunderstorm with a heavy downpour – there is more water than can fit into the pipe. A toxic mix of raw sewage and polluted runoff overflows directly into Rock Creek, the Anacostia or the Potomac at specific locations. These "combined sewer overflows" or CSOs occur whenever it rains more than an inch or there is a short but intense storm.
Green Roof. A light-weight, low-maintenance vegetated roof system that can capture rain that falls on it, while also helping to reduce building cooling costs.
Separated Sewer System (MS4). Today, all developers and builders must separate runoff and sewage into two different systems of pipes. While raw sewage is no longer mixed with runoff, the runoff is still polluted by trash, fertilizers, chemicals, pet waste, etc. Under federal rules, all municipalities must significantly reduce the quantity of pollutants carried by stormwater into local waterways.
Permeable. Having an absorbent, porous surface that allows water to percolate into the soil, thus filtering out pollutants and recharging the water table. Examples include mulch, gravel, pervious concrete, or pervious/permeable pavers.
Permeable/pervious pavers. Specially fabricated paving units designed to replace asphalt and other impermeable paving materials. Interconnected pore spaces within the material channel water into the underlying soil or into a special storage layer which forces slow percolation during periods of heavy rainfall. Permeable pavers are often laid on a bed of sand or gravel to enhance drainage properties.
Pervious. See permeable.
Impervious. Having a solid surface that doesn’t allow water to penetrate, forcing it to run off. Examples are asphalt, concrete and brick.
Runoff. Rain and snowmelt that doesn’t soak into the ground, but instead runs off into the nearest creek, river, or other water body.
Sewershed. A land area in which the sewer pipes drain to a single point.
Stormwater. Rainfall that does not soak into the ground, but instead flows over the land’s hard surface into the nearest water body through storm drain channels.
Streetscaping. Management of runoff along city streets and sidewalks using trees, vegetated swales, curb bump-outs, or other measures to slow the flow of runoff.
Watershed. The land area that drains into a particular body of water, such as a creek or river. What happens in the watershed can affect that body of water.