What's a normal fall for a manhole?

What's a normal fall for a manhole?

• A. 1 inch to 12 inches
• B. 2 inches to 3 inches
• C. 4 inches to 6 inches
• D. 0.5 inch to 1 inch

Answer: (A)

The main idea is that the fall of a manhole is the vertical difference between the upstream invert and the downstream invert inside the manhole. This drop is what keeps wastewater moving by gravity, but it must be within a practical range: enough to drive flow and prevent stagnation, yet not so large that it causes excessive turbulence or requires special design.

A normal fall isn’t a single fixed number—it’s a broad, practical range that covers most standard installations. It accounts for different pipe sizes, slopes, and manhole configurations, from small downward steps to more noticeable drops, while staying within what gravity flow and typical manhole details can handle. If the fall is much larger, you’d need additional design features to dissipate energy and control velocity; if it’s too small, solids can settle and self-cleansing can suffer. Therefore, the broad range from a small amount up to a moderate drop is the best fit for what’s considered normal in practice.