Experimental Study and Numerical Simulation of Scour Inducing Effect on Seepage Underneath Barrages

Flow of water through a pervious foundation produces a seepage force as a result of friction between water and walls of soil pores. Such force is related to the hydraulic gradient due to the decrease in potential head and friction losses. As water upwardly percolates through the exit face, the seepage force tends to lift the soil, by piping or heave phenomenon, due to the excessive hydraulic gradients at the exit face close to the toe of structure. Again, the loss of potential head due to friction, through pores of the under soil, causes uplift pressures upwardly act on the underside of structure floor. Such uplift forces will affect the structure stability against sliding and overturning leading to its failure. On the other hand, seepage flow leads to a loss of considerable quantities of water which is of economic concern for storage structures. Hydraulic structures are often constructed in alluvial streams composed of loose soil; the downstream bed line, of such structures, may be locally lowered, in the form of a hole, due to erosion of bed material, resulting from the interaction between the high velocity of surface flow and the loose material of bed. Such a process called local scour and the formed hole called scour hole. The present paper focuses on the experimental and numerical models to investigate the seepage below hydraulic structures induced by scouring holes. Several scenarios of scour holes with different shape, position, and dimensions have been conducted to clarify the impacts on the seepage characteristics; potential and exit gradients, and seepage discharge. Different shapes, for the scour hole, were considered, these are; segmental circular shape and the trapezoidal shape with its derivatives rectangular and triangular shapes. Effect of the considered parameters, on the seepage characteristics was numerically analyzed using the finite elements method. The model was validated analytically, for some idealized seepage problems that have been studied before. Moreover, some cases were experimentally verified using the viscous flow model and a good agreement was existed. Results of study showed that formation of scour hole greatly affects the seepage characteristics. Thus, a decrease in potential values was obtained, due to existence of scour hole, while values of both exit gradient and seepage discharge were increased. The numerical results are expressed by empirical dimensionless charts, presented in a form to be easily used. Design guidelines are developed to predicate the seepage characteristics in terms of the score hole parameters.