Scour Downstream of Grade-control Structures

A theoretical investigation of local scour downstream of grade-control structures based on two-dimensional jet diffusion and particle stability is experimentally verified. Turbulent jet diffusion reduces fluid velocity near the bed particles and equilibrium scour is obtained when noncohesive bed particles cannot be removed from the scour hole. Equilibrium scour depth is written as a function of velocity, flow depth and particle size. The theoretically derived equation is remarkably similar to the regression equations reported in the literature. The experimental investigation uses a large-scale physical model with unit discharge up to 2.5 m/s (27 sq ft/sec) and scour depths exceeding 1.4 m (4.6 ft). When combined with previous data sets at smaller scales, a total of 231 scour-depth measurements cover a wide variety of conditions: wall to vertical jets, small to large flow submergence, and various face angle slopes. The agreement between calculated scour depths and laboratory measurements is satisfactory considering the wide variety of configurations analyzed.