Comparing labyrinth and ogee spillways at high hydraulic heads – a case study in Zambia

Abstract

A new over-the-dam spillway and 3 m dam raising were selected as the preferred option for resolving matters in the interest of safety for an existing dam in Zambia. The cost of supplying concrete at the dam site had the greatest effect on overall project capital costs, and thus optimising the size of the concrete spillway was critical. Both ogee and labyrinth weir designs were developed for a hydraulic head of 5 m in the Safety Check Flood. The ogee shape design was developed to achieve high efficiency and required a weir width of 81 m. For labyrinth weirs, a limiting factor to the allowable head is the ratio between the water head and weir height above its foundation apron. Using published state-of-the-art research, 60 labyrinth weir configurations were tested, with varying angles, leg lengths and number of cycles. The optimal labyrinth configuration was chosen as the one with the minimum overall spillway width 64 m, and volume of concrete. This configuration provided a 23% potential reduction in concrete volume compared to the ogee. The labyrinth design was taken forward for computational fluid dynamic modelling to prove the concept and for further development into a detailed design.