Detaining only the "first flush"
Detaining only the "first flush"
(OP)
This is probably a silly question, but I've been out of practice for a little bit. How does one detain ONLY the amount of runoff known as the "first flush"? Once the first flush is detained, where does the rest of the water go?
Any help would be appreciated. I'm basically wondering how detention of the "first flush" ONLY is achieved.
Stoddardvilla
Any help would be appreciated. I'm basically wondering how detention of the "first flush" ONLY is achieved.
Stoddardvilla





RE: Detaining only the "first flush"
RE: Detaining only the "first flush"
Maybe the mixing is minimal, I don't know. That is my main concern with detaining the "first flush". I thought of the detention basin with a weir, but I thought mixing would defeat the purpose.
Stoddardvilla
RE: Detaining only the "first flush"
The other biger picture thing is that if you can detain the first flush, in many communities this is typically the whole storm for 80% of the volume of expected runoff that year. First flush is used less, because is that the first storm regardless if it is 4 inches or 0.2 inches.
RE: Detaining only the "first flush"
RE: Detaining only the "first flush"
If the concern is to trap the pollutants related to the runoff from the first 1/2 inch, that is a matter of storm water quality.
In both cases there are retention/detention solutions, I'm just not the best person to get into these details.
f-d
RE: Detaining only the "first flush"
United States Environmental Protection Agency
EPA 832-F-99-019
September 1999
Storm Water Technology Fact Sheet - Infiltration Trench
...the 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) suggests that ...volume be 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.
In Washington D.C., the capture of 1.3 centimeters (0.5 inches) per impervious acre accounts for 40 to 50 percent of the annual storm runoff volume.
The system is sized based upon the design storm which is determined by state regulations (i.e., Maine requires treatment of first half inch of storm and Washington requires treatment of a six month storm). ...the definition and requirements are left up to each state to implement as they see fit.