Stormwater comparison

The US Environmental Protection Agency has created a suggested stormwater permit framework geared to encouraging higher-density, mixed-use development and “low-impact” runoff management techniques. The EPA permit framework is voluntary, but it influences state and local standards. It is being formally considered by West Virginia and Tennessee, Lynn Richards of the EPA’s smart growth program told a gathering of new urbanists in Austin, Texas. Oregon is incorporating parts of the framework. Other states are eyeing it and may seek public comment.

If adopted at the state level, developments would be required to manage stormwater from 90 percent of storm events on site. In many mid-Atlantic states, this can amount to about one inch of rainwater. Projects would get a 10 percent reduction for each of the following five land use types and characteristics (up to a 50 percent total reduction): 1) redevelopment; 2) brownfield redevelopment; 3) at least 7 units per acre; 4) at least 18 units/acre or 2.0 floor area ratio (FAR); 5) a mixture of uses oriented toward transit.

Using the suggested standards, a comparison was done between Atlantic Station, a high-density, new urban brownfield redevelopment in Atlanta and a conventional suburban development with an equal number of residential units and retail square footage on the outskirts of that city (see table). Atlantic Station would be required to manage 3.4 million cubic feet per year of runoff on-site. The conventional development in Cobb and Fulton counties would be required to manage 23.6 million cubic feet/year — nearly seven times as much as the urban project. The new framework takes into account the inherent environmental benefits of the urban project, she says.

The standards that developers currently must meet are vague — most state general permits are written to manage stormwater “to the maximum extent possible,” Richards says. “No one really knows what that means. What many communities and/or states have done is to set specific limits.” The current standards don’t give credit for higher density and lack flexibility to encourage more environmentally friendly management techniques, Richards says.