Design of baffled hydraulic jump stilling basins for dams

Abstract

The hydraulic jump has been studied and used as a primary means of energy dissipation for hydraulic structures for well over a century. By the 1920s and 1930s, baffled hydraulic jump stilling basins were in widespread use as energy dissipators for large dams. These hydraulic jump stilling basins often consisted of toe blocks, a negative step and/or toe curve; one or more rows of baffle blocks; and a solid or dentated end sill. By the 1950's standard design guidance was developed by multiple agencies and universities. A comparison of the standard baffled hydraulic jump guidance illustrates a drastic difference in recommended stilling basin geometry with identical incoming flow conditions. For example, given the same incoming flow conditions, the height of baffle block determined for a U.S. Bureau of Reclamation standard design can be more than twice the block size for a standard outlet works stilling basin determined from US Army Corps of Engineers guidance. The use and/or geometry associated with chute blocks, number of rows of baffle blocks, length of basin, distance to baffle blocks, and end sill geometry have similar discrepancies. The current research includes a systematic physical model evaluation, performing over 400 individual experiments of the most often utilized baffled stilling basin design configurations. These experiments include 15 stilling basin configurations, each being evaluated for six discharge conditions and six tailwater scenarios. In addition, the USACE standard stilling basin configuration was evaluated for the general scour tendencies for discharges below and above the design discharge by means of mobile bed physical modeling. A numbered list of significant findings associated with the current research are provided below. Detailed descriptions of these significant findings and other conclusions and recommendations associated with the research objectives are provided subsequently. 1. The USACE (1992) stilling basin configuration is recommended for incoming Froude Numbers less than 4.5. 2. The Modified Type III stilling basin configuration is recommended for incoming Froude Numbers in the range of 4.5 to 8. 3. The stilling basin length and minimum required tailwater for the USACE (1992) and USBR (1984) can be expressed by unified equations. 4. A toe curve is not recommended for the hydraulic jump basin due to increase in length required and the decrease in jump stability. 5. Intermittent ramps associated with the tapered baffle block configuration are not recommended due to increase in downstream scour potential, decreased tailwater resilience, cost, general lack of observed cavitation damage, and unproven effectiveness in reducing cavitation damage. 6. Macro-scale turbulent structures exiting the stilling basin are the primary phenomenon controlling downstream scour potential. A maximum downstream attack angle of 15-degrees from horizontal was determined for the USACE (1992) stilling basin configuration for incoming Froude Numbers in the range of 3 to 5.