why do waves break in shallow water

John Guiney, chief of the Meteorological Services Division at NOAA's National Weather Service, Eastern Region, offers this answer. The proposition that waves always break in odd-numbered groups may be more anecdotal than fact. Perhaps the best way to approach this question is to provide some information on ocean waves and the processes that lead to their growth, travel and demise as breaking waves along the coast. Wind produces all the waves in the world's oceans. There are two primary types of waves: wind waves and swells. Wind itself forms and sustains the former, which tend to propagate in the direction of the wind that forms them. Three factors govern the size and extent of wind waves: wind speed, the distance the wind blows across the water (also referred to as the fetch) and the length of time it blows. But once wind waves leave the area where the wind generated them, they are known as swells. Swells can propagate freely across the ocean independent of wind direction. is referred to as the wave's period. A swell with a period (the time it takes for a wave's crest and trough to pass a given point) equal to or greater than about 15 seconds can propagate across the ocean surface for thousands of miles with little loss of size or energy until it reaches shallow water in the coastal zone and breaks. Over distance and time, waves that move at nearly the same speed keep pace with one another and form a group.

Wave measurements usually show a tendency for large waves to group together--often referred to by scientists as "groupiness. " Normally, the number of waves in a group range anywhere from three to 15 or more, and it typically consists of smaller waves in the lead, larger waves in the middle and smaller waves again at the rear. This is because waves in the rear tend to move forward, build in size and then diminish as they reach the front. Although individual waves move at twice the speed of the group, they are bound to it by the energy they all share. Eventually, they travel at the group speed, which is defined as half the individual wave speed, in deep water. This state of affairs only changes when wave groups encounter shallow water. When this occurs, the group speed and the individual wave speed become the same. As waves move into more shallow water, their speed decreases while their steepness, or height, increases. Here they are also influenced by the shape of the ocean floor itself. At roughly a depth of 1. 3 times the wave height, the wave breaks and results in surf. When watching the surf at the beach, you will notice that the waves are loosely grouped in periods of high waves alternating with periods of low ones, and that there are waves breaking that are larger than the surrounding waves. This can be caused by sand-bars. Waves that pass over a bar break and reorganize into new, smaller waves as they enter deeper water beyond the bar.

Local winds and terrain effects can also influence waves at the coast and how or where they break. Surfers typically refer to wave groups in the surf zone as a set. These sets occur from every few minutes to once every half hour or more, depending on how far the waves have traveled to reach the coast. Hence, although waves do break in groups or sets, there is no evidence to suggest that the number of waves in a given group is anything but random.
62 comments As a swell approaches the coastline and comes into contact with the sea floor the waves will start to slow down. Some of the waves' energy is lost through contact with the sea floor. The shallower the water becomes the slower they move. As they slow down they have to squash together. (i. e. they shorten their wave period. ) This process is called shoaling and results in increasing wave height. The steeper the sea floor gradient the more pronounced the wave height will increase. The increase in wave height begins to occur at depths of around one half of the wavelength. As the wave moves into increasingly shallow water, the bottom of the wave decreases speed. There comes a point where the top of the wave overtakes it and starts to spill forward the wave starts to break. We're surfing! In general a wave will start to break when it reaches a water depth of 1. 3 times the wave height.

The type of wave that is produced is dependent on different factors. Groundswell is best for creating good waves. The longer wavelength waves will move quickly and get into shallow water before starting to break. The breaking waves will be steeper and faster. Wind swell will tend to break in deeper water and will not pack such a punch. The waves tend to be much more crumbly. Offshore wind is most desirable for creating good waves. The wind blows against the top part of the wave and helps delay the top part from overtaking the bottom part. This results in the waves breaking later than they normally would in calm conditions. When you watch waves or see them in surfing magazines with huge plumes of spray blowing back over the top of the wave, you're looking at offshore surf. An onshore wind will have the opposite effect. The onshore wind pushes the top of the wave forward causing the wave to break before the normal breaking depth is reached. Waves tend to be lumpier and fail to reach their optimum peak. If you've read everything we have written up to this point, you know that it's the action of the sea bed slowing the bottom part of the wave that causes the wave to break. A gently sloping approach causes the bottom of the wave to drag and will result in the top of the wave prematurely overtaking the bottom resulting in the wave breaking in deeper water.

These crumbling waves won't be steep and will lack punch. If you're learning how to surf, then these waves are ideally what you are after. Examples of this type of slope can be seen at average beach breaks all over the place. (We've all surfed them. ) The contrast to the gently sloping sea floor is a steep slope or a reef. The swell approaches the beach / reef at a greater speed. From the diagram below it can be seen that the wave jacks up due to the rapid change in depth creating a higher wave. The breaking depth is reached much later that on the gently sloped bottom. The top of the wave quickly overtakes the bottom and pitches forward. (Often taking the inexperienced surfer with it. ) The waves created by the rapid change in depth are much steeper and hollower, and thus the tube is born! Reef breaks such as are examples of this type of break. Sea floor features are especially important when surfing beach breaks. Surfing a flat beach can be a boring experience. The waves constantly close out, and you can't get a decent ride. The sea floor needs to have different depths at different points of the wave so waves will peel along their length. Big storms and the action of waves moving sand create sand bars which alter the depth of the beach at certain points. Deeper water will run alongside the shallower sand bar giving the depth difference that a peeling wave needs.

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