Remote Sensing of Spilling and Plunging Breakers in the Surf Zone
Abstract
In the surf zone, breaking waves drive mean coastal circulation, transport sediment, and enhance air-sea gas exchange. To predict these important processes, nearshore wave models must parameterize the wave energy flux into the surf zone and its dissipation through breaking. Model skill is high for steady state breaking conditions in the inner surf zone. However, predictions often diverge from in situ measurements in the outer surf zone, where breaking initiates. Therefore, field observations of spilling and plunging breakers near the onset of breaking are needed to develop parameterizations of wave energy dissipation that account for breaker type.
We use LIDAR line-scans and thermal infrared (IR) imagery to detect breaking, classify breaker type, and measure geometric wave parameters in the surf zone on a wave-by-wave basis. From analysis of over 1500 waves observed at the USACE Field Research Facility in Duck, NC, we find that plunging breakers are steeper and more asymmetric than spilling breakers. Plunging breakers exhibit a mean wave slope of 22.8° at the onset of breaking and steepen to 26.0° as breaking develops. Spilling breakers exhibit a mean wave slope of 17.6° at the onset of breaking and steepen to 20.0° as breaking develops. From the IR imagery, we directly measure the extent of active foam along the wave face. Ensemble-averaged wave profiles show that, at the onset of breaking, active foam covers 67% of the wave face for plunging breakers, but only 40% for spilling breakers. As the breakers develop, this percentage increases to 75% for plunging breakers and 57% for spilling breakers. These results support the heuristic understanding of a plunging breaker as the large overturning of a steep wave face and a spilling breaker as the gradual cascade of active foam down a gently-sloped wave face. The significant differences between spilling and plunging breakers through the early stages of breaking suggest that different energy dissipation models should be used for spilling and plunging breakers. Our results also suggest that when evaluating wave energy dissipation models, the evolution of wave face slope and the extent of active foam should be included.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP54B..27C
- Keywords:
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- 4217 Coastal processes;
- OCEANOGRAPHY: GENERALDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERALDE: 4275 Remote sensing and electromagnetic processes;
- OCEANOGRAPHY: GENERALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICAL;
- 4217 Coastal processes;
- OCEANOGRAPHY: GENERALDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERALDE: 4275 Remote sensing and electromagnetic processes;
- OCEANOGRAPHY: GENERALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICAL;
- 4217 Coastal processes;
- OCEANOGRAPHY: GENERALDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERALDE: 4275 Remote sensing and electromagnetic processes;
- OCEANOGRAPHY: GENERALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICAL