Entendendo as ondas
Autor: Manellis | Publicado: 2013-05-11T21:28:00-03:00

Contribuição de Guilherme para o nosso conhecimento surfístico!


One must first understand the basic definitions of wave characteristics and the different words used to describe the life cycle of wind-generated water waves.



Wave height&nbsp;is the vertical distance from trough to crest.
Wave length&nbsp;is the horizontal distance from trough to trough or crest to crest.
Wave period&nbsp;is the time it takes for one full wave length to pass a fixed point.
Seas&nbsp;are waves in the wind generation zone.&nbsp;Seas&nbsp;have a confused nature since they are made of of waves of varying wave periods (lengths).
Swell&nbsp;are waves that have left the generation zone. Waves travel at a speed that increases as the wave period (length) increases. So the longer wave periods race out of the generation area ahead of the shorter periods, a phenomena called dispersion.&nbsp;Swell&nbsp;have a more uniform shape since large areas have similar wave periods (lengths). The&nbsp;wave front, or face of the leading edge of the wave horizontally along the water surface, can stretch horizon to horizon.
Deep water waves&nbsp;refer to seas or swell in water depths greater than&nbsp;half the wave length.
Shallow water waves&nbsp;refer to seas or swell in water depths less than half the wave length.


Surf&nbsp;are&nbsp;breaking waves&nbsp;due to interaction with the water basin bottom when the wave height to depth ratio is roughly 0.8; for example, surf of 8 feet breaks in depths of around 10 feet and surf of 40 feet breaks in depths of around 50 feet. The higher the surf, the deeper the water in which it breaks. This assumes a gradual sea floor drop off, as opposed to abrupt depth changes, such as&nbsp;Teahupoo&nbsp;in Tahiti, where the ratio is higher (depth shallower at breaking).





Spatial Variability
Surf heights vary from location to location, especially in Hawaii where coasts face at various orientations, the sea floor shape is complex, and upstream islands causeshadowing, dependent upon incident swell direction and period. Transformation of wave height from deep to shallow water waves, then on to breakers, is caused by various physical phenomena. The most important are:

Friction. Waves feel the bottom due to circular motion of water particles that are set up as waves travel. In shallow water, these orbits drag on the basin bed, which reduces the wave height and speed. The wider the shallow coastal shelf, the more reduction in wave height before a wave reaches the breaking point.
Shoaling&nbsp;is the shallowing of water. Wave speed decreases as the water depth decreases. In turn the wave length compresses for trains of waves as the adjacent waves in the deeper water catch up to slower waves in shallower water. The bunching of waves increases the wave height, reaching the maximum height just before&nbsp;breaking.
Refraction. If the water depth along a given wave front varies, the wave front will bend toward the shallowest areas due to refraction. This occurs as soon as the wave feels the bottom. A difference in water depth along a wave front can occur if
The&nbsp;direction of travel&nbsp;of the wave front is at an angle to the depth contours.
The coastal sea bed has&nbsp;ridges and valleys, as typical of most areas in Hawaii. The energy along the wave front converges to the shallowest location, where wave height becomes the highest. Wave height can be greatly magnified at the moment of breaking due to the combined effects of&nbsp;shoaling&nbsp;andrefraction&nbsp;. In zones of&nbsp;high refraction, adjacent areas can have little or no breakers.
Refraction is most notable near the surf zone. However, if the waves feel the bottom further offshore, there can be redirection of swell energy, sending more energy to some surf zones and less to others. This aspect is also an important reason why surf can vary from one surf area to another.



There are other factors that affect surf height such as localized currents and winds, and wave-wave interaction (e.g. backwash, which happens when a wave reflected off the shore heads seaward and meets an incoming breaker, or double-ups, which means crests of two waves of differing wave period or direction arrive simultaneously at the same location). These are not considered in this review.
Scientists have models, such as the Simulating Waves Nearshore (SWAN), that can help visualize the primary physical actions of&nbsp;shoaling and refraction. As the surf grows, the location of the breakers occurs in deeper water, shifting to the outer reefs for the north shore of Oahu.&nbsp;Waimea Bay, which has some of the largest waves closest to shore, is roughly 30% lower than in zones of maximum refraction on outer reefs, such as&nbsp;Outer Log Cabins.
Surf height can also vary along the front of any single wave. In zones of high refraction, the difference between the&nbsp;peak face&nbsp;and wave shoulder is more exaggerated.

Temporal Variability
At a fixed surf zone location, wave heights vary with time. For an individual wave, the height is greatest at the moment of maximum cresting just before the top portion of the wave falls forward, excluding wave heights associated with backwash or double-ups. Over a period of time, the heights of waves follow a Rayleigh distribution. Thus the most numerous wave heights are less than the average wave height. In oceanography, statistics are used to define frequency-of-occurrence parameters related to waves:

Significant Wave Height, or H1/3&nbsp;is the average of the highest one-third of all wave heights over a period of time. This is the parameter most often used to describe deep water seas or swell. It has been shown that an experienced observer on a ship watching the seas will estimate the heights close to the H1/3. Waves typically arrive in groups of similar size with a rough estimate of three waves per group. Groups of waves with H1/3 heights arrive on average every 3 +/- 2 minutes.
H1/10&nbsp;is the average of the highest one-tenth of waves. Groups of waves with H1/10 heights arrival on average every 9 +/- 6 minutes.
H1/100&nbsp;is the average of the one percent highest waves. Groups of such waves arrive roughly at 75 +/- 30 minute intervals, and are referred to by surfers as sneaker or clean-up sets. Since larger waves break in deeper water, and the water basin depth increases with distance from shore, H1/100 waves break further out, most often catching the surfers on the shoreward side of the falling crest; hence the name clean-up set. Though infrequent, H1/100 waves are common enough to deserve attention.

Waimea Bay (photo: Kimbal Milikan,&nbsp;Dept. of Oceanography, University of Hawaii)

Given any single one of these parameters,&nbsp;one can calculate the others&nbsp;since they differ by a multiplicative constant as defined by the Rayleigh Distribution.
An application of the Rayleigh Distribution can be seen in the way&nbsp;visual surf observations&nbsp;are made. Reports emphasize the&nbsp;smaller percent of larger waves&nbsp;and are given in a range which estimates the heights of commonly arriving sets with the arrival frequency decreasing as the value within the range increases. Nominally, these observations represent the H1/3 to H1/10, occasionally H1/100 heights.
The amount of wave energy arriving along a shore also has longer scale variations on the order of 30 minutes to a few hours. These groups of groups are referred to as&nbsp;wave envelopes. The number of waves per set and the spacing between wave envelopes is related to the width and proximity of the wave generating zone, with wider and closer sources making more frequent arrivals.


Aqui, o link para um pdf que explica como é feita a medição do tamanho de uma onda.