【Daily Discussion】Public Utilities – Water Resources and Hydropower – 20210331

a. Flow diversion energy dissipation b. Surface flow energy dissipation c. Bottom flow energy dissipation

For gravity dams, the energy dissipation methods used include bottom-flow energy dissipation, spillway energy dissipation, and surface-flow energy dissipation. Bottom-flow energy dissipation involves employing certain engineering measures (such as constructing a stilling basin) to control the position of the water flow; the residual energy is then dissipated through surface vortices and intense turbulence caused by hydraulic jumps. A concrete apron of a certain length is installed downstream of the dam; when the water flowing over the dam passes over this apron, a hydraulic jump occurs, generating vortices that cause the water’s energy to be partially dissipated through aeration and the mutual collision and friction between water molecules. This helps to reduce or prevent severe scouring downstream. Jet dissipator: This energy-dissipation method makes use of the jet apron at the outlet of the water discharge structure to throw the rushing water into the air, where it then falls onto the riverbed farther away from the structure and merges with the downstream water flow. Energy consumption is generally divided into three parts: frictional energy loss due to the flow over the solid boundaries ； The jet frictions with the air in the air, mixes with air, and dissipates its energy through diffusion ； The jet falls into the downstream tailwater, where turbulent diffusion dissipates the energy. Through the nose weir, picket jet energy dissipation can effectively control the location, range, and flow distribution of the jet as it falls onto the downstream riverbed within the picket jet area. It has strong adaptability to variations in tailwater levels, a simple structure, and is easy to construct and maintain. However, the erosion downstream is severe, with a large amount of sediment; the fluctuations in tailwater and the degree of atomization are also high. Turbine flow dissipators are widely used and suitable for various types of structures with medium to high head levels and large, medium, or small flow rates. Surface flow energy dissipation is a method of dissipating excess energy by using steps or funnels at the end of the discharge structure to lift the main stream of the fast-flowing water onto the surface; the remaining energy is then eliminated through the diffusion of the main stream across the surface, as well as through swirling motions at the bottom and on the surface (see figure). Surface flow energy dissipation is divided into two types: stepped surface flow energy dissipation and scoop-shaped surface flow energy dissipation. …Overflow. The methods used to dissipate energy in gravity dams include bottom flow dissipation, spillway dissipation, and surface flow dissipation. Bottom flow dissipation involves using certain engineering measures (such as constructing weirs) to control the water level, and the excess energy is eliminated through surface turbulence and intense mixing that occur as a result of water jumps. A concrete revetment of a certain length is installed downstream of the dam; the water flowing over the dam creates water jumps on this revetment, resulting in turbulence that helps to dissipate some of the water’s energy through air incorporation, collisions between water molecules, and friction, thereby reducing or preventing severe erosion downstream. Jet dissipator: This energy-dissipation method makes use of the jet apron at the outlet of the water discharge structure to throw the rushing water into the air, where it then falls onto the riverbed farther away from the structure and merges with the downstream water flow. Energy consumption is generally divided into three parts: frictional energy loss due to the flow over the solid boundaries ； The jet frictions with the air in the air, mixes with air, and dissipates its energy through diffusion ； The jet falls into the downstream tailwater, where turbulent diffusion dissipates the energy. Through the nose weir, picket jet energy dissipation can effectively control the location, range, and flow distribution of the jet as it falls onto the downstream riverbed within the picket jet area. It has strong adaptability to variations in tailwater levels, a simple structure, and is easy to construct and maintain. However, the erosion downstream is severe, with a large amount of sediment; the fluctuations in tailwater and the degree of atomization are also high. Turbine flow dissipators are widely used and suitable for various types of structures with medium to high head levels and large, medium, or small flow rates. Surface flow energy dissipation is a method of dissipating excess energy by using steps or funnels at the end of the discharge structure to lift the main stream of the fast-flowing water onto the surface; the remaining energy is then eliminated through the diffusion of the main stream across the surface, as well as through swirling motions at the bottom and on the surface (see figure). Surface flow energy dissipation is divided into two types: stepped surface flow energy dissipation and scoop-shaped surface flow energy dissipation. Energy dissipation via hopper surface flow occurs when a hydraulic jump forms within the hopper, followed by the formation of surface flow behind it. This is a hybrid energy dissipation method that combines the characteristics of both bottom flow and surface flow. Surface flow energy dissipation is suitable for medium and low head conditions, where the downstream tailwater is deep, the water level variation is small, and the riverbanks are stable with strong resistance to erosion.

a. Flow diversion energy dissipation b. Surface flow energy dissipation c. Bottom flow energy dissipation