Scour on Alluvial Bed Downstream of Falling Jets

This paper describes a theoretical approach for predicting the local scour downstream of falling jets, due to grade control structures, with the support of laboratory evidences. A number of 63 runs were con- ducted, varying sediment grain size, discharge, downstream water level and drop height. For each run, the scour hole profile was accurately measured in order to derive the bottom shear stress from the local slope, whence the hydrodynamic characteristics of the flow field at the scour bottom were derived from theoretical analyses. These analyses show that the turbulence structure in the scour hole can be subdivided in three zones: i) the impinging jet zone, ii) the transition region and iii) the wall jet zone in which the mean velocity profile is similar. These considerations allow to compute the bottom shear stress distribution along the scour hole di- rectly from the impact jet momentum at the scour bottom. Through the sediment equilibrium equation, the scour hole shape is computed and the maximum scour depth is derived.