Spillways
“The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood=control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as the only flood-control facility.[1]
“One of the most common causes of dam failures is the inability to safely pass flood flows. Failures caused by hydrologic conditions can range from sudden failure, with complete breaching or collapse of the dam, to gradual failure, with progressive erosion and partial breaching”.[2]
“Crest piers, abutments, and approach configurations of a variety of shapes and sizes have been used in conjunction with spillways… Not all of the designs have produced the intended results. Improper designs have led to cavitation damage, drastic reduction in the discharge capacity, unacceptable waves in the spillway chute, and harmonic surges in the spillway bays upstream from the gates. Maintaining the high efficiency of a spillway requires careful design of the spillway crest, the approach configuration, and the piers and abutments. For this reason, when design considerations require departure from established design data, model studies of the spillway system should be accomplished”.[1]
Anatomy of a Spillway
Types of Energy Dissipation
“The design of the energy dissipator probably includes more options than any other phase of spillway design. The selection of the type and design details of the dissipator is largely dependent upon the pertinent characteristics of the site, the magnitude of energy to be dissipated, and to a lesser extent upon the duration and frequency of spillway use. Good judgement is imperative to assure that all requirements of the particular project are met. Regardless of the type of dissipator selected, any spillway energy dissipator must operate safely at high discharge for extended periods of time without having to be shut down for emergency repairs. An emergency shutdown of the spillway facility during a large flood could cause overtopping of the dam and/or create unacceptable upstream flooding”.[1]
Outlet erosion control structures such as headwalls/endwalls, impact basins, USBR Type II or Type III basins, baffled chutes, or plunge pools are all considered energy dissipating devices.[3]
Headwalls/endwalls located at or close to the end of the discharge conduit provide support and reduce the potential for undermining. Headwalls/endwalls are typically made of concrete and should be founded on bedrock or have an adequate foundation footing to provide support for stability. A headwall/endwall can experience undermining and become displaced if not adequately designed. Displacement of the headwall/endwall can lead to separation from the conduit at the joints which could affect the structural integrity of the conduit itself.[3]
A concrete impact basin is an energy dissipating device located at the outlet of the spillway in which flow from the discharge conduit strikes a vertical hanging baffle. Energy dissipation occurs as the discharge strikes the baffle; thus, performance is not dependent on tailwater elevation. Most impact basins were designed by the NRCS and the USBR.[3]
Type II and Type III basins reduce the energy of the flow discharging from a spillway and allow the water to enter the outlet channel at a reduced velocity. Type II basins contain chute blocks at the upstream end and a dentated (tooth-like) endsill. Baffle piers are not used in a Type II basin because of the high velocity water entering the basin. Type III energy dissipators can be used if the entering flow velocity is not high (less than 50 feet per second (Hydraulic Design of Stilling Basins and Energy Dissipators, USBR, 1984)). Type III basins contain baffle piers which are located on the stilling basin apron downstream of the chute blocks. Located at the end of both types of USBR stilling basins is an endsill, which may be leveled or sloped, that creates a tailwater to reduce the velocity of the flow leaving the basin. Baffled chutes require no initial tailwater to operate effectively and are located downstream of the control section. Multiple rows of baffle piers on the chute prevent excessive acceleration of the flow and prevent the damage that occurs from a high discharge velocity. A portion of the baffled chute usually extends below the streambed elevation to prevent undermining of the chute. A plunge pool is an energy dissipating device located below the outlet of a spillway. Energy is dissipated as the discharge falls into the plunge pool as a free-flowing jet. Plunge pools are commonly lined with rock riprap or other material to prevent excessive erosion of the pool area. Discharge from the plunge pool should be at the natural streambed elevation. Typical problems may include movement of the riprap, loss of fines from the bedding material, and scour beyond the riprap and lining. If scour beneath the outlet conduit develops, the conduit will be left unsupported and separation of the conduit joints and undermining may occur.[3]
Types of Spillways (Classified by Operation)
Types of Spillways (Classified by Function)
Examples
Best Practices Resources
Trainings
On-Demand Webinar: Inspection and Assessment of Spillways
Citations:
Revision ID: 2412
Revision Date: 09/07/2022