The first flow of water into the stormwater drains after rain. It often contains a lot of pollutants that built up during the dry time before the rain.
the delivery of a highly concentrated pollutant loading during the early stages of a storm, due to the washing effect of runoff on pollutants that have accumulated on the land.
Pollutant concentrations, including suspended sediments, carried by stormwater in the beginning of a storm. These concentrations are typically higher than at the middle or end of the storm. For purposes of the Massachusetts Stormwater Management Policy, the first flush is the first half-inch of precipitation, and in Critical Areas, the first inch of precipitation.
The first growth of the season often the very best quality due to its relatively slow growth. 'Second Flush' is usually more plentiful, more consistent, but sometimes not such good flavour.
The first half inch of runoff from a rain event is generally referred to as the first flush, in which pollutants within the stormwater conveyance system are transported to the receiving water body. Included in this concentrated flush are the Non Point Source pollutants which have accumulated since the last rain event. Term Acronym Source
Describes the washing action that stormwater has on accumulated pollutants. The first runoff, especially off streets and parking lots, washes them clean and carries pollutants with it. The first one inch of runoff carries 90 percent of the pollution.
The first big rain after an extended dry period (usually summer) which flushes out the accumulated pollutants in the storm drain system and carries them straight to the Las Vegas Wash.
The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will pick up or dissolve pollutants and the "first flush" portion of the flow may be the most contaminated as a result. This is especially the case in small or more uniform catchments, however, in larger or more complex catchments pollution wash-off may contaminate runoff throughout a rainfall event.
It is well known in urban hydrology, that the constituents are normally more concentrated in the first part of runoff. This phenomenon was already described in the beginning of the 20th century (METCALF AND EDDY, 1916) as “first flush†or “first foul flushâ€. The term is often also used to address the first flood after a dry period, which is supposed to contain higher concentrations than a subsequent one.
