Enhancing Spillway Efficiency Discharge Coefficients: Insights from Historical Models and Case Studies of Ogee, Lateral, and Non-Linear Structures
Accurate estimation of spillway discharge capacity remains fundamental to dam safety and hydraulic design. Classical formulations such as those of USBR (1987), Ven Te Chow (1959), USACE (1990) and DWS Calibration Manual were developed under the idealised geometric and hydraulic assumptions. However, many South African dams were designed and tested under complex three-dimensional approach conditions. Asymmetric basing geometries and non-standard spillway layouts. Although numerous physical model studies and subsequent DWS calibrations records exist, these datasets have not previously been synthesised int a consolidated framework to evaluate the systematic discharge behaviours across configurations. This study analyses historical South African physical model studies and available calibration datasets spanning more than four decades to identify consistent tendencies in spillway discharge performance under practical geometric and hydraulic conditions. Sixty dams were compiled, including ogee, side-channel, and non-linear spillways. The datasets were normalised using dimensionless discharge coefficients and unit head ratios to allow consistent comparison across model scales and prototype conditions. Statistical trend analysis was applied to evaluate how observed discharge behaviour compares with established theoretical formulations under varying geometric scenarios. The results demonstrate that ogee spillways exhibit the most consistent and predictable behaviour, with relatively small systematic deviations between theory and observed performance. Approach-flow asymmetry and non-perpendicular inflow introduce measurable efficiency reductions, primarily associated with non-uniform velocity distribution and contraction effects not fully captured in classical two-dimensional formulations. Curved spillways may exhibit slight efficiency enhancement due to flow concentration effects. Lateral (side-channel) spillways show consistent overprediction when ogee-based formulations are applied, confirming that lateral-flow hydraulics cannot be fully represented by traditional crest-based discharge equations. Non-linear spillways (labyrinth and piano-key configurations) display highly site-specific behaviour, and the available dataset remains insufficient to define robust generalised trends. Importantly, the study does not treat model-derived or calibration datasets as definitive prototype truth. Both physical model studies and calibration records contain inherent uncertainties related to scale effects, measurement accuracy, geometric tolerances, and interpretive judgement. The deviations identified are therefore interpreted as performance tendencies within historical South African practice rather than deterministic correction factors to be universally applied to theoretical equations. The principal contribution of this research lies in the consolidation and structured interpretation of historical South African spillway datasets into a comparative analytical framework. The findings provide a diagnostic basis for identifying geometric and hydraulic conditions under which theoretical discharge predictions warrant careful scrutiny or additional verification. In this way, the study advances understanding of spillway discharge behaviour in a national context while reinforcing the continued relevance of classical theory when applied with informed engineering judgement. Overall, the thesis demonstrates that while standard discharge equations remain broadly valid for conventional ogee geometries, their reliability diminishes under asymmetric, lateral, or highly site-specific conditions. The work therefore supports more informed early-stage design assessment and dam-safety review by contextualising theoretical predictions within documented empirical behaviour.
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