The motion of the ocean

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Plunging breakers at Druridge Beach, Northumbria, United Kingdom on Christmas Day 2016. – Photo by Dr JB Wilsdon

Plunging breakers at Druridge Beach, Northumbria, United Kingdom on Christmas Day 2016. – Photo by Dr JB Wilsdon

HOW often do people wave properly these days to each other, with outstretched hands swaying horizontally from side to side? Today we witness a snapping vertical motion with fingers meeting the palm of the hand almost like a tortoise gulping up its food! It was in 1526 that a Mr Tindale first defined the word wave as “the extent of which a body of water rose above the general level of the sea”.

For me, as a Cornishman from South West England, waves have always been part of my life, for my formative years were spent within a stone’s throw of beaches and sea cliffs. Frequently, on Sundays in the summer months, my family would visit a beach at Sennen Cove, near Land’s End, to surf on a belly board.

The surfboard was primitive, made of three-ply wood with a curve up front. My sister and I ‘caught’ the breaking waves, believing erroneously that the seventh wave would always the big one to catch. There, on the same beach several years later, I joined a friend to surf big waves in a kayak but with disastrous consequences – I was flipped over by a wave and nearly drowned. Since then I have a deep respect for the power of all waves, at any time, whilst swimming in the sea.

Generation of waves

It is wind stress – the friction between the wind and the ocean surface – that drags seawater along particle by particle. Gentle ripples in the ocean swell will build up quickly as the wind strength increases. Waves seem to move in a forward direction but the water in the wave actually stays in almost the same place as it oscillates in an elliptical fashion beneath the wave surface. Waves move just like the rollers on a conveyor belt as the seawater merely turns over in an orbital path.

At Sennen Cove, the nearest direct landmass is about 5,000km away on the eastern seaboard of the United States, with nothing in between to initially break the force of wind generated waves. This distance of uninterrupted open water over which winds may blow is known as the fetch of a wave. Many a time I have sat on and swum off sandy Cornish and Sarawakian beaches at Damai and Santubong, as well as in Sabah at Tanjong Aru, and often wondered why, on a sunny day, in calm air conditions, such big waves are pounding me? The answer will be seen later.

In the open sea, waves form an evenly spaced swell, but upon entering shallower water, these waves crowd together losing energy to friction with the seabed. As these waves enter even shallower water they spill over as they topple with considerable force to create ‘breakers’.

As we stand up in shallower water after a swim, so we find our feet are dragged down a beach back into deeper water – the steeper the beach, the greater the drag. This is known as the wave’s ‘backwash’ after a previous wave, the ‘swash’, is receding down the beach slope.

Breaking waves are classified into many types. Gently sloping beaches experience ‘spilling breakers’, which are tall and tumbling. On steeper shorelines, breakers are lower and smoother thus referred to as ‘surging breakers’. Between these two extremes there are ‘plunging’ and ‘collapsing’ breakers. In Hawaii, alongside the world’s biggest ocean, the Pacific, there are ‘rollers’– to the delight of professional surfers – breaking on the shore owing to the vast fetch of the wind over the sea.

Surging breakers on the Portuguese island of Madeira battering the volcanic rock.

Surging breakers on the Portuguese island of Madeira battering the volcanic rock.

Giant waves

As an 11-year-old schoolboy, my father took me as a treat from Penzance, in Cornwall, on a day trip sea voyage to the Isles of Scilly, 64km away. The voyage over was idyllic on calm seas but the return journey of 3.5 hours was a nightmare, as an Atlantic storm passed over the ship. Holding on hard to the deck’s handrails, I succumbed to seasickness as one minute the ship was on the crest of a wave and a few seconds later plunging down into a deep trough before rising on the next wave. This all happened where seven strong sea currents cross each other. Next day, whilst standing at my school’s morning assembly, I was still swaying up and down and fainted.

The steepness and height of those waves were generated by the force of the winds in that Atlantic gale. It is difficult for sailors to accurately measure the height of waves, so instead they rely on the force of the wind by observing the way a wave breaks over the prow of a ship. It was Admiral Beaufort who, in 1805, invented the wind force scale. Often on shipping radio forecasts we hear such terms as for example, ‘South westerly, Force 6’. This scale is graduated from 0 (calm) to 12 (hurricane) force. Wave height is defined as the vertical distance from the crest of a wave to the trough of the next wave.

Freak waves

In February 1933, the USS Rampago steamed into a typhoon with wind speeds of 126km per hour as it crossed the North Pacific Ocean. A monstrous wind-generated wave suddenly smashed down on the ship. This wave was estimated to be 34 metres tall! The biggest wave, accurately recorded by instruments, was on an Atlantic weather recording ship, in 1961. It was 20.4 metres high due to the passage of a hurricane.

Today, weather ships have been mostly replaced by weather buoys, loaded with instruments. In February 2013, such a weather buoy in the North Atlantic Ocean, between Iceland and the UK, registered a record 19-metre high wave – the equivalent height of a six-storey building! This record was caused by a ‘weather bomb’ at 60 degrees north of the Equator where the atmospheric pressure fell by 24 millibars (hPa) in 24 hours and the pent up energy of this cyclone absolutely exploded.

The swash and backwash of waves on a calm day at Damai Beach.

The swash and backwash of waves on a calm day at Damai Beach.

Rogue waves

These are caused by two or more waves merging and are wind driven. This ‘hybrid’ wave lasts for only a minute and is very much taller than the surrounding swell. In October 1986, two angler fishermen were swept off rocks near Land’s End, in West Cornwall, by a rogue wave and only one survived to tell the tale.

Such was the downward force of this breaking wave that what was left of the other fisherman was but a flattened, bloodless body as if a steamroller had driven over it.

The local coroner recorded a verdict of ‘accidental death’ and he also reported that in his 30 years of service he had never seen the like before. Many such waves have led to the mysterious disappearance of large fishing boats and even larger vessels.

Tsunamis

A tsunami on Dec 26, 2004, generated by the 9.2 magnitude earthquake in the Indian Ocean, was 24 metres high as it approached the shores of Banda Aceh in Sumatra. The earthquake was occasioned by the Indian tectonic plate slipping under the Burma plate. This megathrust earthquake sent out shockwaves across the Indian Ocean in the form of gigantic waves not unlike those ripples generated by a stone thrown into a pond. Coastal communities were devastated with huge losses of life in surrounding countries and islands. Many lives were also lost on Langkawi.

The amount of damage a tsunami creates is related to the intensity of an earthquake, the distance of a place from the epicentre and the nature of the coastline. Gently sloping continental shelves allow tsunamis to build to great wave heights, whereas deeper water near shorelines minimises bottom friction and wave size. Narrow V-shaped bays and harbours favour maximum wave height as the wave energy is concentrated. Interestingly, the Japanese word ‘tsunami’ literally means harbour wave.

Rest assured, when you next sit on a beach or go for a swim in the sea anywhere, it has been calculated that only one wave in 300,000 will be four times the average height of the other waves. I hasten to add that you must never ignore the red danger flags that fly on popular beaches these days, banning us from swimming in dangerous sea conditions.

The sea is not only a consumer of sand!