 Impervious Surface Coverage
As a rule, the greater the amount of impervious surface coverage in a watershed, the greater the threat to water resources from non-point source pollution.1 Impervious surfaces are those surfaces through which water cannot drain. Driveways, roads, sidewalks, and rooftops are all examples of impervious surfaces. These surfaces collect pollutants that are emitted into the atmosphere, leaked from vehicles, or from other sources such as runoff from lawns. During storms, accumulated pollutants wash off and are delivered to rivers, streams, lakes, and ponds.
Even relatively little impervious surface cover in a watershed can impact streams. Experts calculate that stream degradation occurs at levels of only 10 percent to 20 percent imperviousness. At present, roughly 20 percent of Gloucester County has an impervious level above 10 percent, according to the NJDEP’s 1995/97 landuse/landcover analysis.
In parts of Gloucester County that were developed before the 1970s, stormwater was shunted directly to a nearby river or stream through a system of storm drains, located within the streets or curbing. These storm drains are connected to underground pipes that empty through outfalls into a waterway. Beginning in the 1970's, regulations required the construction of detention or retention basins in new subdivisions, which hold the runoff for a time and release it gradually to the waterway in order to prevent downstream flooding. This is still the method of managing stormwater from new development in New Jersey.
Neither direct discharge nor detention/retention basins control what gets into the runoff. Consequently, pollutants that wash off impervious surfaces or get dumped into a storm drain will end up in the stream or lake water.
Imperviousness also accelerates the speed at which water runs off, increasing erosion and streambank degradation. Where a detention/retention basin controls the speed, the quantity of runoff that hits a stream may actually be increased, because the basin concentrates water that would otherwise percolate down into the soil. As the runoff arrives at the stream after a rainfall, the force of the increased quantity of water can cause extensive erosion of the banks. This higher volume of water also temporarily alters the level of the stream, which causes major problems for the aquatic organisms that must adjust to this variability and find ways to prevent being swept away downstream. Many outfall pipes that empty into local streams have caused serious erosion that is expensive and difficult to repair.
Over time the force and volume of stormwater has downcut stream channels and made them deeper, so that water levels become lower than the floodplains adjoining them (the floodway and floodfringe areas). During a storm, excess water ordinarily washes across floodplains, which hold and even absorb some of the flow. When the channel and the floodway are no longer connected by a gradual slope, the floodplain cannot serve this function. This is also the case in areas in which buildings have been placed within the floodway or otherwise built too close to the edge of a stream or lake.
- Scheuler, Tom. Site Planning for Urban Stream Protection. Silver Spring, MD: The Center for Watershed Protection, 1995.
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