The replacement of natural soils with impervious surfaces such as roads, parking lots, and rooftops, as well as, the modification of the topographic characteristics of a site, are common changes associated with urban development. Such changes in the land alter the hydrologic characteristics and the natural processes by which rainwater is purified and distributed back into the watershed.
Under natural conditions, rainwater is dissipated through the processes of transpiration, evaporation and percolation. Through transpiration, large quantities of water are intercepted by plant foliage and are then evaporated back into the atmosphere. The remaining water that reaches the surface of the ground infiltrates into the soils and through the process of percolation continues to travel underground until it slowly makes its way into the streams and aquifers. As water seeps into the ground the process of percolation also allows for the removal of pollutants present in stormwater. The ability of water to seep into the ground, as well as, the amount of water that can be retained depends on soil properties such as porosity and permeability. A high porosity soil can hold large amounts of water and usually allows for rapid infiltration. When precipitation reaches the soil surface faster than it can be infiltrated into the ground, water collects at the surface and travels downhill.
The reduction in vegetation and pervious areas minimizes the amount of rain water that percolates into the ground. Such conditions result in increased flow rates and volumes of stormwater which “run off” across land, causing erosion and flooding conditions that can be detrimental to both natural landscape and infrastructure. Moreover, runoff also picks up and carries with it pollutants found at the surface that result from daily activities, such as fertilizer, sediment, oil, etc. Polluted runoff is then received by water bodies directly via storm sewer, deteriorating stream health and natural habitats of aquatic species.
In order to address potential issues associated with stormwater, it is important to evaluate the new hydrologic characteristics of the site under post development condition and design for practices that counteract impacts on its natural process of distribution and filtration of pollutants. Impacts of stormwater after development require special attention and need to be considered from the initial planning phase of every project.
What is Stormwater Management?
Stormwater Management (SWM) is the means for mitigating adverse impacts to the hydrologic cycle resulting from urbanization, through the use of constructed or natural practices. The technical criteria for the design and implementation of such practices include water quality, erosion and flood control as required by law in most municipalities. Currently, Virginia stormwater management policies reflects consistency with the Chesapeake Bay Preservation Act (CBPA), the Virginia Pollution Discharge Elimination System Permit (VPDES), the Erosion and sediment control regulations (Minimum Standards 19, 4 VAC 50-30-40.19) as well as State minimum technical requirements for flood controls (10-yr storm). Each of these regulations requires the implementation of measures such as best management practices (BMP) to diminish the effects of land use changes.
Why is Stormwater Management necessary?
Inevitably, changes in the land result in fluctuation of the hydrologic cycle. As more land cover is replaced with impervious surfaces, less rain can be naturally\ absorbed and treated by the environment. After development, stormwater discharges can increase by more than twice the amount under natural conditions. If not controlled, large quantities of water can cause flooding in our communities and stream channel erosion. Structural damage associated with flooding and stream channel erosion is considered detrimental to value of property. In addition to increased runoff volumes, urbanization also contributes to the degradation of surface water quality due to the amount of pollutants that are washed into the storm sewer system. Untreated water can negatively affect aquatic ecosystems and, in some case, human health. Therefore, stormwater management measures and policies are necessary in order to address impacts of urbanization on water resources, minimize flood damage and significant erosion of channel bed and banks.
How is Stormwater controlled?
Stormwater is controlled through the implementation of Best Management Practices (BMP) which may include dry or wet ponds, rain gardens, trenches, previous pavement, wetlands and other systems. Some of these control measures, such as wet ponds, are developed to accommodate excess water for a period of time, allowing pollutants to settle and then release treated water into receiving streams. These facilities also release water at lower rates to reduce discharges and avoid erosion in bed and banks due to concentrated flows. Other facilities such as rain gardens, intend to mimic hydro-logic processes in which Stormwater naturally infiltrates into the ground.
The implementation of a particular stormwater management facility depends on the characteristics of the development site and shall satisfy all technical criteria established by the local ordinance. Moreover, the ultimate functional goal of the BMP is to address potential problems associated with land development such as flooding, erosion and water pollution.
George Mason University relies on various structural measures to control stormwater and minimize negative impacts associated with land development. Several ponds have been constructed onsite to serve as quality and quantity control for the stormwater on campus. The construction of onsite rain gardens and the replacement of asphalt with pervious pavement are more sustainable approaches designed to work in conjunction with existing controls to help improve water quality and reduce the amount of runoff on campus.