Partitioning sand transport between channels of a deep river channel bifurcation: Implications for river diversion structures and land building in southern Louisiana
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Man-made diversion structures that cut through the levees of the Mississippi River act as channel bifurcations, delivering water and sediment to an otherwise disconnected wetland environment. In this study, the role of diversion depth in controlling the amount of sand that exits from the primary channel during river flooding is examined. We follow the sand fraction because new land built by active subdeltas of the Mississippi River delta is predominately composed of sand rather than mud. Sand transport is not evenly distributed from the channel bottom to the water surface in large rivers. The highest concentrations of moving sand are located near the bed. This suggests that diversions dug to depths tapping the lowermost portion of the water column might export considerably more sand to the neighboring overbank regions for land building purposes. In order to explore the importance of deep diversions for land building we measured properties of the Atchafalaya River - Grand Lake bifurcation near Morgan City, LA, during high discharge in May, 2009. Grand Lake is the entrance point to the Wax Lake outlet channel and the Wax Lake delta. Persistent growth of this delta over the past 30 years highlights the successful land-building properties of this diversion. Bathymetry defining the geometry of this bifurcation, as well as measurements of flow and sediment transport in its Atchafalaya and Grand Lake branches is used to illustrate the effectiveness of a relatively deep diversion for land building. This case study can be used to evaluate the potential of other river diversions as conduits delivering sediment to neighboring wetlands for rebuilding the Mississippi River delta.
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