Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 3Hydraulic Design of ConduitsThe discharge of water through a conduit requires a good understanding of thepurpose for which the structure is being designed. For instance, the invert profile ofthe conduit should be sloped to provide drainage in the downstream direction.Where feasible, the conduit should discharge at an elevation higher than the highesttailwater or at an elevation where there is no influence from tailwater. Free flowconduits should not flow greater than 75 percent full (i.e., 75 percent of the diameteror height of the conduit) at the downstream end, to minimize the risk of surging flowdeveloping in the conduit as the result of inadequate air for open channel flowconditions.This chapter discusses some of the pertinent aspects of hydraulic design of conduits.For detailed guidance on hydraulic design, the reader is directed to references such asReclamation’s Design of Small Dams (1987a), and USACE’s Structural Design andEvaluation of Outlet Works (2003b) and Hydraulic Design of Reservoir Outlet Works (1980).3.1 Outlet worksThe main purpose of an outlet works through an embankment dam is to control therelease of water from a reservoir. An outlet works typically consists of acombination of structures. The outlet works is often comprised of the some or all ofthe following components (this list is not all inclusive, and the type of componentsmay vary, based on project requirements):• Approach channel.—The channel upstream from the intake structure. Thischannel is generally unlined, excavated in rock or soil, and with or withoutriprap, soil cement, or other types of erosion protection.• Entrance structure (typically referred to as an intake structure).—A structure located atthe upstream end of the outlet works. Entrance structures often include gatesor valves, bulkheads, trashracks, and/or fish screens.• Conduit.—A closed channel used to convey water through the embankmentdam.57