[SCG64-P12] Experimental Investigation of Vertical Concentration Profile and Entrainment Rates of Mixed Grain-size Particles in Turbidity Currents
キーワード：flume experiment, mixed grain-size particles, vertical concentrantion profile, entrainment rate
Turbidity currents in the ocean and lakes are driven by excess density originated from suspended sediment. The dynamics of turbidity currents are largely governed by suspended sediment that is entrained from the bed. Therefore, we conducted the flume experiments of turbidity currents in order to obtain better prediction of transport rates of suspended sediment, especially focusing on differences between single and mixed grain-size cases. The vertical profiles of velocity and sediment concentration of turbidity currents containing mixed grain-size particles are the first subject of this research, which is one of the key parameters in morphodynamic models of turbidity currents. Also, a key feature for the prediction of suspended load is the description of the entrainment rate of basal sediment into suspension at the solid-fluid interface, which is the second subject of this research. The lightweight plastic particles were used in our experiments in order to reproduce suspension in relatively small scale flume (4 m long and 15 cm wide). In this research, plastic particles with the gravity of 1.45 were chosen as the model sediment material. As the focus of the experiments reported here was centered on the dynamics of the current body, rather than the head, care was taken to measure only after the current front had passed and the flow had achieved a quasi-equilibrium state.
Firstly, we report vertical profile of concentration of mixed grain-size sediments in experimental turbidity currents. For the experiments using uniform and nonuniform sediment, the vertical profiles suggested that the distribution of concentration of finer particles is less stratified than the coarser particles, and finer particles have more capability to diffuse upward, which tends to approximately uniform distribution above a certain height. Comparing to the result of Sequeiros et al. (2010), the vertical profiles of dimensionless distribution of suspension concentration shows no significant difference between uniform and mixed-grain sediments, which represented the vertical profiles of concentration of particles have identical assessment between turbidity currents and dense saline underflows.
Secondly, we examined the entrainment rate of basal sediment to suspension in turbidity currents, and compared our measurements with the prediction using the empirical formulation proposed by Garcia and Parker (1991, 1993). In the uniform particle experiments, the prediction of entrainment of sediment is consistent with the measurement accurately, whereas in the mixed grain-size particle experiments, it represent less consistent to anticipation. This implies that the new empirical formulation is needed for predicting entrainment rate of the mixed grain-size sediments into suspended load.
References
Garcia, M., and Parker, G. (1991). ‘‘Entrainment of bed sediment into suspension.’’ J. Hydraul. Eng., 117(4), 414–435.
Garcia, M., and Parker, G. (1993). ‘‘Experiments on the entrainment of sediment into suspension by a dense bottom current.’’ J. Geophys. Res., 98(3), 4793–4807.
Sequeiros, O.E., et al. (2010). “Characteristics of velocity and excess density profiles of saline underflows and turbidity currents flowing over a mobile bed.” J. Hydraul. Eng., 136(7), 412-433.
Firstly, we report vertical profile of concentration of mixed grain-size sediments in experimental turbidity currents. For the experiments using uniform and nonuniform sediment, the vertical profiles suggested that the distribution of concentration of finer particles is less stratified than the coarser particles, and finer particles have more capability to diffuse upward, which tends to approximately uniform distribution above a certain height. Comparing to the result of Sequeiros et al. (2010), the vertical profiles of dimensionless distribution of suspension concentration shows no significant difference between uniform and mixed-grain sediments, which represented the vertical profiles of concentration of particles have identical assessment between turbidity currents and dense saline underflows.
Secondly, we examined the entrainment rate of basal sediment to suspension in turbidity currents, and compared our measurements with the prediction using the empirical formulation proposed by Garcia and Parker (1991, 1993). In the uniform particle experiments, the prediction of entrainment of sediment is consistent with the measurement accurately, whereas in the mixed grain-size particle experiments, it represent less consistent to anticipation. This implies that the new empirical formulation is needed for predicting entrainment rate of the mixed grain-size sediments into suspended load.
References
Garcia, M., and Parker, G. (1991). ‘‘Entrainment of bed sediment into suspension.’’ J. Hydraul. Eng., 117(4), 414–435.
Garcia, M., and Parker, G. (1993). ‘‘Experiments on the entrainment of sediment into suspension by a dense bottom current.’’ J. Geophys. Res., 98(3), 4793–4807.
Sequeiros, O.E., et al. (2010). “Characteristics of velocity and excess density profiles of saline underflows and turbidity currents flowing over a mobile bed.” J. Hydraul. Eng., 136(7), 412-433.