Flocculation and transport of cohesive sediment.

摘要

An earlier model for floc dynamics utilizes a constant fractal dimension and a constant yield strength as a part of the model assumptions. However, several prior studies suggest that the fractal dimension of floc changes as floc size increases or decreases. Furthermore, the yield strength of floc is observed to be proportional to floc size and fractal dimension during breakup process. In this research, a variable fractal dimension is adopted to improve the previous flocculation model. Moreover, an equation for yield strength of floc is theoretically and mathematically derived. The newly derived equation is combined with flocculation models. By comparing with laboratory experiments on temporal evolution of floc size (mixing tank and Couette flow), this research demonstrates the importance of incorporating a variable fractal dimension and a variable floc yield strength into the model for floc dynamics. However, it still remains unclear as what are effects of variable fractal dimension and variable yield strength on the prediction of cohesive sediment transport dynamics. The second goal of the present study is to further investigate roles of floc dynamics in determining the predicted sediment dynamics in a tide-dominated environment. A 1DV numerical model for fine sediment transport is revised to incorporate four different modules for flocculation, i.e., no floc dynamics, floc dynamics with assumptions of constant fractal dimension and yield strength, floc dynamics for variable fractal dimensional only, and floc dynamics for considering both fractal dimension and yield strength variables. Model results are compared with measured sediment concentration and velocity time series at the Ems/Dollard estuary. Numerical model predicts very small (or nearly zero) sediment concentration during slack tide when floc dynamics is neglected or incorporated incompletely. This feature is inconsistent with the observation. When considering variable fractal dimension and variable yield strength in the flocculation model, numerical model predicts much smaller floc settling velocity during slack tide and hence is able to predict measured concentration reasonably well. Model results further suggest that, when sediment concentration is greater than about 0.1 g/l, there exists a power law relationship between mass concentration and settling velocity except very near the bed where turbulent shear is strong. This observation is consistent with earlier laboratory and field experiment on floc settling velocity. It is concluded that a complete floc dynamics formulation is important to modeling cohesive sediment transport. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)