Which term describes a section of sewer that is lower than adjacent sections to pass beneath an obstacle?

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

Answer: (D)

A depressed sewer is a section where the invert (the bottom inside of the pipe) is intentionally lowered below the elevations of the adjacent sewer sections so the line can pass beneath an obstacle. In gravity sewer systems, maintaining a steady slope is crucial for sewage to flow by gravity. When an obstacle—such as a road, railway, or other structure—would hinder continuing at the same grade, a depressed section lets the pipe drop to a lower level, then continue at the proper slope after clearing the obstacle. This approach avoids awkward rerouting or pumping and keeps the system flowing smoothly.

It’s different from other terms like a relief sewer, which provides an alternate path during overloads; an outfall sewer, which carries effluent to a downstream receiving water; and a storm overflow sewer, which handles excess stormwater. The essential idea here is the pipe is deliberately lowered compared with adjacent sections to pass beneath an obstacle while preserving gravity flow.