Miles Per Gallon for Raindrops – What is a Performance Based Standard for Managing Stormwater?


Stormwater in Washington, D.C. | Lynette Batt

Stormwater in Washington, D.C. | Lynette Batt

American Rivers has been a leader on clean water issues for some time now, and nowhere is this more evident than in our work on stormwater. In particular, we have been working to ensure that the US Enivironmental Protection Agency (EPA) comes out with a balanced and effective approach to managing stormwater in its anticipated new stormwater rule.

However, unless you deal with stormwater on a daily basis, it is easy to get lost in all the jargon surrounding EPA rules, government requirements and stormwater practices. How do all these government rules ultimately affect my local river and me?

I hope I can provide a little bit of clarity on perhaps the most significant aspect most certainly to be in the new rule: a performance standard.

A performance standard is, quite simply, a benchmark against which actual performance is measured and is expected to meet.  Think of it like the miles per gallon requirement for automobiles. A standard is set that requires all cars of a certain type to get 28 mpg. All new automobiles of that type have to perform to that standard.

It is not a perfect analogy, but it is similar with stormwater. The goal is to reduce the amount of stormwater runoff that flows from a building or parking lot and into the sewers and rivers. You might think the obvious solution is to have a performance standard that requires all buildings keep a certain number of gallons of stormwater from running off their roofs and parking lots.

But this is a challenge given the variation of rainfall patterns across the country – the average rainfall, and thus amount of runoff, in the Southwest is significantly different than that in the Northeast. It would be impractical to have one volume requirement for the whole country.

The solution is quite elegant, actually. In practice, we would like buildings to manage rain water the same way a forest or prairie might – the first 1 to 1.5 inches is absorbed right where it falls by soils and plants. In cities and towns, this “first flush” actually runs off roofs, roads and parking lots into our rivers, carrying pollutants with it.

So any performance standard should strive to require buildings manage that “first flush” on site – not letting it runoff. To make this regionally appropriate, a national performance standard should require a building to manage the 85th-95th percentile storm for its geographic region.

The percentile storm refers to that first flush. For example, in Washington D.C. where I reside, approximately 85% of the storms produce approximately 1.2 inches of water or less. In Georgia, 85% of the storms might produce .9 inches of water or less. By using this percentile approach, we can have one national number that provides flexibility for local weather conditions. In fact, similar performance standards are already being used on new Federal Facilities [PDF] and in states, like Minnesota.

Ideally, this performance standard would be met using any number of practices categorized under the term green infrastructure – they hold or filter rainwater where it falls. These methods, in a limited way, can actually replicate the natural environment in cities and towns. They have multiple benefits beyond stormwater reduction, and in many instances can be cheaper to build (some more specific examples can be found here and here). I think that is a win-win for our rivers and us.