"First Flush" design requirements
"First Flush" design requirements
(OP)
I've been informed that I have to design a site for "first flush" of the initial runoff from the parking areas. I'm having trouble finding info and design requirements on this topic.
Does anyone have any helpful direction for me?
Does anyone have any helpful direction for me?





RE: "First Flush" design requirements
RE: "First Flush" design requirements
Brian
RE: "First Flush" design requirements
Either the first 1/2" of runoff from the whole contributing area or the runoff from the first 1" of rainfall, with DCIA being considered separately, for percolation systems, which percolate within 72 hours with 100% Safety Factor.
For wet detention treatment, the treatment volume is doubled and 1/2 the volume must be released within 60-72 hours. 30% littoral shelf required. anti clogging device on drawddown orifice required.
(For systems that discharge to Outstanding Florida Waters, increase the storage volume by 50%)
Both systems, percolation or wet detention, require a baffle, 4"-6" minimum below the low water level and up to the high water level, unless your contributing area is at least 50% green.
If a wetland is available for treatment, then the first 1/4" of runoff must be intercepted outside the wetland in a 6" deep swale or a baffled sediment sump must be used.
Conclusion, ask the regulators what your local requirements are!!
Clifford H Laubstein
FL PE 58662
RE: "First Flush" design requirements
The EPA states the following:
"...volume based on capture of the first flush, which is defined as the first 1.3 centimeters (0.5 inches) of runoff from the contributing drainage area (SEWRPC, 1991). The State of Maryland (MD., 1986) ...defines first flush as the first 1.3 centimeters (0.5 inches) from the contributing impervious area. The Metropolitan Washington Council of Governments (MWCOG) ... based on the first 1.3
centimeters (0.5 inches) per impervious acre or the runoff produced from a 6.4 centimeter (2.5 inch) storm..."
I am researching this same question and it appears it is not well defined. Researchers have found that by capturing the first half inch, you are not necessarily guaranteed to capture the pollutants. Depending on the nature of the watershed, pollutants and storm event, the pollutant peak discharge may occur at any time and is not always contained within the first flush.
Your state or local agency may have defined "first flush" by statute. You should check there.
RE: "First Flush" design requirements
KRS Services
www.krs-services.com
RE: "First Flush" design requirements