CHARACTERIZATION AND MODELING OF SETTLING, CONSOLIDATION, AND SUSPENSION TO OPTIMIZE SEDIMENT RETENTION OF SEDIMENT DIVERSIONS FOR COASTAL RESTORATION

Although many studies of sediment diversions have been conducted on the Mississippi Delta, relatively less attention has been paid to understanding sediment retention and basic cohesive sedimentation processes in receiving basins. Our research evaluates long-term (up to six months) sedimentation processes through various laboratory experiments, especially cohesive sediment settling, consolidation and resuspension and their impacts on sediment retention. Bulk sediment samples were collected from West Bay, near Head of Passes of the Mississippi Delta, and the Big Mar basin that receive water and sediment from the Caernarvon Diversion in the upper Breton Sound region of Louisiana, USA. A-230-cm tall settling column with nine sampling ports at 15 cm intervals was used to measure the consolidation for four initial sediment concentrations (10-120 kg m^-3) with two salinities (1 ppt & 5 ppt). Samples of sediment slurry were taken from every port at different time intervals up to 15 days or longer (higher concentration needs longer time to consolidate) to record concentrations gravimetrically. A 200 cm long tube was connected to a 50 cm long core chamber to accumulate at least a 10 cm thick sediment column for erosion tests. A dual-core Gust Erosion Microcosm System was employed to measure time-series (0.5, 1, 2, 3, 4, 5, 6 months) erodibility at seven shear stress regimes (0.01-0.60 Pa). Our preliminary results show a significant decrease of erodibility with time and high concentration (120g/L). Coupled resuspension and consolidation tests (concentration of 60g/L) indicate that shear stresses generating the highest turbidity peaks increase from 0.2 Pa after two and three months of consolidation to 0.45 Pa after four months. Accumulative eroded mass decreased with increasing time allowed for consolidation. Salinity impacted on sediment behavior in consolidation experiments. Our study reveals that more enclosed receiving basins, intermittent openings of diversions, or reduced shear stress due to man-made structure all can potentially reduce cohesive sediment erosion in coastal Louisiana. Further results will be analyzed to determine the model constants. Consolidating rates and corresponding erosional changes will be determined to optimize sediment retention in coastal protection.