Doctor of Philosophy (PhD)
Civil and Environmental Engineering
This research investigates and quantifies the effectiveness of salt marsh vegetation in reducing storm-induced waves and surge, and the potential for wetland erosion due to wave action, using field measurements on the Louisiana coast. To quantify wave attenuation and wave energy dissipation by vegetation (Spartina alterniflora), wave data were measured along a transect using pressure transducers during two tropical storms. Measurements showed that incident waves attenuated exponentially over the vegetation. The linear spatial wave height reduction rate increased from 1.5% to 4% /m as incident wave height decreased. The bulk drag coefficient estimated from the field measurements decreased with increasing Reynolds (Re) and Keulegan-Carpenter (KC) numbers. The vegetation-induced wave energy dissipation did not linearly follow incident energy, and the degree of non-linearity varied with the dominant wave frequency. The estimated drag coefficient is shown to be frequency-dependent and is parameterized by a frequency-dependent velocity attenuation parameter inside the canopy. The spectral drag coefficient predicts the frequency-dependent energy dissipation with better accuracy than the integral coefficient. The probability distribution of zero-crossing wave heights attenuated by vegetation was observed to deviate from the Rayleigh distribution and follow the theoretically derived one-parameter Weibull distribution which depends on local wave conditions only. Empirical relationships are developed to estimate the shape parameter from the local wave parameters. Field data collected during Tropical storm Ida (2009) and Lee (2011) showed that the surge attenuated at different rates in two estuaries of different topography. Surge reduction by vegetation was more effective on a large marsh. To quantify the potential for wave action to cause erosion of coastal wetlands, directional wave measurements were collected over a seven-month period. Marsh retreat rates estimated in the study area, using the wave power calculated from the field measurements are on the same order of magnitude of the recent marsh loss monitoring data. The empirical relationships of vegetation drag coefficient and wave height probability distribution function can be used to improve coastal modeling and to estimate characteristic wave heights for the design of coastal defense structures fronted by large swaths of salt marsh vegetation.
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Jadhav, Ranjit S., "Field Investigation of Wave and Surge Attenuation in Salt Marsh Vegetation and Wave Climate in a Shallow Estuary" (2012). LSU Doctoral Dissertations. 3939.