Experimental Investigation of the Effect of Tail Water Level on Discharge Coefficient of Trapezoidal Labyrinth Weirs

Labyrinth weirs are often a favorable design option to regulate water surface elevation in upstream canal and increase flow capacity. Due to their hydraulic performance and geometric versatility, labyrinth weirs have been used extensively in canals, rivers, ponds, and reservoirs as energy dissipaters, flow aerators, and spillways. Nevertheless, it can be difficult for engineer to optimally design, labyrinth weirs due to the complexities of nappe behavior and many geometric and hydraulic variables. A level of sedimentation in upstream and submergence in downstream of the weirs are effective parameters which affects the discharge coefficient, and the performance of these structures. In this investigation effects of 408 tests of different levels of tail water (increase of downstream canal bed level to one third and two third of weir height) and upstream sedimentation level (non-sedimentation and sedimentation level of 90 percent of weir height) on the discharge coefficient of different geometers of trapezoidal labyrinth weirs was investigated experimentally. The analysis of results indicated that increase of tail water level to one third of weir height didn’t affect on the discharge coefficient of weirs. By increasing tail water depth to two third of weir height, air cavity volume under nappe decreased. Therefore, the flow pattern of the nappe changed from leaping to clinging condition and upstream head decreased. As a result, the labyrinth weirs discharge coefficient in non-sedimentation and sedimentation level of 90 percent of weir height increased about 3.3 to 12.2 and 2.1 to 9.2 in comparison with non-submergence tail water for maximum discharge respectively. Finally, the statistical equation for estimation of the discharge coefficient of trapezoidal labyrinth weirs for upstream sedimentation and submerged conditions was developed.