Identifier

etd-04112012-112650

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Dams and hardened levees built by European settlers in the last two centuries have degraded or stopped the cyclic sedimentation patterns that built the Mississippi River Delta Plain over the past 7,000-8,000 years. Man-made diversions of sediment-laden river water along the lower Mississippi, operated during high water periods, are being considered as a primary mechanism for combating the land loss induced by rising sea-level and these modern controls. To date, no study has documented the sediment-delivery potential of a large man-made river diversion during a single high water event. In the spring of 2011, historic flooding on the Mississippi River delivered record river and sediment discharge to the Delta, and the U.S. Army Corps of Engineers opened floodways, including the Bonnet Carré Spillway, to keep the Mississippi River at acceptable stage elevations near New Orleans. The Bonnet Carré Spillway connects the Mississippi River upstream from New Orleans to Lake Pontchartrain. We studied river-sediment flux and fate from the spillway into Lake Pontchartrain as an analog for future river-sediment diversions under consideration at present. The spillway was open from May 9 to June 20, 2011, reaching peak flows > 8,000 m3/s, at the upper end of diversion flow considered for land-building, making the spillway for a short time one of highest discharge waterways in North America. To develop a sediment budget, sediment cores were collected across the ~1500 km2 lake before, during, and after the operation of the spillway. Cores were analyzed for grain size and the cosmogenic radionuclide 7Be. Results show that the plume rapidly dispersed across most of the lake, and deposited a clay to coarse-silt-sized sediment layer with a total mass of ~1.1-3.8 MT. This flux is comparable to the sediment flux measured in the few other modern diversions that are building land, and suggests that such diversions, if operated on a regular basis during high flows when both sediment concentration and discharge are highest, may provide an important tool for conserving and building land in the shrinking Mississippi Delta.

Date

2012

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Bentley, Samuel J. Sr.

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