Implementation of geotechnical and vegetation modules in TELEMAC to simulate the dynamics of vegetated alluvial floodplains

Amongst the most widely used computational fluid dynamics models, some include a sediment transport module that enables the examination of river channel dynamics. However, most ignore two families of processes influencing lateral erosion rates, and thus channel evolution mechanisms: lateral transport of sediment through mass wasting along river banks and valley walls, and soil reinforcement created by plant roots. A few modelling packages consider geotechnical processes, albeit with important limitations. Indeed, most solutions are solely compatible with single-threaded channels, impose a given computational mesh structure (e.g. body-fitted coordinate system), derive lateral migration rates from hydraulic properties, adjust bank morphology solely based on the angle of repose of the bank material, rely on non-physical assumptions to describe certain processes (e.g. channel cut offs in meandering rivers), and exclude floodplain processes. This paper describes the development and testing of two modules that were recently added to the mathematical suite of solvers TELEMAC-MASCARET to address the aforementioned limitations. The first module (GEOTECH) includes an algorithm that scans the computational domain in an attempt to detect potentially unstable slope profiles across the domain or intersecting with water-soil boundaries. The module relies on a fully configurable, universal genetic algorithm with tournament selection to delineate the shape of the surface along which a slump block detaches itself from a river bank or slope by translational or rotational mechanism. Both the hydrostatic pressure caused by the flow and the elevation of the water table are used in the Bishop’s method to quantify slope stability. Another algorithm computes the surface of the coarse fraction of the block material which is deposited at the toe of the slope. The second module (RIPVEG) simulates the evolution of floodplain vegetation, whose properties affect the geotechnical stability of slopes present in the computational domain by imposing a surcharge and increasing soil cohesion near the soil surface. Plants develop in height, weight and rooting depth at a rate that depends on the species and plant age. The two modules, combined with the flow and sediment transport models included in TELEMAC, provide a holistic solution to study the dynamics of a broad range of alluvial river types. The model is currently being tested, calibrated and validated using datasets from meandering rivers.