Technical Bulletin No 3: Implications of the Impervious Cover Model

Recent research has largely confirmed or reinforced the strong relationship between watershed impervious cover and the decline of a suite of stream indicators. The key challenge for local watershed managers is how to handle the numerous planning implications of these relationships.

Even low levels of new land development in a subwatershed will degrade streams and receiving waters to some degree. (Credit: Chesapeake Bay Program)
Even low levels of new land development in a subwatershed will degrade streams and receiving waters to some degree. (Photo credit: Chesapeake Bay Program)

The Impervious Cover Model (ICM) was first proposed in 1994 as a management tool to diagnose the severity of future stream problems in urban subwatersheds. The basic model has been recently modified to reflect more recent research on the relationship between subwatershed impervious cover (IC) and various indicators of stream quality. As might be expected, the ICM has engendered much debate and confusion among planners, engineers and regulators. Most communities continue to struggle with how to influence the location and intensity of subwatershed IC and/or apply techniques to mitigate its impact.

This working paper begins by reviewing the strengths and weaknesses of the broad range of watershed management tools that communities have used to respond to the ICM. The next section outlines new ideas for using the ICM as an urban stream classification system to set realistic and achievable objectives for stream protection or restoration. The third section applies the proposed urban stream classification system to develop integrated subwatershed management strategies for four classes of urban streams — high quality streams, impacted streams, non-supporting streams and urban drainage. These strategies are customized to promote the most effective combination of planning, engineering, economic and regulatory tools within each subwatershed class.The paper concludes with a proposed sub-watershed based permitting approach to provide accountability that a community is providing the maximum degree of stream protection or restoration, given its current inventory of streams.

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