Master of Science in Civil Engineering (MSCE)
Civil and Environmental Engineering
River sediment diversions have been identified as one strategy for creating new land and offsetting Mississippi River delta plain land loss. Numerical modeling is one tool for estimating the amount of land, geomorphic features and ecological benefits from diversions. There are a number of models proposed to estimate sediment diversion land building, ranging from simplistic approaches that provide bulk characteristics and use little computational resources to process-based models that require a large amount of input parameters and computing power. This thesis aims to compare and contrast two approaches to simulating the land building processes in a simplified receiving basin: a 1D spatially averaged model; and a horizontal 2D, process-based Delft3D model. Four scenarios were run: three with varying amounts of non-cohesive sediment; and one with a mixture of non-cohesive and cohesive sediment. A number of simplifying assumptions were made for more direct comparisons of the bulk and detailed delta properties and the computational resources. These included the bulking of cohesive and non-cohesive sediments on deposition are assumed equal; erosion below the pre-delta strata is not allowed; and the river sediment diversion operates continuously at a given flow and sediment concentration. Note that this last assumption was made for easier model comparisons and not how any proposed diversions would be operated. Distributary channel network information, missing in the 1D model but important for ecohydrological processes, is extracted from the 2D model. The 1D model took less than one minute to simulate the same scenario that required over 20 hours on 32 processors using the 2D model. Results showed the 1D model delta radii and areas were always larger, but relatively close, to those simulated by the 2D model, particularly for non-cohesive sediments. The deltas formed from solely non-cohesive sediments had numerous short, but wide, channels and were roughly fan shaped, thus justifying the radial symmetry assumption of the 1D model. The ratios of the 2D to 1D model delta areas were 70% and 55% for non-cohesive and mixed scenarios, respectively. The 2D model results showed that presence of cohesive sediment promoted narrower and weakly sinuous channels that affect delta growth dynamics and result in increased vertical aggradation, thus limiting the area of land built.
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Basyal, Gyan Prasad, "Comparison of Land Building by Mississippi River Diversion Using One and Two Dimensional Numerical Models" (2014). LSU Master's Theses. 2385.
Willson, Clinton S