This paper discusses a computational methodology for modelling rock scour in unlined spillways. Erosion damage mechanisms of unlined spillways were studied by modelling the flow of water and the geomechanical behaviour of the rock mass. Erosive capacity of the spillway bed is determined by computational fluid dynamics simulation to determine the pressure fluctuation and time dependent shear stress of the water flow regime. The resistive capacity of the rock is represented by dynamic discontinuum modelling of jointed rock subjected to hydrodynamic forces obtained from the flow analysis. The discontinuum approach presented in this paper models the rock block ejection or plucking and brittle fracture of blocks without assuming a specific failure mode. In this proposed numerical approach, full-scale flow analysis reduces dependence on hydraulic model studies thereby allowing study of sensitivity of parameters affecting erosive capacity. The geomechanical modelling not only provides an understanding of the mechanism and the extent of damage but also allows study of resistive capacity of mitigation systems (e.g. rock bolts, cut-off walls). ANSYS FLUENT was used for computational fluid dynamics flow analysis and UDEC was used for discontinuum analysis of the jointed rock mass. Erosion processes of the Kariba Dam,Zimbabwe and Canyon Lake Spillway Gorge, Texas were selected as examples. Analyses of these two examples demonstrated that the approach discussed in this paper successfully models the combination of block removal and brittle fracture in the Kariba Dam plunge pool and block removal or plucking during of the Canyon Lake Gorge.
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