Computational fluid dynamics modelling of three-dimensional processes on natural river floodplains

Results are presented from numerical simulations of overbank flows on a natural river floodplain. The model used here solves the three-dimensional Navier-Stokes equations with an RNG k-epsilon turbulence closure. Channel roughness is treated using a wall function approach while a drag-law is used to represent the effects of floodplain vegetation. The model is applied in situations with complex boundary conditions that are specified using a two-dimensional depth-averaged hydraulic model. Simulation results are compared with field measurements of three-dimensional flow velocity and turbulence obtained using an array of Acoustic Doppler Velocimeters (ADV). The model is shown to reproduce the overall spatial patterns in the flow data and the shape of vertical profiles of velocity and turbulent kinetic energy. Differences between model results and ADV measurements reflect small-scale local variability in field conditions and systematic variations in surface roughness. Results illustrate that natural floodplains are characterized by complex, topographically driven and stage-dependent flow structures.