Hydrodynamic modelling of free-surface flows with application to leaky barriers

Abstract

Leaky barriers are styled after natural accumulations of large woody debris. Communities and consortia have been installing them in watercourses with the intention of decreasing flood risk downstream. However, there is a lack of quantitative evidence on their impact at both small and large scales. To address the evidence gap, this thesis uses a fluid dynamics approach, combining hydraulic modelling, Computational Fluid Dynamics (CFD) and physical modelling in a flume. First, leaky barriers are inserted into a 1D hydraulic model for large-scale modelling by modifying a Godunov-type scheme. At the location of a leaky barrier, fluxes from the barrier discharge equation replace the Riemann solver fluxes. Results compare well with 55 flume experiments, but this method only works when the barrier resembles a wellunderstood hydraulic structure, such as the weir and gate combination considered here. Different types of barriers require more fundamental investigations. The majority of the thesis is about such small-scale modelling with CFD, with a focus on extending the CFD software OpenFOAM. A new boundary condition and post-processing tool allow the volume-of-fluid solver interFoam to model open-channel flows more effectively, and this solver is applied to recreate four flume experiments. Finally, a new CFD solver is developed, using a Godunov-type scheme and artificial compressibility. The free surface is captured automatically by the Riemann solver, and the pressure gradient discontinuity at the free surface is captured automatically by a novel reconstruction method. While the results compare well with analytical and experimental benchmark tests, the solver does not reliably converge on 3D unstructured meshes. Once the convergence problem has been overcome, future research will apply the new CFD solver to model different designs of leaky barriers, and the resulting new knowledge can then be used in 1D hydraulic models to model networks of barriers across a catchment.