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STORMWATER INFRASTRUCTURE

These graphics were commissioned by NOAA’s Climate Program office and published on August 28, 2020 in Water Utilities and Climate, an ArcGIS story map written by intern Jack Barker.

A cross section reveals differences between green and gray stormwater infrastructure. Hard surfaces such as streets, drains, and pipes can send polluted runoff into watersheds. Green infrastructure such as bioswales or planter boxes absorb stormwater and reduce pollution.

Cross section of green and gray stormwater infrastructure. Illustration by Fiona Martin / NOAA Climate Program Office.
Cross section of green and gray stormwater infrastructure. Illustration by Fiona Martin / NOAA Climate Program Office.

About 770 cities in the US have “combined sewer systems,” which are designed to collect stormwater, urban runoff, and sewage in the same pipe. In dry weather conditions, all of the wastewater will go to a treatment plant for processing; in extreme precipitation events, the wastewater can overflow into a local watershed.

Explore what can happen in a combined sewer during extreme precipitation in the slider below.

Cutaway Illustration of a combined sewer system in normal day-to-day operations Cutaway illustration of combined sewer system during extreme precipitation event, overflowing into a local water body.
Use the swiper to compare how wastewater and stormwater interact during a heavy precipitation event in a combined sewer system. Notice how the untreated wastewater can flow into the local watershed during a combined sewer overflow. Illustrations by Fiona Martin / NOAA Climate Program Office.

In contrast, “separate sewer systems” are designed with separate networks for stormwater and wastewater. In this setup, rainwater runoff is routed directly to nearby waterways, while sewage is carried to treatment facilities. These stormwater networks—often called Municipal Separate Storm Sewer Systems (MS4s)—are regulated and require permits for discharge. However, aging or damaged sanitary infrastructure can develop cracks, allowing sewage to leak into the stormwater system. When this occurs, untreated wastewater can bypass treatment entirely and enter local rivers or streams—an event known as a “sanitary sewer overflow.”

Explore what can happen in a separate sewer system during an extreme weather event in the slider below.

Cutaway illustration of a sanitary sewer system in day-to-day operations. Cutaway of a sanitary sewer system during an extreme precipitation event. As the system ages, cracks can form, leaking sewage into stormwater overflows heading to local waterways.
Use the swiper to compare how wastewater and stormwater are managed in a separate sewer system during day to day use versus during a heavy precipitation event. Illustrations by Fiona Martin / NOAA Climate Program Office.

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