Development of a finite element model simulating flow and sediment dynamics : application to the Mahakam land-sea continuum (Indonesia)

Land-sea continua present continuous riverine and marine environments that include different interconnected regions of a river such as tributaries, lakes, delta, and adjacent coastal ocean. Although there have been intensive numerical studies on flow dynamics and substances transport in such complex systems, significant efforts related to the representation of the systematic geometry and modelling parameters are still needed to improve the accuracy of simulations, to have better understanding of the flow dynamics processes occurring at different temporal and spatial scales, and to study transport processes of suspended matter. In this context, firstly, different options to parameterize the turbulent shear stresses are investigated in detail by means of the unstructured grid, finite element model SLIM (Second-generation Louvain-la-Neuve Ice-ocean Model, www.climate.be/slim), for accurately reproducing the flow dynamics that are obtained from the laboratory compound open-channels experiments. We found that the Smagorinsky turbulence closure model, which was originally developed to study the dynamics of the atmosphere’s general circulation, finally appears to be well adapted, providing even better results for the laboratory compound open-channels flow than other more classical approaches. This turbulence closure model can be used to parameterize the turbulent shear stresses in both riverine and marine flows. Secondly, in terms of the real application of the Mahakam land-sea continuum, besides the improvements of representing the multiple channels network of the Mahakam Delta in the computational grid and the flow dynamics within the system, various formulations for computing the erosion rate of either fine- or coarse-grained sediments from the riverbed and seabed are investigated, resulting in the conclusion that the Patheniades’ formula can be applied not only in muddy environments as original suggestion of the formula but also in mixture environments of mud and sand like our studied system. In addition, renewal water timescales, i.e. the age, residence time, and exposure time, which are often used to better understand the hydrodynamics and substances transport in coastal regions, are also simulated for the most complicated region of the continuum.