The role of erosion rate formulation on the ignition and subsidence of turbidity current

摘要

An empirical relationship describing the entrainment of suspended sediment is required even if a vertical structure model is used for modeling turbidity currents. Several relationships can be found in the literature for estimating the entrainment rate of suspended sediment. Turbidity currents at laboratory scale are primarily depositional and the model prediction is not sensitive to the specified entrainment relationship. At the field scale however, turbidity currents are more likely to be erosional and the predicted results can be quite sensitive to the entrainment relations used in the model. Here, the four-equation model of turbidity current is utilized to revisit the ignition and subsiding conditions of turbidity currents originally described in the seminal work of Parker et al. (1986). Three different sediment entrainment models are considered. The ignition conditions are studied for a variety of parameters such as sediment size, slope and inflow current thickness. It has been found that steeper slopes and finer sediment gives lower values of ignition velocity and sediment concentration. The ignition Richardson number is found to be strongly dependent on the slope and insensitive to the sediment size. The ignition condition has been found to be quite sensitive to the entrainment relation used in the computation. The phase diagrams describing ignition conditions, and igniting and subsiding regions obtained from different erosion models are generally similar. The auto-suspension generation line dividing the zones of ignition and subsidence are found to be different for different entrainment relationship. As a result, a turbidity current predicted to be subsiding by one entrainment relation could turn out to be igniting when a different entrainment model is used. This has important implication in the modeling of turbidity current as the same numerical model can give completely different prediction depending on which entrainment relation is used.