How much physical complexity is needed to model flood inundation?

Abstract

11Two dimensional flood inundation models are widely used tools for flood hazard mapping and an 12 essential component of statutory flood risk management guidelines in many countries. Yet we still 13 don't know how much physically complexity a flood inundation model needs for a given problem. 14 Here, three two dimensional explicit hydraulic models, that can be broadly defined as simulating 15 diffusive, inertial or shallow water waves, have been benchmarked using test cases from a recent 16Environment Agency for England and Wales (EA) study, where results from industry models are also 17 available. To ensure consistency the three models were written in the same code and share 18 subroutines for all but the momentum (flow) and time stepping calculations. The diffusive type 19 model required much longer simulation times that the other models, whilst the inertia model was 20 the quickest. For flows that vary gradually in time, differences in simulated velocities and depths due 21 to physical complexity were within 10% of the simulations from a range of industry models. 22Therefore, for flows that vary gradually in time it appears unnecessary to solve the full two 23 dimensional shallow water equations. As expected however, the simpler models were unable to 24 simulate supercritical flows accurately. Finally, implications of the results for future model 25 benchmarking studies are discussed in light of a number of subtle factors that were found to cause 26 significant differences in simulations relative to the choice of model. 27

Uncertainty Analysis of a Two-Dimensional Hydraulic Model

“…Until now, in open channel hydraulics, uncertainty analysis has been limited to models that are not as complex as two-dimensional hydrodynamic models [60]. The majority of the approaches for uncertainty analysis are stochastic and most commonly based on the Monte Carlo method.…”

Flood Inundation Modelling of Flash Floods in Steep River Basins and Catchments

“…Different numerical codes can treat some of the key modelling processes differently, such as wetting and drying, implementation of boundary conditions, friction estimation, source terms, etc. This can alter simulation results before different numerical schemes or the level of physical complexity is taken into account [9]. The problem can be avoided by using a single numerical code.…”

“…These include [6]: (i) models that are based on a simplified version of the 2D shallow water equations, (ii) models that solve the full 2D shallow water equations, and (iii) full hydrodynamic 2D models with shock-capturing ability. A number of studies have focused on benchmarking different types of 2D flood inundation models for a range of various test cases and/or real flood events [6][7][8][9][10]. Such studies have outlined the main differences between 2D models with different complexities, including highlighting the pros and cons of each type of 2D hydrodynamic model, and generally establishing what level of model sophistication is appropriate for modelling various flow conditions.…”

Editorial

“…Uncertainty is an inevitable part in any modelling, particularly that involving the natural environment. As discussed above, there are several sources for this: neglect of physical process representation (Neal et al, 2012;Willis et al, 2012); inaccuracy in the estimation of parameters (Beven and Freer, 2001); errors in the discretisation of the equations used (MacDonald et al, 1997;Skeels and Samuels, 1989). Naturally much research is aimed at reducing all sources of errors within modelling, but it is not realistic to imagine that uncertainty will be completely removed.…”

“…Further developments on the use of sub-grid models to increase accuracy on coarser grids will be needed. j There is a need for more quantitative ways of comparing different physical representations and advising users which is most appropriate for a particular situation (Neal et al, 2012;Willis et al, 2012). j It is necessary to continue to reflect uncertainty in modelling.…”