GEOGRAPHY
Waves, Beaches and Coastal Processes
Waves on the ocean or other body of water are driven
primarily by wind. Other sources of energy for water waves
include earthquakes, submarine landslides, and falling objects such
as rocks, ice, and meteorites.
Wave height is the vertical distance between trough and
crest. Typical ocean wind waves range from 0.3 to 5 m in height with
a record recorded height of 34m.
The wavelength is the distance between two wave crests.
Ocean wind waves range from 40 to 400 m in wavelength and move at
speeds of 25 to 90 km/h. Wave motion of the water extends to about
one-half of the wavelength.
When a wave enters water shallower than one-half the
wavelength, its forward motion is slowed by drag on the bottom. This
causes refraction of the wave so that it approaches the shore
line in a nearly perpendicular direction. It may also cause the wave
crest to overtake the trough to form a breaker. Breakers are
collectively called surf.
Wind waves are very efficient agents of size-sorting. Wave
motion can keep silt and clay sized particles in suspension
preventing their deposition in the near-shore environment. Wind waves
are generally not strong enough to transport cobble- and
boulder-sized particles and so the principal size fraction is in the
sand size.
The strip of sediment (usually sand) extending from the low water
line to a cliff or zone of permanent vegetation is a beach.
In temperate regions, the principal mineral in beach sand is
quartz , whereas in tropical regions with reefs the principal
mineral is calcite. Active volcanic islands may have sands
composed of basalt and glass shards.
When waves arrive at an angle ot the beach they may cause a
long-shore current. Sand particles moved by this current is
the long-shore drift.
Sand moved in this manner may block a small bay to form a
bay-mouth bar, may extend past a curve in the coastline to
form a protrusion called a spit, or may form a peninsula,
called a tombolo to connect an off-shore rock to the main
beach.
Because of glacial retreat, sealevel has been rising for the past
15,000 years. The current rise is about 20cm per 100 years or 2
mm/year. Human
population is also rising rapidly in coastal regions.
Emergent coastlines are rising faster than sealevel. This
is typical of New England and Scandanavia where the land is rising
due to glacial rebound and the Pacific coast of the US that is
tectonically active. Emergent coast are typically rocky.
Submergent or drowned coasts are retreating due to rising
sealevel. In temperate latitudes, submergent coasts are typically
low-lying and marked by lagoons and barrier islands (East coast of
the US south of Long Island). In tropical latitudes they are marked
by barrier reefs (CO2 saturation).
Seismic sea waves are called tsunamis. In deep water they
may have a wave height of 60 cm to 2m, but have wave lengths of 100
km or more and travel at speeds in excess of 700 km/h.
Tides are caused by the gravitaional attraction of the
moon, and would form bulges on the proximal and distal points from
the moon that moved around the earth with a 12h period, if there
were no continents. The continents disrupt the wave so that it
propagates unevenly around the world, cancelling in some areas and
multiplying in others. The Gulf coast of the US has almost no tidal
fluctuation, whereas the Bay of Fundy has tides in excess of 15m.
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