Investigating the limitations of the discharge-head relationship of compound crump weirs exceeding their hydraulic capacities

Abstract

The variability of the South African (SA) climate results in abrupt changes in river discharge rates. Compound weirs have been implemented as an attempt to ensure accurate discharge calculations over an extended, but limited, range of flows. It would be financially and practically impossible to design compound weirs that can measure the entire range of flow rates that are experienced in SA rivers to the same degree of accuracy. Extensive research on the accuracy of weirs, within their intended hydraulic capacity, has been done over the years. However, when weirs operate above this capacity, three-dimensional flow is observed as a result of the presence of the divider and flank walls. The observed three-dimensional flow causes uncertainty in the application of the current discharge-head relationships, as these relationships were developed with the assumption of parallel flow lines. In this dissertation, a physical model study was done at the Department of Water and Sanitation hydraulic laboratories. The results indicated that the flow rates, calculated using the current discharge-head relationships, consistently over-estimates the input flow rates. Additionally, a numerical model study using Computational Fluid Dynamics (CFD) software STAR-CCM+ was done. CFD proved to be a valuable tool for extending the domain of the study. A comparison of the physical model and numerical model results is shown with some comments on shortcomings identified in the user-defined volume mesh inputs that could not be addressed in this study due to time and resource limitations.