OBJECTIVE METHOD FOR FORECASTING SWELL WAVES

A number of terms used in dealing with forecasting sea waves will be used again in this process; however, a number of new terms will be introduced. Table 6-3 lists most of these terms with their associated symbol and definition.

As with objective forecasting of sea waves there are a number of different methods for forecasting swell waves. Some of the methods are too technical or time consuming to be of practical use.

When ship operations are conducted outside a fetch area it becomes necessary to forecast swell conditions at that location. Prior to computing swell conditions the height and period of the significant waves departing the fetch area must be determined. For more details refer to Sea and Swell Forecasting, NAVEDTRA 40560.

LEARNING OBJECTIVES: Explain the generation of surf and describe the two changes that occur upon entering intermediate water. Recognize the characteristics of the three types of breakers. Define the terms associated with surf. Describe an objective method for surf forecasting and the calculations of the modified surf index.

Thus far we have discussed the generation of sea waves, their transformation to swell waves, some of the changes that occur as they move, and objective methods of forecasting both waves.

The Navy is greatly involved in amphibious operations, which requires the forecasting of another sea surface phenomena: surf. Senior Aerographers Mates will occasionally be called upon to provide forecasts for amphibious operations, and accurate and timely forecasts can greatly decrease the chance of personnel injury or equipment damage. Therefore, it is important that forecasters have a thorough understanding of the characteristics of surf and a knowledge of surf forecasting techniques.

The breaking of waves in either single or multiple lines along the beach or over some submerged bank or reef is referred to as surf.

The energy that is being expended in producing this phenomenon is the energy that was given to the sea surface when the wind developed the sea waves. This energy is diminished as the swell waves move from the fetch area to the area of occurrence of the surf.

The surf zone is the extent from the water up-rush on the shore to the most seaward breaker. It will be within this area that the forecast will be prepared. When waves enter an area where the depth of the bottom reaches half their wave length, the waves are said to "feel bottom." This means that the wave is no longer traveling through the water unaltered, but is entering intermediate water where changes in wave length, speed, direction, and energy will occur. There will be no change in period. These changes are known as shoaling and refraction. Shoaling affects the height of the waves, but not direction, while refraction effects both. Both shoaling and refraction result from a change in wave speed in shallow water. Now lets look at shoaling and refraction in more detail.

The shoaling effect is caused by two factors. The first is a result of the shortening of the wave length. Wave length is shortened as the wave slows down and the crests move closer together. Since the energy between crests remains constant the wave height must increase if this energy is to be carried in a shorter length of water surface. Thus, waves become higher near shore than they were in deep water. This is particularly true with swell since it has along wavelength in deep water and travels fast. As the swell speed decreases when approaching shore, the wave length shortens, and along swell that was barely perceptible in deep water may reach a height of several feet in shallow water. The second factor in shoaling has an opposite effect (decreasing wave height) and is due to the slowing down of the wave velocity until it reaches the group velocity. AS the group velocity represents the speed that the energy of the wave is moving, the height of the individual wave will decrease with its decreasing speed until the wave and group velocity are equal. The second factor predominates when the wave first feels bottom, decreasing the wave height to about 90 percent of its deep water height by the time the depth is one-sixth of the wave length. Beyond that point, the effect of the decreased distance between crests dominates so that the wave height increases to quite large values close to shore.

When waves arrive from a direction that is perpendicular to a straight beach, the wave crests will parallel the beach. If the waves are arriving from a direction other than perpendicular or the beach is not straight, the waves will bend, trying to conform to the bottom contours. This bending of the waves is known as refraction and results from the inshore portion of the wave having a slower speed than the portion still in deep water. This refraction will cause a change in both height and direction in shallow water.

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