Local scouring at grade-control structures in alluvial mountain rivers

Abstract

Scour holes below 73 grade-control structures (check dams and bed sills) in six mountain rivers located in the eastern Italian Alps have been surveyed. The most likely formative water discharge is used to evaluate jet thickness at each structure, which along with drop height, appears to determine scour hole dimensions, as shown by the consistent trends observed for nondimensional plots of both maximum scour depth and length versus the respective drop ratio. Sediment differences regarding size and lithology apparently play a minor role in determining scour hole dimension. Measured maximum scour depths are well predicted by a semiempirical equation developed through laboratory results, showing an average relative error of 0.13. Scour hole geometry is described by several ratios that are thought to represent approximately invariant characteristics. A new energy-based normalization for scour hole dimensions is proposed as the most suited to evaluate the role of jet geometry and aeration upon scouring efficiency. It is noteworthy that the ratio between the maximum scour depth and the total energy available at a drop tends toward an asymptotic value around unity for increasing drop heights.