Wet-Weather Flow

Water Runoff from Flooding

Projects

Projects List

Introduction
Environmental Issue or Problem
Background
Long-Term Goal and APGs Addressed
Status/Results
Expected Outcomes and Impacts
Contact

Introduction

Wet-Weather Flow is any storm generated flow of water; e.g. rain or snow melt. There are two types of Urban Wet-Weather Flow discharges: Combined Sewer Overflow (CSO) (PDF) (9 pp, 118 Kb, about PDF) and Sanitary-Sewer Overflow (SSO). All are untreated discharges that occur during storm-flow events.

Environmental Issue or Problem

Combined Sewer and Stormwater systems were designed to discharge to nearby receiving waters causes Wet-Weather Flow discharge. The impacts of these discharges were not immediately known at the time of installation, but through research are now apparent.

Many beach closing are caused by Wet-Weather Flows due to high pathogen indicator organism counts of fecal or total coliform and e.coli and enterococci. Research is also determining the effects of Wet-Weather Flow as a source of cryptosporidium and giardia.

Floatables, like plastic bottles, end up in a storm drain and can be discharged to a receiving water, decreasing aesthetic values or potentially harming wildlife. Solids from storm drainage can affect dissolved oxygen in receiving waters, affecting biota, e.g., fish and macroinvertebrates.

Wet-Weather Flow typically carries nutrients which can cause algal blooms in receiving waters.

Background

Wet-Weather Flows have proven to generate a substantial amount of chemical, physical, and biological stress to receiving waters. Control of Wet-Weather Flow pollution is one of the top cleanup priority areas for the Agency. Problem constituents in Wet-Weather Flow include visible matter, pathogenic microorganisms, oxygen-demanding materials, solids, nutrients, and pollutants toxic to receiving water biota.

CSOs represent one of the major Wet-Weather Flow pollution sources. Historically, however, the control of CSO has proven to be extremely complex and costly. This complexity stems partly from past difficulties in quantifying CSO impacts on receiving-water quality and the site-specific variability in CSO volume, frequency, and characteristics. In addition, control costs for communities with CSOs are high.

Pollutants in Wet-Weather Flow discharges from many sources remain largely uncontrolled.

Long Term Goal and APGs Addressed

EPA Goal 2: Protecting America's Waters

Status/Results

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

Expected Outcomes and Impacts

Contact

Michelle Latham, Technology Transfer Specialist
Phone: 513-569-7601
Email: latham.michelle@epa.gov

U.S. EPA National Risk Management Research Laboratory
Andrew W. Breidenbach Environmental Research Center
Water Supply and Water Resources Division
26 West Martin Luther King Drive
Mail Code: MLK 693
Cincinnati, OH 45268