MODELLING BANK DYNAMICS IN GRAVEL-BED BRAIDED RIVERS
Stecca G, 1 2 Measures R,1 Hicks M,1 Zolezzi G,2
1
National Institute of Water and Atmospheric Research, Christchurch, New Zealand
2
University of Trento, Trento, Italy
A distinctive feature of the morphologically active braided and transitional rivers flowing in the
alpine areas and planes of New Zealand is that the banks of individual braids evolve over
relatively short time scales. This implies a strong feedback between in-channel processes, driven
by fluvial transport, and bank processes. Bank retreat can provide significant local sources of
sediment to the river over a single flood event; it can reshape the channel geometry (width and
curvature) and thus interact with bar formation and progradation; and it can affect bifurcations,
driving changes in the flow and sediment transport partition between the downstream channels.
Modelling bank erosion is therefore essential in the prediction of river morphodynamics by twodimensional modelling, even over relatively short time scales.
Aims
Over the last two or three decades, many approaches to modelling bank erosion have been
developed. Here we first aim to analyse some existing models of bank erosion, presently
embedded in two-dimensional morphodynamic models or proposed in the literature, to show their
strengths and weaknesses for the purpose at hand. Then, based on this review, we aim to
propose an original bank erosion algorithm.
Methods and Results
We propose a classification of the existing bank erosion models based on the algorithm used for
identifying eroding banks, the approach used in the computation of bank retreat, the management
of the related domain deformation, and the required grid resolution. We implement these models
in a simplified, one-dimensional cross-sectional morphodynamic model to compare their different
response the flow conditions.
Unfortunately, most existing models of bank erosion are only suitable for single-thread rivers, and
are unfit for application to complex morphologies such as those of braided and transitional rivers.
We therefore propose a new algorithm, able to automatically detect eroding banks and suitable
for general (Cartesian and unstructured) meshes at various mesh resolutions. The algorithm is
tested in a suite of cases, including the widening of a straight narrow channel, the erosion of the
outer bank in a meandering channel over a flood, and the development of a self-sustaining
braiding network.