Comparative Experimental Study on Local-Scour Downstream of Labyrinth Weirs with Different Planforms

A labyrinth weir is an overflow spillway to regulate and control flow in canals, rivers, and reservoirs. The main hypothesis is to effectively increase the crest length, and in doing so, to increase the discharge per unit width of the channel for a given upstream hydraulic head. The aim of the present study was to investigate the local scour downstream of labyrinth weirs with three different planforms with triangular, trapezoidal, and curved apexes. The development of scour hole with time was examined for three free-flow conditions toward the equilibrium state of the bed. The bed scour for each of the three labyrinth planforms was compared with an equivalent standard weir. In terms of flow capacity, primary results indicated that the curved planform is more efficient. With respect to downstream scour, it is generally deeper with the standard weir. The response of labyrinth weirs varies under different hydraulic heads. In general, the triangular, curved, and trapezoidal planforms produce decreasing scour depth, respectively. In low flow conditions, the scour downstream of the trapezoidal planform was 19% and 28% greater than for the triangular and curved planforms, respectively. Corresponding values were 22% and 7% for mean flows, and 11% and 5% for high flows. The impacts of Froude number, upstream head, and time on the scour depth were also examined. Semi-empirical relationships are presented to estimate the scouring time, maximum scour depth, and location for the three labyrinth planforms tested herein. The estimated relative error for maximum scour depth and its location is 5% and 10%, respectively.