As Austin looks to update its stormwater policies as part of CodeNext to help reduce flooding and water pollution, the debate can get real technical, real fast. For folks who aren't knee-deep in water policy like I am, here's a hopefully simple guide to key terms in the world of stormwater.
As Austin looks to update its stormwater policies as part of CodeNext to help reduce flooding and water pollution, the debate can get real technical, real fast. For folks who aren’t knee-deep in water policy like I am, here’s a hopefully simple guide to key terms in the world of stormwater.
Stormwater management: the term generally used by professionals to refer to “drainage,” the term that’s usually used by the public, but which only refers to one component of stormwater management. I like to used this term, because it covers all of the stuff below.
On-site vs. off-site: Where is the stormwater managed, on the property or off? Off-site was the preferred option for most of the past century. Most developments were designed so that stormwater would drain away as runoff, and to do so as quickly as possible, and to flow into the municipal drainage system of curbs, culverts, and pipes (which is often called “gray infrastructure” because it’s made of concrete). However, more development (meaning more impervious cover) pours more runoff into the municipal system, which can’t always be expanded quickly or cheaply enough to handle this increased load, which is why on-site stormwater management has become more common.
Detention vs. retention: Detention means temporarily holding some stormwater on-site before it’s discharged as runoff. Retention means permanently holding a portion of stormwater and not discharging it off-site at all.
Flood mitigation: This has historically been the only goal for on-site stormwater management, which is what people usually mean when they refer to “drainage” or “detention.” This is almost always accomplished with detention ponds or tanks. The goal is to slow down the rate at which runoff leaves a property. Impervious cover doesn’t just increase the amount of runoff, it also increases the speed — more runoff leaves a developed property in a shorter amount of time. On-site detention is designed to reduce the amount of runoff that will flow off-site at any one point. (This is why the criterion in the Pre/Post Rule in CodeNext 2.0 was “peak flow rate.” In 3.0 the criterion has been changed to “runoff conditions.”
Pollutant removal: This is the goal for what’s referred in Austin’s code and others as “water quality.” When runoff flows over impervious cover, it picks up whatever’s on those surfaces — roofing chemicals, automotive fluids, etc. It can also pick up bacteria from pet and wildlife poo when flowing over lawns or parks. “Water quality treatment” means that a portion of runoff (much less than the detention volume) has to pass through an on-site feature that will remove some of these pollutants.
Austin’s treatment of choice for the past 20+ years has been sand filters, which capture most pollutants but not bacteria. One easy way to think of green stormwater infrastructure (GSI) features is that they’re like sand filters with an additional element: plants, which can remove bacteria. (Rain gardens and bioswales are constructed with a sandy base layer, usually consisting of about 2/3 sand and 1/3 compost.) This is why they’re called “green infrastructure.” Water passes through a rain garden more slowly than through a sand filter, which means that the rain garden needs to be bigger (generally, about twice as big) in order to treat the same amount of runoff.
Detention vs. retention, part 2: Sand filters (like detention ponds) only detain runoff temporarily before discharging it off-site. GSI features are usually designed to retain some runoff, meaning that this amount is never discharged and has to be either disposed of on-site (by infiltrating into the soil, evaporating into the air, or being absorbed into the plants) or else re-used on-site (by using the water collected in rain harvesting tanks for landscape irrigation or other on-site uses). GSI features are sometimes designed with an outlet so that runoff is only detained, not retained. In our response to Draft 3 of CodeNext, we’re aiming for retention when it’s possible. More on that soon.