Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 3—Hydraulic Design of Conduitswhere trash cleaning is ordinarily not required, a submerged structure may beappropriate.The conduit entrance may be placed vertically, inclined, or horizontally,depending on intake requirements. Where a sill level higher than the conduitlevel is desired, the entrance can be a drop intake similar to the entrance of adrop inlet spillway. A vertical entrance is usually provided for intakes at theconduit level. In certain instances, an inclined intake structure may be placedalong the upstream slope of the dam or along the reservoir bank upstream ofthe dam.The designer should exercise caution in the design of the trashracks for the intakestructure. As releases are made through the intake structure, debris can accumulateon the trashracks. Continued accumulation of debris will gradually begin to clog thetrashracks to a point where the internal and external hydrostatic pressures on theintake structure and conduit are no longer balanced. Unless these structures havebeen properly designed to resist this type of loading, the pressure differential maycause a collapse of the structures. Another concern the designer should be aware ofis the accumulation of sediment and debris in the reservoir. Severe storms can washtree stumps and other large debris (e.g., logs) into the reservoir. Also, if a forest fireoccurs in the watershed, this can cause mud, ash, and debris to enter the reservoir.For trashracked intake structures, this debris can accumulate on the trashracks andclog them. For intake structures without trashracks, this can result in plugging of theconduit. The lack of regular testing of gates and valves and reservoir flushing cancontribute to this situation. For flood control conduits, the clogging or plugging canresult in loss of discharge capacity, which could lead to overtopping of theembankment dam. Clogging and plugging can also affect the operation ofdownstream turbines (ICOLD, 1994a).Reclamation’s Design of Small Dams (1987a, p. 452) provides a good source ofinformation concerning terminal structures. The following has been adapted fromthat reference:Terminal structures.—The discharge from a conduit, whether it be pressure orfree flow, will emerge at a high velocity, usually in a nearly horizontal direction.If erosion-resistant bedrock exists at shallow depths, the flow may bedischarged directly into the river. Otherwise, it should be directed away fromthe toe of the embankment dam by a deflector. Where erosion could be apotential problem, a plunge basin may be excavated and lined with riprap orconcrete.When more energy dissipation is required, the hydraulic jump basin is mostoften used for energy dissipation of discharges. However, flow that emerges inthe form of a free jet, as is the case for valve-controlled outlets of pressure73