A CFD Account of the Mean Flow and Turbulence over Breached Embankment Dam

Abstract

We measure and compute the flow over a breached dam, identifying the main topological features of the mean flow (separation and stagnation surfaces, attached vortexes), secondary currents and boundary layer development. The Navier-Stokes solver is based on the RANS with a k-ω SST closure model and a rough-wall correction, implemented in OpenFoam. The geometry of the breached dam corresponds to a fixed instant of a model dam tested at LNEC. It depicts a homogeneous dam at the transition between the first and second erosion stages (the breach has started to deepen). Validation data is provided by PIV measurements obtained at IST. We show that the development of the boundary layer over the crest is unlikely to be described by the Kármán momentum integral, as it is influenced by the breach crest wall curvature, secondary currents and strong lateral flow convergence. Three-dimension separation is observed in the plunging pool. Two different attached vortices develop at the interface between the dam body and horizontal fixed wall. The first is associated to an adverse pressure gradient at the lower end of the boundary layer. The second seems originated by the flow that spreads outward from the stagnation point of the plunging flow. The attached vortices rotate in opposite directions but may both contribute to underscoring. Strong turbulence production is observed in the plunging pool. A counter rotating vortex pair is observed in the flow exiting the dam breach channel. We discuss the significance of these structures for hydraulic erosion and underscoring. We also provide recommendations for CFM modelling of dam breaching.