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We use in situ measurements of suspended mud to assess the flocculation state of the lowermost freshwater reaches of the Mississippi River. The goal of the study was to assess the flocculation state of the mud in the absence of seawater, the spatial distribution of floc sizes within the river, and to look for seasonal differences between summer and winter. We also examine whether measured floc sizes can explain observed vertical distributions of mud concentration through a Rouse profile analysis. Data were collected at the same locations during summer and winter at similar discharges and suspended sediment concentrations. Measurements showed that the mud in both seasons was flocculated and that the floc size could reasonably be represented by a cross‐sectional averaged value as sizes varied little over the flow depth or laterally across the river at a given station. Depth‐averaged floc sizes ranged from 75 to 200 microns and increased slightly moving downriver as turbulence levels dropped. On average, flocs were 40 microns larger during summer than in winter, likely due to enhanced microbial activity associated with warmer water. Floc size appeared to explain vertical variations in mud concentration profiles when the bed was predominately composed of sand. Average mud settling velocities for these cases ranged from 0.1 to 0.5 mm/s. However, Rouse‐estimated settling velocities ranged from 1 to 3 mm/s at two stations during winter where the bed was composed of homogeneous mud. These values exceeded the size‐based estimates of settling velocity.

 

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. 

An inexpensive and compact underwater digital camera imaging system was developed to collect in situ high resolution images of flocculated suspended sediment at depths of up to 60 meters. The camera has a field of view of 3.7 × 2.8 mm and can resolve particles down to 5 . Depending on the degree of flocculation, the system is capable of accurately sizing particles to concentrations up to 500 mg/L. The system is fast enough to allow for profiling whereby size distributions of suspended particles and flocs can be provided at multiple verticals within the water column over a relatively short amount of time (approximately 15 min for a profile of 15 m). Using output from image processing routines, methods are introduced to estimate the mass suspended sediment concentration (SSC) from the images and to separate identified particles into sand and mud floc populations. The combination of these two methods allows for the size and concentration estimates of each fraction independently. The camera and image analysis methods are used in both the laboratory and the Mississippi River for development and testing. Output from both settings are presented in this study.