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Wet-Weather Flow Research

Picture of flood water.
Water Runoff from Flooding
Research Projects
  • Projects List

Wet-Weather Flow (WWF) is any storm made surge of water - rain or snowmelt. During extreme weather events, this water may overwhelm the wastewater collection system, resulting in overflows. All discharges that occur during storm-flow events are untreated. This can cause environmental damage through flooding and contamination. WWF pollutants discharges from many sources remain largely uncontrolled. There are two types of urban WWF discharges:

  • Combined Sewer Overflow (CSO)
  • Sanitary-Sewer Overflow (SSO)

Control of WWF pollution is one of the top EPA cleanup priority areas. Problems caused by Wet-WWF include:

  • floatables, like plastic bottles, decrease aesthetic values or potentially harm habitat and wild life,
  • beach closing due to high pathogen indicator counts, such as e. coli and enterococci,
  • solids from storm drainage can affect dissolved oxygen in receiving waters, affecting habitat and wildlife
  • algae blooms from nutrients can have adverse affects on the health of people and marine organisms, as well as the "health" of local and regional economies,
  • pollutants toxic to receiving water’s habitat and wildlife.


EPA researchers are currently:

  • developing methods for repairing aging infrastructure for Sanitary-Sewer Overflow (SSO)
  • researching treating Combined Sewer Overflow (CSO) and stormwater prior to discharge
  • looking at incorporating hydrology into onsite design to reduce or eliminate offsite discharge, this Best Management Practice (BMP) is often termed Low Impact Development (LID)
  • controlled testing of swales

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