Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 3—Hydraulic Design of Conduitsthrough joints or cracks. A steel pipe liner is normally used with pressureconduits.• Access.—Access for inspection and maintenance can be seldom (i.e., either thereservoir must be drained, or divers must perform inspections after gates orvalves are closed, or the conduit must be unwatered after an upstream bulkheadis installed). At some sites, a submerged upstream gate or valve is provided forclosure to facilitate access for inspection. Arrangement 4 is discouraged unlessthe embankment dam is low hazard with minimal downstream consequences.• Emergency closure.—Emergency closure is typically not possible upstream of thecontrol point, unless a submerged intake structure with a mechanically orhydraulically operated gate or valve is provided.• Risk.—This type of arrangement is considered to have more risk thanarrangements 1, 2, and 3. If the conduit develops a defect upstream from thedownstream control structure, a high pressure differential will exist due to theinternal hydrostatic pressure from the full reservoir head and the lack ofexternal hydrostatic pressure. A conduit defect in the area upstream from thecontrol structure could result in water flowing out of the conduit. In thisarrangement, no emergency closure exists upstream from the control structure.Even if an upstream emergency closure gate is provided in a submerged intakestructure, a leak from a defect in the conduit may not be identified in time toprevent an embankment dam failure.3.2 SpillwaySpillways utilizing conduits are generally shaft or drop inlet type. These types ofspillways typically consist of an entrance (crest) structure with or without controldevices, a conduit, and a terminal structure. Figure 39 shows an example of a dropinlet type of spillway called a morning glory. The drop spillway is often referred to asa “principal spillway.” Figure 40 shows an example of a riser structure for a principalspillway. A spillway provides flood control regulation for floods, either incombination with an outlet works, or as the only flood control facility. Typically, thespillway is used to release surplus water or floodwater that cannot be contained inthe allotted reservoir storage space. The discharge capacity of a spillway conduit isdetermined by the results of flood routings and is influenced by the flood surchargevolume available above the spillway crest. Where little flood surcharge volume isavailable, the spillway must be large enough to pass the peak of the flood. If thereservoir has a large storage capacity above the normal water surface, a portion of theflood volume can be retained temporarily, and the spillway discharge capacity may beconsiderably reduced.67