Why Stormwater Management?

Land development is a vital part of the economic and social well-being of the community, and there should be no unnecessary impediment to the reasoned expansion of the urban area. That is not to say that land development is not an unmixed blessing. Problems arise in its wake, and some are severe and might extend well beyond the boundaries of the host community. Although these problems are unavoidable, the land development industry has a moral obligation to minimize its impacts on its neighbors and the community to the greatest extent possible. A problem that is most readily identified with land development is the resulting changes in rainfall runoff from the postconstruction site. These changes are caused, in part, by the necessary conversion of pervious surfaces to impervious ones. Inexcusably, in regions akin to northeastern Ohio, the offsite stormwater changes are not merely the result of the nature of the development but more the result of how runoff is dealt with in the design phase of the project. In those regions, the design community is ardently reluctant to fire up the practice that its proponents refer to as efficient drainage design. Efficient drainage design makes no pretense of managing stormwater or mitigating adverse impacts; it is strictly a disposal system. It has only one objective: the prevention of even the most temporary ponding within the finished development. The practice attempts to capture the entire rainfall as it arrives and to rapidly transfer it to storm sewers. The pipes, designed to be as efficient and frictionless as possible, speed the concentrated volume of water to a downstream point on the boundary of the site. Once upon a time, the stormwater was then simply released to go its merry way according to the laws of gravity and its own nature. Fairly recently, however, within the last quarter-century or so, the downstream flooding that the method causes could no longer be plausibly denied by the design community, and detention, intended merely to reduce the postconstruction peak flow, was added to the practice. As a disposal system, the practice, as its name implies, is very efficient; that’s the good news. The bad news is that, rather than minimizing the drainage problems inherent to land development, it magnifies them. The potential for downstream flooding and flood damage increases. The potential for water pollution and declining water quality increases. Finally, because the system relies on totally artificial components, the cost to the owner or developer–and, ultimately, to the community–is maximized. In spite of its documented history of failure, some segments of the design community stubbornly insist on the continued use of efficient drainage design. They do so for several reasons. For example, the method is simple to design and implement, requiring little imagination, creativity, or actual engineering. In addition, local drainage regulations are based on the practice and are difficult to update to current concepts and state-of-the-art technology. Finally, because the practice is based on engineering standards, it provides the design community with a refuge from responsibility and liability. Across most of the country, however, efficient drainage design has long been abandoned as the method of choice in all but the most isolated instances. For some time, the consensus of popular as well as scientific thinking has held that water is a valuable resource to be managed rather than a nuisance to be disposed of. Decades ago, in areas with a strong interest in water quality, such as the Pacific Northwest and the Chesapeake Bay area, forward-thinking engineers pioneered–or actually reinvented the ancient art of–stormwater management. Stormwater management is not an enhanced version of efficient drainage design. It takes a completely opposite approach to the issue of rainfall runoff. Rather than one, stormwater management has three concurrent and equally important objectives: the protection of water quality, the preservation of the hydrologic cycle (i.e., water quantity in all its forms), and finally flood prevention both within and downstream of the site of development. In place of the traditional engineering standards that tell the designer what to do, stormwater management follows performance standards that tell the designer only what the objectives are. The designer, relying on expertise and experience, determines the best means of achieving the objectives. Performance standards allow for the greatest flexibility in design, but they also assign responsibility. The designer’s work is judged by how well it performs, not how well it conforms. Where efficient drainage design produces a totally artificial system, stormwater management utilizes natural processes to the greatest extent possible. It does not attempt to remove all rainwater from the site but seeks to retain as much as possible on-site. It does not attempt to concentrate flows but to disperse them. Rather than accelerating flow velocity as much as possible, it strives to slow the flow. Instead of forcing all runoff onto impervious surfaces and into pipes, it minimizes impervious surfaces and extends the time surface water has to interact with the soils. Rather than prohibiting the temporary ponding of water on the site, it encourages judicious ponding for hydraulic, hydrologic, and environmental reasons. Stormwater management uses a mix of artificial and natural filtration practices, such as grassed swales, bioretention cells, media filters, sand filters, filter strips, and constructed wetlands, to reduce pollutants and protect water quality–its first objective. Stormwater management also uses a mix of artificial and natural infiltration practices, such as infiltration basins, infiltration strips, subsurface infiltration systems, and prolonged contact between water and soil, to protect water quantity within the hydrologic cycle–its second objective.The combination of filtration and infiltration practices reduces both postconstruction peak flows and surface runoff quantity, thereby serving as a mechanism for flood prevention–the approach’s third objective. A fringe benefit is that by reducing peak flows and runoff quantities, the method also reduces the size and extent of storm sewer systems and flood control basins, which, in turn, reduces development costs. When structural stormwater management practices are used in conjunction with such nonstructural, administrative practices as riparian setbacks, open space, green or conservation development, reduced impervious surfaces, and elimination of directly connected impervious surfaces, the overall effectiveness increases significantly. Why stormwater management? The more relevant question is, why not?