Graduate Thesis Or Dissertation
 

Effects of Wave Nonlinearity and Vertical Variation of Vegetation Density on Wave Attenuation

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  • The understanding and prediction how coastal wetland vegetation attenuates wave action has received renewed attention with global climate change impacts and increasing populations on coastlines. Conventional attenuation models utilize empirical drag coefficients, but these coefficients exhibit a wide range of values, and there is some uncertainty in the relative importance of plant flexibility, wave nonlinearity, and vertical density variations of the above ground biomass. Better knowledge of how to predict wave attenuation accurately considering wave nonlinearity, biomechanical properties, and vertical density variations will improve parameterizations to evaluate the ecosystem services provided by coastal vegetation. This dissertation focuses on three core aspects that evaluate and improve existing analytical and numerical models for predicting wave attenuation by vegetation: (a) effects of wave steepness and relative water depth on wave attenuation by emergent vegetation, (b) effects of vertical variation in vegetation density on wave attenuation, and (c) modeling wave attenuation induced by the vertical density variations of vegetation. Chapter 2 presents a quarter scale physical model experiment to investigate the effects of wave nonlinearity on the attenuation of irregular waves propagating through a stand of uniform, emergent vegetation in constant water depth. The experimental conditions spanned relative water depths from near shallow to near deep water waves (0.45 < kh <1.49) and wave steepness from linear to nonlinear conditions (0.03 < ak < 0.18). The wave height to water depth ratios were in the range 0.12 < H[subscript s]/h < 0.34, and the Ursell parameter was in the range 2 < Ur < 68. Frictional losses from the side walls and bottom were estimated and removed from the wave attenuation in the vegetated cases to isolate the influence of the vegetation. The normalized wave height attenuation for each case was fit to the decay equation of Dalrymple et al. (1984) to determine the damping factor α, which was then used to calculate the bulk drag coefficient C[subscript D]. This paper shows that the damping factor is dependent on the wave steepness ak across the range of relative water depths from shallow to deep water and that the damping factor can increase by a factor of two when the wave steepness ak approximately doubles. In turn, this causes the drag coefficient C[subscript D] to decrease on average by 23%. The drag coefficient can be modeled using the Keulegan-Carpenter number using the horizontal orbital wave velocity estimated from linear wave theory as the characteristic velocity scale. Alternatively, the Ursell parameter can be used to parameterize C[subscript D] to account for the effect of wave nonlinearity, particularly in shallow water, for vegetation of single stem diameter. Chapter 3 uses a physical model experiment to investigate irregular wave attenuation through emergent vegetation with variations in stem heights to mimic the variations of above ground biomass found in nature. The experiment was conducted with six peak periods, six incident wave heights, and two schemes of vegetation with different heights over a constant water depth. This paper details the influence of height characteristics where short waves are more sensitive to a layered vegetation scheme and highlights limitations of the Dalrymple et al. (1984) approach when there are significant variations in stem height. In addition, this chapter highlights that the effect of vertical biomass distributions has shown the order of magnitude in C[subscript D] about 140% to 280% change between uniform height vegetation and varying height vegetation from the shallow water condition to deep water condition in contrast to the 23% change in C[subscript D] due to nonlinear effects in Chapter 2. This experiment shows that recalibrating the vegetation drag coefficient, C[subscript D], can account for variations in stem height only in an ad-hoc manner. Chapter 4 uses the physical model experiment from Chapter 3, a phase-averaged spectral wave model, SWAN, and a phase-resolving RANS-type numerical model, NHWAVE, to investigate the attenuation of irregular waves propagating over emergent vegetation with variations in stem heights. The physical model data were used to calibrate the drag coefficients for SWAN and NHWAVE, which includes separate, calibrated coefficients for frictional and vegetated drag. This chapter details the influence of vertical variation in plant morphology characteristics where short waves are more sensitive to a layered vegetation scheme and highlights the benefit of the vertical layer schematization of vegetation density in NHWAVE compared to SWAN when there are significant variations in stem height. The most important finding is that NHWAVE does not need to vary C[subscript D] between the two treatments of vegetation with uniform density of vegetation (Type A) and vertical density variations of vegetation (Type B), whereas SWAN required changes to C[subscript D] when there were changes to the vertical distribution of biomass. The NHWAVE model was used to estimate the vertical structure of vegetation-induced turbulent kinetic energy budget. It is found that the turbulence is reduced near the edge vegetation and has higher turbulent kinetic energy at the top of the canopy in the vertical variations of vegetation.
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