Forward-modeling of co-evolution of turbidity currents,sediment transport,and cyclic steps in the Rio Muni Basin

摘要

Previous quantitative studies of field-scale cyclic steps are mostly based on analysis of field data.Such studies have shed light on the erosion/deposition patterns over these morphological features as well as the magnitudes of the turbidity current parameters back estimated using the measured geometry data.However,it remains unclear to what extent such back estimated hydraulic features and erosion/deposition patterns can be numerically reproduced by process-based numerical models.Here,a twodimensional layer-averaged fully coupled turbidity current model is applied to forward-model co-evolution of turbidity currents,sediment transport,and cyclic steps in the Rio Muni Basin off the West Africa margin,where nine cyclic steps featuring a transition from the upstream erosion to the downstream deposition were identified.Numerical case studies indicate that large and bankfull turbidity currents do not form the erosion-deposition transition because they are likely to fall into the high-speed regime that favors whole-scale erosion.In this regard,threshold values for the current velocity were derived to distinguish high/low-speed regimes of turbidity currents.It is shown that smaller turbidity currents,which do not follow the bankfull assumption,and,thus,fall into the low-speed turbidity current regime,may produce cyclic steps by current deceleration.While values of the derived physical parameters of the smaller turbidity current differ quantitatively from those of previous back estimated values,their qualitative variation trends are basically the same.