Which feature protects the spillway from erosion in an extended detention basin?

• A. Maintenance access road
• B. Riser
• C. Armored spillway
• D. Emergency spillway

Answer: (C)

An armored spillway is designed to protect the spillway from erosion by providing a hard, durable surface that can withstand the high-velocity flow and scour that occur when water is released from the basin. In an extended detention basin, the spillway must convey significant energy during storms, and without protective armor the leading edges and bed could erode, undermining the structure. Armor—such as concrete lining or riprap—disperses energy, reduces scour, and maintains the spillway’s integrity over time.

Maintenance access road is for access and maintenance purposes, not erosion protection. A riser is part of the outlet control structure that helps regulate water levels, but it isn’t specifically about preventing spillway erosion. An emergency spillway provides an overflow path during extreme events, but it’s not the feature that protects the main spillway from erosion.