Numerical modelling of separated flow in river bends: model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank

The occurrence of separated flow in sharply curving river bends, and the associated deposition of fine-grained sediment, is widely recognized, but the factors controlling the existence and extent of separated flow have not been clearly identified. A three-dimensional, fixed lid computational fluid dynamics program was used to simulate time-averaged flow in idealized channel bends in order to investigate geometric controls on flow separation. The code was applied first to a field site; it successfully modelled the qualitative features of the flow, including a separation zone, and was insensitive to choice of turbulence closure or exclusion of lateral variation in water surface topography. The simulations of idealized bends demonstrate that the existence and extent of concave bank flow separation can be significantly influenced by changes in bend planform, point bar topography and upstream planform.