Wind, water and waves are pertinent subjects for coastal people. I’ve discussed wind and water in previous columns. My subject today is waves, specifically waves in water.
Waves move energy through water. We like to think waves push water forward. However, in true waves, the water just moves in a circle under the surface, as the wave passes. One can feel this circular motion when standing in nonbreaking waves.
Breaking waves are unstable waves and present a special case discussed later.
There are three basic types of waves in water — deep-water waves, shallow-water waves and waves that are transitioning between the two.
The circular motion of the water under a deep-water wave does not extend to the seafloor. Submarines can cruise in deep water totally unaffected by waves on the surface.
Shallow-water waves “feel the bottom” so to speak. That circular motion under the wave actually hits the seafloor.
This interaction of the wave with the seafloor causes things to happen, and our surf is one of the results.
As waves approach the long, gentle slope up to the shore here in Galveston, they slow down. The crest increases in size (height) and the trough decreases in depth. It’s because of the “law of conservation of energy.”
You will have to take my word on this because, I promise, you don’t want to “wade” through the explanation (pun intended).
This process is called “wave shoaling.” As the height of the wave builds, the wave becomes steeper and the crest of the wave begins to overtake the lower portion of the wave, which is being slowed by friction against the rising seafloor.
A plunging breaker is born as the faster moving crest curls forward crashing before the slower base.
The “breaking” completely disrupts the circular flow under the wave and water is shoved forward, as you can clearly feel in breaking surf.
At breaking, the height of a wave is about 75 to 80 percent of the depth of the water. Waves can break on an offshore bar, reform and break again nearer the beach.
Waves rarely approach perfectly parallel to the beach, thus the point of breaking often appears to travel along parallel to the beach as each successive part of the wave hits the depth that causes breaking.
So, I suggest you walk out into the surf to the waves breaking farthest from the beach and note the depth contour of the seafloor as you walk out.
Then sit on the beach and watch the surf. You will start to see the order in what could be mistaken for chaos.
You will likely get a soothing and hypnotic experience to boot.