Go with the flow? Reservations about river morphology models

We think that we have a good idea how sediment is transported in river- and tidal systems, but is that really the case? The Netherlands leads the way in the development of models to predict the formation of these kinds of systems, yet physical geographer Anne Baar has shown that these models still have their shortcomings. “That’s not a reproach of the people who create the models, but it does mean that we understand far less about basic, fundamental processes than we’ve believed so far. We need to conduct many more measurements and much more research, and we need experts to explain and interpret the results of the models.”

Deeper than the real world

Knowledge of river- and tidal systems is crucial in areas such as water management, dredging for shipping channels, and adaptation to climate change. Models known as ‘morphological models’ help researchers predict how a river or estuary will behave in the future. Anne Baar has studied the models that describe how current channels develop over time, and she discovered a disbalance between the channel cross-section and the transport of sediment along the sides of the channel. “According to the models, the channels would be deeper over time than they were in reality. In order to compensate for that effect, model users increase the amount of sand that rolls down on the side-slopes of the channel, but that doesn’t accurately reflect the actual erosion of the sand banks and the amount of sand that is transported.” When studying river- or tidal systems, one must therefore clearly define the goal of the model. “Because you can’t simultaneously correctly predict both the process of channel formation and the amount of sediment transport.”

Dependent on funding

Morphological models consist of a grid of cells, in which the model calculates the sediment transport in each cell. The calculations depend on a vast number of variables. “But the designers of the models are in turn dependent on data from the field, and those data have to come from research institutions which are dependent on government funding. Hydrological engineers and scientists realise that their models aren’t perfect, but they have to make do with the data available. So that means we have a dangerous degree of uncertainty in several practical studies for the general good, but we’ve been cutting the budgets for the fundamental research that is needed to eliminate that uncertainty. Nevertheless, the Dutch models currently available are still the best in the world.”

Two models of the same bend in the Western Scheldt. The channel in the standard model is deeper than the one in the calibrated model. The standard model therefore suggests that the channel need not be dredged to keep it deep enough for shipping, which is not the case in reality.

Publication

A.W. Baar, M. Boechat Albernaz, W.M. van Dijk & M.G. Kleinhans, ‘Critical dependence of morphodynamic models of fluvial and tidal systems on empirical downslope sediment transport’, Nature Communications 10 (2019), https://www.nature.com/articles/s41467-019-12753-x