Modelling floods in hydrologically complex lowland river reaches

This paper considers the modelling of lowland river reaches which contain c omplex within-reach hydrological interactions. It is clear that river and f loodplain flow are the most important processes in terms of flood modelling in lowland systems, although there are often important lateral inflows fro m catchments and hillslopes bounding the floodplain and from interactions b etween the river and the floodplain, which can all affect the propagation o f the flood wave. Previous models have often either considered a complex re presentation of the fluvial processes with no representation of the hydrolo gical inflows into the reach (Bates, P.D., Anderson, M.G., Price, D.A., Har dy, R.J., Smith, C.N., 1996. Analysis and development of hydraulic models f or floodplain hows. In: Anderson, M.G., Walling, D.E., Bates P.D. (Eds.), F loodplain Processes. Wiley, Chichester, pp. 215-254), or have simulated a r ange of catchment processes with a poor representation of the river and flo odplain (Abbot, M.B., Bathurst, J.C., Cunge, J.A., O'Connell, P.E., Rasmuss en, J., 1986. An introduction to the European Hydrological System-Systeme H ydrologique Europeen, SHE, 2. Structure of a physically based, distributed modelling system. Journal of Hydrology, 87, 61-77). Hence, this paper devel ops a modelling approach based on a two-dimensional finite element hydrauli c 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 m odel is able to predict flood hydrographs for a series of flood events, und er a range of different hydrological conditions, down a reach of the River Severn, UK. Furthermore, the comparison of results from simulations using h ydrological representations of different degrees of complexity suggest that there are restrictions on the necessary complexity of the hydrological com ponents depending on the application of the model and the available validat ion data. Simple approaches to the reach scale hydrology may be sufficient if only the bulk outflow hydrograph is required by the user, however more c omplex spatially and temporally distributed models appear to be required if predictions of the hood inundation extent are desired. The simulations rai se the issue of the application of distributed models and attempt to provid e a framework for future research. The results suggest that there is a need for the validation of the internal predictions of distributed models of fl ood flow, and suggests a need for field data of river and floodplain intera ctions within long lowland river reaches. (C) 1999 Elsevier Science B.V. Al l rights reserved.