Impervious Surfaces

The management of storm water runoff from sites after the construction phase is important in controlling the impacts on urban water quality. The increase in impervious surfaces such as rooftops, roads, parking lots, and sidewalks due to land development has a number of effects on aquatic systems. First, increases in imperviousness create a corresponding increase in the total volume of storm water runoff from a site. This increase in runoff volume leads to degradation of stream channel habitat and increases the occurrence of flooding. Loss of natural cover has also been associated with stream warming and loss of aquatic biodiversity in urban areas. Runoff from impervious areas can also contain a variety of pollutants that are detrimental to water quality, including sediment, nutrients, road salts, heavy metals, pathogenic bacteria, and petroleum hydrocarbons. Efforts to control the impacts of imperviousness and stormwater runoff are typically in the form of structural and nonstructural management practices that either detain or treat storm water to reduce pollutant levels and control the volume of runoff or prevent initial pollutant loadings.

Best Management Practices

Structural features such as wet ponds, infiltration basins, and sand filters are the traditional techniques used to treat storm water runoff from developments, and are generally incorporated into projects independent of other design considerations for the project. These structures are commonly referred to as “best management practices” (BMPs). These BMPs generally work by capturing and holding a portion of the runoff and then releasing it slowly over a sufficient period of time to promote settling out of pollutants. This results in improved water quality and reduces the “peak” flow from the site. Also categorized as structural BMPs are grassed channels/swales and filter strips that treat storm water runoff by using vegetation to filter and settle pollutants. Swales and filter strips also function to attenuate post-development peak discharge rates for small storm events when compared to concrete channels as a result of runoff velocity reduction and limited infiltration.

Detention controls, or dry detention ponds, fill up during storms but they discharge completely and are dry during the periods between storms. With proper design and maintenance, dry ponds can be used for recreation when they are dry. Retention controls are known as wet ponds since they maintain a permanent pool of water between storms. The banks and outlet of a wet pond are constructed to provide for storage and subsequent slow release of storm water runoff. Wet ponds look much like any other pond, and with careful attention to design and maintenance, they can serve as an attractive water feature in residential and commercial developments. Infiltration basins also collect storm water runoff, but rather than release the water gradually, they hold the water and allow it (and any pollutants) to infiltrate into the ground. Proper design and construction of infiltration basins are vital to achieve capture of polluted runoff in the soil path and prevent percolation into ground water.

Structural Management Practices to Reduce Runoff

Land intensive or expensive structural management practices can be avoided or reduced in size if the amount of runoff and entrained pollutants are minimized by the design of the project. Generally, it is cheaper and more effective to implement design elements that prevent or reduce the generation of storm water runoff, and/or pollutants at the source. Design aspects such as minimizing impervious surfaces, building in the least sensitive areas of the site, preserving natural streams and riparian buffers, directing runoff over vegetated areas, and providing open space all help to reduce the need for structural controls.