Dr. Jeffrey A. Nittrouer is awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the University of Illinois, Urbana-Champaign. Dr. Nittrouer will use field observation studies from the lower Mississippi River coupled with physical modeling to provide an understanding for sediment transport dynamics where a river nears its marine outlet. Data to be collected include measurements for water and sediment transport during low- and high-water discharge, which will help constrain spatial and temporal sediment flux disparity, and the transport stress associated with both water discharge regimes. Additionally, measurements for channel bed sediment composition and timing of sediment flux will be used to describe causes for measured changes in channel morphology and planform character of the lowermost Mississippi River.
This research will provide analyses for the timing and movement of sediment and associated particulates from meandering fluvial systems to the deltaic and marine environment. This is a particularly important area of study, because decelerating water flow as a river nears its outlet has important influences for the routing of sediments through source-to-sink sedimentary systems. This information will have direct applications for recent efforts to mitigate coastal wetland loss in southern Louisiana. Wetlands are environmentally sensitive systems that, amongst many ecological services, naturally buffer storm surges that threaten urban populations and infrastructure.
Students with interest in geoscience and river system dynamics at the host institution will be trained to operate state-of-the-art instruments and software programs that collect and analyze data from the lowermost Mississippi River. Additionally, students will be educated in the physical theory behind fluvial hydrodynamics and sediment transport. Educational seminars will be held to discuss river and coastal wetland issues that abound in southern Louisiana, and will be followed by a field trip that will trace the Mississippi River, starting from the Midwest and ending at its outlet in Louisiana.
As a NSF post-doctoral research award, the broad objective of this project was to evaluate the time and space properties of bed material sediment transport within the lowermost 500 kilometers of the Mississippi River, which is the section of river that extends from the Old River Control structure to the river-marine outlet at the Gulf of Mexico. Over this portion of the river, there are profound changes in the hydrodynamic flow regime as the system nears the Gulf of Mexico receiving basin; because flow regime is coupled with sediment transport, the objective of this project was to identify the linkages between dynamic river flow and the timing and magnitude of bed material (predominantly sand) transport and delivery to the downstream delta. The main finding from this study is that non-uniform flow (backwater conditions) within the lowermost section of the Mississippi River profoundly influences the transfer of the river’s coarse sediment load to the adjacent ocean receiving basin. The significant and specific results of this project include: 1) identifying coarse bed material sediment deposition far upstream of the delta (located at river-ocean interface), arising due to decreasing sediment transport capacity associated with backwater flow; 2) that the removal of this coarse sediment prevents channel bed from maintaining full alluvial cover, thereby rendering the lower portion of the river channel subject to erosion as the river scours it’s the underlying substrate of the channel bed; 3) that the grain size of the bed material sediment feeding the downstream delta is relatively small (primarily very fine and fine sand), due to the deposition and sequestration of medium sand and coarser sediment. Therefore, despite the abundance of coarse sediment within the Mississippi River basin, this material never reaches the river-ocean interface; and 4) the preferential deposition of coarse sediment at the transition to backwater flow facilitates higher the rates of lateral migration and channel avulsions. These research findings have important implications for society as well as academic interests in geological sciences. For example, lowland river deltas require sediment dispersal in order to mitigate ongoing land drowning due to relative sea-level rise. This particular research project has identified the time and space properties for bed material sediment transport—critical for building and maintaining the subaerial portion of the river delta—within the lowermost Mississippi River. This land is critical for attenuating water surge associated with significant storm events, and therefore delta land is needed to protect societal infrastructure located on river deltas (e.g., city of New Orleans). Insofar as important academic findings, this project has identified the critical role that backwater hydrodynamics play for regulating sediment discharge to the adjacent ocean receiving basin. Importantly, backwater hydrodynamics filter coarse-grain bed material sediment, thereby prohibiting this sediment from reaching farther downstream and contributing to the construction of active deltaic sediment deposits. Furthermore, the preferential deposition of sediment drives changes in channel kinematics, such as rates of lateral migration, and the tendency for the river channel to avulse. This information is important for informing models that predict delta stratigraphy and evolution over time and space. For example, where coarse-grain sediment deposition arises at the transition to backwater flow, over geological time scales, the fluvial stratigraphy will trend toward amalgamated channel sand bodies because of the preferential deposition of bed material sediment. Additionally, this location serves as a nodal point for naturally occurring river avulsions (whereby the channel suddenly and catastrophically shifts course) due to the super elevation of the channel bed with respect to the adjacent floodplain. Avulsions will relocate the river depocenter at the coastline by many tens to hundreds of kilometers, and therefore avulsions serve as an important natural driver for shifting the location of sediment deposition at the river-marine interface. This post-doctoral research was conducted at the University of Illinois, as part of a collaboration between the Civil Engineering and Geology departments. The execution of this research involved the training of many graduate students as well as several undergraduate students. All students benefited from training, development, and education arising from this project. For example, students participated in field campaigns and modeling research, thereby building skills and learning methods for coupling quantitative analyses informed by field research that sought to collect data to inform and validate models. During the two-year project, the principle investigator participated in numerous presentations and interactions at yearly and specialized academic meetings; was invited to present findings of this project at roughly 10 universities as well as several industry companies; published several peer-reviewed science papers (some of which included press coverage from international media in regards to the publication findings); and held multiple classroom activities to inform and educate students in both the Civil Engineering and Geology departments at the University of Illinois.
