A Clearer Picture Of How Rivers and Deltas Develop

By adding information about the subsoil to an existing sedimentation and erosion model, researchers at Delft University of Technology (TU Delft, The Netherlands) have obtained a clearer picture of how rivers and deltas develop over time. A better understanding of the interaction between the subsoil and flow processes in a river-delta system can play a key role in civil engineering (delta management), but also in geology (especially in the work of reservoir geologists). Nathanaël Geleynse et al. recently published in the journals Geophysical Research Letters and Earth and Planetary Science Letters.

Many factors are involved in how a river behaves and the creation of a river delta. Firstly, of course, there is the river itself. What kind of material does it transport to the delta? Does this material consist of small particles (clay) or larger particles (sand)? But other important factors include the extent of the tidal differences at the coast and the height of the waves whipped up by the wind. In this study, researchers at TU Delft are working together with Deltares and making use of the institute's computer models (Delft3D software). These models already take a large number of variables into account. Geleynse et al. have now supplemented them with information on the subsoil. It transpires that this variable also exerts a significant influence on how the river behaves and the closely related process of delta formation.

These are the results of the model for the sedimentary composition of the subsoil at the horizontal white lines in Figure 2. The initial situation (soil elevation) is indicated by the dotted black lines. The figure on the left represents a situation in which only sand is present in the subsoil at the beginning of the model simulation. The clay visible in this delta was therefore carried along by the upper-current river. The figure on the right shows a situation in which a great deal of clay is present in the initial subsoil, in addition to sand. The figure in the middle shows a situation in which there is more of a volumetric balance between sand and clay in the initial subsoil. The blue line indicates the calculated water surface. The figure presents the information as if the viewer is looking downstream. Credit: Geleynse et al

Room for the River

The extra dimension that Geleynse et al. have added to the model is important to delta management, among other things. If – as the Delta Commission recommends – we should be creating "Room for the River", it is important to know what a river will do with that space. Nathanaël Geleynse explains: "Existing data do not enable us to give ready-made answers to specific management questions ... nature is not so easily tamed ... but they do offer plausible explanations for the patterns and shapes we see on the surface. The flow system carries the signature of the subsoil, something we were relatively unaware of until now. Our model provides ample scope for further development and for studying various scenarios in the current structure."