Diversion of Mississippi River Water Downstream of New Orleans, Louisiana, USA to Maximize Sediment Capture and Ameliorate Coastal Land Loss

Abstract

Boat-based field data and monitoring station data from the tidal reach of the Mississippi River are utilized to examine the sediment capture of large (>1,400 cms) proposed water and sediment diversions from the channel to build and sustain wetlands in the Mississippi delta. The purpose herein is to suggest the importance of siting the diversion relative to river morphology and operating it to optimize sediment capture. At the site of a proposed diversion near Myrtle Grove, LA, water and sediment data suggest that the washload (fine) fraction is strongly weighted toward the rising limb of individual freshets (e.g., flows >16,990 cms, 1 to 5 events/y over the last 49 y) based on daily turbidity records. Significant variability in suspended fines exists between freshets depending on whether they are the first peak of the water year, and on their tributary source. Much of the sand fraction in suspended load, since it is derived from the underlying bed (e.g., bed material load), is strongly tied to water discharge in this reach, and can be accurately predicted by ratings curve. An analytical model is presented that is utilized to test efficiency of sand capture in a diversion based on ADCP backscatter data calibrated by isokinetic water samplers. Using observations from the diversion site, the model predicts a 30 % more efficient sand capture for a 1,416 cms diversion (0–10 m withdrawal depth) at a discharge of 27,250 cms (March 2011) on the proposed lateral bar diversion site, relative to a thalweg site on the opposite bank. This suggests that the proximity of large dunes, and the turbulence induced by them, is a primary control on sand resuspension in the water capture zone, which in turn plays a strong role in efficiency of diversion sand capture.