Spillways: Spilling the Right Way

Unlined earthen spillways are common features of small earthen dams. They can be principle, emergency or auxiliary spillways made to pass rare flood flows around the embankment. While many spillways may never pass a significant flow or volume of water, one large event could result in significant erosional consequences.

Historically there was little engineering design of the spillways for small dams. The spillway was often situated in the dam borrow area or incorporated as part of an auxiliary dam. Today there has been much more research conducted on spillway performance and there are many tools available for spillway design and evaluation. Some of the most popular tools are discussed in this article. Spillways should be designed to experience flow from a known recurrence interval storm and earthen spillways can expect to suffer some erosion damage during events that cause them to flow. However, where erosion enlarges enough to destabilize the structure or cause an uncontrolled release, it could lead to a dam failure with downstream consequences.