Urban fluvial flood modelling using a two-dimensional diffusion-wave treatment, part 2 : development of a sub-grid-scale treatment.

This paper develops and tests a sub-grid-scale wetting and drying correction for use with two-dimensional diffusion-wave models of urban flood inundation. The method recognizes explicitly that representations of sub-grid-scale topography using roughness parameters will 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 than 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 a coarser mesh process representation and increases the sensitivity of model predictions to variation in n