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.

. 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.

OBJECTIVE TECHNIQUE FOR FORECASTING SURF

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 Aerographers 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.

FORECASTING THE MODIFIED SURF INDEX

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 J o i n t S u f f M a n u a l, COMNAVSURFPAC/COMNAVSURFLANTINST 3840.1. By following the listed procedures on the Modified Surf Index Calculation Sheet the Aerographers 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 Weapons Systems and Naval Warfare (U), (S)RP1.

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