Scour of unlined dam spillways

[摘要] ENGLISH ABSTRACT: The scour process of unlined spillways is an important research topic of value in engineering practice.In South Africa numerous unlined spillway dams have experienced severe erosion. This led, in somecases, to the costly concrete lining of spillways for erosion protection. On the other hand, the erosionof unlined spillways can lead to damage to, and even failure of dams and consequently can affect publicsafety, properties, infrastructure and the environment. In this regard, methods to predict erosion ofunlined spillways are therefore needed as tools in the risk management and design of existing unlinedspillways as well as future spillway structures.The prediction of the rock scouring process is challenging and empirical formulas have been establishedto predict incipient conditions for scour. These empirical methods however do not predict the rate ofscour or the ultimate equilibrium rock scour.The key objective in this study was to investigate the applicability of a non-cohesive two-dimensional(2D) sediment transport hydrodynamic mathematical model to simulate unlined spillway scour. Aphysical model flume test was set up to simulate rock scour represented by uniformly sized polyethylenecubes. The flume slope and discharge were varied in the different test scenarios. The 2D mathematicalmodel correctly predicted the extent and location of scour as observed in the laboratory tests. Temporalchanges in the scour formation were also predicted with reliability. This was achieved by only calibratingthe hydraulic roughness of the 2D model, and by specifying the rockparticle settling velocity andmaterial density. The simulation results were satisfactory, providing an accurate and detailed erosionprediction. From this, the mathematical modelling was validated by using a field case study.The results obtained with the mathematical model were promising for non-cohesive cases and could beapplied to field prototype cases if the rock joint structure is known. This would typically apply in faultzones, where the joints would give an idea of the rock size to be used in the mathematical model. Ingeneral, where jointed rock is more massive and acts cohesive , rock parameters describing criticalscour conditions for the rock in terms of stream power are required to be built into the mathematicalmodels.

[发布机构] Stellenbosch University