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Muddy sediment constitutes a major fraction of the suspended sediment mass carried by the Mississippi River. Thus, adequate knowledge of the transport dynamics of suspended mud in this region is critical in devising efficient management plans for coastal Louisiana. We conducted laboratory tank experiments on the sediment suspended in the lower reaches of the Mississippi River to provide insight into the flocculation behavior of the mud. In particular, we measure how the floc size distribution responds to changing environmental factors of turbulent energy, sediment concentration, and changes in base water composition and salinity during summer and winter. We also compare observations from the tank experiments toin situobservations. Turbulence shear rate, a measure of river hydrodynamic energy, was found to be the most influential factor in determining mud floc size. All flocs produced at a given shear rate could be kept in suspension down to shear rates of approximately 20 s−1. At this shear rate, flocs on the order of 150–200 μm and larger can settle out. Equilibrium floc size was not found to depend on sediment concentration; flocs larger than 100 μm formed in sediment concentrations as low as 20 mgL−1. An increase in salinity generated by adding salts to river water suspensions did not increase the flocculation rate or equilibrium size. However, the addition of water collected from the Gulf of Mexico to river-water suspensions did enhance the flocculation rate and the equilibrium sizes. We speculate that the effects of Gulf of Mexico water originate from its biomatter content rather than its ion composition. Floc sizes in the mixing tanks were comparable to those from the field for similar estimated turbulent energy. Flocs were found to break within minutes under increased turbulence but can take hours to grow under conditions of reduced shear in freshwater settings. Growth was faster with the addition of Gulf of Mexico water. Overall, the experiments provide information on how suspended mud in the lower reaches of the Mississippi might respond to changes in turbulence and salinity moving from the fluvial to marine setting through natural distributary channels or man-made diversions.
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The Flocculation State of Mud in the Lowermost Freshwater Reaches of the Mississippi River: Spatial Distribution of Sizes, Seasonal Changes, and Their Impact on Vertical Concentration Profiles
Abstract 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.
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- Award ID(s):
- 1801142
- PAR ID:
- 10441379
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 128
- Issue:
- 7
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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