Date of Award
Master of Science (MS)
Louisiana coastal saltmarshes are disappearing at an exponential rate. Even though several researches indicate that vegetation on coastal marshes protect the coast from erosion by providing soil reinforcement, wave buffering, sediment trapping and overall hydrologic regime control, the complex nature of coastal erosion process makes it too difficult to quantify the erosion resistance provided by vegetation. This research is focused on the study of soil binding ability provided by smooth cord grass, Spartina alterniflora, flourishing in dredged soil of Sabine Refuge Marsh Creation Project (CS-28) in Louisiana.
Field vane shear test was conducted to obtain the in-situ un-drained shear strength of soil vegetated by Spartina alterniflora. Direct shear tests on the rooted soil samples collected from the site were performed to investigate the overall effect of the roots on the shear strength of the soil.
Laboratory tensile strength tests of individual roots of different diameters were done to study the mechanical reinforcement provided by the roots. Two different root reinforcement models were used to study the correlation between the root-induced cohesion and the root tensile strength. The first model is a perpendicular root reinforcement model by Wu et al. (1979), which considers simultaneous mobilization of tensile strength of all roots crossing the shear plane. The second model is the Rip Root Model by Pollen and Simon (2005), which assumes progressive failure of bundled roots. Results from the vane shear test and the direct shear test demonstrated that the roots of Spartina alterniflora significantly increase the shear strength of the soil. The increased shear strength for a location varied depending upon the root area ratio (RAR), depth of the soil sample, and tensile strength of the roots. The laboratory test results were compared with the outcomes from root reinforcement models to verify if the models can be effectively used to calculate net root reinforcement. Analysis using both the root reinforcement models shows overestimation of root induced cohesion and a new correlation was proposed. The discrepancies were because these models do not account for all the factors involved in the root-soil matrix.
Results obtained from a validated “Delft3D Wave-FLOW” coupled model encompassing the hydrodynamics of Lake Calcasieu estuarine system, with Hurricane Ike wave forcing condition was analyzed. Four different grid conditions were considered in the WAVE-FLOW coupled model and time-period of peak storm surge was selected to study the extreme bed stress conditions. Maximum bed shear stress and water velocity during the peak period of the hurricane was obtained at those locations from where the undisturbed soil samples were collected during the field visit. The study was done by comparing the ratio of shear strength of the soil to shear stress developed for these locations. For the different grid conditions, the ratio of shear strength to shear stress was found to be relatively higher in those grids with the presence of vegetation. This trend suggests that the significant amount of wave energy is dissipated by the presence of shoots of the vegetation.
Results from experimental tests, analytical tests and numerical analysis show the importance of Spartina alterniflora in enhancement of the shear strength of the soil and in reduction of the erosive power of the waves. Thus, the study justifies the use of this marsh vegetation as a vital asset in marsh creation projects.
Baral, Sujan, "" (2018). Thesis. 9.