Study on sedimentation will help planners develop effective river diversions
December 4, 2012 | Posted by Delta Dispatches in 2011 Mississippi River Flood, 2012 Coastal Master Plan, Diversions, Reports, Science

By Alisha A. Renfro, Ph.D., Staff Scientist, National Wildlife Federation

This year, drought conditions throughout most of the country have left the Mississippi River flowing at a near all-time low. This is a stark comparison to 2011, when heavy rains and a large snowmelt in the spring sent record levels of water and sediment flowing down the river. At the Old River Control Structure north of Baton Rouge, the flow of the river is split, with 70 percent continuing down the Mississippi to the Bird’s Foot delta, and the remaining 30 percent flowing down the Atchafalaya River. During the 2011 flood, the flood protection levees and the opening of the Morganza and Bonnet Carré spillways successfully shunted water safely past the high population centers in the region. However, this event was a missed opportunity to capitalize on the influx of fresh water and sediment and to reconnect the river with sediment-starved wetlands of Louisiana.

In a recent study published in Nature Geoscience, research led by Federico Falcini, Ph.D. examined the link between the historic 2011 river flood and sediment accumulation in nearby wetlands. Their analysis suggested that the natural dynamics of the coastal system coupled with man-made alterations to the river system influenced the amount of sediment deposited in the wetlands. This work shows that under river flood conditions, diverting the flow of the river into shallow basins adjacent to the river could contribute significantly to sediment deposition in the wetlands and therefore contribute to wetland growth.

Mississippi River sediment plumes, as viewed from space (May 17, 2011). Credit: NASA Modis imagery

In the study, the sediment plumes of the Mississippi and Atchafalaya Rivers were tracked using satellite imagery from the 2011 flood event to understand where the sediment went once it exited these rivers. The Mississippi’s sediment plume exited the river in focused jets of sediment-laden water due to the confinement of much of the river’s flow between artificial levees. This plume moved past the coastal current and into the deeper waters of the Gulf of Mexico, limiting the amount of sediment that could be deposited in the near-shore area and adjacent wetlands. In contrast, the Atchafalaya’s sediment plume exited the river and moved along a broad, near-shore area, mixing with waters from the Gulf of Mexico and creating conditions that were likely to favor sediment deposition.

A comparison of sediment accumulation during the 2011 flood in nearby marshes shows a trend that corresponds to the difference in behavior of the two river plumes. Sediment accumulation was highest at marsh sites near the Atchafalaya River, which supports the idea that its sediment plume spreading out over a large area in relatively shallow water, promoting increased sedimentation in the region. Sediment accumulation in wetlands near the mouth of the Mississippi River was substantial, but significantly lower than near the Atchafalaya. While the Mississippi River carried a larger sediment load during the 2011 flood event, much of the sediment was lost to the deeper waters of the gulf.

Louisiana’s 2012 Coastal Master Plan identifies several sediment diversions that are key to restoring the important coastal Louisiana landscape. The success of these diversions will depend on a variety of factors, including location and operation. However, this new research confirms that fine sediments introduced into shallow water can substantially contribute to sediment accumulation in wetlands. In order to restore the rapidly deteriorating wetlands of coastal Louisiana, it is critical to reintroduce the sediment that once built this productive region.

2 Responses to Study on sedimentation will help planners develop effective river diversions

  1. Cecil W. Soileau, P.E., D.WRE says:

    The Plume you see in the aerial photographs is nothing more than clay particles that have little tendency to settle out and make land. Clays tend to remain in suspension for a very long time, up to 3 months, in laboratory samples, and are dispersed by wave activity, even when they final reach to bed of the waterbody.

    On the other hand, it is the sands, and coarse silts, that create land, and those particles settle out in the deeper part of the river near Venice, La. The Mississippi River bed has been aggradating at least 1 foot a year between Venice and Head of Passes, since 1992, and is only kept open to navigation due to dredging. This is due to nearly 1/5 of the flow going out the east bank of the river between Bohemia, at River Mile 44, and Venice. One fifth of the river flow below New Orleans is about 250,000 cfs, which is comparable to Wax Lake Outlet into Atachafalaya Bay. Yet very little accretion is seen taking place in Breton Sound, where the clay plume is apparent, beginning at Bohemia. This is due to the river diversion into Breton Sound taking place too high in the water column, where there are few sand particles in suspension.

  2. Alisha Renfro says:

    Thank you for your comment. You are absolutely correct, the plume that can be seen in satellite imagery is almost entirely comprised of silt and clay sized sediment. Sand, which is very important for building landing and creating a viable marsh platform, is largely deposited in the Mississippi River channel. And just as you said the sand deposited near the Bird’s Foot creates problems for the navigation industry and requires frequent and expensive dredging. While it is absolutely critical that any sediment diversion constructed along the Mississippi is designed to maximize the capture of sand, I think the study discussed in the above blog post points out some important factors that influence deposition of fine sediments.

    In the study, they found that the sediment deposited in the marshes near the Atchafalaya and Mississippi River during the 2011 flood had similar characteristics (bulk density, median grain size, percent organic matter), but deposition was higher in marshes near the Atchafalaya compared to marshes near the Mississippi. They attributed the higher deposition to the shallower Atchafalaya Basin, the longer residence time of the sediment-laden water with marine water in the nearshore, and the thorough mixing of the plume with marine water. This research emphasizes the importance of prioritizing diversion projects along the Mississippi that introduce river sediment high up in the adjacent shallow basins to lengthen the residence time and retain as much of that fine sediment as possible.

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