TSUNAMI WAVE BREAKING BEHAVIOR ON A REEF WITH A ...
TSUNAMI WAVE BREAKING BEHAVIOR ON A REEF WITH A ... TSUNAMI WAVE BREAKING BEHAVIOR ON A REEF WITH A ...
1. Abstract This study is part of the University of Hawaii at Manoa’s HIREEF 2 project and aims to analyze the breaking behavior of tsunami waves over a reef shape similar to the ones present on tropical islands like Hawaii. In addition, energy dissipation due to bottom roughness was investigated and proven to be significant. When roughness was in the zone before breaking, waves were smaller, broke sooner, and crashed sooner than waves that propagated over roughness which had been placed after breaking. When looking at breaking characteristics in comparison to previous research where only a mild slope was considered, the waves in this experiment had a higher breaking index (wave height at breaking/water depth at breaking) and a lower water depth at breaking due to the geometry of the bottom surface, indicating that bathymetry plays a role in wave breaking. In the future, better understanding of the breaking behavior and energy dissipation of tsunami waves due to roughness and reef geometry will lead to more accurate numerical models that can then be used as engineering tools to save both infrastructure and lives in tsunami hazard zones. 2. Introduction and Literature Review Tsunamis occur when underwater earthquakes, landslides, or volcanic eruptions create massive displacements of water that travel towards the shore at alarming speeds. When these massive waves reach the coastline, they are capable of causing enormous damage to infrastructure and killing thousands of people. It is crucial that we study tsunami events and wave propagation not only to improve our understanding of the phenomena, but so we can create improved building codes that can lead to enhancing human safety and improving our coastal management. The HIREEF 2 project is aimed at studying how tsunamis and structures are affected by bathymetry specific to the Hawaiian region and other islands. This particular part of the HIREEF project works to understand the effect of roughness and reef geometry on the breaking of these waves. By observing the wave breaking characteristics, the wave energy dissipation due to roughness and geometry can be estimated. Tsunamis are characterized by long wave lengths, and can therefore be simply modeled as solitary waves with theoretically infinite wavelengths (Young et al 2008). The hydrodynamic similarities between these two types of waves have been proven and solitary waves are therefore used in most experiments relating to tsunamis, including this one. This experiment differs from previously completed studies by Grilli et al. (1997) in that roughness is introduced by using timber planks installed in the bottom of the wave flume, as well as island reef bathymetry as seen by the steep slope and flat reef configuration of the tank. Figure 1: Bathymetry of the Hawaiian Islands. (SOEST, 2009) 2
3. Methods 3.1 Facility This experiment was carried out at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in the Two-Dimensional Wave Flume (Figure 2). The tank measures 104m long by 3.7m wide by 4.6m deep, and a large stroke piston-type wavemaker is used to generate long wave tsunamis. The bottom of the flume is movable and therefore able to be configured in many different ways by placing concrete slabs at various depths and slopes along the tank. With the wavemaker as the datum (x=0), the bathymetry for this experiment was configured such that the first 28.6m was flat, the middle 25.6m was sloped upwards and the last 35.8m was again flat. The slope of the middle section remained at a constant 1:12 ratio throughout the entire duration of the HIREEF 2 project. This slope was considered to be steep as necessitated by the characteristics of a reef shape. Since tsunamis and reefs alike have a drastic range of sizes, it would be unreasonable to try to quantitatively scale the experiment in comparison to the real world. Therefore the project was not aimed at mimicking a particular tsunami at a specific location. Figure 2: Wave flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University. Figure 3: Diagram of wave flume and wave variables: water depth from tank bottom (ho), wave height from free surface (Ho), wave depth at breaking (hb), wave height at breaking from free surface (Hb), distance of breaking (db), distance of crashing (dc). The wavemaker location is represented by x=0. 3
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- Page 6 and 7: Table 2: Bedform nomenclature and r
- Page 8 and 9: Figure 7: The bedforms over the ree
- Page 10: Figure 10: The breaking index vs. S
3. Methods<br />
3.1 Facility<br />
This experiment was carried out at the O.H. Hinsdale Wave Research Laboratory at Oregon State<br />
University in the Two-Dimensional Wave Flume (Figure 2). The tank measures 104m long by 3.7m wide<br />
by 4.6m deep, and a large stroke piston-type wavemaker is used to generate long wave tsunamis. The<br />
bottom of the flume is movable and therefore able to be configured in many different ways by placing<br />
concrete slabs at various depths and slopes along the tank. With the wavemaker as the datum (x=0), the<br />
bathymetry for this experiment was configured such that<br />
the first 28.6m was flat, the middle 25.6m was sloped<br />
upwards and the last 35.8m was again flat. The slope of the<br />
middle section remained at a constant 1:12 ratio<br />
throughout the entire duration of the HI<strong>REEF</strong> 2 project. This<br />
slope was considered to be steep as necessitated by the<br />
characteristics of a reef shape. Since tsunamis and reefs<br />
alike have a drastic range of sizes, it would be unreasonable<br />
to try to quantitatively scale the experiment in comparison<br />
to the real world. Therefore the project was not aimed at<br />
mimicking a particular tsunami at a specific location.<br />
Figure 2: Wave flume at the O.H. Hinsdale Wave<br />
Research Laboratory at Oregon State University.<br />
Figure 3: Diagram of wave flume and wave variables: water depth from tank bottom (ho), wave height from free surface<br />
(Ho), wave depth at breaking (hb), wave height at breaking from free surface (Hb), distance of breaking (db), distance of<br />
crashing (dc). The wavemaker location is represented by x=0.<br />
3
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