Overbank Deposition of Sand and Mud within Two Point Bars Bound by the Mississippi River Levee System: Implications for Coastal Restoration Sediment Budgets
Semester of Graduation
Master of Science (MS)
Geology and Geophysics
As ongoing Mississippi River sediment diversion projects are being developed and planned, the availability of sand and mud for use in the lower Mississippi River Delta is being calculated and budgeted. This project attempts to quantify the amount of sand and mud being lost from the lower Mississippi River past the Old River Control Structure in the remaining unleveed floodplains, an area that encompasses 270 km2. Two land masses were chosen along each bank of this reach of the Mississippi River: Raccourci Island (west bank) and the Cat Island National Wildlife Refuge (east bank). Seventeen sediment cores were collected and have been analyzed for the presence of Cesium-137, a radioactive isotope that resulted from the fallout of nuclear weapons testing, and Lead-210, a natural Uranium-series radioisotope. Cores were also analyzed for grain size and bulk density. The combined geochronological, grain size, and density results allow the spatial distribution of sediment mass accumulation rates to be determined, and a key motivation of this project is to ascertain how much sediment is being trapped annually by these floodplains that retain a natural connection to the river.
Sediment accumulation rates ranged from 0.217 to 1.04 cm yr-1. Conversion of sediment accumulation rates to mass accumulation rates was conducted and resulted in a range of 0.073 to greater than 1.48 g cm-2 yr-1. A regional mass accumulation rate was determined using natural neighbor interpolation and characterizing deposition according to geography and topography. These methods were based on elevation for cores greater than 500 meters from open water and based on distance from a channel for cores within 500 meters of an open channel. A range of total sediment mass loss was calculated to be 1.58 to 3.03 MT yr-1. Additional pathways for sediment storage in the region include growth of the Narrows lacustrine delta and an additional 82 km2 of unleveed floodplains and river islands south of Saint Francisville to Baton Rouge. The estimated sediment storage to these two pathways are 0.45 and 0.64 MT yr-1, respectively, for a final total range of 2.46 to 4.27 MT yr-1.
Magliolo, Christopher, "Overbank Deposition of Sand and Mud within Two Point Bars Bound by the Mississippi River Levee System: Implications for Coastal Restoration Sediment Budgets" (2017). LSU Master's Theses. 4326.