"First Flush" design requirements

[theclipper]

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?

 

[dicksewerrat]

what they are talking about is a pond to hold the first runoff from the lot. They may have a storm event and/or a time frame that it should hold, like the first 15 minutes of the 10 year event.

 

[jugglerbri]

The Denver area has set standards for detaining the first flush of a storm and basis it on the land use and area. They also have a reduction factor for sites that limit the amount of directly connected impervious areas (dcia). You should contact the reviewing agency to get the requirements for the site.

Brian

 

[gibfrog]

In SW Florida the "first flush" is calculated thus -
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

 

[cvg]

"the highest concentrations of contaminants are often contained in the "first flush" discharges, which occur during the first major storm after an extended dry period"

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.

 

[KRSServices]

I have found that recently, at least up here, most of the detention/retention parameters are the result of having to provide a comprehensive environmental impact assessment on the storm runoff to the regulatory authority. Every project is now different in terms of what is now required and is reflected accordingly by the amended statutes. While I grumble due to increased costs, some of the requirements imposed make sense from an environmental standpoint (not that I have taken to hugging trees as a hobby).

KRS Services

http://www.krs-services.com

 

[dicksewerrat]

I can remember back to 1969 while in college that the 'first flush' was a topic of some interest. Then it was to design a system to take the 'first flush' from a rain event out of the storm drain system and divert it to the sanitary system. tho I haven't seen a design for that yet.