Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 3—Hydraulic Design of ConduitsMore restrictive filling rates (e.g., first filling) may be required when thereservoir exceeds historically high levels. The designer must assume that firstfilling can be sudden and unexpected, and the outlet works must have sufficientcapacity to accommodate this type of event. The Picketberg and Wister Damcase histories in appendix B are examples of embankment dam failures thathave occurred upon first filling of the reservoir.• Diversion.—The outlet works may be utilized for the diversion of stream or riverflows during construction of the embankment dam. The sizing of the outletworks conduit is based upon the size of the flood that might be reasonablyexpected to occur during construction. Historically, diversion flood capacitiesare in the range of 5, 10, or 20 years, with consideration given to larger floodlevels, if the consequences of failure during construction are large.• Operational.—The outlet works is typically used to pass downstream releaserequirements, such as irrigation releases, environmental enhancement forwetlands, fisheries, or water quality, and municipal and industrial releases.• Flood control.—The outlet works may be sized to restrict the amount of flow thatcan pass through the system, thereby storing excessive flood flows in thereservoir and limiting flood flows downstream of the embankment dam.3.1.1 Arrangement of control featuresDepending on the requirements of the project, the outlet works may be controlled oruncontrolled. Controlled outlet works are used at multipurpose reservoirs thatprovide storage for conservation, irrigation, etc. and for single-purpose flood controlprojects in which control of the discharge is required. Uncontrolled outlet works areused at some flood control reservoirs, where predetermined discharges (varying withthe head) are required to meet the flood control requirements. The type and size ofthe controls depend on the purposes that the outlet works will serve.The location of the control features within the outlet works affects the riskassociated with internal erosion and backward erosion piping incidents.Downstream control features can allow pressurized conditions to occur in theupstream portion of the conduit. Pressurized conditions create a greater potentialfor water escaping under pressure, potentially eroding the surrounding earthfill orfoundation soils. Careful consideration is required in selecting the location ofcontrol features.Control of the outlet works discharge is accomplished by gates and valves. The gatesand valves are typically motor operated, hydraulically operated, or manually operated.Operators for gates and valves are typically required to have backup systems to openthem under emergency conditions. Regulating gates and valves are used to control59