River flood inundation under climate change : assessment of the relative effects of changes in plant growth and flood regime on conveyance

Aquatic vegetation are major controls of river flow. In-stream vegetation located in river channels increase local channel resistance by reducing flow velocities which, in turn, raises river levels and exaggerates flood magnitudes. In the UK, climate change is expected to increase the amount of in-stream vegetation occupying river channels whilst delaying the occurrence of peak biomass to coincide with intensified storms which are predicted to occur during the autumn and winter months. This creates a 'perfect storm' where high river flow interacts with high vegetation coverage with the potential to exacerbate flooding. This thesis has investigated the interaction between in-stream vegetation and river flow for a natural chalk river during flood events. To estimate future flow-vegetation interactions during flood events a three-dimensional numerical model was developed which was representative of the natural topography of the River Blackwater, UK. The model used double-averaged Navier-Stokes equations to simulate the influence of vegetation drag on flow conveyance. The model was calibrated using measurements of in-stream vegetation-flow interactions and compared against values reported in the literature and those produced with coefficients set to unity. The model was shown to be able to successfully simulate complex flow structures reported in the field data. A scenario-based approach was used to simulate changes to peak flow, seasonal flow regimes, and the changing channel cover of in-stream vegetation. The results consider the impact of vegetation on flow conveyance in terms of flow depth, flow velocity, turbulence generation, and vegetation geometry. The results showed that future increases in vegetation patch size will have a considerable impact of mean flood levels. The extent to which vegetation patches influence flood levels was found to change with flow rates; in-stream vegetation exhibited a greater impact on flood-water conveyance at lower flow rates. The morphological differences between the studied vegetation species Sparganium erectum and Sparganium emersum, resulted in different distributions of the local velocity and turbulence and the trailing morphology particular to the S. emersum species was shown to have a greater impact on mean flood levels. This study provides a detailed characterisation of the effects of vegetation on flood events under climate change. The results are finally discussed in terms of implications for river management strategies future research.