Energy Dissipation: Difference between revisions
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{{Website Icon}} | {{Website Icon}} | ||
==Best Practices Resources== | ==Best Practices Resources== | ||
{{Document Icon}} | {{Document Icon}} [[Design Standards No. 14: Appurtenant Structures for Dams (Spillways and Outlet Works)]] | ||
{{Document Icon}} [[Hydraulic Design of Spillways (EM 1110-2-1603)]] | |||
==Trainings== | ==Trainings== | ||
{{Video Icon}} | {{Video Icon}} | ||
Revision as of 18:30, 7 September 2022
“Terminal structures located immediately downstream of the conveyance feature include stilling basins, energy dissipaters, and flip buckets. These structures are intended to dissipate or manage the kinetic energy of the flow, so it can be returned to the river or stream without significant scour or erosion that could damage or fail the dam and appurtenant structures”.[1]
The high energy and velocity of flow coming down a spillway can be destructive in more ways than one if not adequately contained and dissipated. Flow from a spillway with high velocity can cause erosion at the toe of the dam, and if left unchecked, can destabilize the dam embankment or base of the concrete monolith. High velocity flows can also damage downstream natural channels causing excessive sediment transport and negatively affecting the waterways downstream of the dam. High velocity flows can also cause cavitation damage of the spillway or stilling basin components, degrading concrete and steel, and causing structural instabilities.
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”.[2]
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]
Examples
Best Practices Resources
Design Standards No. 14: Appurtenant Structures for Dams (Spillways and Outlet Works)
Hydraulic Design of Spillways (EM 1110-2-1603)
Trainings
Citations:
Revision ID: 2423
Revision Date: 09/07/2022