Page 221 - NS-2 Textbook

216 NAUTICAL SCIENCES

Waves are normally described by certain terms. The
top of a wave is called the crest, while the lowest part,
usually between two waves, is called the trough. The
height of a wave is the vertical distance between the crest
and the trough, while the length of the wave (the wave-
length) is the horizontal distance between two successive
crests. The length of time it takes for a complete wave
(successive crests or troughs) to pass a given point is
called the period of the wave. Normally wind waves have
short periods, ranging from 2 to 5 seconds. Swells far in
advance of a major storm may have a period of from 12
to 15 seconds. The period of a tSlmami wave ranges from
The state of the expected surf is always an important consideration
10 minutes to as much as an hour. in the planning of an amphibious assault. Beaching a landing craft
properly is difficult when the surf is heavy.
BREAKERS AND SURF
Waves that break (fall over) when they hit bottom in Such wearing down and changing of the coastal outline
shallow water are called breakers. A line of breakers along and makeup is called erosion. Repeated ocean action
a shore is called a SlIIj, or swj !ine. There are three kinds against exposed rocky headlands, and especially sandy
of breakers. The kind is determined by the slope or gra- shores, constantly remodels beaches and topography
dient of the bottom. near the shore.
A spilling breaker develops where there is a mild, grad- In some cases the waves may lift up huge rocks bod-
ual, almost flat bottom shape. The breaker is slight and ily, break off rocky outcroppings, and throw them ashore.
can be seen advancing as a line of foam toward the beach. At other times the steady grinding of erosive sands
A plunging breaker occurs where there is a steep bot- wears away sediments and soil, creating cliffs that may
tom slope, such as occurs with a coral reef a mile Of so eventually crumble. Occasionally whole sand beaches
offshore. Such a gradient creates the often huge surfs off may be washed to sea or moved and deposited else-
Australia, South Africa, and Hawaii that are the joy of where. People who have had the misforhme of having a
surfers. The plunging breaker creates an advancing ver- beach cottage undermined or washed away on the east-
tical wall of water called swt ern seaboard or on the Gulf of Mexico during gales and
A surgillg breaker occurs where there is a very steep hurricanes know what this means.
bottom slope with sudden rock formations such as along Waves and currents produced by waves cause most
the coasts of Alaska, Chile, Norway, Maine, and much of major shoreline changes. It is estimated that shorelines
California. These formations are very close to the conti- of the United States are being worn away at the rate of
nentallandmass. The waves crash into the bottom rocks, about 1 foot each year. Cape Cod, Massachusetts, may
and the breaker explodes in a surge of foaming, turbu- be eroded away completely in about five thousand years
lent water. It is extremely dangerous to be near such a if the present rate of erosion by waves continues, for
coastline in bad weather, and it is rarely safe to swim in example.
such areas. Many people are swept into the sea by sud- On the other hand, waves and currents cause sedi-
den surging breakers and drowned each year. ments to accumulate in other places. The great Missis-
Knowledge of sea waves, s"\vell J and surf conditions sippi River Delta continues to grow into the Gulf of Mex-
is crucial to naval and marine amphibious operations. ico from sediments carried down the river from interior
Surf conditions must be predicted accurately in order to North America. TItis endless struggle between construc-
determine when troops and vehicles from amphibious tion and destruction of the surface of the Earth is one rea-
landing craft can be safely landed. A four-foot surf is con- son that geology and oceanography are so interesting.
sidered to be the" critical" height for normally safe am- In addition to the pounding of water itself against
phibious landings on an average beach. Above that the shore, small fragments of rocks and sand carried by
height, boats may broach-that is, turn broadside to the the waves also scour away beaches and wear down the
beach after grounding. Broaching can cause damage to shoreline. Seaward of breakers, fine grains of sand and
propellers and bring sand into engine intakes. pebbles constantly move back and forth like sandpaper
on a tabletop, in a continual grinding action. Often, this
BEACH AND COASTLINE EROSION erosion effect is concentrated nlore in one area of the
shore than in another. For example, a line of incoming
Coastal landforms owe their shapes to the action of ocean waves that encounters an island or a landmass jut-
waves, tides/ and currents on coastal rocks and sediments. ting out into the sea tends to change its direction of for-