ABSTRACT

Turbid rivers and density currents carry, distribute, and deposit considerable quantities of fine muddy sediment within rivers, coastal regions, and reservoirs. The muddy sediment in these flows has the potential to flocculate, and knowing and predicting the floc size is critical for predicting mud movement. Flocs are notoriously difficult to measure. Imaging of flocs either within a turbulent suspension or in a separate settling chamber are methods widely considered to be the most accurate ways to measure floc size. The benefit of imaging flocs within the suspension is that the measurements are made within the conditions that gave rise to those particular flocs. The drawback is that it is not possible to make measurements in suspensions with concentrations > 400 mg/L. Transferring a suspension sample to a settling chamber allows for imaging of flocs from suspensions with higher concentration. But, it also removes flocs from the environment in which they were formed, possibly leading to floc growth or breakup. In this study, we compare these two methods to determine whether or not the flocs imaged in a settling chamber are representative of the flocs found in a turbulent suspension. For the experiments, flocs are formed from kaolinite and montmorillonite clay mixed with saltwater at different concentrations and mixing conditions. The suspension is then imaged within the mixing tank, and samples from the mixing tank are imaged in a settling chamber. Results show that flocs imaged in the settling chamber tend to be slightly smaller than those imaged in the mixing chamber, though the differences are minimal if care is taken in the transfer process. Additional trends in the difference between the two methods with turbulent shear rate and concentration are discussed.