FORECASTING SURFLEARNING 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.
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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 Aerographer’s 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 let’s 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.  When a wave enters water that is shallower than half its wave length, the motion of the water near the bottom is retarded by friction. This causes the bottom of the wave to slow down. As the water becomes more shallow the wave speed decreases, the wave length becomes shorter, and the wave crest increases in height. This continues until the crest of the wave becomes too high and is moving too fast. At this point the crest of the wave becomes unstable and crashes down into the preceding wave trough; when this happens the wave is said to be breaking. The type of breaker (that is, whether spilling, plunging, or surging) is determined by the steepness of the wave in deep water and the slope of the beach. Figure 6-9 depicts the general characteristics of the three types of breakers.
SPILLING BREAKER.— Spilling breakers occur with shallow beach slopes. The water at the crest of a wave may create foam as it spills down the face of the wave. Spilling breakers also occur more frequently when deepwater sea waves approach the beach. This is because the shorter wavelength of a sea wave means that the wave is steeper in the deep water and that the water spills from the crest as the waves begin to feel bottom. Because the water constantly spills from the crest in shorter wavelength (shorter period) waves, the height of spilling waves rarely increases as dramatically when the wave feels bottom, as do the longer period waves forming at the crest and expanding down the face of the breaker.Figure 6-9.-General characteristics of spilling, plunging, and surging breakers.PLUNGING BREAKER.— Plunging breakers occur with a moderately steep bottom. In this type of breaker, a large quantity of water at the crest of a wave curls out ahead of the wave crest, temporarily forming a tube of water on the wave face before the water plunges down the face of the wave in a violent tumbling action. Plunging breakers are characterized by a loud, explosive sound made when the air trapped in the curl is released Plunging breakers are more .SURGING BREAKER.— Surging breakers are normally seen only with a very steep beach slope. This type of breaker is often described as creating the appearance that the water level at the beach is suddenly rising and falling. The entire face of the wave usually displays churning water and produces foam, but an actual curl never develops.Remember that refraction occurs when a wave train strikes a beach at an angle, and this action causes a mass transport of water parallel to the beach in the same direction as the wave train. This mass transports called the longshore current or littoral current.Many of the craft used in amphibious operations are small and, because they are designed to land upon the beach are not sea-worthy. Owing to the size of landing craft, significant breaker height, maximum breaker height, breaker period, breaker type, the angle of breakers to the beach, the longshore (littoral) current speed and the number of lines of surf can have a dramatic effect on amphibious operations and are of vital importance.The following are some terms that will be used extensively in surf discussions and should be understood by the forecaster:Breaker height - the vertical distance in feet between the crest of the breaker and the level of the trough ahead of the breaker.Breaker wave length - the horizontal distance in feet between successive breakers.Breaker period - the time in seconds between successive breakers. This is always the same as the deepwater wave period.Depth of breaking - the depth of the water in feet at the point of breaking.Surf zone - the horizontal distance in yards between the outermost breakers and the limit of wave uprush on the beach.Number of lines of surf - the number of lines of breakers in the surf zone.Deep water wave angle - the angle between the bottom contours and the deep water swell wave crests.Breaker angle - the angle between the beach and the lines of breakers. It is always less than the deep water wave angle.Wave steepness index - ratio of the deep water wave height to deep water wave period squaredBreaker height index - ratio of breaker height to deep water wave height.Breaker type - classification of breaker as to spilling, plunging, or surging.Breaker depth index - ratio of depth of breaking to deep water wave height.Width of surf zone - horizontal distance in yards between the outermost breakers and the limit of wave uprush on the beach.Refraction index - ratio of depth of breaking to the deep water wave length.Coefficient of refraction - percent of breaker height that will actually be seen on the beach after refraction occurs.Longshore current - current parallel to beach due to breaker angle, height, period, and beach slope
Figure 6-10  provides an example of the surf worksheet that may be used in a surf forecasting procedure. The steps in the method conform to steps on the worksheet.Equipped with an understanding of the terms discussed above, the surf forecast worksheet, figure 6-10, and the step-by-step procedures listed in Surf Forecasting, NAVEDTRA 40570, the Aerographer’s Mate can prepare accurate surf forecasts. The presentation to the user can be made in any manner that is agreed upon; however, figure 6-11 illustrates one of the most commonly used methods.The Modified Surf Index is a dimensionless number that provides a measure of likely conditions to be encountered in the surf zone. The Modified Surf Index provides a guide for judging the feasibility of landing operations for various types of landing craft. The Modified Surf Index Calculation Sheet, breaker, period, and wave angle modification tables are listed in the Joint Surf Manual, COMNAVSURFPAC/COMNAVSURFLANTINST 3840.1. By following the listed procedures on the Modified Surf Index Calculation Sheet the Aerographer’s mate obtains an objective tool to be used by on-scene commanders.The Joint Surf Manual also lists modified surf limits for various propeller driven landing craft. The modified surf index is not applicable for the Landing Craft Air Cushion (LCAC). LCAC operations use the significant breaker height.For more information on amphibious operations, see Environmental Effects on Weapon’s Systems and Naval Warfare (U), (S)RP1.
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FORECASTING THE MODIFIED SURF INDEX