Stormwater Management

A basic principle of sustainable design is that buildings should be able to supply all their water needs from the annual rainfall on the project site and from recycling the wastewater generated by a project, effectively getting several uses out of the same amount of rainwater. At the present time, it's not always possible for projects to use all of the rainfall on a site, so they need to reduce the impact of new development on downstream flows from stormwater running off a site.

The problems with pollution of lakes, rivers and the ocean from urban runoff were mentioned previously. In addition, new development can tax existing stormwater collection systems with increased runoff and higher peak flows from paved surfaces. On a large scale, urban development has led to greater flooding, both from higher runoff and from more people locating in flood plains.

As a consequence, the LEED system deals explicitly with stormwater management by rewarding projects that reduce both the rate and quantity of stormwater from a site and those projects that improve the quality of runoff generated from site runoff. LEED rewards projects on previously undeveloped sites that keep runoff from the most frequent storms (those that occur, on average, every two years or more frequently) to pre-development conditions, by instituting various measures such as detention or retention ponds, onsite infiltration (using permeable paving), bioswales (vegetated runoff ditches), green roofs and using native and adapted vegetation instead of turf.

On sites that are already more than 50% impervious (for building

Bioswales can be integrated with other architectural design features to take rainwater from buildings, either to remove from the site or to recycle for building water uses, as in this project at Portland State University's Epler Hall, a LEED-NC Silver-certified building, designed by Mithun architects.

Bioswales can be integrated with other architectural design features to take rainwater from buildings, either to remove from the site or to recycle for building water uses, as in this project at Portland State University's Epler Hall, a LEED-NC Silver-certified building, designed by Mithun architects.

additions or renovations in most developed urban areas), LEED rewards those that reduce stormwater runoff by 25% or more from prevailing conditions. To this end, green roofs are a great aid, especially on limited urban sites that usually don't have extensive areas for new plantings.

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