Graduate Thesis Or Dissertation
 

Prototype-Scale Physical Model Study of Wave Attenuation by an Idealized Mangrove Forest of Moderate Cross-shore Width

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  • The use of natural and nature-based features for coastal hazard mitigation, particularly emergent vegetation such as mangrove forests, have become increasingly popular. However, the protection that these systems can provide has not been fully quantified for engineering design, and the uncertainties in parameterized equations have not been fully defined. In particular, laboratory investigations of wave attenuation by vegetation have typically been conducted at reduced scales using Froude similitude. The drag coefficients derived from these studies have been shown to be a function of the Reynolds number, for which Reynolds similitude cannot be maintained when Froude similitude is achieved in reduced scaled tests. Therefore, the purpose of this thesis is to conduct a prototype-scale physical model study that can quantify the wave attenuation by an idealized mangrove forest and to develop scaling relations to utilize previous and future reduced-scale tests. For the study, a prototype-scale physical model was constructed in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory, Oregon State University. The model trees were constructed based on field observations and parameterizations from Ohira et al. (2013), where the diameter at breast height, DBH, was 0.1143 m, and the diameter of the roots, DRoot, was 0.0286 m with 14 roots per tree. Two model forest densities of 0.75 and 0.375 trees/m2 were tested for a forest with 18 m cross-shore width. Half of the experimental tests included a vertical wall on the landward side of the model forest. The wall was intended to represent a coastal structure, and the pressure forces on the wall were measured to determine the model forest’s effect on attenuating wave forces. Regular and random waves were tested at 4 water depths that ranged from shallow inundation to storm surge conditions within the model forest. Transient wave cases were also tested for the two lower water depths of the study. In total, 298 tests were completed for the study. A methodology using LiDAR measurements was developed to quantify the projected area of the idealized mangrove forests. The LiDAR results had an error of 5% from the known value for the model tree trunk section. These measurements were used to find an effective diameter, De. The method can be practically applied for field applications where destructive or traditional measurements using calipers would be cumbersome or unfeasible. The results of the regular and random wave cases for the tested layouts without the wall are presented with the focus on estimating the wave attenuation and parameterized drag coefficient, CD. For random wave cases, wave height decay coefficients for the model trees, α ̃_m were calculated for each case. After removing the attenuation effects of the flume walls and bathymetry, the α ̃_m values were found to be affected by the water depth and forest density. An average ratio of 2.0 was also found between the high-density (HD) to low-density (LD) forests α ̃_m values. This was the same value as the HD/LD tree density ratio, N, ratio indicating a possible linear relationship between α ̃_m and N. Furthermore, wave height attenuation of 14 to 28% and 6 to 16% were found for the HD and LD forests, respectively, where the highest attenuation coincided with the lowest water depth and highest measured projected area. The drag coefficients, CD, for the study, ranged from 0.40 to 3.75 and were related to the Reynolds number, ReU,De , in the range 4.5E3 < ReU,De < 2.9E4. The largest CD values was associated with the lowest water depth. Uncertainties for CD values for the study were also found based on the variability seen in the measured mean projected area per unit height per tree, At,m, of the model forest and the best fit for the wave height decay coefficients, α ̃_m. The uncertainty from these two parameters were combined to overall uncertainty for the CD values. By scaling the Reynolds number according to Froude similitude, it was possible to rescale previous studies to agree well with the CD estimates for both regular and random wave cases, with the Reynolds number estimated using the diameter at breast height, DBH, and the depth averaged velocity, U. An empirical equation of the form CD = a1 + (a2/ReU,DBH)a3 gave a best fit to the combined data with values a1=0.70, a2=26,000, a3=1.0 with R2=0.63 in the range 4.9E3 < ReU,DBH < 1.9E5. The coefficients of a1=0.6, a2=30,000, a3=1.0 and R2 =0.63 are suggested for engineering design so that the asymptote is 0.6, consistent with the work of Sarpkaya and Isaacson (1981) for waves on vertical piles.
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  • The material of this thesis is based upon work supported by the National Science Foundation under the awards 1519679, 1661315, and 1825080. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.
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