Which sewer is built to carry flows that are in excess of the capacity of an existing sewer?

Which sewer is built to carry flows that are in excess of the capacity of an existing sewer?

• A. Storm overflow sewer
• B. Depressed sewer
• C. Outfall sewer
• D. Relief sewer

Answer: (D)

When a section of sewer is at or near capacity, a relief path is added to take the extra flow. This relief sewer serves as a bypass, carrying flows that exceed the capacity of the existing line so the original sewer doesn’t surge or back up. It helps prevent manhole surcharges, basement flooding, and sewer overflows by diverting the excess during high-flow conditions, such as heavy rainfall.

A storm overflow sewer, by contrast, is used to route extra stormwater away from the sewer system, often releasing to a receiving water, and isn’t specifically designed to relieve a particular downstream sewer’s capacity. A depressed sewer is simply a deeper or lower-invert conduit to fit under obstacles or maintain slope, not to carry excess flow. An outfall sewer is the main trunk sewer that discharges to a lake, river, or treatment facility, not a bypass for capacity relief. So the relief sewer best matches the idea of carrying flows in excess of an existing sewer’s capacity.