Density and Land Use

Sept. 1, 2009

As many land use planners and stormwater managers have now acknowledged, high-density development isn’t the evil specter it was once considered in terms of stormwater and impervious surface.

True, high-density developments bring a great deal of impervious surface to a watershed. The tradeoff is that by putting housing and commercial space into fewer but denser footprints, we can reduce the total amount of impervious cover.

Lower-density developments require more roads, longer driveways, and more sidewalks–all contributing to greater imperviousness. Ultimately the collective footprints of the buildings themselves will be larger as well, since most will be one- or two-story dwellings, where a high-rise condominium or office building can have dozens of floors. For a detailed comparison of the differences, see Randel Lemoine’s article in our October 2007 issue, “An Evaluation of the ReducedEnvironmental Impact From High-Density Development“, comparing the overall impervious surface of residential developments with 38 residences per acre to those with only five per acre.

There’s a catch, though: For the high-density approach to confer a stormwater-related advantage–or any other advantage, for that matter–the land that would have been used for sprawled-out development must be preserved as open space. If too many high-density developments are approved and built, we’ve only compounded the problem of impervious surface, with no benefit.

This allocation of land use to maintain the balance between high-density development and open space is beyond the scope and authority of the stormwater department. The best-intentioned developers can’t do it either working on a site-by-site basis. Achieving that balance requires zoning changes and coordination among many different departments and

It’s exactly this balance that the practice of Smart Growth aims to achieve–and it takes into account much more than stormwater. It considers, among other things, where potential new residential developments lie in relation to public transportation and how walkable new neighborhoods are.

Kaid Benfield, the director of the Natural Resources Defense Council’s Smart Growth Program, recently posted a slide show online showing some excellent examples of neighborhoods that have successfully incorporated Smart Growth, from Seattle to the South Bronx. He also cites a current debate in Virginia, which has been criticized for proposing stormwater offsets that seem to encourage low-density development, without regard to the overall effect on water quality. Even though credit-earning low-impact development measures like pervious pavement, green roofs, or rain gardens can reduce runoff and pollutants site by site, the credits seem to encourage developers to take the less-expensive route of incorporating these measures in low-density rural developments on previously open land.

How well coordinated is the planning processes in your city, county, and state? Has Smart Growth taken hold? Share your experiences here.

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