Abstract
Existing flocculation models for cohesive sediments are classified into two groups: population balance equation models (PBE) and floc growth models. An FGM ensures mass conservation in a closed system. However, an FGM determines only the average size of flocs, whereas a PBE has the capability to calculate a size distribution of flocs. A new stochastic approach to model the flocculation process is theoretically developed and incorporated into a deterministic FGM in this study in order to calculate a size distribution of flocs as well as the average size. A log-normal distribution is used to generate random numbers based on previous laboratory experiments. The new stochastic flocculation model is tested with three laboratory experiment results. It was found and validated with measured data that the new stochastic flocculation model has the capability to replicate a size distribution of flocs reasonably well under different sediment and carrier flow conditions. Three more distributions (normal; Pearson type 3; and generalized extreme value distributions) were also tested. From the comparison with results of different distribution functions, it is shown that a stochastic FGM using a log-normal distribution has a comparative advantage in terms of simplicity and accuracy.
Highlights
The flocculation process, a series of aggregation and disaggregation processes, is one of the most important characteristics of suspended cohesive sediment such as mud and organic matter
This study focuses on the distribution of floc size in the equilibrium state
This paper presents a stochastic floc growth model (FGM) to calculate the size distribution of flocs
Summary
The flocculation process, a series of aggregation and disaggregation (so called breakup) processes, is one of the most important characteristics of suspended cohesive sediment such as mud and organic matter. Collisions of suspended sediment are caused by Brownian motion, differential settling, and turbulent shear [2,3,4]. Many laboratory studies on the effects of turbulent shear have been conducted utilizing a mixing tank and Couette viscometer in order to control the intensity of turbulent shear (e.g., [9,10,11,12]). From these studies, it is apparent that the flocculation process of aggregation (mainly due to particle collision) and breakup is significantly governed by turbulent shear
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