Benchmark study of numerical model grids to study historic floods of the river Rhine

Up until now, structured curvilinear grids are commonly used for hydraulic modelling. However, this grid type has several disadvantages such as staircase representations along closed boundaries and unnecessary high resolution in sharp inner bends. A so called ‘flexible mesh’ can overcome these limitations, since different shapes of grid cells can be used. However, model performance is not directly clear. Several different grid types are compared based on model performance and computation time. This shows that an unstructured grid with curvilinear grid cells in the summer bed and triangles in the floodplains is the most appropriate mesh for 2DH hydraulic modelling. A curvilinear grid in the summer bed ensures high resolution in the channel cross direction, while less grid cells are needed compared to a complete triangular grid. This has a beneficial effect on computation time

Modelling the effect of time-dependent river dune evolution on bed roughness and stage

This paper presents an approach to incorporate time-dependent dune evolution in the determination of bed roughness coefficients applied in hydraulic models. Dune roughness is calculated by using the process-based dune evolution model of Paarlberg et al. (2009) and the empirical dune roughness predictor of Van Rijn (1984). The approach is illustrated by applying it to a river of simple geometry in the 1-D hydraulic model SOBEK for two different flood wave shapes. Calculated dune heights clearly show a dependency on rate of change in discharge with time: dunes grow to larger heights for a flood wave with a smaller rate of change. Bed roughness coefficients computed using the new approach can be up to 10% higher than roughness coefficients based on calibration, with the largest differences at low flows. As a result of this larger bed roughness, computed water depths can be up to 15% larger at low flow. The new approach helps to reduce uncertainties in bed roughness coefficients of flow models, especially for river systems with strong variations in discharge with time