Numerical Modeling of Particle Driven Density Currents on Gentle Slopes
Type:
Presented during:
CEDA Dredging Days 2021
Authors:
J.C. Goeree, J.T.M. Wijnands, H.U. Osterchrist, and C. van Rhee
Abstract
Particle driven density, or turbidity, currents, occur in both man-made and natural environments.
Turbidity currents exist because of the density difference between the surrounding water and the suspended sediment. Natural causes of turbidity currents are earthquakes, failure of submarine slopes or other geological disturbances. Also dredging and sub-marine mining operations can cause turbidity currents. For instance, a density plume of fine sediments caused by mining activities at the bottom of the deep-sea.
The dispersion of fine sediments over a wide (horizontal) area, due to dredging and sub-marine mining activities, have a potential negative impact on the environment. These fine sediments can cover the seabed burying sea-life.
The main objective of this paper is to model and investigate the conditions which are favorable to sustain a density current. With this model, it is possible to determine the dispersion of particles over the seabed.
A semi-empirical model is developed which yields the front-velocity of the turbidity current. The front-velocity governs the degree of dispersion of the fines, i.e. run-out length of the debris. Moreover, the amount of dispersion of fines depends on the particle settling velocity and the degree of turbulence of the flows. Furthermore, two numerical models are employed, the C++ OpenFOAM library and the 2DV model, for determining the run-out length of turbidity currents.
Three particle sizes were used, ddpp = 50 ? 100 ? 150 µµµµ, as input for both the semi-empirical and the numerical model. Other input parameters, such as mixture density, were kept constant in the calculations. The results show that turbidity currents consisting of smaller particle sizes can be sustained over long distances. The run-out length of the deposit, for larger particles, decreases. The calculated run-out lengths for both the semi-empirical model and the numerical models correspond well for all three particle sizes.
Keywords: Turbidity Currents, Particles, Sediment, CFD, Dredging, Mining, Numerical.