Predominance of wind-wave transport and resuspension of fine river sediments in a great lake: A Delft3D modelling study of lake Turkana, East African Rift

Depositional models for clastic sedimentation in large lakes, notably in rift lakes, emphasize on downslope river and gravity-driven processes. Wind-driven waterbodies (WWB), a recently-defined category of lakes, display features created by wave related processes and wind-induced water circulation such as beach ridges or spits along the coasts, as well as sediment drifts, sedimentary shelf progradation and erosional surfaces in deeper, offshore domains. A coupled hydrodynamic, wave and sediment transport three-dimensional Delft3D model was established for Lake Turkana, East Africa, one of the world’s greatest lakes in order to test the WWB validity for fine sediments. Using available data, the model is forced for 1.5 years with river liquid and solid discharge, as well as wind data, in order to simulate cohesive sediment transport and resuspension. The model simulates stratification due to salinity, wave generation and dissipation, and sediment advection and resuspension by waves and currents, with multiple cohesive sediment fractions. Model results were compared with remote sensed imagery and with available in-situ sediment deposition rates, reproducing the general surface suspended sediment patterns, and agreeing with the mass deposition rates data from the literature.

By creating scenarios in which certain processes were switched off, the contribution of waves resuspension, wind-induced currents, salinity-induced stratification, and river jet, in resuspending and transporting sediment along the lake could be investigated. With just the wind and/or the river influence, most of the sediment deposition occurs in the first 10 km from the river mouth and at depths from 0 to 10 m. When waves are switched on, significant quantities of sediments can be resuspended by waves, and most of the sediments are deposited in the first 30 km from the river mouth, at depths from 10 to 30 m. This study provides insights on sediment transport in the Lake Turkana, and similarly in great lakes in general, supporting waves as the main agent transporting fine sediments away from river mouths into deeper areas, as opposed to river-plume derived transport.