Modelling floods in hydrologically complex lowland river reaches

Abstract This paper considers the modelling of lowland river reaches which contain complex within-reach hydrological interactions. It is clear that river and floodplain flow are the most important processes in terms of flood modelling in lowland systems, although there are often important lateral inflows from catchments and hillslopes bounding the floodplain and from interactions between the river and the floodplain, which can all affect the propagation of the flood wave. Previous models have often either considered a complex representation of the fluvial processes with no representation of the hydrological inflows into the reach (Bates, P.D., Anderson, M.G., Price, D.A., Hardy, R.J., Smith, C.N., 1996. Analysis and development of hydraulic models for floodplain flows. In: Anderson, M.G., Walling, D.E., Bates P.D. (Eds.), Floodplain Processes. Wiley, Chichester, pp. 215–254), or have simulated a range of catchment processes with a poor representation of the river and floodplain (Abbot, M.B., Bathurst, J.C., Cunge, J.A., O'Connell, P.E., Rasmussen, J., 1986. An introduction to the European Hydrological System—Systeme Hydrologique Europeen, SHE, 2. Structure of a physically based, distributed modelling system. Journal of Hydrology, 87, 61–77). Hence, this paper develops a modelling approach based on a two-dimensional finite element hydraulic model of river and floodplain flow, which is linked to a series of simple hydrological models that simulate catchment runoff, surface and subsurface hillslope runoff, and floodplain infiltration. Simulations show that the model is able to predict flood hydrographs for a series of flood events, under a range of different hydrological conditions, down a reach of the River Severn, UK. Furthermore, the comparison of results from simulations using hydrological representations of different degrees of complexity suggest that there are restrictions on the necessary complexity of the hydrological components depending on the application of the model and the available validation data. Simple approaches to the reach scale hydrology may be sufficient if only the bulk outflow hydrograph is required by the user, however more complex spatially and temporally distributed models appear to be required if predictions of the flood inundation extent are desired. The simulations raise the issue of the application of distributed models and attempt to provide a framework for future research. The results suggest that there is a need for the validation of the internal predictions of distributed models of flood flow, and suggests a need for field data of river and floodplain interactions within long lowland river reaches.