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

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Table 2: Bedform nomenclature and roughness coefficients Bedform configurations Name Description k (cm) w (cm) Roughness Ratio (w/k) BR1 Small bedform, closely spaced, on flat reef 3.8 30 7.89 BR2 Large bedform, closely spaced, on flat reef 7.6 30 3.95 BR3 Large bedform, widely spaced, on flat reef 7.6 68 8.95 BR4 Small bedform, widely spaced, on flat reef 3.8 68 17.89 BS1 Small bedform, closely spaced, on beach slope 3.8 30 7.89 BS2 Large bedform, closely spaced, on beach slope 7.6 30 3.95 BS3 Large bedform, widely spaced, on beach slope 7.6 68 8.95 BS4 Small bedform, widely spaced, on beach slope 3.8 68 17.89 BN No bedform, smooth concrete beach and reef 0 0 0 3.5 Video Capture Videos were taken of the breaking and crashing of the waves at 30 fps. The breaking properties of wave height at breaking (Hb), distance of breaking (db) and distance of crashing (dc) were manually determined and recorded by using a 0.5m x 0.5m grid that was painted on the back of the tank wall as a reference (Figure 6). The breaking point was characterized as the point where "the wave front has a vertical tangent" and the crashing point as where the "touchdown of breaker jet on the free surface" occurs (Smith 2002). A database was made of all the movies and breaking information, and this contribution to the project as a whole is considered to be significant. Figure 6: Wave breaking, bedform configuration, and grid used to measure wave parameters. 6
4. Results Since the wave breaking geometry was determined manually using a grid that was of a low resolution, the measurements could only be made to within 0.25m, giving a maximum error of ±0.125m. Since the scale of this experiment was on the order of tens of meters, the possible maximum error was not considered significant. Because a normal digital camera was used, the video only captured the movement at 30 fps. This could cause a small source of error since the wave breaking point could have occurred between frames. If the breaking point was between two frames, the difference in the point of breaking between the frames was no more than 0.125m, again making the error insignificant. All relationships and graphics were made using Ho’ as the independent variable except for those relationships being compared to the previous literature. Following the standard method to describe hydraulic experiments, all variables were turned into ratios. While the original intent was to use all of the trials in every different experimental setup, it was difficult to determine the breaking point when the waves were very small (Ho’= 0.05 or 0.1). After calculating the slope parameter So (Eqn 2), the reason for difficulty was found. Waves with Ho’ ≤0.15 were considered surging instead of plunging (Table 1), indicating that these waves did not have a defined breaking point. Consequently, surging waves were not considered in this study. Out of the eight bedform configurations, three were left out of data analysis due to time constraints. Blank surface trials were also omitted as they were run after this paper was written. While data from all water levels was analyzed, only data from water level 20cm is shown in the majority of this report. 4.1 Roughness The roughness altered the breaking behavior of the waves. The bedforms slowed the waves, causing them to lose energy and consequently affecting the breaking parameters (Hb, hb, db, dc). The flat reef bedforms had minimal affect on the breaking parameters however since they were placed beyond the zone where the waves usually broke. The wave height at breaking (Hb) was significantly lower when the bedforms were on the slope (Figure 7) and the waves also broke and crashed sooner than when the roughness was on the reef flat (Figures 8 and 9). Of the different bedform configurations on the slope, BS2 had the lowest roughness coefficient and therefore was considered the roughest (Table 2). As seen in Figures 8 and 9, the waves that propagated over BS2 had the smallest breaking and crashing distances, indicating that the energy had dissipated the most. The difference in roughness in BS1 and BS3 was very similar, and therefore they behaved almost identically. Since the reef roughness didn’t affect the breaking behavior significantly, BR3 and BR4 also behaved similarly. The blank trials would have been a useful addition to this analysis as they would have given insight into exactly how the bedforms affected the waves. 7
Page 1 and 2: TSUNAMI WAVE BREAKING BEHAVIOR ON A
Page 3 and 4: 3. Methods 3.1 Facility This experi
Page 8 and 9: Figure 7: The bedforms over the ree
Page 10: Figure 10: The breaking index vs. S

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