The Mississippi River and Atchafalaya River are experiencing a historic flood of major geological significance. South Louisiana has one of the fastest land loss rates on any region on Earth, with much of the loss due to a lack of sediment delivery into the coastal zone. This flood is likely to deliver large amounts of sediments to the coastal zone, and therefore could potentially contribute to land building along the South Louisiana coast. This research is a rapid response to investigate whether the massive release of freshwater from the Atchafalaya River in late May, 2011 will lead to the development of a freshwater-dominated continental shelf environment and whether sediments released by the flood will initiate the development of a large mobile mud belt that will extend from the mouth of the Atchafalaya River westward along the Chenier Plain Coast. These hypotheses are being tested by examining the physical and geological oceanography of the inner continental shelf and determining the patterns of recently deposited sediments. The proposed research provides a rare opportunity to study the critical processes of sediment supply and land building in a coastal ecosystem. Broader impacts include support of two institutions in EPSCoR states and student training.
Intellectual Merit and Broader Impact. The goal of this work was to understand the hydrodynamics and sediment dynamics along the Atchafalaya and Mississippi River Shelves under a major river discharge event. Sediment from these rivers plays a critical role in the development of the Mississippi River Delta and the Chenier Plain, the two great features of coastal Louisiana. As such, these research projects are critical to understanding the fate and transport of material that could be used to restore Louisiana’s coast. This work involves data collection of several research cruises. A cruise was conducted on the Atchafalaya River Shelf (ARS) from May 28-31, 2011 that used a suite of instruments to examine hydrodynamic and related sediment dynamics in this system. We deployed a pole-mounted acoustic Doppler current profiler (ADCP) to determine the velocity distribution of water on the continental shelf, and the flux of fresh and salt water across the continental shelf. We conducted CTD casts at 50 stations, which are used in determining the depth, salinity, dissolved oxygen, and turbidity along the ARS. A Laser-In situ Scattering and Transmissiometry (LISST) was also deployed; this instrument determines the distribution of sediment particles in the water column. Finally, a series of Niskin bottles were attached to this system, and we collected water samples from them at each station for total suspended solids. A complimentary cruise was conducted on the Mississippi River Shelf from June 5-7, 2011, deploying similar instruments. Additionally, a second cruise on the ARS was conducted on the R/V Calliou Boca on June 3 and 4, 2011 to further elucidate the distribution of freshwater on the Atchafalaya River Shelf. Finally, a cruise was conducted on the R/V Calliou Boca on June 26 and 27, 2011 on the Atchafalaya River Shelf to collect sediment cores. These cores were analyzed for the naturally occurring, particle reactive radionuclide 7Be. 7Be has a short half-life (53 days), and as such, is an excellent indicator of event scale sediment dynamics. Major findings from the ARS section study can be described as follows. First, we noticed a very large pool of freshwater along the continental shelf of the Atchafalaya River. The 2 ppt isohaline extends up to 30 km from the edge of Atchafalaya Bay. Second, we noticed intense stratification of waters along the continental shelf. The salinity distribution of these waters transitioned from less than 2 ppt to greater than 20 ppt over a spatial scale of less than 1 meter. Third, we noticed that these different layers of water moved in opposite directions, essentially revealing a pattern of estuarine flow outside the bounds of the classical estuary. Fourth, we noticed that sediment depositions along the Atchafalaya River Shelf were five to ten times greater than long-term sediment accumulation rates that have been measured in this region. Also, we noticed that sediments were transported to the west, under the influence of the long-shore transport. The Mississippi River mouth and the Atchafalaya Bay have very different response to the river discharge as the sediment coming from the AB is trapped by the extensive shallow bay and shelf, allowing much longer time for the sedimentation and transport on the shelf vs moving to the deep ocean much more quickly at the MR mouth. This project provided partial support for a paper that PI Kolker recently published in Estuarine, Coastal and Shelf Science on sediment dynamics in the West Bay Mississippi River Diversion. The formal citation information for that paper is: Kolker, A.S., Miner, M.D., Weathers, H.D., 2012. Depositional dynamics in a river diversion receiving basin: The case of the West Bay Mississippi River Diversion. Estuarine, Coastal and Shelf Science 106, 1-12. We were invited to submit a paper to a special issue of the journal "Continental Shelf Research," on event driven sedimentation. That paper will focus on the work on the Atchafalaya River Shelf that was described above. The authors on that paper will include PIs Kolker and Li along with several of their colleagues and students. It is anticipated that this paper will be published mid, 2013. Also, we have a paper accepted for publication in Nature – Geoscience, as a result of this project: Falcini, F., N. S. Khan, L. Macelloni, B. P. Horton, C. B. Lutken, K. L. McKee5, R. Santoleri, S. Colella, C. Li, G. Volpe, M. D’Emidio, A. Salusti, D. J. Jerolmack. Connecting the historic 2011 Mississippi River flood to wetland sedimentation on the Delta, Nature Geoscience, accepted Sept. 5, 2012. We are still working on the data and anticipate to have more results.
