That’s where they unveiled a case study involving a unique approach to estimating first flush on a site for stormwater treatment infrastructure.
Ready? Let’s dig in.
What is first flush?
First flush is the initial runoff from a rain. In stormwater management circles, the first flush is important because that’s when the pollutants are most concentrated.
Picture a parking lot. Cars and trucks sit on the pavement, dripping engine grease and other chemicals onto the paved surface. The chemicals mostly stay on that paved surface…until it rains. Then, the rainwater washes those chemicals, along with dirt and debris to the nearest low point and eventually to your community’s storm sewers.
State stormwater regulations require that communities evaluate and treat the first flush to remove those highest concentrations of harmful substances before they reach rivers and streams.
Why the first flush is so important
“To reduce the impacts of runoff on urban streams, the EPA expanded the Clean Water Act in 1987 to require municipalities to obtain permits for discharges of stormwater runoff. As a result, many communities have adopted regulations requiring developers to install stormwater management practices that reduce the rate and/or volume and remove pollutants from runoff generated on their development sites.” (Center for Watershed Protection)
The first flush is typically used for estimating design criteria for infrastructure to improve water quality. This means that the capital investment required for water quality enhancements is directly related to the first flush rainfall volume induced runoff volume and peak flow rates.
How does the industry calculate first flush and why can it be inadequate?
Standards are predominantly based on rainfall depth statistics and a prescribed rainfall volume.
One of the common practices in stormwater management is to calculate the first flush using what’s called the Rational Method to estimate the peak discharge rates that are used to determine treatment requirements.
However, the Rational Method has drawbacks: it assumes that the rainfall is uniformly distributed across the drainage area and evenly distributed. It doesn’t take soil moisture into account, either.
These assumptions can impact a community’s compliance costs unnecessarily. Stormwater funds are scarce and communities want to stretch them as far as possible. Overestimating the first flush pollutants means designing and building stormwater management infrastructure that’s bigger than you need.
Localized rainfall, runoff and moisture measurements
Because runoff volume and peak flows vary continuously with rainfall volume, rainfall intensity, watershed runoff characteristics, and antecedent moisture conditions, it’s more accurate to measure the rainfall and flows locally.
That’s the approach Kuhns and Ulasir presented at the Michigan Stormwater Floodplain Association Conference: using localized continuous model to establish design flows.
First, they used a hydrologic model calibrated to local rainfall data and local flow data. There are several continuous simulation models available:
· EPA SWMM
These models use continuous modeling routines that establish the non-linear relationship between rainfall, temperature, and runoff.
First Flush Design for Stormwater - Michigan Stormwater FloodPlain Association from OHM Advancing Communities