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Since Sellin (1964) reported the difficulties involved with discharge estimation in channels with floodplains, a significant amount of research has been undertaken into the hydraulic behaviour of compound channels. There is however a lack of empirical data relating to cases where urban development has taken place on the floodplain, given the large amount of urbanisation that has taken place on the banks of many rivers, it is obvious that this is an area of research that needs attention. This investigation addressed this need by investigating a compound channel with roughness simulating an idealised urban area on the floodplain. The aim was to quantify the effects of changes in roughness configuration on discharge and flow resistance. The research programme included 202 tests utilising a compound cross section and 129 tests in a channel of rectangular geometry. Symmetrical and asymmetrical arrangements of roughness were investigated in the compound channel, with simple channel testing providing a data set against which compound channel results could be compared. The longitudinal bed slope was set at 1.85x 10-3 for all compound and simple channel test cases. The research highlights the effects of large scale vorticity, previously unrecognised in floodplain research. This vorticity acts with several other factors and has the effect that, when the roughness element length to streamwise spacing length ratio was unity, the roughness coefficient of the total channel reduced as rows of roughness were added behind an initial row of roughness placed at the main channel edge. This was found to be caused by increases in the main channel and smooth floodplains capacity. A consideration of this vorticity together with other reasons for observed behaviour has been presented. An extensive sensitivity analysis has been carried out considering the effect varying the longitudinal and transverse roughness element spacing has on these phenomena.

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This thesis investigates geomorphological processes within the forested floodplain of the Highland Water, a small, lowland river in the New Forest, southern England. Geomorphological processes were monitored (a) before restoration, in order to define reference conditions, and (b) after restoration, in order to monitor the performance of the restoration against the reference conditions. The results demonstrate that the restoration was successful at moving the restored system towards target reference conditions by re-connecting the channel and floodplain, and consequently floodplain geomorphological dynamics were increased after restoration. However, the restored floodplain was considerably more connected and more dynamic than an upstream semi-natural reference reach, indicating that the restored channel was perhaps undersized. Floodplain channels were an important geomorphological feature observed on semi-natural floodplains, particularly in association with hydraulically effective wood jams. Experiments into sedimentation and erosion showed that overbank flow scoured the surface and distributed sediment, and rates of erosion and deposition were higher within floodplain channels than elsewhere on the floodplain surface. These channels were therefore a major control over the spatial distribution of energy and materials on the floodplain at the patch, feature and reach scale (10-1 to 102 m). The formation of in-channel wood jams, which force flow overbank, relies on the accumulation of wood. Experiments to investigate transport of small wood recorded travel distances ranging from 0 to over 1000 m. Shorter travel distances were associated with higher in-channel geomorphological diversity, particularly the presence of in-channel wood jams. This thesis therefore provides a greater understanding of the geomorphological processes operating on a forested floodplain in conjunction with monitoring the performance of a river restoration project that incorporated a forested floodplain.

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The River Amazon has the largest drainage basin in the world; however, spatial and temporal water fluxes in the floodplain are poorly understood. With few in situ gauging stations, all situated on the main channel, understanding flow dynamics of the floodplain is difficult. The work in this thesis uses ICESat (Ice, Cloud and land Elevation Satellite) to observe changes in water level in the channel and floodplain. To ensure the suitability of ICESat for this purpose, a proof-of-concept study was conducted on the Mississippi and Danube rivers. This study found that ICESat was capable of accurately observing water levels (mean bias and standard deviation of -16 ± 16 cm). These results allowed ICESat to be used with confidence in the Amazon. The Amazon main channel gauges are not tied to a common datum. ICESat was used to level six gauges within the study area to a common datum to be used as verification data. With the aid of Landsat imagery, ICESat was used to investigate the floodplain, identifying water level dynamics in both floodplain lakes and channels with unprecedented detail. Seasonal changes in water level, as well as spatial patterns, were clearly observable. Water levels in the main channel and floodplain were compared to ENVISAT altimeter data and showed good agreement, except at low water when the greatest difference was seen. The ICESat data were then used to create maps of water flow directions and gradients across the floodplain. These ICESat-derived flow characteristics were used to assess simulations from the hydrodynamic model LISFLOOD-FP and identify whether the model is able to reproduce observed floodplain water levels, as well as determining possible causes for discrepancy. Overall, detailed analysis of ICESat data has provided unprecedented detail of water fluxes in the Amazon, advancing understanding of the dynamics of large basin floodplain inundation processes.

This research is based on two contexts. First, the research is concerned with a much understudied aspect of flood inundation: upland environments with topographically complex floodplains. Although the presence of high resolution topographic data (e.g. LiDAR) has improved the quality of river flood inundation predictions, the optimum dimensionality of hydraulic models for this purpose has yet to be fully evaluated for situations of both topographic and topological (i.e. the connectivity of floodplain features) complexity. In this research, we present the comparison of three treatments of upland flood inundation using: (a) a one-dimensional model (HEC-RAS) with the domain defined as series of extended cross-sections; (b) the same 1 D model, but with the floodplain defined by a series of storage cells, hydraulically connected to the main river channel and other storage cells on the floodplain according to floodplain topological characteristics; and (c) a 2D diffusion wave treatment, again with explicit representation of floodplain structural features. The three models were tested on a 6 km upland reach of the river Wharfe, UK. The results showed that both the extended cross-section and storage cell modes were conceptually problematic. They also resulted in poorer model predictions, requiring incorrect parameterisation of the main river to floodplain flux in order to approach anything like the level of agreement observed when the diffusion wave treatment was assessed. The research concludes that a coupled 1 D-2D treatment is likely to provide the best modelling approach, with currently available technology, for complex floodplain configurations. Second, as a substantive issue, the research is concerned with the effects of channel configuration change (i.e. in-channel morphology) and climate change on flood inundation; the aspect on which mush less attention has been paid. The results show that for the 1 in 0.5 flood event in this study reach, there is a rise of 5.7% and 12.2% to the inundation extent due to 2-year in-channel change '(i.e. 10cm rise in bed level) and climate change (i.e. scenario 2050) respectively. The sensitivity of inundated area changes to climate change is strongly conditioned by in-channel sedimentation.

High-resolution data obtained from airborne remote sensing are increasing opportunities for representation of small-scale structural elements (e. g. walls, buildings) in complex floodplain systems using two-dimensional (2D) models of flood inundation. At the same time, 2D inundation models have been developed and shown to provide good predictions of flood inundation extent, with respect to both full solution of the depth-averaged Navier-Stokes equations and simplified diffusion wave models. However, these models have yet to be applied extensively to urban areas. This study applies a 2D raster-based diffusion wave model, either loosely-coupled or tightly-coupled to a ID river flow model, to determine patterns of fluvial flood inundation in urban areas using high-resolution topographic data. The aim of this study is to explore the interaction between spatial resolution and small-scale flow routing process, through model validation and verification. The model assumes that the prime source of the flood is fluvial: pluvial floods and floods associated with urban drainage systems are not addressed. The topographic data are based upon airborne laser altimetry (LiDAR) obtained for the City of York, U.K. Validation data were available in the form of inundation patterns obtained using aerial photography at a point on the failing limb of the flood event. Inflow data is provided either by a loosely-coupled or a tightly-coupled ID river flow model. The model was used to simulate a major flood event which occurred in the year 2000 in the City of York on the River Ouse at 4 different sites. Applications of the basic model showed that even relatively small changes in model resolution have considerable effects on the predicted inundation extent and timing of flood inundation. Timing sensitivity would be expected given the relatively poor representation of inertial processes in a diffusion wave model. Compared with previous work, sensitivity to inundation extent is more surprising and is associated with three connected effects: (i) the smoothing effect of mesh coarsening upon input topographical data; (ii) poorer representation of both cell blockage and surface routing processes as the mesh is coarsened, where the flow routing is especially complex; and (iii) the effects of (i) and (ii) upon water levels and velocities which in turn determine which parts of the floodplain the flow can actually travel to. The combined effects of wetting and roughness parameters can compensate in part for a coarser mesh resolution. However, the coarser the resolution, the poorer the ability to control the inundation process as these parameters not only affect the speed but also the direction of wetting. Thus, high resolution data will need to be coupled to more sophisticated representation of the inundation process in order to obtain effective predictions of flood inundation extent. A sub grid scale wetting and drying correction approach was developed and tested for use with 2D diffusion wave models of urban flood inundation. The method recognises explicitly that representations of sub grid scale topography using roughness parameters ill provide an inadequate representation of the effects of structural elements on the floodplain (e. g. buildings, walls) as such elements not only act as momentum sinks, but also have mass blockage effects. The latter may dominate, especially in structurally complex urban areas. The approach developed uses high resolution topographic data to develop explicit parameterization of sub grid scale topographic variability to represent both the volume of a grid cell that can be occupied by the flow and the effect of that variability upon the timing and direction of the lateral fluxes. This approach is found to give significantly better prediction of fluvial flood inundation in urban areas as compared with traditional calibration of sub grid-scale effects using Manning's n. In particular, it simultaneously reduces the need to use exceptionally high values of n to represent the effects of using coarser meshes, whilst simultaneously increasing the sensitivity of model predictions to variation in n. Finally, the model was coupled (tightly) to a one-dimensional solution of the Navier-Stokes equations. This showed that significantly better representation of urban inundation could be achieved in a tightly-coupled formulation as a result of better representation of boundary condition effects.