Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Control features.—Typically gates or valves located in the intake structure,conduit, gate chamber, or a downstream structure.• Terminal structure.—A structure located at the downstream end of the outletworks. Terminal structures often include gates or valves and may include sometype of structure to dissipate the energy of rapidly flowing water and to protectthe riverbed from erosion.• Discharge channel.—The channel downstream from a terminal structure. Thischannel conveys releases back to the “natural” stream or river. This channelmay be excavated in rock or soil with or without riprap, soil cement, or othertypes of erosion protection.For guidance on the design and construction of entrance and terminal structures, seesection 3.4. Design and construction guidance on approach and discharge channels,control features, and gate chambers are outside the scope of this document.Additional guidance relating to various components of an outlet works is available inreferences, such as Reclamation’s Design of Small Dams (1987a), and USACE’sStructural Design and Evaluation of Outlet Works (2003b) and Hydraulic Design of ReservoirOutlet Works (1980).Discharge requirements through the outlet works may fluctuate throughout the year,depending upon downstream water needs or reservoir flood control requirements.Outlet works typically serve a number of different purposes (Reclamation, 2001b):58• Emergency evacuation.—The outlet works should be sized to meet establishedreservoir evacuation guidelines that apply for the State in which theembankment dam was constructed or for the agency/organization responsiblefor the dam. For example, Reclamation (1990b, p. 13) specifies depths andvolumes of the reservoir to be evacuated during specified timeframes based onthe levels of “risk” (potential for an incident to occur at the dam) and “hazard”(level of downstream consequences as the result of misoperation and/oruncontrolled release of part or all of the reservoir). Deviation from establishedevacuation guidelines may be justified for existing reservoirs if (1) the riskassociated with first filling of the reservoir has passed, or (2) the risk reductionfor increasing evacuation capacity does not justify the cost of modifications.• Reservoir filling rates.—The first filling of a reservoir is a critical time. Someembankment dams have failed due to hydraulic fracture caused by the pressureof water as it penetrates the embankment dam too rapidly. The rate ofreservoir filling is generally regulated to monitor the response of theembankment dam to increasing hydrostatic loading. The outlet works may berequired to pass some portion of the reservoir inflow to keep filling rates withinthe desired range. Typical filling rates are in the range of 0.5 to 2 feet per day.