Date of Award

Spring 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Advisor

Jay X. Wang

Abstract

Considering the past history and future risks of hurricanes in the USA, well understood storm protection plans are needed to shelter the important areas of the population and economy, especially within southeastern Louisiana. It is extensively assumed that marshes offer protection from hurricane though the degree of this protection is not well measured or understood due to the complex physics involved in this overall system. Moreover, marshes experience significant erosion while serving as a barrier for important areas. Consequently, a particular method to quantify the effects on marshes during a coastal hurricane period is necessary to mitigate major marsh loss.

A study comprised of experimental work and numerical simulation was undertaken to evaluate the effect of marsh vegetation on resisting hurricane induced erosion and erosion of the marsh itself. Local vegetation Spartina alterniflora was selected as principal marsh vegetation for this study. Contribution from Spartina alterniflora had been analyzed from two different directions such as contribution of roots and contribution of shoots.

The overall research was divided into three different phases. The first phase was the laboratory experiments of collected soil samples with and without roots of Spartina from the study area (Cycle-1 of CS-28 project). Direct shear tests were perfouned on the samples to study the effect of roots on soil shear strength. Tensile strength of the roots was also studied. In the second phase, Delft3D wave flow coupled model was applied on the Louisiana coastal marsh near Calcasieu Lake to assess the contribution of marsh vegetation in reducing hurricane induced wave and current actions. The objective of this phase was to develop an integrated wind, current, wave modeling system for the Louisiana coast under hurricane conditions. Hurricane Ike in 2008 was chosen as an example to study the marsh's contribution during hurricane. The wave flow coupled model was generated covering a significant part of Calcasieu Lake, surrounding marshes and a part of the Gulf of Mexico. The coupled model was calibrated and validated against observed data gathered from NOAA and CPRA observation stations. Later after validation, Hurricane Ike forcing condition was introduced to the wave flow coupled model. Moreover, to originate the extreme scenario, the hurricane was introduced by excluding the precipitation and flooding effect of a previous hurricane named Gustav that made landfall 13 days prior to Hurricane Ike. Delft3D vegetation model was also analyzed to investigate the effect of a hurricane on vegetated mud bed. In the third phase, based on the experimental results from the tensile and direct shear tests and hurricane stress results from Delft3D analysis, slope stability analyses were performed for 16 different scenarios by utilizing Slope/W to predict erosion of vegetated and non-vegetated mud surface during different phases of a hurricane.

Experimental results suggested that the marshes do have the potential to enhance soil shear strength. Results suggested that the additional cohesion developed from plant roots played a vital role in enhancing shear strength of marsh soil, especially near the surface. A correlation between Spartina alterniflora root tensile strength and root cohesion was proposed for dredged soil. The validation of the coupled wave flow model showed that the water level computed by Delft3D agrees fairly well with the measured data. Results from Delft3D vegetation model study indicated a major reduction in the current velocity in presence of the Spartina alterniflorashoot system. Results from the hurricane induced wave flow model showed that the wave induced bed shear stress up to 90 Pa can be the result while hurricane reached its peak time.

It was found that the edge and flat soil mass of the marsh reacted differently under hurricane induced wave and current action especially when time dependent analysis is considered. It was also observed that the presence of a shoot system around the weak spot reduces bed shear stress significantly, especially while the marsh bed is submerged or under a low wave energy field. Yet, completely exposed vegetation during the peak of a hurricanes was found to be most vulnerable and supposed to experience severe mass erosion/marsh shears.

It was also noticed from the erosion prediction analysis that the hurricane damage could have been severe if there was no prior hurricane before Hurricane Ike. From the summary of erosion prediction analysis output, it was observed that the uprooting or mass erosion only occurred during two scenarios among sixteen scenarios. Near the marsh edge, mass erosion occurred during the hurricane landfall with the condition that the marsh edge was above water prior to hurricane impact. On marsh flat, mass erosion occurred during the peak of the hurricane when analyzed with drought condition prior to the hurricane.

The combined experimental and numerical analysis of Louisiana coastal marsh under hurricane-induced waves and currents provided useful insights of actual scenarios and probable cases. The findings could be used effectively in the design and construction of future marsh creation projects in Louisiana.

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