Numerical Modelling

In this page we present the outcomes of our morphodynamic modelling of braided rivers.

The videos on this page describe the evolution of a vegetated braided river reach driven by vegetation spread, as predicted by our 2D numerical model. The two videos feature slight differences in the setup of empirical models for vegetation evolution and boundary conditions but show very similar outcomes.

In both videos, the first part of the movie shows the braided channel development under high flow, starting from a narrow straight channel, in a bare sand bed (without any vegetation). The second part allows vegetation to grow on bars and banks during periods of low flow while intervening periods of high flow remove vegetation. Note that the time scale for vegetation growth periods has been accelerated relative to the high flow periods. Alternation of low-flow periods which permit vegetation spread and encroachment and of (shorter) high-flow period in which the bed is mobilised and vegetation partially removed is representative of a dam-impacted flow regime. By the end of the simulation the channel has become less braided.

Note that the first phase (development of a braided network under high flow) is representative of the "natural state" of a braided river, whereas the second phase (vegetation spread and transition to a meandering configuration) represent what could happen under a dam-impacted flow regime, when extended low-flow periods favour vegetation encroachment across the floodplain and floods at formative discharge are too short and infrequent to clear vegetation. This is thus a good conceptual prototype of what was observed in the Waitaki River.

The numerical model reproduces the outcomes of a physical experiment at laboratory scale [Tal and Paola, 2010] which analysed morphological change in a braided river under the effect of vegetation spread (alfaalfa seeds). However what we show here is purely the result of numerical simulation, not a time lapse of the laboratory experiment prototype.

We used the GIAMT2D numerical model [Stecca, 2012; Siviglia et al., 2013], which we modified for this study by including parametrisation of secondary currents, bank erosion, and vegetation processes. The vegetation module builds upon the concepts put forward by Bertoldi et al. [2014].

After training our model in the reproduction of the experiment of Tal and Paola, 2010, we applied the same approach to model the Waitaki River in New Zealand's South Island.

Starting from the closure of Waitaki Dam in 1936, the Lower Waitaki River experienced dramatic morphological and ecological change (see Hicks, 2006, for detail). Spread of alien vegetation (mainly crack willows, originally introduced by settlers) across the river floodplain was favoured by the dam-dampened flow regime. By 1950, vegetation encroachment had occurred. The action of trees favoured transition to an almost single-thread morphology.

More information about this simulation is available the notes to the youtube video.