HR: 16:30h
AN: H24A-05    [Abstracts]
TI: Quantitative Measurements of Bedform Transport Rates and Sand Sheet Character in the Lower Mississippi River
AU: * Nittrouer, J A
EM: jnittrou@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, 120 Dinwiddie Hall, 6823 St. Charles Avenue, New Orleans, LA 70118 United States
AU: Allison, M A
EM: malliso@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, 120 Dinwiddie Hall, 6823 St. Charles Avenue, New Orleans, LA 70118 United States
AU: Campanella, R
EM: rcampane@tulane.edu
AF: Tulane University, Center for Bioenvironmental Research, New Orleans, LA 70118 United States
AB: Channel sand volume and downstream flux in the Mississippi River have important implications for proposed mitigation projects (dredging and pipelines) that seek to utilize this resource for replenishing neighboring barrier islands and restoring Louisiana's deteriorating wetlands. This study quantifies bedform migration-induced sand flux through the lower river on daily and seasonal timescales, and evaluates the sedimentary character of the bedload component. Observations and measurements were conducted along three study grids (Audubon Park, English Turn and Venice) over a range of river discharges between April 2003 and January 2005. Two multibeam bathymetric profiles of the study grids were conducted 24 h apart to document bedform migration, and stratigraphy and thickness of the sand layer were confirmed by CHIRP seismic profiling. Downstream transport is evaluated from bed elevation changes for a 1 m grid after correcting for river stage, and utilized to calculate bedload sand fluxes for larger, averaged grid cells after visual examination confirmed dunes had migrated <1 wavelength. Algorithms were formulated to remove spurious grid cells created by vessel motion, navigation and swath-matching errors. Initial data analysis indicates flux rates conform to expected trends: values are proportional to river discharge and are higher in the channel thalweg of straight reaches relative to shallower water. Bedform size also increases with river discharge and spatial changes in flux rates; height ranges from <1 m to 10 m, and wavelength from 10 m to 100 m. Seasonal trends in sand sheet thickness are evident, particularly in deeper meander reaches, where aggradation occurs at low flow and scour is observed during high flow. At highest discharges observed (35,000 m³/sec), bedform troughs bottom out on exposed relict fluvio-deltaic strata that the river has incised (i.e., sediment starved). A spatially uniform grab sampling effort (250 samples) provided grain size data of the active sand sheet for the lower 135 km of the river. A downstream trend of decreasing grain size (2.6 to 2.0 φ) is likely due to the finest sand fraction leaving suspension and becoming bedload as river gradient and velocity progressively decrease. Locally, grain size increases in the deepest portions of the channel as a result of enhanced flow scouring the fine and medium sand fraction.
DE: 1815 Erosion and sedimentation
DE: 1894 Instruments and techniques
DE: 4558 Sediment transport
SC: Hydrology [H]
MN: 2005 Joint Assembly