Abstract
Abstract. High resolution airborne laser data provide new ways to explore the role of topographic complexity in hydraulic modelling parameterisation, taking into account the scale-dependency between roughness and topography. In this paper, a complex topography from LiDAR is processed using a spatially and temporally distributed method at a fine resolution. The surface topographic parameterisation considers the sub-grid LiDAR data points above and below a reference DEM, hereafter named as topographic content. A method for roughness parameterisation is developed based on the topographic content included in the topographic DEM. Five subscale parameterisation schemes are generated (topographic contents at 0, ±5, ±10, ±25 and ±50 cm) and roughness values are calculated using an equation based on the mixing layer theory (Katul et al., 2002), resulting in a co-varied relationship between roughness height and topographic content. Variations in simulated flow across spatial subscales show that the sub grid-scale behaviour of the 2-D model is not well-reflected in the topographic content of the DEM and that subscale parameterisation must be modelled through a spatially distributed roughness parameterisation. Variations in flow predictions are related to variations in the roughness parameter. Flow depth-derived results do not change systematically with variation in roughness height or topographic content but they respond to their interaction. Finally, subscale parameterisation modifies primarily the spatial structure (level of organisation) of simulated 2-D flow linearly with the additional complexity of subscale parameterisation.
Highlights
Roughness elements on a floodplain comprise both ground surface irregularities and vegetation elements
This paper explores the role of topographic complexity considering a spatially and temporally distributed subscale parameterisation, where the roughness parameterisation scheme varies with the amount of high resolution geometric detail included in the topographic DEM
The roughness height (z0) can be determined as a function of the amount of topography contained within the discretised mesh, which depend on the mesh resolution (m) of topographic and roughness parameterisation
Summary
Roughness elements on a floodplain comprise both ground surface irregularities (i.e. topographic variability) and vegetation elements (trees, bushes, logs, stumps, etc.). Roughness parameterisation must account for energy losses due to geometric variability of the surface produced at scales finer than those represented in the mesh (discretisation scale). Both concepts are physically linked by a scale-dependency, which may strongly influence two-dimensional hydraulic modelling results (Nicholas, 2001; Horritt et al, 2006). With a coarser model resolution, smaller topographic variations will need to be parameterised, either explicitly through a porosity type treatment (e.g. Yu and Lane, 2006b) or upscaling of a roughness parameter (Clifford et al, 1992). The main problem of assessing spatial subscale effects upon flow is that, in practice, roughness parameterisation must account for discrepancies between the intrinsic scale of the surface variability and the scale represented in a mesh, and for the discrepancies between the intrinsic scale of the flow process and the processes explicitly represented in the numerical solution
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