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Optimum vegetation height and density for inorganic sedimentation in deltaic marshes

Abstract

River deltas support a disproportionate percentage of the world’s population and some are drowning as sea level rises1. Resilient deltas theoretically balance relative sea-level rise with vertical growth from surface sedimentation2,3,4,5,6,7. Vegetation generally enhances inorganic sedimentation and resiliency in some settings, such as tidal saltwater marshes8, but the effect of vegetation on freshwater marshes in river deltas is less clear. Here we use a hydrodynamic numerical model9 to simulate deposition in a river delta with varying vegetation characteristics and water discharge and show that vegetation does not always enhance sedimentation on a freshwater marsh. For a given flood, we find that intermediate vegetation height and density are optimal for enhancing both sand and mud deposition, whereas tall or dense vegetation causes sand to remain in the river channel, reducing marsh sedimentation. A multivariate regression analysis of remote-sensing data from Wax Lake Delta, Louisiana, USA shows that the delta exhibits a hydrodynamic response to vegetation in agreement with model predictions. Because most sediment is delivered to freshwater deltaic marshes by infrequent storm and flood events, we further suggest that the timing of such events relative to seasonal vegetation growth determines the integrated effect of vegetation on delta resiliency.

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Figure 1: Plan view image of the delta configuration used in this study.
Figure 2: Modelled water fluxes under vegetated and non-vegetated conditions.
Figure 3: Modelled sediment fluxes for marsh platforms under vegetated and non-vegetated conditions.
Figure 4: NDVI and flood hydrograph time series for Wax Lake Delta.

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Acknowledgements

This work was financially supported by an NSF Frontiers in Earth Systems Dynamics grant (EAR-1135427), NSF grant OCE-1329542, and funds from the Alfred P. Sloan Foundation. We would like to thank S. Fagherazzi for insightful conversations about vegetation and sedimentation.

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Correspondence to William Nardin or Douglas A. Edmonds.

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Nardin, W., Edmonds, D. Optimum vegetation height and density for inorganic sedimentation in deltaic marshes. Nature Geosci 7, 722–726 (2014). https://doi.org/10.1038/ngeo2233

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