Learning Goal 8c: Explain how wave and beach-slope characteristics determine thetypes of breaking waves.
Waves are formed out in the open ocean and can travel vast distancesbefore breaking on a distant coastline. The energy carried by thesewaves and the way they break against the shoreline has dramatic impactson erosion and how shorelines are shaped over time.
As waves approach the shore, the bottom of the wave meets the oceanfloor. As they drag across the bottom, the front waves slow down, and wavelength is reduced. The following wavesstart to build up behind the slow ones, and as the wavelengths getshorter, the wave energy gets transferred upwards, increasing wave height. The friction along the bottomslows the base of the wave down while the water at the surfacecontinues forward. When the wave steepness(the ratio between wave height and wavelength) exceeds a ratio of 1:7, it becomesunstable and breaks. The slope of the sea floor greatlyinfluences how quickly the sea floor affects the waves as the waves get closer to shore, and therefore how the waves break.
Source: UBC EOSC 114
Local tides and the direction in which the ocean swell approaches the shore can also influencethe way in which waves break. If ocean swell approaches the shore at anangle, which it typically does, waves reaching the shallow water firstwill break first. The break will then progress parallel to theshoreline as the rest of the wave gradually reaches shallower water. Wewill touch on this again in Learning Goal 8d on Anticipating Swell.
Hopefully, you will never be sailing through breaking waves, as itwould cause significant damage to your boat and likely injure those onboard (see video below). However they are an important hazard toconsider when sailing in coastal areas where breakers can also formover reefs or sand bars.
There are three basic types of breaking waves: spilling breakers, plungingbreakers, and surging breakers.
Spilling Breakers
Spilling breakers occur as waves travel across a gently slopingbottom (i.e., gently sloping sea floor near the beach). The wave breaks long and slow, losing its energy as white waterspilling from the crest down the front of the wave.
Source: Mendax - Own work (Original text: selbsterstellt), CC BY-SA 2.0 de, https://commons.wikimedia.org/w/index.php?curid=40273068
Plunging Breakers
Plunging breakers occur as waves approach moderate to steep bottoms.The wave becomes steeper than a spilling breaker and the crest falls asa well-defined curl, falling forward with considerable energy. The tubethat forms as these waves hit the shore at an angle and progress acrossthe shoreline is what surfers love.
Source: Mendax - selbst erstellt auf der Grundlagephotographischer Aufnahmen im Wellenkanal, CC BY-SA 2.0 de, https://commons.wikimedia.org/w/index.php?curid=40273009
Surging Breakers
Surging breakers occur when long wave period,low amplitude waves approach moderatelysteep shores. The wave doesn’t spill or curl; it builds up and thenslides rapidly up the beach with less foam or spray than the other twobreakers.
Source: Mendax - Own work (Original text: selbsterstellt), CC BY-SA 2.0 de, https://commons.wikimedia.org/w/index.php?curid=40273146
Extra Info for Everyone & Additional Resources: (non-required material)
Water Encyclopedia – Waves: http://www.waterencyclopedia.com/Tw-Z/Waves.html
Videos: (non-required material)
Sailboat takes on plunging breakers (not smart, do not try this!): https://www.youtube.com/watch?v=ml5wBgV7iMs
Mavericks – big waves with plunging breakers in California (surfing videos):
Keywords: wavelength, wave height, wave steepness,swell, spilling breaker, plunging breaker, surging breaker, waveperiod, amplitude
Image credits: Government sources for the tablesabove are indicated in or near the table captions, and in the ExtraInfo box.
UBC ATSC 113 - Weather for Sailing, Flying & SnowSports. Copyright © 2016-2019 by Samantha James & Roland Stull.
Last updated Mar 2019.
As an oceanography enthusiast with a deep understanding of wave dynamics and coastal processes, I'll delve into the concepts mentioned in the article about Learning Goal 8c, which explores how wave and beach-slope characteristics determine the types of breaking waves.
The article discusses the journey of waves from the open ocean to their breaking point on the coastline and emphasizes the crucial role of wave and beach-slope characteristics in this process. Let's break down the key concepts:
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Wave Formation and Energy Transfer: Waves, initially formed in the open ocean, carry energy that influences erosion and shapes shorelines. As waves approach the shore, the interaction with the ocean floor becomes critical. The drag on the bottom causes the front waves to slow down, reducing wavelength. This process transfers energy upwards, increasing wave height.
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Wave Breaking Mechanism: Wave steepness, defined as the ratio between wave height and wavelength, plays a pivotal role. When this ratio exceeds 1:7, the wave becomes unstable and breaks. The sea floor slope significantly affects how quickly waves break as they approach the shore.
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Local Factors Influencing Wave Break: Local tides and the direction of ocean swell also impact how waves break. The angle at which the swell approaches the shore causes waves to break sequentially, with those reaching shallow water first breaking first. This phenomenon progresses parallel to the shoreline.
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Types of Breaking Waves: The article categorizes breaking waves into three types:
- Spilling Breakers: Occur on gently sloping bottoms, breaking long and slow.
- Plunging Breakers: Happen on moderate to steep bottoms, featuring a well-defined curl with considerable energy.
- Surging Breakers: Occur with long wave periods and low amplitude on moderately steep shores, building up and sliding rapidly up the beach.
Understanding these concepts is crucial, especially for activities like sailing, where navigating through breaking waves can pose significant hazards to boats and passengers.
In conclusion, the dynamics of wave breaking are complex and influenced by various factors, including wave characteristics, ocean floor slope, and local conditions. This knowledge is essential for anyone navigating coastal areas or studying coastal processes.
For those interested in further exploration, additional resources and videos are provided, offering a more in-depth look into wave dynamics and the dangers associated with sailing through breaking waves.
