Urban fluvial flood modelling using a two-dimensional diffusion wave treatment, part 1: mesh resolution effects

High-resolution data obtained from airborne remote sensing is 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 devel-oped 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 paper applies a 2D raster-based diffusion wave model to determine patterns of fluvial flood inundation in urban areas using high-resolution topog-raphic data. The aim of this paper is to explore the effects of spatial resolution upon estimated inundation extent and flow routing process. This is important as the complexity of urban surfaces is such that chang-ing data resolution may have a major effect upon surface representation and hence flow routing. Similarly, few applications to date have explicitly considered the timing of the inundation process. 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 (Li-DAR) obtained for the City of York, U.K. A series of image processing steps are used to pre-prepare the topographic data. A one-dimensional hydraulic model of the Ouse from Skelton (upstream of the city) through to Naburn Weir (downstream of the city) is used to provide estimates of flux from the river to the floodplain for a major flood inundation event (estimated to have a return period of greater than 100 years) in November 2000. Validation data were available in the form of inundation patterns obtained using aerial photography at a point on the falling limb of the flood event. Model response shows that even relatively small changes in model resolution have considerable effects on the predicted inundation extent and the 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: (1) the smoothing effect of mesh coarsening upon input topographical data; (2) poorer representation of both cell blockage and surface routing processes as the mesh is coarsened, where the flow routing is especially complex; and (3) the ef-fects of (1) and (2) 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 compen-sate 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 wet-ting. Thus, high resolution data will need to be coupled to more sophisticated representation of the inun-dation process in order to obtain effective predictions of flood inundation extent. This is explored in a companion paper.