11.5 Sustainable stormwater management  (Page 5/26)

In assessing flood risks, it is important to realize that managing the volume and rate of urban stormwater being discharged from developed areas does not affect the total amount of stormwater that is being discharged to a river or stream within a watershed – they only affect the timing of when a storm's precipitation will be discharged to the waterway ( NRC, 2008 ). Both the conventional and the newer, more sustainable, ways of managing stormwater discussed below seek to delay the time it takes for stormwater runoff to reach a waterway in order to reduce the water levels and flow velocities of the receiving streams after a storm. Slowing the rate by which stormwater is being contributed to a stream spreads out the peak of the resultant flood levels over a longer time period, allowing many flood risks to be substantially reduced.

Conventional stormwater management

Urban stormwater is traditionally managed by the construction of engineered stormwater facilities, such as storm sewers and detention basins, as part of the land development process. These engineering processes are specifically designed to modify the natural hydrology    of a site. For example, when land is being developed, the parcel is usually graded for development and stormwater infrastructure is installed to channel the stormwater from individual lots into a separate stormwater sewer system connected to a detention basin where it is retained until it can be discharged off-site. Site preparation also includes elevating building sites so that they are constructed on slightly elevated "pads" to encourage stormwater to flow away from building foundations and toward the streets. After reaching the street, stormwater is then directed to the stormwater sewers by curbs and gutters.

Conventional stormwater detention facilities were historically built to reduce off-site flood risks, and were not expressly designed to reduce off-site water pollution risks. Any stormwater detention that was provided was only temporary, often providing an insufficient retention time to allow the natural attenuation of any pollutants that were carried by the runoff into the detention basin – unlike the natural attenuation processes occurring in a river or riparian wetland (where ambient pollution levels are gradually reduced through dilution, oxidation, chemically binding to rocks and soils, being gobbled up by microorganisms, etc.). Stormwater is usually detained on-site after a storm only for a period of hours or, at most, days and then released to a waterway. Some of the particulate contaminants in the stored runoff might settle out if they are large or heavy enough to do so during that short time, some might infiltrate into the soils in the bottom of the detention basin, and some pollutants might be taken up by grass lining the basin, but many pollutants still end up being carried into the waterway along with the released stormwater.

Since the 1990s, environmental protection agencies have begun to consider the water pollution impacts of releases from stormwater detention facilities, after the Clean Water Act was amended to require states to treat stormwater discharges from detention basins as a type of direct source and to require that NPDES permits be phased in for discharges from Municipal Separate Stormwater Sewer Systems ("MS4") in cities and urban areas above certain population thresholds ( NRC, 2008 ). The NPDES permits issued under the U.S. Environmental Protection Agency's (U.S. EPA) MS4 program now require the water pollution loads from stormwater detention basin discharges to be assessed through the creation and adoption of local stormwater management plans and that the contaminants carried by the stormwater runoff to the basins for later re-release to a waterway be better managed and reduced through the adoption of local BMPs. MS4 permit regulations issued by state environmental protection agencies usually involve the issuance of a "general permit" by the agency, applying to all applicable Municipal Separate Stormwater Sewer Systems located within the state’s designated urban areas.