TSUNAMI WAVE BREAKING BEHAVIOR ON A REEF WITH A ...

3.2 Instrumentation
Instruments were used for this particular aspect of the project to
determine the free surface profile of the water as a wave passed. Fourteen
resistance-type parallel wire wave gauges and eight sonic wave gauges
(Figure 4) recorded the height of the waves over time and were positioned
down the length of the tank in the horizontal x-direction. The
measurements from these instruments were used to record the actual
wave height Ho and compare the values to the experimental Ho, the
number that was entered into the wavemaker computer. The actual wave
height values were typically 3-5% lower than the wave height input in the
computer, and therefore the difference was not taken into account in this
analysis.
Figure 4: Wave measurement
instrumentation: sonic wave
gauge (left) and wire wave
gauge (right)
3.3 Wave Trials
In order to gather a broad spectrum of data, waves were run at combinations of 9 different wave height
ratios and 5 different water depths (0cm, 5cm, 10cm, 20cm, and 30cm above the flat reef). Trials of
wave height ratios equal to 0.1, 0.2, 0.3, 0.4, and 0.5 were run twice in order to test repeatability. By
combining these variables in different ways, different aspects affecting the waves could be isolated.
Waves were run every 20-30 minutes in order to allow the water to settle.
3.4 Breaking Index and Breaker Type
There are four ways in which breaking waves can be classified. These “breaker types” are spilling,
plunging, collapsing, and surging. Spilling waves are characterized by a crest that becomes unstable and
then cascades down the shoreward face of the wave. In plunging breakers, the crest of the wave curls
over the face of the wave and falls into the base, what surfers refer to as pipelines. In collapsing waves,
the crest remains stable and unbroken while the lower part of the shoreward face steepens and then
falls to produce a turbulent flow. Surging waves practically don’t break, as the front face of the wave
just advances towards the beach. The surf similiarity parameter, ξo (Eq. 1), is typically used to classify
the waves based on wave height Ho and period Lo (Demirbilek and Vincent, 2002). However, the slope
parameter So (Eqn 2) can also be used and has a 99.9% correlation to the surf similarity parameter. This
is the parameter that was used in this experiment as it is in a form that can be used for solitary waves by
substituting an approximation for Lo (Eqn 3) since solitary waves are infinite in wave length. (Grilli et al.
1997). The slope parameter was used because it is derived from the Boussinesq equation and therefore
considered to be a mathematically derived as opposed to experimentally. After calculating So, the
following guidelines were used in determining the breaker types of the waves in this experiment:
surging breaking= 0.3