Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 39.—A drop inlet spillway conduit through an embankment dam.Ideally, the spillway should be designed to operate with crest control throughout theentire expected range of discharge. However, the range of expected discharge isbased on the current hydrologic data. Spillway flood flow rates may change due toupdated probable maximum precipitation quantities; changes in the basin runoffcharacteristics could vary significantly with time; and the project operation may berevised at a future date, which may result in an increase above the original spillwaydesign flow. Any of these factors, separately or in combination, could be sufficientto cause a spillway designed for crest control to shift to conduit control in the upperrange of expected discharge. The spillway conduit is considered a closed channeland generally takes the form of a vertical or inclined shaft connected to a horizontalor sloped conduit. In most cases, a spillway conduit is designed to flow partly fullthroughout the entire length. Another condition that could cause the control shift atessentially any discharge is partial plugging of the conduit. Plugging could occureither by external debris (i.e., logs or ice) or an internal problem resulting fromcavitation damage. To ensure free flow in the conduit, the ratio of flow depth toconduit diameter or height should be limited to about 75 percent or less. Some siteshave utilized log booms or specially designed trashracks that reduce surface currentsto prevent floating debris from entering the conduit. Additionally, air vents may berequired to ensure adequate air supply is provided to prevent unstable flow in theconduit. In areas where high velocity flow may occur, aeration of the flow may berequired to prevent cavitation damage.68