Improving the Range of Hydraulic Performance of Type III Stilling Basins
The hydraulic performance of Bureau of Reclamation (Reclamation) Type III stilling basins may be improved over a broader range of flow conditions with the installation of a ramp in between the baffle blocks. During a physical hydraulic model study funded by the U.S. Army Corps of Engineers in 2008, the performance of a modified Reclamation Type III stilling basin unexpectedly improved with the installation of a 3H:1V (horizontal:vertical) sloped ramp in between the baffle blocks. The ramp, which was installed to reduce cavitation potential on the stilling basin floor, allowed the basin to be more stable over lower tailwater elevations. The modeled stilling basin contained a stepped spillway with no chute blocks at a 2.5H:1V slope.
It is unclear whether the stepped chute or spillway slope contributed to the success of the ramp, so further research is warranted to identify under what conditions this phenomenon holds true.
Need and Benefit
Improving the acceptable range over which Reclamation Type III stilling basins can successfully operate will improve infrastructure reliability. When adequate tailwater is not available downstream of a stilling basin, unstable flow surging develops as the toe of the hydraulic jump is pushed downstream to the face of the baffle blocks. When the stilling basin can no longer contain the hydraulic jump, heavy turbulence and high velocities occur downstream of the basin, leading to scour and the potential for undermining of the structure. This situation can cause costly repairs, an out-of-service period for operation, and a risk to public safety. The design of a stilling basin that functions at lower tailwater elevations may also limit the amount of excavation needed for construction in some cases.
Providing a modification for the Reclamation Type III stilling basin to improve performance at low tailwaters has broad application within Reclamation, within the Federal Government, and in the private sector. Reclamation's Engineering Monograph No. 25, "Hydraulic Design of Stilling Basins and Energy Dissipaters," published originally in 1958, is the industry standard for the design of stilling basins. In general, this document recommends tailwater elevations equivalent to the conjugate flow depth or higher. For river conditions where insufficient tailwater is available, an alternate stilling basin design will be directly applicable. Nathan Cox, Hydraulic Engineer with the U.S. Army Corps of Engineers, and Michael Phillips, Senior Project Engineer with URS Corporation, are both supportive of further investigations into improving the stability of Reclamation stilling basins.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Performance of Type III Stilling Basins – Stepped Spillway Studies (final, PDF,
By K. Warren Frizell and Connie Svoboda
Report completed on May 01, 2012