Undercurrent: The Alden Blog


Industrial fluid dynamics insights

Energy Dissipating Devices: Total Dissolved Gas Production at High Head Dams - Part 3
In Parts 1 and 2 of this series we talked about production of total dissolved gas (TDG) at high head dams, and the use of air supply systems to mitigate cavitation damage to spillways and spillway modifications. In this part, we’ll discuss the structural designs used to reduce TDG during flow release from spillways. Super-cavitating roughness elements, or baffle blocks for short, are designed to break up flow as water travels down a spillway. Depending on the placement of the blocks on the spillway surface, they can also help spread out the jet of water laterally as it exits the spillway, resulting in a larger impact area on the tailrace. Why does this help reduce TDG production? As we discussed in Part 1, dissolved gas supersaturation in the tailrace is a function of how long it takes the bubbles from the aerated flow to reach the water surface. By spreading out the impact zone of the spillway jet, and decreasing the amount of energy the jet has, plunge depth of the jet is reduced and the aerated flow can rise ...

Air Supply Ramps and Cavitation: Total Dissolved Gas Production at High Head Dams – Part 2
Part 1 of this series outlined how high concentrations of total dissolved gas (TDG) can occur downstream from high head dams when their spillways are open, and how this TDG can be harmful or even fatal to fish. Alden has been involved in several recent projects for which the objective was to reduce TDG downstream of high head dams. Alden performed the hydraulic and structural design of roughness elements that break up the high velocity jet of flow discharged from the spillway. TDG production is reduced by these roughness elements because they cause the jet to spread out and thereby reduce the plunge depth in the receiving water, which reduces TDG. The roughness elements work very well at reducing plunge depth, but they can cause cavitation, which can damage the spillway surface and the blocks themselves. The design and implementation of the roughness elements will be topic of another article. The present article focuses on reducing the potential for cavitation on the roughness elements. Alden designed roughness elements have been installed on spillways at Cabinet Gorge and Boundary Dams. Cabinet ...