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)
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