Lower Mississippi River Sediment and Ecohydrology Studies

Description

Background: The development of the Mississippi River and its floodplain for navigation and flood control has been ongoing since the 18th century, with the most concerted efforts occurring as a result of the Flood Control Act (FCA) of 1928 (U.S. Congress 1928) following the Great Flood of 1927. The Mississippi River and Tributaries (MR&T) Project that was spawned from the FCA of 1928 has produced a massive, comprehensive system for flood control and channel stabilization that includes levees, channel improvements, and floodways, as well as tributary reservoirs and other basin improvements. Additionally, the development of the river for safe and dependable navigation has generated a substantial engineering effort involving river training structures, meander cutoffs, and dredging. The historical, present-day, and future morphology of the Mississippi River reflects an integration of all these engineering interventions (and the process-responses they have triggered in the fluvial system), combined with natural drivers of channel change and evolution, including floods and droughts, hurricanes, neotectonic activity, geologic outcrops, climatic variability, climate change, and relative sea-level rise. Understanding how these various factors affect river morphology and its short- and long-term evolution is a complex challenge that must be addressed by the river engineers and scientists responsible for managing this system for flood control, navigation, and ecosystem restoration. With the complex requirements in navigation, flood risk reduction, and ecosystem restoration, all with multiple stakeholders, future Mississippi River management will require the most advanced knowledge available. The Mississippi River Geomorphology & Potamology (MRG&P) Program was developed in recognition of this challenge. The MRG&P Program is a joint effort of the U.S. Army Corps of Engineers (USACE), St. Louis, Memphis, Vicksburg, and New Orleans Districts, conducted with the oversight of the Mississippi Valley Division (MVD) and benefiting from technical contributions from the Engineer Research and Development Center (ERDC). The statement of need described herein is just one of many components of the MRG&P Program. Brief Description of Anticipated Work: Objective 1: Refine and develop consensus on sediment transport and loads in the Lower Mississippi River. Over the past decade, great strides have been made in understanding the sediment transport capability of the Lower Mississippi and Atchafalaya rivers (LMAR). Several different methods of analyses have been developed using somewhat different data sets. Mize, et al. (2018) recently published a paper using a method called Weighted Regressions on Time, Discharge and Season (WRTDS) which suggests that the amount of suspended sediment moving through the LMAR has declined in recent decades. However, other reports show no trend (Little and Biedenharn, 2014) or a slightly increasing trend since 1993 (Horowitz, 2010). There is a vital need to articulate the uncertainty and differences within the sediment data and analysis methods for managers, modelers, and future research efforts. This effort will involve an analysis of the available sediment transport data for the LMAR and an investigation of the uncertainty in the data and methods. This work effort could also involve organizing and hosting a well-focused workshop of the leading researchers in this area with the goal to produce a consensus publication. This work effort could also include evaluation of methods which estimate sediment loads based on surrogate information such as discharge or turbidity. Consideration may also be given to better quantification of the hysteresis behaviors exhibited in both sediment concentrations and water surface elevations. Objective 2: Refine a total Sediment Budget for the Lower Mississippi River. A sediment budget was recently completed for the LMR which focused on the measured suspended sediment data. While this is a valuable contribution, it is only tells part of the story of the Mississippi River sediment loads because it only addresses the fine loads (wash loads) in the system. The delivery of the bed material is a much more complicated process, but is one that needs to be fully investigated. This is a critical piece of information because most of the researchers who have published studies in the journals have ignored or misrepresented the role of the bed material loads. This type of study would not only focus on measured suspended sediment data, but would also link sediment sources from bank caving, tributaries, and the watershed with the energy in the channel system as reflected by slope and stream power calculations at long term gaging stations. These historical changes in sediment supply and stream power will be correlated with the observed morphologic trends in the river. The result would be a conceptual sediment budget that would be extremely valuable in understanding the historical behavior in the system as well as serving to better understand the delivery of sediments through the present-day system. Objective 3: Development of an Eco-Geomorphology Approach to Assessment and Management of the Lower Mississippi River (LMR). This objective would involve an effort to begin to assemble various ideas and develop a research plan to bring together and integrate the geomorphology, hydrology and ecology of the LMR floodplain. An interdisciplinary technical study team will be formed to organize and guide studies of fluvial geomorphology and ecological processes in the channel and floodplain of the LMR. The study team will develop a comprehensive assessment plan that will include proposed work units and how they are linked together based on established riverine concepts. Applying historical trends and recent research findings, as well as decades of first-hand professional knowledge of the LMR, the study team will develop an initial framework for a long-term basic research program in eco-geomorphology of the LMR, and then organize a Workshop consisting of experts in river geomorphology, sediment transport, hydrology and hydraulics, and aquatic ecology to provide additional insights for the program. Final products will include a conceptual framework of the overall approach, proceedings of the workshop, and draft statements of work that together provide a long-term basic research program in eco-geomorphology of the LMR, with primary goals of identifying/addressing knowledge gaps and drawing inference across the research disciplines.

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