A comparison of one‐ and two‐dimensional approaches to modelling flood inundation over complex upland floodplains

Abstract

Abstract:A much understudied aspect of flood inundation is examined, i.e. upland environments with topographically complex floodplains. Although the presence of high-resolution topographic data (e.g. lidar) has improved the quality of river flood inundation predictions, the optimum dimensionality of hydraulic models for this purpose has yet to be fully evaluated for situations of both topographic and topological (i.e. the connectivity of floodplain features) complexity. In this paper, we present the comparison of three treatments of upland flood inundation using: (a) a one-dimensional (1D) model (HEC-RAS v. 3Ð1Ð2) with the domain defined as series of extended cross-sections; (b) the same 1D model, but with the floodplain defined by a series of storage cells, hydraulically connected to the main river channel and other storage cells on the floodplain according to floodplain topological characteristics; (c) a two-dimensional (2D) diffusion wave treatment, again with explicit representation of floodplain structural features. The necessary topographic and topological data were derived using lidar and Ordnance Survey Landline data. The three models were tested on a 6 km upland reach of the River Wharfe, UK. The models were assessed by comparison with measured inundation extent. The results showed that both the extended cross-section and the storage cell 1D modes were conceptually problematic. They also resulted in poorer model predictions, requiring incorrect parameterization of the main river to floodplain flux in order to approach anything like the level of agreement observed when the 2D diffusion wave treatment was assessed. We conclude that a coupled 1D-2D treatment is likely to provide the best modelling approach, with currently available technology, for complex floodplain configurations.

Rural land management impacts on catchment scale flood risk

“…The reason why water flow across the floodplain is rapid is that the topography of the reach is so steep that water level in storage units quickly exceeds spill height. A similar observation was made by Tayefi et al, (2007) for the Upper Wharfe in Yorkshire. A sensitivity analysis of channel and floodplain roughness was carried out by Tayefi et al, (2007).…”

“…A similar observation was made by Tayefi et al, (2007) for the Upper Wharfe in Yorkshire. A sensitivity analysis of channel and floodplain roughness was carried out by Tayefi et al, (2007). It was expected that as channel roughness increases, then downstream peak discharge is decreased and delayed.…”

“…Lane and Richards (2001) argue that the term "validated" is used to prevent the criticism from the public that model results are unreliable. Another approach to evaluating models is to use a benchmarking approach which compares different models for the same network or catchment (Tayefi et al, 2007).…”

“…This expected result did not occur in the 1D-Storage model due to rapid floodplain conveyance. Tayefi et al, (2007) hydraulic models in simulating situations with out of bank flows (Sellin et al, 1993;Bates and Anderson, 1993), as the topography may significantly impact localised flow processes. These types of problems would not occur if a 2D hydraulic model was used, as these include inertia and advection terms which improve the representation of the floodplain processes and storage (Horritt and Bates, 2002).…”

A coarse-grid approach to representing building blockage effects in 2D urban flood modelling

“…The constant upstream inflow was 1 m 3 /s and the downstream boundary water depth was fixed at 0.2 m; the Manning roughness was 0.06, which was widely used for 2D modelling Tayefi et al, 2007;Yu and Lane, 2006b). The longitudinal profile of water depths, obtained by HEC-RAS, UIM 5M, and UIM 10M simulations, along street centres are displayed in Figure 6.…”