Advertisements
What are waves?
- Waves are forward movement of fluid due to oscillation of particles caused by a force.
- Despite the large amount of energy it may carry forward, there is little actual forward motion of individual water particles in a wave,
- Waves can occur on the free surface of all water bodies.
- Caused by gravity, wind, underwater volcanoes earthquake, landslides or explosions and by movement of a ship or a fish.
- Here confined to sea water oscillations caused by frictional drag caused by a wind force over the sea surface.
- Wind waves vary from small ripples to huge rogue waves.
- Waves can travel thousands of miles before reaching land.
Wave parameters
- Top of wave is “crest”, bottom is “trough“.
- Difference between crest and trough is the “height“
- Distance two consecutive crests is “wave-length”.
Formation of waves – Fore & Aft Motion
- As wind blows over water it tries to drag water surface with it.
- The surface cannot move as fast as air, so it rises.
- When it rises, gravity pulls the water back’
- Momentum of falling waters reaches below the surface.
- Water pressure from below pushes falling water up again.
- This tug of war between gravity and water pressure cause the up-down movement of fluid.
- There is also a small component of to and fro motion of water in a wave.
- The wave may travels forward for thousands of mile but but the water particles don’t.
- Otherwise all the oceans would empty onto the shore!!!!!
- A wave transfers energy from one water molecule to the next causing water particles to move in a circular pattern as shown by the bottle movement on a water surface.
- This circular motion of water particle does not occur over the entire depth of water.
- There are multiple circles of water particle movements where circles grow smaller with increasing depths as seen in the next diagram to the right.
- At a depth about equal to half the wavelength, the motion stops.
- As they propagate across the open ocean, wind-generated waves maintain a constant speed, which is unaffected by depth until they reach shallow water
Factors Affecting Waves
- Wind speed
- Wind Duration
- Length of wind fetch ( Distance of water over which wind blows)
- Depth of water
- Set and drift of water current
Arrival on shore
- As ocean waves approach a shore, the lower part begins to interact with sea floor.
- This slows the waves down and causes crests in series of waves to bunch up—an effect called shoaling.
- Period of waves does not change instead waves gain height and eventually break.
Height of Waves
- Waves in a given area have various heights.
- Significant wave height (SWH) is used in weather reporting.
- SWH is an average height of highest one-third of the waves in a given time period, normally of 20 minutes.
- In relation to SWH, other waves are
- Most frequent wave height 0.5 x SWH
- Average 0.6 x SWH
- Highest 10% 1.3 X SWH
- The Highest wave in 1175 waves 1.9 X SWH
Rogue Waves
- Every 30,000th wave is still larger than 1.9X SWH.
- One of the harmonica of the 30th wave are exceptionally large emerging suddenly in the open ocean without warning and are strong enough to topple even large ships.
- These waves are called “Rogue waves”, “Freak waves”, “Monster waves”, “Killer waves” or “King waves”
- Scientists still don’t know exactly how rogue waves occur, But is generally concurred that when crests, troughs, and length wave harmonics coincides with swell and the flow of current, a Rogue wave occurs.
- However there is no method for predicting them.
- Individual “rogue waves” sometimes occur, up to heights near 30 meters and have broken ships and sunk them.
- Southern coast of Africa where the swell due to the Westerlies and the Aghullas current reinforce sea waves, is notorious for Rogue waves.
- Famous Examples of Rogue waves are
- The S.S. Waratah and M.S. München vanishing off Cape Town
- S.S. Edmund Fitzgerald disappearing on Lake Superior.
- In 1995, an oil rig in the North Sea recorded an 84-ft.-high (25.6 m) wave that appeared out of nowhere.
- In 2000, a British oceanographic vessel recorded a 95-ft.-high (29 m) wave off the coast of Scotland.
- In 2004, scientists from the European Space Agency (ESA), as part of the Max Wave project, used satellite data to show that freak waves higher than 10 stories were rare but did occur on the oceans
Methods of Measuring Wave Height
- On ships, waves are measured by observations and use of Beaufort tables.
- Techniques used ashore for wave measurement are:-
- Wave pole on a fixed structure like a lighthouse.
- ‘Electronic poles’ known as wave staffs used for precise engineering applications.
- Wave-rider buoys measure wave movement along three axes including the wave direction
- Radar or laser altimeters.
- From satellites using Doppler radars.
Types of Waves
Three different types waves caused by wind are:-
- Capillary waves, or ripples
- Seas
- Swells
Capillary waves or ripples
- They are caused by surface tension of the water over which the wind is blowing.
- The restoring force that allows them to propagate is surface tension.
- Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops.
The Seas
- Waves generated and affected by local winds are called a wind sea or Sea Surface waves (colloquially Sea)
- Seas are larger and last longer than ripples.
- Formation of Sea is influenced by following 4 factors:
- Wind speed,
- Fetch – distance of open water over which the wind is blowing
- Time duration the wind has blown over a given area,
- Depth of water .
- The greater each of these variables, the larger the seas.
The Swell
- Swell is the wind wave caused by the momentum given to sea water by long sustaining winds.
- Swell persists even after the wind has ceased to blow.
- Swell can also been generated by TRS or an underwater earthquakes at a distance.
- Occasionally, there could be multiple Swells traveling at different speeds and directions which create difficult conditions for handling a ship.
Effect of Waves
- Waves resist ships ahead movement causing drop in speed.
- Axis of Propeller is most efficient when horizontal. During pitching, it gets inclined which reduces its efficiency.
- At times, due to heavy pitching, the propeller races causing speed reduction and more seriously engine damage.
- Slamming not only impedes ships forward movement but also damages the hull structure and a prudent master will reduce speed even though it means longer time at sea and increased fuel consumption.
Breaking of Waves
- Some waves “break”.
- A wave breaks when its base can no longer support its top, causing it to collapse.
- A wave breaks when:-
- It runs into shallow water when their wave height H is larger than 0.8 times the water depth h.
- Two wave systems oppose and combine forces
- The slope of a wave is too great — the ratio of the wave height H to the wavelength λ — exceeds about 0.17.
- Wind grows strong enough to blow off the crest