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.