Implementation of geotechnical and vegetation modules inTELEMAC to simulatethe dynamics of vegetated alluvial floodplains

Amongst the most widely used computational fluid\ndynamics models, some include a sediment transport module that\nenables the examination of river channel dynamics. However,\nmost ignore two families of processes influencing lateral erosion\nrates, and thus channel evolution mechanisms: lateral transport\nof sediment through mass wasting along river banks and valley\nwalls, and soil reinforcement created by plant roots. A few\nmodelling packages consider geotechnical processes, albeit with\nimportant limitations. Indeed, most solutions are solely\ncompatible with single-threaded channels, impose a given\ncomputational mesh structure (e.g. body-fitted coordinate\nsystem), derive lateral migration rates from hydraulic properties,\nadjust bank morphology solely based on the angle of repose of\nthe bank material, rely on non-physical assumptions to describe\ncertain processes (e.g. channel cut offs in meandering rivers), and\nexclude floodplain processes. This paper describes the\ndevelopment and testing of two modules that were recently added\nto the mathematical suite of solvers TELEMAC-MASCARET to\naddress the aforementioned limitations. The first module\n(GEOTECH) includes an algorithm that scans the computational\ndomain in an attempt to detect potentially unstable slope profiles\nacross the domain or intersecting with water-soil boundaries.\nThe module relies on a fully configurable, universal genetic\nalgorithm with tournament selection to delineate the shape of the\nsurface along which a slump block detaches itself from a river\nbank or slope by translational or rotational mechanism. Both the\nhydrostatic pressure caused by the flow and the elevation of the\nwater table are used in the Bishop’s method to quantify slope\nstability. Another algorithm computes the surface of the coarse\nfraction of the block material which is deposited at the toe of the\nslope. The second module (RIPVEG) simulates the evolution of\nfloodplain vegetation, whose properties affect the geotechnical\nstability of slopes present in the computational domain by\nimposing a surcharge and increasing soil cohesion near the soil\nsurface. Plants develop in height, weight and rooting depth at a\nrate that depends on the species and plant age. The two modules,\ncombined with the flow and sediment transport models included\nin TELEMAC, provide a holistic solution to study the dynamics\nof a broad range of alluvial river types. The model is currently\nbeing tested, calibrated and validated using datasets from\nmeandering rivers.\n

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