Numerical Modelling of Braided River Morphodynamics: Review and Future Challenges

Abstract

Numerical morphological modelling of braided rivers is increasingly used to explore controls on river pattern and for applied environmental management. This article reviews and presents a taxonomy of braided river morphodynamic models and discusses the challenges facing model development and use, illustrating these challenges with a case example. The taxonomy is contextualised by an initial discussion of the physical mechanisms of braiding. The taxonomy differentiates between reach-scale and catchmentscale models. Reach-scale models are usually physics-based, which are further divided based upon the mathematical approach used to solve equations (analytical or numerical) and their dimensionality (1D, 2D or 3D). Cellular automata models are one type of numerical model that replace at least some physical processes with expedient rules. A 2D physics-based approach encapsulates sufficient process complexity to provide behavioural predictions. Predictions from catchment-scale landscape evolution models have potential for providing boundary conditions. Future progress in physics-based modelling needs to address three challenges: (i) representation of f low and sediment transport; (ii) temporal and spatial scaling; and (iii) model calibration, sensitivity, uncertainty and validation. The key problem for addressing these is the dearth of laboratory or natural experiment datasets. To show that progress can be made by comparing reach-scale predictions to high-resolution observations, a case study of monitoring and modelling, conducted in the Rees River, New Zealand, is presented. Hydraulic predictions of cellular automata and shallow water equation (Delft3d) models are compared to observed inundation extent. The efficacy of high-resolution, multi-temporal morphological data for assessing 2D physics-based morphodynamic model predictions is also demonstrated.

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

“…To date, field data on braided river channel topography is limited by low spatial and temporal frequency and a lack of simultaneous bedload transport rates (Bertoldi et al, 2010;Church and Ferguson, 2015;Williams et al, 2016b;Vericat et al, 2017). These data and corresponding analysis can inform, assess, and assist in developing numerical models of braiding morphodynamics and bedload flux (Bertoldi et al, 2009a;Williams, 2012;Church and Ferguson, 2015;Lugo et al, 2015), which require calibration and validation with high-resolution 3-dimensional surveys of braided rivers (Williams et al, 2016a;Redolfi et al, 2017). These data and corresponding analysis can inform, assess, and assist in developing numerical models of braiding morphodynamics and bedload flux (Bertoldi et al, 2009a;Williams, 2012;Church and Ferguson, 2015;Lugo et al, 2015), which require calibration and validation with high-resolution 3-dimensional surveys of braided rivers (Williams et al, 2016a;Redolfi et al, 2017).…”

Evolution of grain size distributions and bed mobility during hydrographs in gravel‐bed braided rivers

“…For example, these data can be used to estimate bedload sediment yield and channel stability, which in turn are used to inform channel, reservoir, and infrastructure design (Powell et al, 2001;Ryan et al, 2002). In addition, GSD and bed mobility data are necessary for the effective numerical modelling of sediment entrainment and channel morphodynamics (Wilcock and McArdell, 1997;Powell et al, 2001;Williams et al, 2016aWilliams et al, , 2016b and can help define the disturbance regimes and substrate quality of gravel-bed rivers for benthic organisms and fish (Wilcock and McArdell, 1997;Haschenburger and Wilcock, 2003).…”

“…Defined by their multiple anabranch channels and ephemeral bars, braided rivers have a complex morphology producing spatially and temporally variable bedload transport rates commonly observed in the field (Ashworth and Ferguson, 1986;Powell and Ashworth, 1995;Mao and Surian, 2010;Williams et al, 2015) and in physical models (Ashmore, 1988;Hoey and Sutherland, 1991;Hoey, 1992;Ashmore and Church, 1998). While there has been some success characterizing bedload transport functions at the reach scale using temporal averages (Ashmore, 1988;Bertoldi et al, 2009;Williams et al, 2016a), the complex morphology and hydraulics as well as rapid morphological change makes bedload transport rates and bed material mobility in braided rivers inherently difficult to measure directly, or to predict using classic hydraulically-driven bedload functions (Davies, 1987;Powell and Ashworth, 1995;Bertoldi et al, 2009;Mao and Surian, 2010;Kociuba and Janicki, 2015;Recking et al, 2016).…”

Model-Based Analysis of Macrophytes Role in the Flow Distribution in the Anastomosing River System

“…Most previous research has used numerical models in multichannel rivers that focused on braided rivers exploring flow-vegetation interactions [11][12][13] as well as sediment transport as a control on channel pattern [14,15]. A broad review evaluating the modeling frameworks appropriate for simulating the morphodynamics of braided rivers was given by Williams et al [16]. The review covers a whole range of models varying from 1D to 3D and from reach to catchment scale.…”