Stormwater runoff is commonly defined as the water from rainfall or snowmelt that flows over the land and into surface waters. In a natural environment, much of this water would be absorbed by the soil from where it would nourish plants, recharge groundwater, or feed surface waters over time. In a built environment, however, this process of infiltration is inhibited by impervious coverage, including street paving and rooftops. Urban development increases impervious coverage, and reduced infiltration results in greater volumes of runoff. Furthermore, while on its way to lakes and streams, runoff can become contaminated with sediments and chemicals that are harmful to aquatic environments and, by extension, potentially threatening to public health. Runoff can also increase the probability and intensity of flooding.
The relationship between urbanization in watersheds and the health of aquatic ecosystems is by now a well-researched subject, and the results confirm a strong link between the two. While initial studies focused on the effect of stormwater runoff on ecosystems and groundwater aquifers, more recent research seeks to find ways to minimize these impacts.
While some overlap does occur, most stormwater management techniques fall into two different categories: remediation and prevention. Remediation techniques attempt to manage stormwater runoff once it leaves the boundaries of a parcel of land. These techniques are generally characterized by large, centralized systems that attempt to manage stormwater over a wide area. Remediation is a part of the traditional "divert and drain" approach currently in use in many cities. Examples of this type of technique would include catch basins, filtration systems, infiltration ditches, and street cleaning.
Prevention techniques attempt to manage stormwater at the parcel level. These techniques address the issue of reduced infiltration due to impervious coverage. The goal of this approach is to return as much area as possible to pre-development hydrologic conditions. This is accomplished with low impact design (LID) methods such as rain gardens, green roofs, and pervious pavement. As with the goal of greater energy conservation, impervious coverage reductions over the long term can be achieved through the overall design of new urban development. Just as the principles of transit-oriented development provide guidelines that will lead to reduced vehicle miles traveled, water sensitive design has the potential to produce significant reductions in stormwater runoff.
Even while prevention techniques focus on impervious coverage on the parcel level, it is important to maintain a city-wide perspective as well. For example, one would expect denser areas within a city to have a higher percentage of impervious surface area. Rather than imposing unrealistic requirements on these areas, a city plan should recognize that higher density in one area can accommodate impervious coverage reductions in other parts of the city. Our study incorporates this principle by employing three different standards for impervious coverage for high, medium, and low density districts.
Stormwater management is now an important component in many comprehensive plans, and solutions have incorporated a variety of techniques depending on a particular community's goals. It is important for a community to define its goals, as these will ultimately inform the management techniques that are most appropriate. The City of Madison as a whole should not lose sight of the importance of its water resources. Madison is a place whose history, identity, and character are inextricably linked to its lakes. While a concept like impervious coverage may not sound glorious, what is at stake is the protection of Madison's waters, their integrity as ecosystems, and their recreational and aesthetic contributions to the City.