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How can increased stormwater runoff caused by expanding construction projects be controlled and managed?
As development throughout the world booms, so does the potential for a variety of environmental problems and challenges. One such challenge facing engineers, developers, and regulators is how to deal with the increase in stormwater runoff on construction sites.
Leading the charge to promote environmentally-sound stormwater management practices, the United States Environmental Protection Agency and other regulatory agencies began to study and promote effective stormwater management practices in the 1970s. This study and independent research by product manufacturers have resulted in the development of new products that can efficiently handle runoff created by small and large construction projects, better protecting the future of the environment.
Land use changes, usually those directed toward urbanization, have the potential to increase runoff from any given site. As the percentage of impervious unvegetated surface increases, so does the total runoff volume and the peak rate of flow.
When the peak rate of runoff increases, greater volumes of water are directed "downstream" in a shorter period of time. Often, the increases in peak flow are capable of overloading storm sewer systems, causing accelerated erosion within roadside ditches and streams, and contributing to downstream flooding problems.
The increased volume of runoff associated with development can also negatively affect groundwater elevations. Since each foot of rainfall occupies three to four vertical feet in the soil, it is easy to see why after time, groundwater supplies in and around some urban areas are dwindling. This occurs in urbanizing area with impervious surfaces because the natural tendency of water is to run off to surface water bodies and streams, rather than filtrating the soil and percolating to the groundwater table. Less water percolating through the soil also means that there is less dilution of contaminants that may travel with groundwater at higher than predevelopment concentrations. The harmful effects of post development runoff rates can be mitigated through properly engineered stormwater management.
The rate at which runoff leaves any given site can be determined and managed through the use of controlled-flow outlets. However, if the rate of runoff is slowed, storage space must be provided to temporarily hold or detain runoff that would have otherwise contributed to increases in peak volumes at offsite destinations. Customarily, this storage has been provided by large diameter pipe, concrete vaults, drywell, grass swales and surface basins. Recently, with the exorbitant commercial and industrial land prices in many areas, subsurface storage has been gaining popularity with designers. Most subsurface systems allow the land above the storage areas to be utilized for parking, recreation or green space. In the late 1980s, plastic chambers, due to their light weight and relatively low cost, were added to the designer's arsenal. In addition to saving valuable land for other uses, subsurface systems eliminate the safety concerns associated with traditional methods of stormwater control. Without the "attractive nuisance" factor there is no need for fencing.
Increased volume and velocity of uncontrolled stormwater can also transport pollutants, such as sediment, fertilizers, pesticides, hydrocarbons and other organic compounds and metals into water bodies or streams. Although this may not seem significant, increased sediment and pollutant loads can adversely effect fish habitat and other aquatic life. The physical forces of increased flow may also be sufficient to disrupt aquatic habitat that is already established.
A stormwater management plan should ideally be carried out within a given drainage area or watershed. The land uses and land condition within a given watershed will often manifest themselves in the condition of downstream channels, wetlands and water bodies. In the mid-to-upper reaches of most watersheds, it is usually desirable to detain water to ensure that post-development runoff rates do not exceed predevelopment rates. In lower reaches of watersheds, faster runoff rates should be encouraged to prevent the peak flows from upper and lower areas within the watershed from coinciding. Only a full hydrologic analysis of the watershed and downstream areas can tell the story.
In the past, stormwater runoff was viewed as a non-point source of pollution. Today, stormwater runoff is thought of as a point of source pollution. This has prompted the United States Environmental Protection Agency (EPA) to undertake a stormwater quality improvement program under the National Pollutant Discharge Elimination Systems (NPDES) that is authorized under Sections 402 and 405 of the 1972 Federal Water Pollution Control Act and the 1987 Clean Water Act. Authority for the NPDES program is delegated to states.
In each state, municipal planning and zoning authorities, conservation and wetland commissions, the various agencies of the State Department of Environmental Protection, and Conservation Districts assist in overseeing the regulations and the permitting processes that address stormwater runoff quality area by area. In addition, Soil and Water Conservation Districts, as well as Water Management Districts, may be involved with stormwater management regulations aimed at sustaining acceptable levels of water quality and encouraging groundwater recharge.
The best solutions for the future will result from the development of regional stormwater management planning and implementation strategies. New stormwater runoff management measures that are implemented need to focus on regional water quality and quantity, erosion control and the issue of downstream flooding. Local, state, national and international environmental agencies and the development community, working together to develop, implement and monitor the progress of stormwater management, will ultimately uncover the most environmentally-sound solutions for the 21st century and beyond.