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Discussion papers | Copyright
https://doi.org/10.5194/esurf-2018-17
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Apr 2018

Research article | 13 Apr 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth Surface Dynamics (ESurf).

Modelling braided river morphodynamics using a particle travel length framework

Alan Kasprak1,2, James Brasington3, Konrad Hafen2,4, Richard D. Williams5, and Joseph M. Wheaton2 Alan Kasprak et al.
  • 1US Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff, Arizona, 86001, USA
  • 2Department of Watershed Sciences, Utah State University, Logan, Utah, 84322, USA
  • 3Te Waiora, Institute for Freshwater Management, University of Waikato, Hamilton 3240, New Zealand
  • 4Water Resources Program, University of Idaho, Moscow, Idaho, 83844, USA
  • 5School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK

Abstract. Numerical models that predict channel evolution are an essential tool for investigating processes that occur over timescales which render field observation intractable. The current generation of morphodynamic models, however, either oversimplify the relevant physical processes, or in the case of more physically-complete CFD based codes, have computational overheads that restrict severely the space-time scope of their application. Here we present a new, open-source, hybrid approach that seeks to reconcile these modelling philosophies. This framework combines steady-state, two-dimensional CFD hydraulics with a rule-based sediment transport algorithm to predict particle mobility and transport paths which are used to route sediment and evolve the bed topography. Data from two contrasting natural braided rivers (Rees, New Zealand and Feshie, United Kingdom) were used for multi-scalar model verification incorporating reach-scale quantitative morphological change budgets and volumetric assessment of different braiding mechanisms. The model was able to simulate eight of ten empirically observed braiding mechanisms from the parameterized bed erosion, transport, and deposition. Representation of bank erosion and bar edge trimming necessitated the inclusion of a lateral channel migration algorithm. Comparisons between simulations based on steady effective discharge versus event hydrographs represented as a series of steady states were found to only marginally increase the predicted volumetric change, with greater deposition offsetting erosion. A decadal-scale simulation indicates that accurate prediction of event-scale scour depth and subsequent deposition present a methodological challenge because the predicted pattern of deposition may never catch up to erosion if a simple path-length distribution is employed, thus resulting in channel over-scouring. It may thus be necessary to augment path length distributions to preferentially deposit material in certain geomorphic units. We anticipate that the model presented here will be used as a modular framework to explore the effect of different process representations, and as a learning tool designed to reveal the relative importance of geomorphic transport processes in rivers at multiple timescales.

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Short summary
We present a modelling framework for the prediction of gravel-bed braided river evolution. The model is unique in that it simplifies sediment transport through the use of empirically-derived relationships between channel form and flood-scale particle transport distances, or path lengths, which ultimately allow for longer duration model runs with reduced computational demand. We use field surveys on two braided rivers to validate the model at the event, annual, and decadal timescales.
We present a modelling framework for the prediction of gravel-bed braided river evolution. The...
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