Reduced-complexity flow modelling of large sand-bed rivers

NICHOLAS, A.; AMSLER, M.; ASHWORTH, P.; BEST, J.; HARDY, R,; LANE, S.; ORFEO, O.; PARKER, N.; PARSONS, D.; REESINK, A.; SMITH G, SAMBROOK; SANDBACH, S.; SIMMONS, S.; SZUPIANY R,

University of Exeter

Conferencia; Annual Conference of the British Society for Geomorphology; 2008

British Society for Geomorphology

Reduced-complexity (RC) river models implement simple, efficient rules to represent the processes of flow and sediment transport. Such models have the capability to simulate channel evolution and associated fluvial deposits over extended time periods (e.g. centuries or longer). However, the simple process rules used by these models, particularly their treatment of flow hydraulics, may limit their ability to provide a physically-based understanding of process-form feedbacks and channel behaviour. To date, this issue has received relatively little attention, and few studies have sought to evaluate the physical realism of the process rules within RC models. This poster provides an assessment of the potential for simulating flow characteristics within large sand-bed rivers using a RC cellular modelling approach. An established cellular routing scheme is modified to: (i) take advantage of the very low water surface gradients that are characteristic of such rivers; (ii) incorporate an iterative calculation procedure to compensate for the simplicity of the routing equations; (iii) account for the effects of upstream flow direction on local flow routing; and (iv) include a flexible structure that enables routing to be conducted to a variable number of downstream cells. The routing scheme is evaluated at a 2 km wide, 5.5 km long mid-channel bar confluence on the sandy braided Río Paraná, Argentina, using a combination of bathymetric survey and acoustic Doppler current profiler (aDcp) data (at 12 cross-sections throughout the confluence and upstream anabranches) and output from a two-dimensional hydraulic model that solves the depth-averaged shallow water equations in conservative form. Attention focuses on: (i) the potential to derive realistic patterns of flow velocity, unit discharge and shear stress using the RC flow routing scheme; and (ii) the sensitivity of the routing scheme results to the orientation of the channel geometry within the cellular model domain. Results demonstrate that a range of routing scheme structures and parameterisations are capable of reproducing the flow characteristics observed in the field and those derived using the more sophisticated hydraulic model. Further development and assessment of the routing scheme will be conducted using aDcp data obtained for a wider range of channel geometries, and by inter-comparison with output derived from three-dimensional Computational Fluid Dynamics modelling