Frequently Asked Questions (FAQ)

What does "first flush" mean?

In different parts of the country, the term "first flush" can mean different things. In more humid parts of the country where it can rain almost any day of the year, the term usually refers to the first few minutes or hours of a storm. The common perception is that during this initial time, the concentration of pollutants in the runoff is greater than during later parts of the storm. The reason for this phenomenon would be the deposition and "build up" between storms of various pollutants on the drainage area land surface, followed by the pollutants' subsequent "wash off" during the storm. Accordingly, the Standard Urban Stormwater Mitigation Plans under the Los Angeles Municipal Stormwater Permit require that, for specific types of property development and redevelopment, the runoff from the first 3/4 inch of rainfall for every storm be either treated or infiltrated. Almost 85 percent of all the rain storms that have occurred in Los Angeles have been 3/4 inch or less in total rainfall.

This description of first flush generally holds true in more arid parts of the country as well. But in parts of the country like the southwest where there are definite rainy and dry seasons, additional first flush phenomena can be expected during the first storm or storms of the season. That is, due to the extended build up of pollutants over a number of months without rain, pollution concentrations overall can be expected to be higher for the season's first few storms than for the rest of the season.

This expectation of higher concentrations at the beginning of the storm season in Southern California does not hold for all pollutants, however. While the seasonal first flush pattern was observed for stormwater toxicity, monitoring conducted under the Los Angeles Municipal Stormwater Permit has shown that peak stormwater bacteria counts do not follow any "first flush" patterns when it comes to month of year, size of storm, or even location.

  94-2000 Integrated Receiving Water Impacts Report