Scour Downstream of Grade‐Control Structures

Abstract

Grade-control structures prevent excessive channel-bed degradation in alluvial channels. The erosive action of flowing water, however, causes significant downstream local scour, which may undermine these structures. Structural design considerations must therefore include adequate protective measures against local 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 was verified experimentally. 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 used a large-scale physical model with unit discharge up to 2.5 sq m/s and scour depths exceeding 1.4 m. 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.