Letting it All Out: Hydraulic Design of Outlet Works

The purpose of an outlet works is to regulate or release water impounded by a dam. This is done for a variety of reasons, including (1) passage of storm or run-of-river inflow; (2) releasing flow to meet demands downstream; or (3) draining the reservoir (in the case of a low-level outlet). In some cases, the outlet works might also be referred to as the principal or service spillway, if it is the primary outlet used to control the reservoir level.

This article will discuss the major components of outlet works, the hydraulic analyses required to size and design each component, and general design considerations associated with outlet works of small dams. Structural analyses are an important component of outlet works design but they are not addressed in this article.