Local rock scour downstream large dams

This paper draws some examples from research works carried out in Argentina on local scour downstream from ski jump structures and from hydraulic jump energy dissipators. An equation of local scour downstream from flip buckets, intentionally oversimplified so as to be of use as an initial scour estimate, is presented. Knowledge of unit discharge and fall height from reservoir level to tailwater level is required. It should be pointed out that during the design stage many parameters are unknown, such as the size of blocks formed by rock fracture at different depths near the jet impact. For preliminary calculations, this equation shows acceptable performance of data furnished by other authors, including data that was published after the equation was formulated. The river bed is formed by local erosion downstream from hydraulic jump energy dissipators in large flatland dams, where rocks undergo severe pressure fluctuations. This problem was analyzed using fluctuating pressures (statistic amplitude and frequency values) in the base of a hydraulic jump stilling basin, downstream from a continuous end sill, taking into account amplification of the process and the trend to induce important depressions. When the riverbed is made up of granular material, the end sill collaborates for a controlled local scour near the structure. If the riverbed is composed of large rocks, prototype experimental data demonstrates that an end sill is not a good solution. It increases flow fluctuations and concentrates the turbulent energy around a dominant frequency, removing bigger blocks and favoring local scour. A methodology to calculate the weight of eventual protecting blocks or anchorage bars to avoid removal is proposed. It takes into account the incident Froude Number of the jump and the relationship between the block’s width and length. The authors propose an unconventional. approach to estimate maximum local scour depth downstream of stilling basins with baffled piers (for forced energy dissipation) and not sufficient tailwater depth. The critical flow condition over the end sill of an energy dissipater generates a new hydrodynamic condition. The accelerated flow acts on the river bottom as a submerged jet, as observed for the hydrodynamic configuration of ski-jump spillways, but with very low hydraulic head.