Posted by Jonah on December 30, 2001 at 21:42:03:
For all those beach divers (and boat divers). This is a great write up on how waves work. It's from www.swell.com.
"WAVES
Waves and swell are created by wind. Around the earth, we have areas of high air pressure and areas of low air pressure in the atmosphere. Think of the air as liquid, as water. The areas of high pressure are constantly trying to fill the areas of low pressure. If you have an area of high water right next to an area of low water with no barrier between, the high water will flow to fill the area of low water. The transition of airflow from high pressure to low pressure is wind.
When the wind blows over the ocean, it creates small ripples on the surface. As these ripples grow, the wind gets better friction on the ocean surface. After a period of time, these ripples grow into small waves or chop on the water. As the wind increases and continues to blow, the chop transforms into small waves, then into larger waves and then, if all goes well, into huge waves.
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ENERGY
Simply put, waves are created when wind transfers its energy from the air to the water. Wave generation requires three variables: wind velocity, wind duration and wind fetch. The harder the wind blows, the longer the time it blows and the greater the distance it blows, the bigger the waves. Limitation of any one of these variables will severely restrict the development of wave heights and the transfer of energy into the water.
As waves grow larger, the distance between waves will become greater, signifying more and more energy being transferred deeper into the ocean. As more energy is transferred deeper into the water, the waves have better ability to sustain that energy as they travel great distances across the oceans. The most common way to measure wavelengths is by measuring swell period, which is the time between successive wave crests as they pass a stationary point on the ocean surface, such as a buoy.
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WAVE DECAY
Waves decay and get smaller the farther they travel. In the middle of a storm there is a confused mix of sea state. Various waves of different heights, directions and swell periods turn the ocean surface into a chaotic mess. We call this the wave spectrum. All of these waves are the result of different cycles of the storm, with the short-period waves generated by current winds in the local area and the longer period waves generated by winds earlier in the storm's life that have had a longer time to develop.
As the waves move out of the storm area, they decrease greatly in size within the first thousand miles (more than 60 percent) and slowly thereafter. This is caused by three factors: short-period waves and chop dissipating rapidly once outside of the wind-generation area; directional spreading of waves as they move away from the storm at different angles and the separation of waves as they travel forward at different speeds after leaving the storm area. This initial wave-decay process allows the underlying long-period waves to move out from beneath the messy short-period sea state in the middle of the storm. Once these longer period waves break free from the storm's confusion, they are easily identified as a more organized wave train, which we call swell
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DIRECTION
Where the wind or swell is coming from. In the marine community, directions are always identified as the direction the swell or the wind is "coming from," not the direction it's headed. Degrees used are true degrees with north at 0 or 360 degrees (and then moving clockwise), east at 90 degrees, south at 180 degrees and west at 270 degrees. Northeast may be anywhere between 0 and 90 degrees, southeast between 90 and 180 degrees, southwest between 180 and 270 degrees and northwest between 270 and 360 degrees.
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SWELL PERIOD
The most overlooked three-dimensional variable. Most surfers look at waves from a two-dimensional perspective: wave height and direction. But waves need to be analyzed from a three-dimensional perspective, which also includes the swell period. The swell period variable is the X-factor. It's the make or break variable and plays a huge role in the eventual size of a swell. This is why:
1. Wave decay and travel. The longer the swell period, the more energy the wind has transferred into the ocean. Long-period swells are able to sustain more energy as they travel great distances across the ocean. Short-period swells (less than 14 seconds between wave crests) are steeper as they travel across the ocean and, therefore, are more susceptible to decay from opposing winds and seas. Long-period swells (greater than 14 seconds) travel with more energy below the ocean surface and are less steep so they can easily pass through opposing winds and seas with very little affect.
2. Conserving energy. Swells travel as a group of waves or a "wave train." As the swell moves forward, the wave in the front of the wave train will slow down and drop back to the rear of the group while the other waves move forward by one position. Then the next wave in front moves back and another takes its place -- much like a rotating conveyor belt that is also moving forward. It's a process somewhat similar to the "drafting" technique used by bicycle racers and car racers, and it enables wave trains to conserve their energy as they travel great distances across the oceans. Working together to sustain energy.
3. Wave speed. The speed of a swell or a wave train can be calculated by multiplying the swell period times 1.5. For example, a swell or a wave train with a period of 20 seconds will be traveling at 30 knots in deep water. (Knots are nautical miles per hour. One knot equals 1.2 mph on land.) A swell with a period of 10 seconds will travel at 15 knots. The individual waves actually move twice as fast as the wave train or the swell, and a single wave's speed can be calculated by multiplying the swell period times three. So individual waves with a period of 20 seconds travel at 60 knots in deep water. Again, think of the wave train like a rotating conveyor belt that is also moving forward.
4. Forerunners. Long-period waves move faster than short-period waves, so they will be the first to arrive. Forerunners are the initial long-period waves that travel faster than the main body of the swell. Usually, forerunners are pulses of energy with periods of 18 to 20 seconds or more. A wave train's peak energy will usually follow in the 15- to 17-second range. The swell period will steadily drop during the life cycle of the swell as it arrives on the coast. The farther a swell travels, the greater the separation of arrival time between the forerunners and the peak of the swell. Often the forerunners will only be inches high but can be measured by buoys and other sensitive oceanographic instruments. To the naked eye, forerunners are very hard to see; sometimes you can pick them out as slight bumps on a jetty or other rocks. Surfers with a sharp eye can often sense forerunners as the "ocean seems to be moving" with extra surging and currents. Even though forerunners may only be inches high, they constitute a large amount of energy. LOLA uses real-time buoy data to separate these tiny forerunners from the rest of the swell in the water so we can identify the first signs of a new swell -- before we can see it at the beach.
5. 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.
6. Shoaling. 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.
7. Refraction. 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."
The swell model at http://cdip.ucsd.edu/models/wave.model.shtml shows swell direction strength and period.
Have a great New Year,
Jonah