Waves
The winds not only drive surface currents, it also causes waves. Waves appear on the
surface as a series of crests and troughs, moving in the direction of the wind.
Waves are defined by the following:
Wave height H is the vertical distance
between the top of a wave, or crest, and
the bottom of the preceding wave, or
troughs.
Wavelength L is the distance between
successive crests or troughs.
Period T is the time it takes for one
wave to pass a given point.
Velocity V is the speed at which a crest,
or other specified point, travels.
Velocity (V) = wavelength (L)/Period (T)
Although waves look like they are moving along, this is only an illusion (‫)وهﻢ‬. A
floating object such as a boat or a bird does not move forward in a wave train but
moves up and down with each passing wave. This is because, when under a wave
crest, the water moves up and forward; while under the troughs, it moves down and
back; thus, on the whole, water particles don't go anywhere at all as the wave passes,
but move in circles or orbits.
One could imagine wave action is like the snapping of a rope (The above right Figure).
When you snap a rope, the rope itself does not move forward. The movement of your
hand produces mechanical energy that is transferred in waves along the length of the
rope. Similarly, a wave starts with the energy of the wind pushing on the water.
Mechanical energy is transferred to each successive wave.
When waves are symmetrical, water particles move in orbits. The diameters of these
orbits decrease with increasing water depth and become insignificant at depths greater
than ½ L.
Breaking Waves
What causes a wave to crash, or break, on the beach? As a wave approaches the shore,
it enters shallow waters. As the bottom of the wave makes contact with the seafloor,
the wave slows (due to friction), which decreases its wavelength, too. This occurs
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when the water depth is about one-half the wave’s wavelength. When the water depth
is less than one-half the wavelength, the top of the wave—which moves faster than its
bottom— is effectively pushed upward and the wave height increases. These effects
are due to the insufficient space available for complete orbits to occur. As a result, the
faster top pitches (‫ )ﻳﻨﺤﺪر‬forward and crash. This action produces a type of wave
known as a breaker.
Longshore currents and rip currents
Longshore Current A current located in the surf zone and running parallel to the
shore as a result of waves breaking at an angle on the shore. It also called littoral
current.
Longshore currents form because waves are continuous and, in most cases, approach
the shore at an angle. When a wave enters shallow water it is slowed by the rising
sandy or rocky bottom and eventually breaks. As one wave meets the shore and
breaks, another wave is right behind it, preventing the broken wave from flowing
backward. This causes a “build-up” of water at the shoreline. This “build-up” of water
is then forced to form a current that flows parallel to the shore close to the water’s
edge. Longshore currents affect shorelines by redistributing sand and sediment along
their path. This redistribution is known as littoral drift and responsible for extensive
erosion and transport of beach sands along outer coast beaches.
Rip currents
Occasionally longshore currents suddenly run offshore in a dangerous, jet-like flow of
water that typically extends from near the shoreline out past the line of breaking
waves called rip currents.
Clues for identifying a rip current
•
•
•
Difference in water color (Suspended sediments may be transported back to
sea in the rip current.)
Line of foam, seaweed, or debris moving out to sea.
An area of confusing waves
Formation of rip current
1. The orientation of the coastline
2. The angle of incoming waves
3. The presence of man-made coastal structures
4. The flow through channels in sandbars
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Shaping the coastline of Palestine
The coastline of Palestine forms a small section of a larger concave system (a “littoral
cell”) that extend from Alexandria, Egypt to the Bay of Haifa, North of Palestine.
This littoral cell forms the southern east corner of the Levantine ‫ﻲ‬
ّ ‫ ﺷ ﺮﻗ‬Basin (the next
picture). The Nile River, especially its sediment yield originating from Africa’s
mountains, has shaped this entire coastline, including the coastline of Palestine, over
the last 15,000 years. The Nile sand transported by northern east (NE) directed wave
driven longshore currents along the entire concave coastline in an anti-clockwise
direction.
Since the building of the Aswan dams the sand supplied to Palestine's coastal system
is derived mainly from erosion of the Nile Delta and from sands offshore Egypt that
are stirred up by storm waves. The sands are transported by longshore and offshore
currents along the coasts of northern Sinai and Palestine. Their volume gradually
declines northward with distance from their Nile source. The longshore transport
terminates in Haifa Bay where some sand is trapped, and the rest escapes to deeper
water by bottom currents and through submarine canyons.
ِAkko
Haifa
Mediterranean
Long-Shore currents and the local erosion problem:
New structures along the
coastline, like breakwaters,
and commercial ports causes
blocking the along shore sand
transport and causes an erosive
effect on the coast downstream
(the northern coast). An
example is the recently
constructed
Gaza
fishing
harbor (the right picture), that
has locally disturbed the
coastal
erosion
and
sedimentation
pattern,
resulting in local coastal sand
erosion problems. The planned
Gaza Sea Port will even more
increase this problem.
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Tsunami
Tsunami (seismic sea waves) is long, fast waves produced by earthquakes and other
seismic disturbances of the sea floor.
Tsunami is a Japanese word meaning means “harbor” (津, tsu) “waves” (波, nami).
They were once called tidal waves, but they have nothing to do with the tides. They
are produced instead by earthquakes and other seismic disturbances of the sea floor,
so they are more properly called seismic sea waves. When such a disturbance occurs,
it can produce very long, fast-moving waves. Tsunamis may have wavelengths of 240
km and can travel at over 800 km/hr —as fast as a jet airplane. In the open ocean,
tsunamis are not very high, usually less than 1 m. Most of the time ships at sea don’t
even notice the passing of a tsunami. Tsunamis usually get higher when they approach
shore and may reach as much as 20 to 30 meters high.
Just before the wave hits, water along the beach is suddenly sucked away and then the
giant wave approaches with a loud noise. After the tsunami breaks on the shore, there
is another tremendous rush of water back to the sea.
A few tsunamis occur almost every year, but most are not very damaging.
Occasionally, however, the waves grow huge and cause great death and destruction.
A tsunami watch is issued whenever there is an earthquake stronger than 6.75 on the
Richter scale.
Fig.
Diagram illustrating how Tsunami’s form
Since tsunamis are unexpected, especially in developing countries, it can be so
destructive. On Sunday, 26 December 2004, the greatest earthquake in 40 years about
150 kilometers off the west coast of northern Sumatra Island in Indonesia. The
earthquake generated a disastrous tsunami that caused destruction in 11 countries.
The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India,
Thailand and other countries with waves up to 30 m (100 feet) high. It caused serious
damage and deaths as far as the east coast of Africa, with the furthest recorded death
due to the tsunami occuring at Port Elizabeth in South Africa, 8 000km (5 000 miles)
away from the epicentre. Anywhere from 228,000 to 310,000 people are thought to
have died as a result of the tsunami.
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Tides
According to Newton’s Law of Gravitation; the force of gravity acting between any
two bodies is proportional to the product of the masses of the two bodies and
inversely proportional to the square of the distance between them.
Both the sun and the moon exert significant gravitational attraction on the ocean, but
the moon exerts twice the gravitational attraction and tide-generating force as
the sun because it is closer to the earth (384,400 km instead of 149,600,000 km).
Tidal motion can be measured throughout the ocean, but it is especially noticeable at
the shoreline in the form of tidal currents and vertical motion.
The extent of the tide is largely determined by the difference in gravitational
attraction on either side of the earth. On the side closer to the moon the gravitational
attraction pulls water toward the moon. On the opposite side of the earth, a minimum
of gravitational attraction combines with the earth's spin to produce a net excess of
centrifugal force, creating a tidal bulge away from the earth. Corresponding depressions (low tide) will exist on parts of the earth between the bulges, where there is
no net excess of gravitational pull relative to centrifugal force. Because the moon
"passes over" any point on the earth's surface every 24 hours, 50 minutes, or once
each tidal day, ideally there should be two low and two high tides per day. Because
the moon's position relative to the earth's equator shifts from 28.5° N to 28.5° S, the
relative heights of high and low water differ geographically owing to changing
vectors of gravitational attraction.
Types of Tides
•
Spring tides ‫ اﻟﻤ ﺪ اﻟﻌ ﺎﻟﻲ‬occur when Earth, Moon and Sun are aligned in a
straight line, thus, the gravitational force exerted by the sun amplifies that of
the moon, and maximal tidal range is achieved producing very high, high tides
and very low, low tides. This occurs at the full and new moons.
•
Neap tides ‫ اﻟﻤ ﺪ اﻟﻤﻌﺘ ﺪل‬Occur when sun, earth, and moon form a right angle,
which happens when the moon at the first and third quarters, the
gravitational effects tend to cancel each other out, and neap tide occurs, with
the minimum vertical range.
Two spring tides and two neap tides occur each lunar month (approximately 29.5
days).
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Tides in the Real World
Given the daily rotation of the earth, one might expect two equal high tides each
day, separated by 12 hours. However, tides in the real world behave somewhat
differently. The presence of irregularly shaped basins, the tilt of the earth's axis, and
the Coriolis effect tend to cause significant deviations from this idealized expectation.
We can recognize three major patterns to tidal cycles:
•
Diurnal tides ‫ اﻟﻤﺪ واﻟﺠﺰر اﻟﻴﻮﻣﻲ‬have one high tide and one low tide each lunar day.
•
Semidiurnal tides ‫ اﻟﻤ ﺪ واﻟﺠ ﺰر اﻟﻨﺼ ﻒ ﻳ ﻮﻣﻰ‬have two high tides and two low tides
each lunar day, and each tide is the same height as the previous one.
•
Mixed tides ‫ اﻟﻤ ﺪ واﻟﺠ ﺰر اﻟﻤﺮآ ﺐ‬also have two high tides and two low tides each
lunar day, but each tide is a different height than the previous one.
In Gaza Strip, so far, no systematic records are available of tides. However, at
Ashdod, some 40 km North of Gaza City, the following tidal levels are given as
below:
•
•
•
•
Mean high water springs (MHWS)
Mean high water neaps (MHWN)
Mean low water neaps (MLWN)
Mean low water springs (MLWS)
0.6 m
0.4 m
0.1 m
0.0 m
Effect of Tides on the Life Cycles of Marine Organisms
The incoming tide signals the final chapter in the life cycle of many marine organisms
as their remains are washed up on the shore. But the rising tide also heralds the
beginning of life for other life-forms. For the grunion (Leuresthes tenuis), a small fish
(15 cm), life begins at high tide.
Grunion run
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Fertilization
During the spring and summer, thousands of these silvery fish swim up onto the sandy
beaches, carried in by the high tide. This so-called grunion run occurs at night during
the new moon and full moon when the tide is highest. The female grunions wiggle
into the sand and lay thousands of eggs as the males deposit sperm around them.
Afterward, the fish are swept back into the sea by the water. The spawning is timed so
exactly that it occurs only on the second, third, and fourth days that follow a new or
full moon.
After the grunion eggs are fertilized, they incubate in the sand for two weeks until the
next new or full moon occurs. At that time, the waters of the high spring tides will
reach the eggs and wash them out of the sand. The eggs then begin hatching into tiny
grunions as they are carried seaward by the outgoing tide.
El Niño-Southern oscillation (ENSO).
Condition in which warm surface water moves into the eastern pacific, collapsing
upwelling and increasing surface water temperatures and precipitation along the west
coast of North and South America.
Normal Conditions
Normally, strong trade winds push warm equatorial waters across the Pacific,
resulting in storms and high rainfall on the west side of the Pacific, and cold
upwelling on the east side.
El Niño Conditions
In an El Niño year, the trade winds diminish and reverse. This allows the warm
equatorial water to flow back east, resulting in more storms and rain in the eastern
Pacific, and reduced amounts of upwelling.
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