Laboratory Study on Particle Size Effects on Constant-volume Particle-driven Gravity Currents
In this dissertation, particle size effects on propagation behavior, sediment deposition patterns and suspended sediment concentrations in constant-volume of particle-driven gravity currents were mainly investigated. The gravity currents were generated by a lock exchange in a two-dimensional tank, and the propagation behavior and particle characteristics of depositions and suspended sediments were investigated using various instruments and techniques such as image processing technique, filtration technique, a laser diffraction size analyzer, and an Acoustic Doppler velocimetry.
The main finding of this study is that particle size and turbulence control the dynamics of the particle-driven gravity currents and the deposited and suspended sediment characteristics in these currents. In this study, two propagation phases were proposed based on turbulent Reynolds number ReL and particle settling behaviors. The critical turbulent Reynolds number ReLc was estimated to be the order of one (~ 2 in our estimates). The effects of particle size on the particle settling velocity were relatively small in the early propagation stage (we called the turbulent dominated settling (TDS) propagation phase) due to strong turbulent flow and were significant in the later propagation stage (we called the gravity dominated settling (GDS) propagation phase) because of decreasing turbulence effects. The vertical concentration profiles at the current head were nearly linear within the TDS phase and changed to an exponential profile during their propagation; especially, the concentration profile quickly decreased in large particles. In addition, horizontal particle sorting that is size grading of particles towards the flow direction was more significant rather than vertical particle sorting.