Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Specialty contractors are not required, if quality engineering oversight isavailable during construction.The disadvantages of a siphon include:• Inability to handle flows greater than the designed capacity even though designhead exceeds the design level.• Inefficient flow at heads below 1 to 1¼ times the diameter of the pipe, causingmore frequent fluctuation in the water surface when compared to pipe-and-riserspillways.• Not cost effective (or, in many cases, feasible) for large watersheds. Generallylimited to small drainage basins with relatively small peak inflows.• A siphon designed for automatic operation may require excavation into theembankment dam to locate it below the anticipated reservoir water level.• If the siphon extends above the reservoir water level, it will require some meansof initiating the siphonic action.• Can be susceptible to vandalism unless protective measures are taken.• Inability to drain the entire reservoir and limited ability to drain reservoir deeperthan about 20 to 25 feet.• Some underwater work may be required for construction of the siphon.• A siphon is not recommended in colder climates. Siphons are susceptible toblockage with ice unless special provisions are implemented during design andconstruction. Siphon piping may require removal from the reservoir duringwinter to avoid damage from ice loadings. Otherwise, the ice surrounding thesiphon may have to be broken up daily.• The theoretical lift on the upstream side of the siphon is less for sites at higherelevations compared to those at sea level.• Best suited for low head operations to avoid cavitation potential.• Storms or snow may prevent site access for personnel to operate the siphon.For examples of siphons constructed at embankment dams, see the Crossgate andSugar Mill Dams case histories in appendix B.280