An investigation into the effect of lateral hillslope inputs on floodplain hydraulic model predictions
Charlton, Rosemary Anne
PublisherUniversity of Bristol
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In recent years there has been a growing interest in the contemporary floodplain environment which has come from a number of fields including civil engineering, hydrology, geomorphology and ecology. A major advance in civil engineering has been the development of two-dimensional hydraulic models capable of a high degree of spatial representation. These models were originally developed for engineering applications although recent developments, such as their application to longer reach lengths, mean that these models are very powerful predictive tools with potential for application in many different fields. Two-dimensional floodplain hydraulic models can be viewed as a platform for further development through the incorporation of additional components to represent specific processes. For the case of the application of these models in hydrology, whilst the models provide a good representation of floodplain processes in a hydraulic context, catchment hydrology is essentially treated as a black box. The only input to the system is the upstream input hydrograph (occasionally rainfall over the floodplain surface and tributary inflows are included) and output only occurs at the downstream boundary. The floodplain is assumed to be impermeable and any input from the hillslopes bordering the reach is ignored. This investigation examines the significance of contributions to the floodplain from the hillslopes bordering the reach. In order to do this, the zero flux boundary condition at the hillslope-floodplain interface is relaxed. A two-dimensional floodplain inundation model, RMA-2, is set up for a 14 km reach of the River Culm in Devon. A distributed hillslope hydrology model, VSAS3 is set up for a section of the hillslopes bordering the reach. This model is coupled to RMA-2 using a simple external coupling mechanism whereby water produced by VSAS3 is applied to elements along the edge of the RMA- 2 finite element mesh. A sensitivity analysis is carried out using this coupled scheme to identify some of the range of hillslope environments which may contribute a significant volume of lateral inflow to the floodplain. Five key hillslope parameters are selected and altered over a range of values. It has been shown that hillslope inflows can have a significant effect on the predictions made by RMA-2, both in terms of changes to the predicted output hydrograph and localised changes in depth and inundation extent. It has also been shown that the timing of the hillslope inflow peak relative to the arrival of the floodwave from upstream is of great importance. The addition of inflows has also been found to affect the calibration of the floodplain inundation model.