Abstract
This paper investigates the challenge of representing structural differences in river channel cross-section geometry for regional to global scale river hydraulic models and the effect this can have on simulations of wave dynamics. Classically, channel geometry is defined using data, yet at larger scales the necessary information and model structures do not exist to take this approach. We therefore propose a fundamentally different approach where the structural uncertainty in channel geometry is represented using a simple parameterisation, which could then be estimated through calibration or data assimilation. This paper first outlines the development of a computationally efficient numerical scheme to represent generalised channel shapes using a single parameter, which is then validated using a simple straight channel test case and shown to predict wetted perimeter to within 2% for the channels tested. An application to the River Severn, UK is also presented, along with an analysis of model sensitivity to channel shape, depth and friction. The channel shape parameter was shown to improve model simulations of river level, particularly for more physically plausible channel roughness and depth parameter ranges. Calibrating channel Manning’s coefficient in a rectangular channel provided similar water level simulation accuracy in terms of Nash–Sutcliffe efficiency to a model where friction and shape or depth were calibrated. However, the calibrated Manning coefficient in the rectangular channel model was ∼2/3 greater than the likely physically realistic value for this reach and this erroneously slowed wave propagation times through the reach by several hours. Therefore, for large scale models applied in data sparse areas, calibrating channel depth and/or shape may be preferable to assuming a rectangular geometry and calibrating friction alone.
Highlights
There has been substantial interest in simulating river hydraulics at regional to global scales, most notably for the purpose of flood hazard and risk assessment
There is currently no clear definition of what constitutes a large scale hydraulic model, for the purpose of this paper it will be assumed that the model has a structure that can be applied to simulate water levels and flows over an entire continent
This would appear to indicate that the additional channel shape information is unnecessary, the model with mean observed geometry calibrated to a friction far closer to the first guess for this reach
Summary
There has been substantial interest in simulating river hydraulics at regional to global scales, most notably for the purpose of flood hazard and risk assessment. Approaches to regional or global scale river and floodplain simulation (Alfieri et al, 2013; Neal et al, 2012a; Paiva et al, 2011; Sayama et al, 2012; Winsemius et al, 2012; Yamazaki et al, 2011) often make a number of simplifications from the one- or two-dimensional shallow water models widely used at the reach scale (e.g. river lengths of 10–100’s of km) After validating the method using a simple test case, the influence of channel shape friction and depth on river dynamics was investigated with the aid of a test case from a 60 km reach of the River Severn, UK
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