Method for Quantifying Floating Marsh Strength and Interaction with Hydrodynamics

Louisiana possesses over 350,000 acres of unique floating vegetated systems known as floating marshes or flotants. Due to their buoyant nature, floating marshes are susceptible to high energy changes in the hydrodynamic environment that may result from proposed river diversion projects which introduce flow to areas that are typically somewhat isolated. The overall goal of this research is to improve the understanding of how exposed flotants deteriorate under increased hydrodynamic stresses. More specifically, this thesis aims to answer how the material limits of floating marshes can be measured and how the mats interact with hydrodynamics. The two primary objectives are: 1) Develop a technique for accurate, in-situ measurement vegetative mat root-soil matrix material properties; and 2) Develop a means for predicting floating marsh washout (critical velocities) through numerically modeled derived empirical relationships. The device constructed to capture the tensile properties of the vegetative mats, called the Marsh Mat Tensile Strength Tester (MMTST), successfully produced full stress-strain profiles including the Young’s modulus, yield stress, and ultimate strength of a root-soil matrix (sod). The estimated mean Young’s modulus, yield stress, and ultimate strength values (sod) were found to be 31.95 kPa, 9.58 kPa, and 9.91 kPa, respectively. Next, flows around 25 idealized mat geometries were simulated with 2-D & 3-D Fluent models. Mat-specific drag coefficients (Cd,m) were found ranging from 1.084-1.645 depending on mat aspect ratio. An equation developed for predicting Cd,m successfully estimated the modeled drag coefficients with a mean percentage error of 2.33%. A finite element analysis (FEA) was performed on the 25 mat shapes using the predicted drag forces and the material properties measured by the MMTST. By applying various failure criteria (Fc), a correlation was found between the modified mat width-to-length aspect ratio () and critical velocity (Vc). The critical velocities ranged from 0.31-1.48 m/s depending on mat aspect ratio and material properties. The general equation developed for predicting floating marsh failure due to flow, in the form: Vc = f(,Fc), performed well with a mean percentage error of 3.33% relative to the unique values directly extracted from the FEA