Originally Posted by barnakiel
Well, one can feel the swell changes as soon as it runs over the shelf!
So to say, it affects early, but it takes pretty shallow water for the swell to break. Many sources quote this happens @ water's depth
1.3 of wave's height. However - it breaks earlier on ebb later on flood, etc. etc..
As waves enter shallow water, they slow down, grow taller and change shape.
At a depth of half its wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen. Although their period (frequency) stays the same, the waves slow down and their overall wave length shortens.
The 'bumps' gradually steepen and finally break in the surf when depth becomes less than 1.3 times their height.
Note that waves change shape in depths depending on their wave length, but break in shallows relating to their height.
The following is excerpted from ➥ SURFOLOGY
Swell period and ocean depth:
The depth at which the waves begin to feel the ocean floor is one-half the wavelength between wave crests. Wavelength and swell period are directly relative, so we can use the swell period to calculate the exact depth at which the waves will begin to feel the ocean floor. The formula is simple: take the number of seconds between swells, square it, and then multiply by 2.56. The result will equal the depth the waves begin to feel the ocean floor.
A 20-second swell will begin to feel the ocean floor at 1,024 feet of water (20 x 20 = 400. And then 400 x 2.56 = 1,024 feet deep). In some areas along California
, that's almost 10 miles offshore
. An 18-second wave will feel the bottom at 829 feet deep; a 16-second wave at 656 feet; a 14-second wave at 502 feet; a 12-second wave at 367 feet; a 10-second wave at 256 feet; an eight-second wave at 164 feet; a six-second wave at 92 feet and so on.
As noted above, longer period swells are affected by the ocean floor much more than short-period swells. For that reason, we call long-period swells ground swells (generally 12 seconds or more). We call short-period swells wind
swells (11 seconds or less) because they are always generated by local winds and usually can't travel more than a few hundred miles before they decay. Long-period ground swells (especially 16 seconds or greater) have the ability to wrap much more into a surf spot, sometimes 180 degrees, while short-period wind
swells wrap very little because they can't feel the bottom until it's too late.
When waves approach shallower water near shore, their lower reaches begin to drag across the ocean floor, and the friction slows them down. The wave energy below the surface of the ocean is pushed upward, causing the waves to increase in wave height. The longer the swell period, the more energy that is under the water. This means that long-period waves will grow much more than short-period waves. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can grow to be a 15-foot breaking wave (more than five times its deep-water height depending on the ocean floor bathymetry).
As the waves pass into shallower water, they become steeper and unstable as more and more energy is pushed upward, finally to a point where the waves break in water depth at about 1.3 times the wave height.
A 6-foot wave will break in about 8 feet of water. A 20-foot wave in about 26 feet of water. A wave traveling over a gradual sloping ocean floor will become a crumbly, slow breaking wave. While a wave traveling over a steep ocean floor, such as a reef, will result in a faster, hollower breaking wave. As the waves move into shallower water, the speed and the wavelength decrease (the waves get slower and move closer together), but the swell period remains the same.
Waves focus most of their energy toward shallower water. When a wave drags its bottom over an uneven ocean floor, the portion of the wave dragging over shallower water slows down while the portion wave passing over deeper water maintains its speed. The part of the wave over deeper water begins to wrap or bend in toward the shallower water -- much the same as how waves wrap and bend around a point like Rincon or Malibu. This process is called refraction.
Deep-water canyons can greatly increase the size of waves as the portion of the swell moving faster over deep water bends in and converges with the portion of the swell over shallower water. This multiplies the energy in that part of the wave, causing it to grow into a larger breaking wave as it nears shore. The effects of a deep-water canyon just offshore
is often why we see huge waves along one stretch of beach, while maybe just a few hundred yards down the beach the waves are considerably smaller. This happens at spots such as Black's and El Porto in Southern California, and Maverick's in Northern California.
Remember, the longer the swell period, the more the waves will be affected by the ocean floor bathymetry, the more they will wrap into a spot and the more the waves will grow out of deep water.