When the wind blows across the water, it changes the water's surface, first into ripples and then into waves. Storms can make enormous waves, particularly if the wind, blows in the same direction for any length of time. In this chapter, you can learn what waves are and how they behave. |
Waves have a major influence on the marine environment and ultimately on the planet's climate. | ||
Waves travel effortlessly along the water's surface. This is made possible by small movements of the water molecules. This chapter looks at how the motion is brought about and how waves can change speed, frequency and depth. | ||
The wind blows over the water, changing its surface into ripples and waves. As waves grow in height, the wind pushes them along faster and higher. Waves can become unexpectedly strong and destructive. | ||
As waves enter shallow water, they become taller and slow down, eventually breaking on the shore. | ||
In the real world, waves are not of an idealised, harmonious shape but irregular. They are composed of several interfering waves of different frequency and speed. | ||
Water waves bounce off denser objects such as sandy or rocky shores. Very long waves such as tsunamis bounce off the continental slope. | ||
Tsunamis are caused by deep earthquakes which disturb the water above them, causing a wave front to radiate out at high speed. Tsunamis are unpredictable and can cause considerable damage. Mega tsunamis may occur when asteroids hit the ocean or when volcanoes erupt. | ||
Seiches are standing waves in lakes, harbours and enclosed oceans. Bores are rapidly moving waves, caused by spring tides entering narrowing rivers. | ||
Internal waves are an interesting phenomenon that cannot be observed from above. They propagate along layers caused by thermoclines, underlying fresh water and the like. They can cause sizeable undersea waves. | ||
Waves cause damage to the coast. Whereas healthy dunes and beaches are able to repair themselves, the rocky shore is not. This section looks at how waves cause damage to the rocky shore. |
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For the creatures in the sea, ocean currents allow their larvae to be dispersed and to be carried great distances. Many creatures spawn only during storms when large waves can mix their gametes effectively.
Coastal creatures living in shallow water experience the brunt of the waves directly. In order to survive there, they need to be robust and adaptable. Thus waves maintain a gradient of bio-diversity all the way from the surface, down to depths of 30m or more. Without waves, there would not be as many species living in the sea.
Waves pound rocks and make them erode faster, but sea organisms covering
these rocks, delay this process. Waves make beaches by transporting sand from
deeper down towards the shore and by washing the sand and removing fine
particles. Waves stir and suspend the sand so that currents or gravity can
transport it.
Waves are oscillations in the water's surface. For oscillations to exist and to propagate, like the vibrating of a guitar string or the standing waves in a flute, there must be a returning force that brings equilibrium. The tension in a string and the pressure of the air are such forces. Without these, neither the string nor the flute could produce tones. The standing waves in musical instruments bounce their energy back and forth inside the string or the flute's cavity. The oscillations that are passed to the air are different in that they travel in widening spheres outward. These travelling waves have a direction and speed in addition to their tone or timbre. In air their returning force is the compression of the air molecules. In surface waves, the returning force is gravity, the pull of the Earth. Hence the name 'gravity waves' for water waves.
In solids, the molecules are tightly connected together, which prevents them
from moving freely, but they can vibrate. Water is a liquid and its molecules
are allowed to move freely although they are placed closely together. In gases,
the molecules are surrounded by vast expanses of vacuum space, which allows them
to move freely and at high speed. In all these media, waves are propagated by
compression of the medium. However, the surface waves between two media (water
and air), behave very different and solely under the influence of gravity, which
is much weaker than that of elastic compression, the method by which sound
propagates.
If each water particle makes small oscillations around its spot, relative to
its neighbours, waves can form if all water particles move at the same time and
in directions that add up to the wave's shape and direction. Because water has a
vast number of molecules, the height of waves is theoretically unlimited. In
practice, surface waves can be sustained as high as 70% of the water's depth or
some 3000m in a 4000m deep sea (Van Dorn, 1974).
Note that the water particles do not travel but only their collective energy
does! Waves that travel far and fast, undulate slowly, requiring the water
particles to make slow oscillations, which reduces friction and loss of energy.
In the diagram some familiar terms are shown. A floating object is observed to move in perfect circles when waves oscillate harmoniously sinus-like. If that object hovered in the water, like a water particle, it would be moving along diminishing circles, when placed deeper in the water. At a certain depth, the object would stand still. This is the wave's base, precisely half the wave's length. Thus long waves (ocean swell) extend much deeper down than short waves (chop). Waves with 100 metres between crests are common and could stir the bottom down to a depth of 50m. Note that the depth of a wave has little to do with its height! But a wave's height contains the wave's energy, which is unrelated to the wave's length. Long surface waves travel faster and further than short ones.
Water waves can store or dissipate much energy. Like other waves (alternating electric currents, e.g.), a wave's energy is proportional to the square of its height (potential). Thus a 3m high wave has 3x3=9 times more energy than a 1m high wave. When fine-weather waves of about 1m height pound on the beach, they dissipate an average of 10kW (ten one-bar heaters) per metre of beach or the power of a small car at full throttle, every five metres. (Ref Douglas L Inman in Oceanography, the last frontier, 1974). Attempts to harness the energy from waves have failed because they require large structures over large areas and these structures should be capable of surviving storm conditions with energies hundreds of times larger than they were designed to capture.
Waves have a direction and speed. Sound waves propagate by compressing the
medium. They can travel in water about 4.5 times faster than in air, about 1500m
per second (5400km/s, or mach-4.5, depending on temperature and salinity). Such
waves can travel in all directions and reach the bottom of the ocean (about 4km)
in less than a second. Surface waves, however, are limited by the density of
water and the pull of gravity. They can travel only along the surface and their
wave lengths can at most be about twice the average depth of the ocean (2 x 4
km). The fastest surface waves observed, are those caused by tsunamis. The
'tidal wave' caused by an under-sea earthquake in Chile in May 1960, covered the
6000 nautical miles (11,000km) to New Zealand in about 12 hours, travelling at a
speed of about 900 km/hr! When it arrived, it caused an oscillation in
water level of 0.6m at various places along the coast, 1.4m in Tauranga Harbour
and 2.4m in Whitianga harbour. Note that tsunamis reach their minimum at about
6000 km distance. Beyond that, coriolis forces bend the wave fronts to focus
them again at a distance of about 12,000 km, where they can still cause
considerable damage.
The rougher the water becomes, the easier it is for the wind to transfer its energy. The waves become steep and choppy. Further away from the shore, the water's surface is not only stirred by the wind but also by waves arriving with the wind. These waves influence the motion of the water particles such that opposing movements gradually cancel out, whereas synchronising movements are enhanced. The waves start to become more rounded and harmonious. Depending on duration and distance (fetch), the waves develop into a fully developed sea.
Anyone familiar with the sea, knows that waves never assume a uniform,
harmonious shape. Even when the wind has blown strictly from one direction only,
the resulting water movement is made up of various waves, each with a different
speed and height. Although some waves are small, most waves have a certain
height and sometimes a wave occurs which is much higher.
Going back to the 'wave motion
and depth' diagram showing how water particles move, we can see that all
particles make a circular movement in the same direction. They move up on the
wave's leading edge, forward on its crest, down on its trailing slope and
backward on its trough. In shallow water, the particles close to the bottom will
be restricted in their up and downward movements and move along the bottom
instead. As the diagram shows, the particle's amplitude of movement does not
decrease with depth. The forward/backward movement over the sand creates ripples
and disturbs it.
Since shallow long waves have short crests and long troughs, the sand's forward
movement is much more brisk than its backward movement, resulting in sand being
dragged towards the shore. This is important for sandy beaches.
Note that a sandy bottom is just another medium, potentially capable of guiding gravity waves. It is about 1.8 times denser than water and contains about 30-40% liquid. Yet, neither does it behave like a liquid, nor entirely like a solid. It resists downward and sideways movements but upward movements not as much. So waves cannot propagate over the sand's surface, like they do along the water's surface, but divers can observe the sand 'jumping up' on the leading edge of a wave crest passing overhead (when the water particles move upward). This may help explain why sand is so easily stirred up by waves and why burrowing organisms are washed up so readily. |
Surf breakers are classified in three types:
Spilling breakers are a familiar sight on most beaches. They arise from long waves breaking on gently sloping beaches. There are several rows of breakers. |
Plunging breakers can occur on steeply sloping beaches. There is only one row of breakers. |
Surging breakers surge over steeply sloping (but not vertical) beaches or rocks. Waves break one at a time. Photos Van Dorn, 1974 |
When waves break, their energy is absorbed and converted to heat. The gentler the slope of the beach, the more energy is converted. Steep slopes such as rocky shores do not break waves as much but reflect them back to sea.
When approaching a gently sloping shore, waves are slowed down and bent towards the shore.
When approaching a steep rocky shore, waves are bounced back, creating a
'confused sea' of interfering waves with twice the height and steepness. Such
places may become hazardous to shipping in otherwise acceptable sea conditions.
When wave fronts approach a gently sloping beach on an angle, they slow down in the shallows, causing them to bend towards the beach. If the beach slopes gently enough, all breakers will eventually line up parallel to the beach. | When a beach is steep, the wave fronts get bent and then reflected back. Sometimes part of the energy is absorbed and the remaining energy reflected. |
This drawing shows how waves are bent around an island
which should be at least 2-3 wave lengths wide in order to offer some
shelter. It causes immediately in the lee of the island (A) a wave
shadow zone but further out to sea a confusing sea (B) of interfering
but weakened waves which at some point (C) focuses the almost full wave
energy from two directions, resulting in unpredictable and dangerous
seas. When seeking shelter, avoid navigating through this area.
Recent research has shown that underwater sand banks can act as wave lenses, refracting the waves and focussing them some distance farther. It may suddenly accelerate coastal erosion in localised places along the coast. Drawings from Van Dorn, 1974. |
Earthquakes
and tsunamis could be monitored by sea-based monitoring stations. By
placing these along seismically active zones, they could warn
immediately after a potential earth quake occurred and when a tsunami
wave passes by. By placing sensors on the bottom of the sea, large waves
could be detected. Normal storm waves do not reach deep enough but a
tsunami's long wave would, even though its amplitude might be very
small. By means of satellite communication, the early warning signals
could be transmitted to a tsunami co-ordination centre, with direct
connections to coastal tsunami warning centres.
The map shows all major earthquakes of the 1990s as
tabled below (Source: Scientific American, May 1999): |
|
Also read Tsunami by Frank I Gonzalez. Scientific
American, May 1999. (Available from the Seafriends library)
Visit Dr
George Pararas-Carayannis pages on earthquakes and tsunamis.
Mega-tsunamis Based on findings and projections of British geologist Simon Day, the BBC television programme, in October 2000, screened a disturbing documentary about the possibility of a mega tsunami arising from a collapse of the Cumbre Vieja volcano on the island of La Palma in the Canarias archipelago. The resulting shock wave could send a massive wave all across the Atlantic Ocean, to swamp large areas of America's east coast. Headlines in newspapers ran like this: It was a scene straight from a disaster movie but a disaster on such an epic scale that even the most flamboyant Hollywood director would hesitate to suggest it might ever happen. Imagine being transported in your tiny fishing boat on the crest of a wave 1,600ft high - three times the height of Blackpool Tower- over forests and glaciers and living to tell the tale. That is what happened to fishermen Howard Ulrich and his son Sonny on a July evening in 1958 in Lituya Bay, Alaska.On 8 July 1958 a 7.5 magnitude (others say 8.3) earthquake occurred along the Fairweather Fault, running along a trench near Lituya Bay. As a result, a massive land slide originating from 1000m altitude, while possibly shifting 30 million m3 of earth, plunged into the bay. The wave it caused, denuded the sides of the bay up to 516m high near the slide, and the rest of the shoreline between 200 and 30m high as it moved away towards the entrance. Simon Day extrapolated that if the entire west flank of the Cumbre Vieja mountain did the same, but with 1000 times more earth, it would send 650m shockwaves across the ocean, that would still be 40-50m high when reaching the USA, 6500 km away. But computer models do not agree, and tsunami expert Charles Mader advised that the wave would have a short wave length (less than 10 minutes), rapidly decaying to a deep water wave before it reached the US. |
The La Palma story helps us to brush up our knowledge of the physics of waves (see above). Whether originating from an underwater land slide, the explosion of a volcano or the impact of an asteroid, waves still obey the same physical laws.
Impulse and energy: There are two aspects of a disturbance that we need to distinguish: suddenness (velocity, v) and volume (mass, m). The two combined create impulse or impact (v x m), and kinetic energy (v x v x m). Impulse determines how much matter will be moved by the disturbance (water and earth and buildings), which is often more destructive than the energy content of the disturbance. Think for instance of the huge energy contained in normal moon tides, which causes no harm because tides move slowly.
Physical limitations: The critical element is how a
wave (a surface wave) moves through a narrow channel of 6000 km long but only
4km deep, the Atlantic Ocean. And there's an obstacle in the middle as well, the
Mid-atlantic Ridge. Such waves are physically limited to travel no faster than
700-800 km/h, giving them a wave period of around 2 minutes (120 sec) and a wave
length of 26 km (see equations above). This narrow channel soon dampens the
quicker components of a disturbance. Think for instance of a sudden movement,
like throwing a pebble into a pool. As the pebble displaces water outward, it
also produces a wave moving inward and upward, which dissipates most of the
energy and impact. So, as far as underwater tsunamis are concerned, the
long-distance component is proportional only to the amount of earth shifted (in
2 minutes). An underwater slip or slump consists of a volume of mud sliding down
hill. Since the velocity of such incidents is roughly the same, their impact
depends on mass only.
Impacts from outside, however, can produce larger waves.
Asteroids are a point in case. They travel at speeds between 10-70 km/s (usually
around 20 km/s=70,000 km/h), such that a 1km3 asteroid can produce a 70-90m deep
water wave 100 km away from impact, but such impacts occur about once in 100,000
years. (Please note that not all scientists agree on these figures)
Diminishing with distance travelled: Once the wave is on its way, it fans out over 180 degrees, becoming weaker as it 'dilutes' over a larger area, while encountering resistance as well. The wave weakens roughly inversely to distance travelled, thus at 1000km distance, the wave is 100 times (computers say 200 times) smaller (its energy 10,000 times less) than it was at 10km distance. The 1964 tsunami in Alaska shifted some 500 km3 of earth, starting a deep water wave of 30m, which diminished to 0.3m at 1500 km distance, then increased to 3-6m as it ran up the shallows. This wave caused substantial damage to boats, piers and the business district in Crescent City, California. The 1960 Chilean earthquake may have shifted over 1500 km3 (my estimate), causing extensive coastal damage locally and as far away as Hawaii (15 hours) and Japan (22 hours), but was hardly noticeable in New Zealand (18 hours later).
Mountain slide: According to Simon Day, in the case of the Cumbre Vieja, as much as 500 billion tonnes (200 km3) of earth could suddenly slide into the ocean, creating 650m waves locally. In his model he used a solid wedge, which slid rapidly, but natural slumps break up and slide more gradually. Since earth is between 2 and 3 times 'heavier' above water, land slides can acquire 2-3 times more momentum from their mass, and another similar amount from their higher speed, totalling perhaps 10 times. This would suggest that the La Palma island mega tsunami would equate to an under water slump of no more than 2000 km3.
Meteorites: Meteorite impact studies suggest that such an impact (equal to 10,000 Mt TNT) equates to that of a 500m diameter asteroid (0.037 km3). Such waves would diminish to less than one metre at 6000 km, but would still be capable of causing much damage through their variable 'run-up' effect.
Run-up: Tsunamis cause more or less damage depending on how they run up the coast. As they enter shallow water, the waves rise and slow down. Depending on the shape of the sea bottom and that of bays, their size increases between two (normal) and forty times (very abnormal). A 1m wave could thus rise to 2m (normal rise, within normal tidal range) or 40m, causing extreme damage very locally.
Conclusion: although scientific knowledge and computer
models are not able to disproof Simon Days' findings, common sense
evaluation of the situation makes 40-50m tall waves swamping America and
Britain, highly unlikely.
Seiches and boresOscillations of lake water levels were first studied in Lake Geneva in Switzerland, where they are called seiches. Seiches are standing waves that slosh to and fro in deep lakes, from one end to another. Changes in barometric pressure or other disturbances may start such standing waves. The speed and length of standing waves is given by the basin's depth and the distance between shores. In this manner each enclosed body of water has its own standing wave characteristics.At the entrances to semi-enclosed harbours, waves or an incoming tide may start oscillations that bounce to and fro between the shores of the harbour. Such oscillations can be started by large wave trains, gusty winds and may damage moored boats. When a spring tide comes in on a gently sloping shore which narrows
into a river entrance, the tide currents can become strong enough to
displace outflowing water and to rise up, forming a fast moving vertical
wall of water. Such a wave, characterised by a surge of water moving
swiftly upstream, headed by a wave or series of waves, is called a bore
(Old-Nordic bara=wave. A bore is also called an eagre). |
Internal wavesInternal waves are a class of gravity waves to be found almost entirely under the surface. They propagate along the boundaries of layers of water with differing densities:
|
Because of the small difference in density between such layers, the corresponding restoring force for gravity waves is much less than that for surface waves (which is the weight difference between water and air). From the speed equation for gravity waves, it follows that internal waves move much more slowly but at a fixed rate, which also depends on the depth of the boundary layer. For instance, for a thermocline at 10m with a difference of 0.15% density, the wave velocity would be 0.4m/s (1.4 km/hr). For a fresh water layer of 5m depth, the wave velocity would be 1.3 m/s (6.1 km/hr).
Internal waves require very little energy to be set in motion. The tidal
current flowing over a sea bottom discontinuity could create packets of internal
waves. Internal wave amplitudes of tens of metres and periods of up to 12 hours
have been measured in the open ocean. Internal waves can also produce standing
waves (like seiches) in enclosed bays. Because these waves are difficult to
observe, very little is known about them.
The wave phase
velocity of gravity waves in a two layer ocean is given by:
c x c = g x d x (p2 - p1) / p2From this formula it follows that the wave velocity of internal waves depends both on the thickness of the layer and the relative difference in density between the two layers. It is interesting to note that the water particles above the layer move clockwise whereas those below move counter clockwise. |
Although they did not know what caused it, seamen were familiar with a strange phenomenon, called deadwater. When travelling into a fiord, or near an ice shelf, their slow ships seemed to come to a halt, and even at full power they would only make very slow headway. Later, scientists discovered its cause, an internal wave created by the ship's movement. It appears as if the ship is travelling uphill against the heavier salt water crest. A 1000 tonne ship could experience it as a 20 tonne drag, because salt water of 3.5% is about 2% denser than fresh water. |
Internal waves arising from temperature or saline
differences, can reach magnitudes of 40m, bringing deep nutrient-rich
water right into the shallow light zone where it causes sudden and dense
plankton blooms. The polar explorer Fridtjof Nansen, leader of the Norwegian North Polar expedition to the Arctic in 1893-1896 reported the following experience aboard the small research vessel Fram, as he was tracking the ice dirft across the Arctic: On tuesday, August 29th, 1893, the Fram got into open water in the sound between the Isle of Taimur and the Almvist Islands and steamed in calm water through the sound to the north-east. . . . We approached the ice to make fast to it, but the Fram had got into dead-water, and made hardly any way, in spite of the engine going at full pressure. It was such slow work that I thought I would row ahead to shoot seal. . . . the speed must have reduced to 1 - 1.5 knot in the dead-water. . . The water at the surface was almost fresh, whereas through the bottom-cock of the engine room we got perfectly salt water. |
In 1963, the nuclear submarine USS Thresher was lost
with all hands on board. Prior to the sinking there had been no
indication of equipment malfunction or unusual storm weather. While
submerged, submarines attain neutral buoyancy by flooding or jettisoning
seawater from a series of ballast tanks. An effective way for a
submarine to avoid detection by suface vessels is to dive and cruise
silently along density discontinuities (pycnoclines), which tend to
reflect the engine noise downward and sonar pulses from above upward.
Navy scientists speculate that the USS Thresher was probably cruising
along a pycnocline when it encountered a large internal wave. Because of
its neutral buoyancy, it is thought that the submarine suddenly slid
down the wave's back side, down to greater depths. Unable to compensate
for this sudden fall, the submarine exceeded its design depth and
imploded with loss of all life. Source: Paul R Pinet: Oceanography.1992. |
How waves damage the rocky shoreWhen waves roll along in deep water, the water particles hardly move at all, relative to each other. But once a wave enters shallow water, the situation changes. At its foot, the wave meets a boundary that won't move and as a consequence, the water particles move relative to that boundary. At its top, the wave breaks, sending rushes of water forward. At these two points, the bottom and the top, the coast appears under severest attack of the waves. Depending on the amount of exposure and depth, the rock face is ground to a shape of minimal erosion, such that any point deviating from this shape, would erode faster than its surroundings. Although affected by the vagaries of rock hardness and homogenity, a typical coastal profile emerges.In deep water, the profile plunges down vertically to over 20m depth. These steep rock walls bounce waves back into sea, without absorbing much of their energy at all. So erosion rates near the surface are low. In moderately deep water, a drop-off may exist but the slope becomes more gradual. Platforms are a consistent feature of the shore. In the diagram with actual north-facing shore profiles, one can see a platform developing in the sea urchin habitat, and perhaps also one lower down. Because the urchins graze their patch thoroughly, the stoneleaf algae (Lithothamnia sp) thrive, protecting the rock. What are the forces of the waves that carve these shapes? |
When a mass of water is on the move, it is much harder to stop than air. Water is about 800 times heavier than air. So it can turn and lift boulders. Enormous pressure waves can develop when water is squeezed into a crack or cave. But most damage is caused by the friction of water, as it races along a rock face (shearing). Creatures attached to the rock may be ripped and their bodies washed up on a beach. At the bottom, where the sand or pebbles can move freely, rocks and organisms become sand-blasted and erosion here is high during large storms. The freely moving sand may be deposited on deep rock flats, smothering the attached organisms.
Ironically, scientists discovered that the hardness of the rock has little bearing on its rate of wear [Taylor, 2000]. Such findings go against all logic, but nature can be perplexing. What is often overlooked, is the protection afforded by organisms living on the rock. Even a thin scum of algae prevents water and sand from touching its surface, and although the living skin may suffer damage, it is capable of repairing itself. The 'living skin' idea could explain why soft rock is protected relatively better (because it is easier to attach to), than hard rock such as granite (is too smooth for attachment). It could explain the existence of platforms (because these are the best form for sun-loving plants) and why the shoreline erodes so slowly despite the enormous forces occurring in the wave zone. It could also explain why shaded coasts wear faster (because the protective film grows slower), and why coastal erosion is increasing everywhere in the world (because protective organisms are disappearingdue to pollution and mud).
Among the rock-protecting creatures one can find both animals (that don't need sunlight) and plants (that do need sunlight). Animals: barnacles, flea mussels, greenlipped mussels. Plants: various matting algae, pink paint (Lithothamnia sp), large algae.
Amongst the living skin, also creatures can be found that are capable of drilling into soft rock. They do so by means of scraping, assisted by excreting acids. Particularly limestone-rich rocks can be attacked in this way.
Reader please note that the above are my own observations
that have not been confirmed by scientific method. Floor Anthoni
Taylor, Anna.
Geography Dept, Univ Canterbury: Erosion of shore platforms, East Coast,
South Island, New Zealand. International Coastal Symposium 2000, Rotorua,
New Zealand.
These
live paua shells (abalone Haliotis iris), living close to the
bottom of a shallow cave in northern New Zealand, have been abraded by
shingle, showing their finely polished nacre. It was like entering
Aladdin's cave of treasures. The rocks were finely polished too. By night, pauas leave their sleeping places to browse the rocks that are exposed to sunlight by day. |
Close-up of a polished paua shell. Notice that the rock is covered by a rock-hard pink alga, affectionately called 'pink paint' (Lithothamnia sp.). It is apparently hardy and resilient enough to repair damage from both the grazing of paua and abrasion from shingle. This most amazing rock lining lives from shallow rock pools, down to 50m depth. Here it thrives in the darkness of this cave. |
Close-up of pink paint (Lithothamnia sp) on the side of a rock at 20m depth. The edges of each plant (leaf) can clearly be distinguished. Top left, part of a kelp's holdfast (Ecklonia radiata) is seen. A number of top shells can be distinguished, grazing the pink paint. | These young kelp plants had settled in shallow water (6m), the domain of the sea urchins. During a storm (cyclone Fergus), their canopies were stripped off, causing almost full mortality. It is amazing to see how sharp the boundaries of storm damage are. Only one metre deeper, none of the kelp plants was affected. |