Hydraulic Properties and Local Scour Downstream of Permeable Grade-Control Structures

Abstract

A series of laboratory experiments were carried out to study the hydrodynamics of flow through and over permeable grade-control structures with different dimensions and degrees of permeability. The local erosion downstream of the grade-control structures was measured at different flow regimes, and the effects of geometry and permeability of the grade-control structures were evaluated. Three flow regimes were observed in free-flow conditions, named as the through-flow regime, combined fully through-flow and overflow regime, and the transition between the two regimes. Semiempirical equations were proposed based on the theory of orifices and weirs to estimate the stage-discharge relationships over and through the permeable grade-control structures. The maximum scour depth at the downstream was predicted by implementing dimensional analysis and multivariable regression technique for both partial through flow (i.e., transition regime) and the combination through-flow and overflow regime. A series of numerical simulations was also performed to model the hydraulics of flow through and over grade-control structures. The numerical model was validated with the experimental results, and detailed information such as instantaneous and time-averaged velocity profile, bed shear stress, and turbulent kinetic energy were extracted from the validated numerical model. The numerical results clearly showed that the effect of crest length on jet deflection and formation vortex region at the downstream of structure was significant. The numerical outcomes indicated the existence of three zones in the surface jet above the grade-control structure, named the entrance zone, the upper roller zone, and the upper–lower rollers interaction zone. Numerical results confirmed a direct correlation between the structure’s length and the geometry of local scour hole and indicated that by increasing the crest length by three times, the maximum scour depth decreased by approximately 25%.