First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.:216
First flush effect
The term "first flush effect" refers to rapid changes in water quality (pollutant concentration or load) that occur after early season rains. Soil and vegetation particles wash into streams; sediments and other accumulated organic particles on the river bed are re-suspended, and dissolved substances from soil and shallow groundwater can be flushed into streams. Recent research has shown that this effect has not been observed in relatively pervious areas.:216
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. This is referred to as "first flush flood." There are various definitions of the first flush phenomenon.
Because the reference of the first flush is not always clear, the terms "concentration-based first flush" (CBFF) and "mass-based first flush" (MBFF) have been introduced.
Apart from this definition, there are a number of rating parameters in literature to determine the occurrence of a first flush.
In the context of rainwater harvesting, a first flush diverter is a simple device that is designed to protect a storage cistern from contamination by first flush runoff. This leads to a higher quality of water captured, and less silting of the cistern over time in dusty areas. The diverted first flush water is used for irrigation or other purposes in a fashion similar to greywater. Although many commercial versions are available, these devices are frequently constructed of spare pipe when the cistern is initially installed or thereafter. See Texas Manual on Rainwater Harvesting for calculations on sizing.
- Combined sewer
- Nationwide Urban Runoff Program - U.S. research project
- Nonpoint source pollution
- Sanitary sewer overflow
- Urban runoff
- Brad Lancaster - Rainwater harvesting educator and author
- Metcalf, Leonard; Eddy, Harrison P. (1916). American Sewerage Practice: Disposal of Sewage III. New York: McGraw-Hill. p. 154.
- Alex Maestre and Robert Pitt; Center for Watershed Protection (2005)."The National Stormwater Quality Database, Version 1.1: A Compilation and Analysis of NPDES Stormwater Monitoring Information." Report prepared for U.S. Environmental Protection Agency, Washington, DC. September 4, 2005.
- Australian Government, Department of the Environment and Heritage. Canberra. "Glossary of Terms."
- Gupta, K.; Saul, A.J. (1996). "Specific Relationships for the First Flush Load in Combined Sewer Flows". Water Research 30 (5): 1244–1252. doi:10.1016/0043-1354(95)00282-0. ISSN 0043-1354.
- Geiger, W. (1987). "Flushing Effects in Combined Sewer Systems". Proc. 4th Int. Conf. Urban Drainage, Lausanne, Switzerland: 40–46.
- Sansalone, John; Christina, Chad M. (November 2004). "First flush concepts for dissolved solids in small impervious watersheds". Journal of Environmental Engineering 130 (11): 1301–1314. doi:10.1061/(ASCE)0733-9372(2004)130:11(1301). ISSN 0733-9372.
- University of Hawai`i Agricultural Extension. Manoa, HI (2008). "Rainwater Catchment Solutions: First-Flush Diverters." Brochure.
- Texas Water Development Board. Austin, TX (2005). "Texas Manual on Rainwater Harvesting." 3rd ed. pp. 8-9.
- Hager, Mary Catherine (2001). "Evaluating First-Flush Runoff." Stormwater: September-October 2001.
- Alex Maestre, Robert Pitt, and Derek Williamson (2004). "Nonparametric Statistical Tests Comparing First Flush and Composite Samples from the National Stormwater Quality Database." In Models and Applications to Urban Water Systems, Vol. 12 (edited by W. James). Computational Hydraulics Int., Guelph, Ontario. pp. 317 – 338.