Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams3.1.1.2 Arrangement 2—Intermediate control without downstream accessIn this type of arrangement (figure 34), gates or valves are located at an intermediatepoint (typically at or upstream of the embankment dam centerline) between theintake and the terminal structures. The specific aspects of this arrangement are:• Flow conditions.—Pressure flow exists upstream of the intermediate control, andopen channel flow exists downstream of the intermediate control.• Design considerations.—The internal pressure upstream from the intermediatecontrol is approximately equal to the full reservoir head. The internal andexternal hydrostatic pressures will be closely balanced, and the potential forleakage into or out of the conduit will be minimized. As external hydrostaticpressure around the conduit diminishes with increasing distance from thereservoir, there may be excess internal pressure, and the conduit must be keptwatertight to avoid leakage through joints or cracks, which could allow water tobe forced out of the conduit and into the surrounding soils. Normal practicelimits the length of the pressure portion of a conduit to that part of the conduitupstream from the crest of the embankment dam or to approximately theupstream third of the dam. The upstream conduit should be designed to resistthe full external hydrostatic pressure when it is dewatered for inspection ormaintenance. The use of a steel liner for the upstream conduit should beconsidered, whenever there is concern regarding the watertightness of apressure conduit.• Access.—Access for inspection and maintenance of the downstream conduit canbe limited, since the gates or valves located at the intermediate point must beclosed. Once closed, the downstream conduit can be accessed. However, theupstream conduit will remain inundated. Access to the upstream conduitrequires bulkheading of the conduit entrance. Access to the gates or valves(normally located within a structure called a gate chamber) is typically providedthrough an access shaft from the crest of the embankment dam.• Emergency closure.—Emergency closure is possible at the intermediate controlpoint. Typically, this type of arrangement provides tandem gates or valveslocated at the intermediate control point. The upstream gate or valve serves asa guard, and the downstream gate or valve provides regulation.• Risk.—This type of arrangement is typically used for high embankment damswith significant to high downstream consequences. This arrangement isconsidered to have more risk than arrangement 1, but less risk thanarrangements and 3 and 4, since the ability exists to provide closure at anintermediate location. Since the external and internal hydrostatic pressures are64