C. Zwolanek1*, E. Holzenthal2 , Y. Ding3, B. Johnson4
1 Oak Ridge Institute for Science and Education, USA; 2-4 U.S. Army Engineer Research and Development Center
* 1: clara.zwolanek@yale.edu ; 2: Elizabeth.R.Holzenthal@usace.army.mil ; 3: Yan.Ding@usace.army.mil ; 4: Bradley.D.Johnson@usace.army.mil
Introduction
Mangrove species—such as Rhizophora, Avicennia, and Sonneratia—have evolved unique characteristics that enable them to survive in dynamic coastal settings [1, 2, 3]. Their structurally complex root systems of mangroves play a critical role in the use of mangroves as coastal protection [4]. The dense, above-ground root systems act as physical barriers to incoming waves and storm surges, reducing their flow velocity and wave energy before it reaches the shoreline [1, 2, 3]. Many experimental studies have quantified the dissipative force due to vegetation, allowing for the development of numerical spectral wave models, such as SWAN, WAVEWATCH III, and CSHORE [5, 6, 7, 8, 9].
CSHORE is a numerical model that computes phase-averaged wave transformations, mean currents, sediment transport, and morphological change in the coastal nearshore. Users can set wave and boundary conditions, bathymetry, as well as vegetation to compute wave energy dissipation and the effect of vegetation-induced drag forces; however, its calculations have yet to consider depth-varying vegetation [9]. The goal of this study was to determine the effect of depth-varying morphology on the vegetative dissipation term (v).
Objective and Methods
Using the field observation and parameterizations of the Rhizophora mangrove species from Ohira et al. (2013), Kelty (2022) built a prototype-scale physical model of mangroves and placed the model mangrove forest into a wave flume [4, 10]. Various wave conditions were run for high-density and low-density prototype mangrove forests with varying wave periods, wave heights, and regularity. Data collected by an acoustic doppler velocimeter (ADV) stack was filtered and used to calculate their associated drag coefficients (CD) using the frequency-dependent dissipation term defined by Chen and Zhao (2012) [11]. The measured velocities were correlated to the expected velocities defined by linear wave theory to determine the effect of spatially varying vegetation.
Results
The mean coherence between measured horizontal velocity and that defined by linear wave theory exceeded 0.80 for all ADVs across vegetation layouts. The high coherence indicated depth-varying vegetation did not significantly dissipate wave energy beyond that expected of linear wave theory. The bottom most ADV had a much lower coherence. This discrepancy may be due to the amount of noise present in trials at lower water levels.
Due to the high coherence between η and u, the velocity over the water column follows linear wave theory, allowing the velocity of the entire water column to be calculated with linear wave theory over depth despite the presence of depth varying vegetation. The lack of additional variation in velocity over depth allows for the approximation of CD to be calculated without separating the water column into defined sectors based on the vegetation's variation. CD may therefore be removed from the wave energy flux dissipation by vegetation calculation and simplified into a term relying on a depth averaged area instead of a multi-layer integrated model [11]. The removal of CD from the integral allows for a more efficient calculation of the wave energy dissipation due to vegetation without sacrificing the calculation's accuracy.
References
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