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Energy Dissipation

Water discharging downstream of an outlet works can erode the downstream channel. Reducing the flow velocity is usually accomplished by constructing an energy dissipation structure. The design of an energy dissipation structure can vary from simple to complex.


Energy Dissipation Structures Types

Select a structure type from below to learn more:

In the hydraulic jump stilling basin, water flowing at higher than critical velocity is forced into a hydraulic jump to dissipate energy in the resulting turbulence. The hydraulic jump basin is often favored by designers since it has been extensively studied and is well documented. A properly designed basin can usually dissipate 50 to 70 percent of the energy in the flow within the basin. Tailwater is required for successful operation of hydraulic jump stilling basins.

The impact basin directs the flow into a stationary concrete baffle located within the structure that diverts the flow in all directions, causing the energy in the flow to be dissipated. The impact basin is often used in low head situations and is considered to be more effective than the hydraulic jump basin to dissipate energy, resulting in smaller and more economical structures. The impact basin requires little or no tailwater for successful performance

The plunge basin is commonly used with a cantilevered outlet pipe that is either gated or free flowing. Water scours plunge basins to a depth that is related to the height of the fall, the depth of the tailwater, concentration of the flow, and the erodibility of the bottom of the basin. The abrading action from flow into the basin may be extremely aggressive. The basin lining must be designed to ensure that it will provide acceptable performance for the life of the project.

In a stilling well, the incoming flow can be directed vertically downward into the bottom of the well or horizontally into the well. The energy dissipation is achieved by the expansion in the enlarged stilling well, the impact of the fluid on the base and walls in the stilling well, and the change in momentum resulting from redirection of flow. The flow rises up and emerges from the top of the well, which is often flush with the outlet channel.

The conduit outlet expansion is a relatively low cost energy dissipation structure that is designed to contain the hydraulic jump within the confines of a flared transition structure between the outlet conduit and the downstream channel. The conduit outlet expansion requires tailwater control and may be used for discharges with Froude numbers up to 2.0. The expansion is self-cleaning and does not tend to accumulate debris. The conduit outlet expansion is typically used on low head structures.

Riprap consists of a protective blanket of rock that is usually placed by machine to achieve a desired configuration. In some cases, the riprap is grouted in place. The riprap should be placed on top of adequate bedding. Properly anchored concrete blocks may be used as an end treatment downstream of the outlet works conduit or stilling basin. A concrete block system consists of a matrix of interconnected block units sufficient for erosion protection. Units are connected by geometric interlock and/or cables, geotextiles, or geogrids, and typically include a geotextile for subsoil retention.

The baffled drop is used to provide dissipation of energy at changes in grade downstream from the energy dissipator. This serves a useful purpose in the outlet channel but is rarely used solely as an energy dissipator. Thus, the baffled drop should be considered an ancillary design feature. The baffled drop is often used in low head applications where widely fluctuating tailwater conditions must be accommodated. The baffled drop may become uneconomical for large flows and significant drops due to wide sections and the numerous blocks involved.

From Outlet Works Energy Dissipators, FEMA 2010 🔗