Graduate Thesis Or Dissertation
 

Effect of an Idealized Mangrove Forest of Moderate Cross-shore Width on Loads Measured on a Sheltered Structure and Comparison with Predicted Forces

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z316q872m

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  • As coastal communities face increasing chronic and acute hazards, nature-based coastal engineering solutions have experienced a rapid growth in popularity and interest. Recent works on this topic have shown that “Green Infrastructure” may be effective at mitigating coastal hazards and therefore provide sustainable adaptation alternatives to traditional engineering solutions such as seawalls and breakwaters. However, the amount of protection that green infrastructure can provide has yet to be quantified broadly, which makes it difficult to propose engineering solutions using these systems. Additionally, the intrinsic randomness of nature introduces a variety of uncertainties that have not been quantified, further limiting the use of green infrastructure in engineering design. Work to date on emergent vegetation, such as mangrove trees, has focused primarily on reduced-scale models in a laboratory setting and centered on wave height attenuation. Little existing research has examined the impacts of emergent vegetation on the attenuation of wave forces. The purpose of this study is to quantify the wave force attenuation by an idealized mangrove forest at a prototype scale and provide guidance to engineers in the design of mangrove forests for this purpose. For our study a 1:1 scale physical model of a mangrove forest was built in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University. A test wall was placed behind the model mangrove forest, representing a coastal building. Regular and random waves at four different water depths were generated by a wavemaker, propagated through the forest, and impacted the test wall on which the wave generated pressures were measured. Wave heights before and after the model forest were used to compare with the integrated wave pressures (forces) and assess attenuation caused by the mangrove forest at two different stem densities. The model trees were constructed based off existing studies that defined and quantified the characteristic parameters of a mangrove forest such as average diameter at breast height, average diameter of the roots and the density of the forest in trees per meter squared. Two forest densities and a control case with no trees were tested. The densities and other parameters of our forest were derived from the aggregate of previous publications. A field study was also completed as part of this experiment which examined six forests, both undisturbed and restored forests, near the Port St. Lucie inlet in Jupiter, Florida. In the companion work to this document, (Kelty, 2021) the drag coefficient for use in existing wave height attenuation models was determined in this same experiment as well as a methodology using Light Detection and Ranging (LiDAR) to quantify the engineering parameters of the forest. This document combined the advances made in (Kelty, 2021) regarding wave height attenuation with existing, and well known models for wave forces estimation such as (Goda, 2010), to see if these two bodies of work could be used in concert to model the wave forces attenuation properties of mangrove trees. Our experiment found that as waves propagate through the mangroves, the wave heights are reduced, but the wave period and wavelengths remain relatively constant. Additionally, the hydrodynamic pressures underneath the wave are not significantly changed. With these results, our study shows that engineers can model the wave heights with the drag coefficients quantified in (Kelty, 2021) and then use the new wave heights as an input into existing empirical models for wave-induced forces on coastal structures. The accuracy of the method of (Goda, 2010) ranged from -49% to 6% for regular wave cases and -2% to 47% for random wave cases. Additionally, the trends in the predicted forces from (Goda, 2010) aligned well with the trends that we observed in our data. Additional work by (Cuomo, Allsop, Bruce, & Pearson, 2010) for breaking waves also showed excellent agreement with our data for random waves. Their predicted values had an accuracy range between 8% to 26% where our test cases fell within the range of parameters tested in their experiment. Our study found that the underlying assumptions upon which these existing works were built hold true even with the presence of an offshore mangrove forest, showing that the existing wave forces estimation tools and works investigating wave height attenuation from mangroves can be combined and used in the design of green infrastructure engineering solutions.
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Related Items
  • Prototype-Scale Physical Model Study of Wave Attenuation by an Idealized Mangrove Forest of Moderate Cross-shore Width
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  • The material of this thesis is based upon work supported by the National Science Foundation under the awards 1519679, 1661315, 1825080 and 2037914.
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