FORECASTING
SURF
LEARNING
OBJECTIVES:
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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.
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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.
Figure
6-8.-Sea and swell graph 7.
Table
6-3.-Sea Wave Terminology
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.
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
Figure
6-9.-General characteristics of spilling, plunging, and surging breakers.
is
released Plunging breakers are more commonly
associated with swell waves that approach the beach with much longer wavelengths.
The shortening of the wavelength as the wave feels bottom causes a great mass of
water to build up in the crest in a short time. Longer period swell waves may
double in height when feeling bottom.
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:
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Breaker
height - the vertical distance in feet between the crest of the breaker and
the level of the trough ahead of the breaker.
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Breaker
wave length - the horizontal distance in feet between successive breakers.
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Breaker
period - the time in seconds between successive breakers. This is always the
same as the deepwater wave period.
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Depth
of breaking - the depth of the water in feet at the point of breaking.
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Surf
zone - the horizontal distance in yards between the outermost breakers and
the limit of wave uprush on the beach.
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Number
of lines of surf - the number of lines of breakers in the surf zone.
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Deep
water wave angle - the angle between the bottom contours and the deep water
swell wave crests.
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Breaker
angle - the angle between the beach and the lines of breakers. It is always
less than the deep water wave angle.
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Wave
steepness index - ratio of the deep water wave height to deep water wave
period squared
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Breaker
height index - ratio of breaker height to deep water wave height.
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Breaker
type - classification of breaker as to spilling, plunging, or surging.
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Breaker
depth index - ratio of depth of breaking to deep water wave height.
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Width
of surf zone - horizontal distance in yards between the outermost breakers
and the limit of wave uprush on the beach.
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Refraction
index - ratio of depth of breaking to the deep water wave length.
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Coefficient
of refraction - percent of breaker height that will actually be seen on the
beach after refraction occurs.
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Longshore
current - current parallel to beach due to breaker angle, height, period,
and beach slope
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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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