Computer modelling of flood flows and floodplain sedimentation
Sweet, Robert John
PublisherUniversity of Exeter
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Past studies of overbank flow and sediment deposition have identified the importance of basin scale and reach scale controls. In addition, floodplain topography has been identified as an important control on overbank flow sequences and associated sediment transport and deposition. Recently, the adoption of combined numerical modelling and field-based approaches have provided an effective means of quantifying reach scale processes operating in floodplain environments. However, there is a need to investigate these processes both at the reach and catchment scale. In this study, a twostage procedure has been developed in an attempt to investigate these processes at a range of spatial scales. First, an existing two-dimensional hydraulic model that solves the depth-averaged shallow water equations was used to predict distributed flow depths and velocities over high-resolution topographic grids representing each of the study reaches. These grids were generated from detailed field survey data of floodplain topography collected using a Global Positioning System (GPS) within three study catchments where between six to eight contrasting reaches were chosen to investigate basin and reach scale morphological controls on overbank flows. The results of the hydraulic model indicate three stages in floodplain inundation. First, initial inundation of low-lying areas adjacent to channel margins. Second, a rapid increase in inundation extent via interconnected low-lying areas as threshold discharges are exceeded. Third, complete inundation where floodwater is conveyed as a single unit in a downstream direction. These patterns were also observed using ground and oblique aerial photography of floodwater inundation patterns. The second stage in the procedure utilised a sediment transport and deposition model that was developed using these hydraulic data. This model was used to estimate patterns and rates of floodplain sedimentation at a reach scale and then extended to the catchment scale. Parameter combinations used in this stage were investigated using a Generalised Likelihood Uncertainty Estimation (GLUE) framework and predictions were compared with estimates of medium-term sedimentation rates derived from 137Cs analysis of floodplain sediment cores. The results of the sediment transport and deposition model indicate that within reach variability of floodplain sedimentation is influenced by small-scale local topographic controls. Typically, 15 % of the inundated floodplain area receives ~50% of the total sedimentation amount and reflects the low-lying areas adjacent to the channel. In contrast, 40% of the inundated floodplain area receives < 20% of the total sedimentation, reflecting distal floodplain areas that are inundated for short periods of time.