Detaining only the "first flush"

[nutbutter]

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

 

[francesca]

The rest of the storm flow bypasses, e.g. by passing over a weir on the detention basin. However, detention of the first flush is mostly used on things like grease traps in commercial developments. Still, the bypass principle applies, you just have to be creative in how you get your flow to bypass.

 

[nutbutter]

Wouldn't some mixing occur in the detention basin since flow may be entering with a significant velocity? Wouldn't this mixing make it so the "first flush" mixes with additional stormwater, so that in effect, the first flush is not actually being detained as required?

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

 

[blueoak]

Put it offline.
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.

 

[cvg]

in theory, the first flush contains most of the pollutants and should be retained (not detained) in the bottom of the basin. For larger runoff events, the flows may mix. In practice, the first flush doesn't necessarily contain all the pollutants, due to mixing on the surface of the ground / pavement or due to the duration / intensity of the rainfall. The entire runoff event may contain the pollutants which are mobilized during the first flush. Depending upon the sizing for your basin, you may be able to retain all of the runoff from most storms. For instance, the policy here is to retain the entire 100-year, 2-hour runoff from onsite. This

 

[fattdad]

I think the answer to this question depends on nature of the real concern. If you are trying to match the pre-development to the post-devleopment hydrograph, then the first flush may be retention of the "initial extraction" volume (i.e., the amount of water that would normally to immediately into the topsoil/forest duff layer). This is often considered to be the first 1/2 inch (at least in central Virginia). After than you basically have full runnoff.

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

 

[cvg]

first flush retention is a requirement by the EPA to trap pollutants, and not related to pre vs post development peak runoff. As can be seen from the excerpts below, EPA and the states define it a bit differently...

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.