Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 3—Hydraulic Design of Conduitsusually closely balanced upstream of the intermediate control, the developmentof a defect in the conduit in this area will be less of potential problem.3.1.1.3 Arrangement 3—Upstream controlIn this type of arrangement (figure 35), the gates or valves are located at orimmediately downstream of the intake structure. The specific aspects of thisarrangement are:• Flow conditions.—Open channel (free-flow) flow exists throughout the conduitdownstream from the gates or valves.• Design considerations.—Designed for external loadings and outside waterpressures on the conduit. Near full reservoir head will be exerted on theexterior of the conduit until adequate thickness of impervious embankment isprovided over the conduit. Due to large external hydrostatic pressure, theconduit must be kept watertight to avoid leakage through joints or cracks,which could allow embankment materials to be carried into the conduit.• Access.—Access for inspection and maintenance is greater than arrangements 1,2, or 4 (i.e., closing the gates or valves allows inspection of almost the entireconduit. An upstream bulkhead must be installed to inspect the upstream sideof the gates or valves and the remaining portion of conduit). In most cases, thistype of arrangement requires an intake tower and access bridge for gate or valveoperation or bulkhead installation, which add significant design andconstruction costs, especially in areas with potentially high seismic activity.Figure 38 shows an example of a footbridge. Sometimes submerged intakestructures containing gates or valves have been used instead of intake towers.• Emergency closure.—Emergency closure is provided at the intake structureupstream of the regulating gate or valve.• Risk.—This type of arrangement is considered to have more risk thanarrangements 1 and 2, but less than arrangement 4. If the conduit develops adefect downstream from the intake structure, a high pressure differential willexist due to the external hydrostatic pressure from the full reservoir head andno internal pressure within the conduit. A conduit defect in the areadownstream from the intake structure could result in water flowing into theconduit. In this arrangement, no emergency closure exists downstream fromthe intake structure. Another factor for the higher risk assignment is thepotential for the free-flow conduit not being properly sized and operationresulting in a pressurized condition. If the conduit is properly sized andoperated, this arrangement does not have the concern with high pressure flowbeing forced out of the conduit then into the surrounding fill.65