The Long-Term Outlook of the Mississippi-Atchafalaya Bifurcation: A Convergence of Engineering, Economics, and Deltaic Evolution
Doctor of Philosophy (PhD)
Geology and Geophysics
The most recent and currently active delta lobe of the Mississippi River (MR) is the Atchafalaya-Wax Lake lobe, which was initiated approximately 400 years ago as a result of MR stream capture by the Atchafalaya River (AR). This capture process accelerated in the early to mid-1900s but further progress was prevented by construction and operation of the Old River Control Structure (ORCS) Complex. Many recent studies indicate that MR system below the ORCS is on a retreating geologic trajectory due to contributing factors such as sea level rise, subsidence, faulting, and declining hydraulic stream power. Diversions along the Lower MR currently being planned would partially alleviate these risks by altering the Lower MR hydraulics and capturing land-building sediment.
This study uses both 1D and 2D numerical modeling techniques to evaluate the long-term effects of current water and sediment regulation mandates and alternatively explore the long-term land-building potential of progressively continuing the avulsion at the MR-AR bifurcation in a controlled manner. Riverine hydraulic and sediment transport modelling was performed using Delft 3D coupled with a spreadsheet which used a 1D spatially averaged equation to calculate delta growth. A 150-year model duration was used to account for flow and system adjustments, implementation of current restoration plans, and to sufficiently analyze delta response trends into the future. The study tests 12 scenarios (1 No Change scenario, 8 increased AR flow scenarios, and 3 increased MR flow scenarios) where flow adjustments are made gradually over time in 5% increments.
The 150-year resulting land areas built for the MR vs. the AR at discrete flow percentages demonstrate a clear land-building capacity advantage in the AR. For common flow percentages, the AR builds 1.7 times more land at 30% of the total river flow and 3.0 times more land at 70% of the total river flow. Reconnaissance-level cost impact analysis on river levees and navigational dredging to accommodate flow, stage, and sediment load alterations indicate that flow diversion of up to 60% of total river flow into the AR could result in a cost-effective flood alleviation and deltaic land-building strategy over the next 150 years.
Andrus, Thomas Mitchell, "The Long-Term Outlook of the Mississippi-Atchafalaya Bifurcation: A Convergence of Engineering, Economics, and Deltaic Evolution" (2020). LSU Doctoral Dissertations. 5218.
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