The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study
2.1. Conceptual Multichannel Deltaic Systems
- Extensive P. australis. In this landscape, the discharge is constrained by long, narrow channels that bifurcate repeatedly. This landscape is intended to represent conditions similar to what in the Bird’s Foot Delta has occurred in Main Pass from 1922 to the present  that may persist if P. australis dieback is a temporary phenomenon (Figure 4D–F).
- Minimum P. australis extent. In this landscape, the discharge is constrained only by short, wide channels that rarely bifurcate. This landscape is intended to represent conditions similar to what occurred in Main Pass during 1884  that may occur again if P. australis dieback is extreme and persistent (Figure 4B).
- Poor P. australis extent. In this landscape, the discharge is constrained by channels that are approximately one-third as complex, in terms of length and bifurcation, as in condition (i). This landscape is intended to represent the conditions that occurred in Main Pass during 1905  and that may occur if the P. australis dieback is fairly severe and persistent (Figure 4C).
- Good P. australis extent. In this landscape, the discharge is constrained by channels that are approximately two-thirds as complex, in terms of length and bifurcation, as in condition (i) (Figure 4D–F). This landscape is intended to represent conditions that may occur if the P. australis dieback is moderately severe and persistent.
2.2. Conceptual Multichannel Process-Based Model
4. Sensitivity Analysis
- Tide: With no tides, the sedimentation rate, compared with the base case, increased for the first several years but then decreased afterwards. Interestingly, the final sedimentation was very close to the base case with the tide turned on. The tide affected the evolution of the system, but this effect diminished over decades.
- Wind: There was no significant difference between the wind case and the base case with no wind. It seems that the normal wind fields (excluding extreme events such as hurricanes and cold fronts) have little effect on long-term sedimentation processes.
- Waves: The effect of the waves was substantial. Both scenarios showed a reduction of sedimentation when the waves were turned on. The most likely explanation is that enhanced bottom shear stresses by the waves made the bed material more erodible.
- Sea level rise: The sea level rise caused an increase of the water level and, equivalently, a decrease of the river current speed given the same river discharge. The reduced current velocity increased the sedimentation in the system.
- River discharge: The system was very sensitive to river discharge. An increase of discharge accelerated the current, which moved more sediment out of the system and caused less sedimentation and more erosion.
- River sediment concentration: A change of the river sediment concentration changed the riverine sediment supply. It was obvious that the more sediment supply came with more sedimentation and vice versa.
- Sediment median grain size (D50): Under the same hydrodynamic conditions, the transport rate for coarser sediments was smaller than for finer sediments, which can result in more sedimentation for coarser sediments.
- Bed level: Similar to the river discharge, the system was also very sensitive to a bed level change. Given the same river discharge, an increase in the bed level decreased the current velocity, which increased the sedimentation.
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Conflicts of Interest
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|Level||NL (# of Links)||NV (# of Vertices)||NO (# of Outlets)||Complexity Index|
|Factor||Base Case||Option 1||Option 2||Impact on Sedimentation|
|(Option 1)||(Option 2)|
|Waves 2||Off||On (Fetch/depth limited)||On (Young–Verhagen)||Decrease||Decrease|
|Sea level rise 3||Off (0 m)||On (+0.2 m)||/||Increase||/|
|River discharge||10,800 m3/s||12,960 m3/s (+20%)||8640 m3/s (−20%)||Decrease||Increase|
|River sediment concentration||50 mg/L||60 mg/L (+20%)||40 mg/L (−20%)||Increase||Decrease|
|D50||150 µm||160 µm||140 µm||Increase||Decrease|
|Bed level||8.0 m||9.6 m (+20%)||6.4 m (−20%)||Increase||Decrease|
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Hu, K.; Meselhe, E.; Nyman, J.A. The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study. Water 2021, 13, 1407. https://doi.org/10.3390/w13101407
Hu K, Meselhe E, Nyman JA. The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study. Water. 2021; 13(10):1407. https://doi.org/10.3390/w13101407Chicago/Turabian Style
Hu, Kelin, Ehab Meselhe, and J. Andrew Nyman. 2021. "The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study" Water 13, no. 10: 1407. https://doi.org/10.3390/w13101407