Search for: All records

Due to the flocculation process, suspended mud aggregates carried by rivers to the coastal ocean are thought to undergo changes in size and shape in response to environmental drivers such as turbulence, sediment concentration, organic matter (OM), and salinity. Some have assumed that salt is necessary for floc formation, and that mud, therefore, reaches the estuary unflocculated. Yet mud flocs exist in freshwater systems long before the estuarine zone, likely due to the presence of OM acting as a floc‐promoting binder. Therefore, it is important to consider how salinity affects flocculation, if at all, in the presence of OM. Here, we used experiments to examine the flocculation of a natural mud with and without OM. Results showed that the rate of floc growth and equilibrium size both increase with salinity regardless of the presence or absence of OM. However, the response of both to salinity was stronger when OM was present. In deionized water, natural sediment with OM was seen to produce large flocs. However, the size distribution of the suspension tended to be bimodal. With the addition of salt, increasing amounts of unflocculated material became bound within flocs, producing a more unimodal size distribution. Here, the enhancing effects of salt were noticeable at even 0.5 ppt, and increases in salinity past 3–5 ppt only marginally increased the floc growth rate and final size. Data from the experiment were used to develop a salinity‐dependent model to account for changes in floc growth rate and equilibrium size.

 

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.