Ocean waves — their beauty and dangers

AS a young lad, I experienced the amazing force of breaking waves when my parents took me and my sister to Sennen Cove beach in West Cornwall, England, during our summer holidays.

On a wide sandy beach backed with sand dunes, we shared a prototype body board made of plywood with a curled up front. It was sheer joy to ‘catch’ a breaking wave which would bring us right up onto the beach.

Several years later whilst on a summer vacation from university, a former school friend took me to the same beach to experience surf kayaking in his brother’s clinker built wooden kayak.

The waves breaking offshore were of considerable height and I watched my friend Colin quickly turn the kayak to catch a big wave, which took him ashore.

It seemed so simple. Now it was my turn in the handmade kayak fitted with an easily removable spray deck attached to my body.

The first time I paddled out over smaller waves and quickly turned the kayak to catch a big breaker. I felt so ecstatic as I rode the crest of the wave and landed safely on the shore.

The second time, more confident now, I was slow in turning the kayak and a huge breaking wave flipped me over in deep water. I had been taught to corkscrew my body to automatically detach the spray deck from the kayak and this I immediately did, but to my surprise, my feet hit the sandy seabed in a depth of four metres and I could see daylight above the sea surface.

I thought that I was about to meet ‘my maker’ and desperately swam upwards until I broke the surface and swam ashore.

My brain was outwitted by the force of nature for I was lucky to be alive and to survive to recall this tale!

Understanding waves

Basically, waves are the means by which mass energy from the oceans and seas make their final landfall. They are mostly wind driven or propagated by sudden earth movements.

The latter are not dissimilar to the action of a stone thrown into a pond thus creating ripples and are only occasional disturbances. It was Aristotle, in the 4th century BC, who first recorded wind-wave connections.

The general profile or form of waves can be recorded in terms of their height (vertical distance from trough to crest) and the frequency of waves (measured in the number of waves per minute) and the fetch of a wave (distance of open water in which the wave travels). The waves that capsized my kayak had a fetch of 5,060km from the east coast of North America.

The strength of a wind combined with the length of time over which the wind blows lead to the gradual build-up of a wave. Large waves over five metres high are only generated in stormy seas with gale force winds of over 17 metres per second.

These occur in the cyclonic storm zones of the North Atlantic and North Pacific Oceans and especially in the ‘Roaring Forties’ areas of the southern hemisphere where the recent Olympic surfing championships were held on the French Polynesian island of Tahiti.

Types of sea waves

Waves are oscillations on the surface of seawater and are manifestations of kinetic energy which is transferred through the seawater. As waves approach the shore, their profiles are modified by the resistance offered by the sloping seashore where the wave energy is lost to friction.

The motion of the wave is obstructed by the seabed, but the rest of the wave continues to move forward at its usual speed thus causing the wave to lean forward as it gradually reaches the shore.

When the wave’s steepness ratio reaches 1:7, the crest outruns the moving trough, and the entire wave collapses on itself forming a breaking wave.

Spilling or ‘mushy’ waves occur on gently sloping beaches where the waves’ energy is lost to friction and thus the crest slowly spills, and gentle waves are formed.

These waves take longer to break and with much less force than other wave types. Plunging breakers move over a steeply inclined ocean floor and the crest of the wave curls; thereby trapping a pocket of air underneath it.

When reaching the steeper gradient of the shore, they ‘explode’, and their energy is dissipated over a short distance.

Driven by offshore winds, these waves possess high energy and move very fast and thus, are dangerous to unsuspecting beachgoers for they swash up the beach and then have a powerful backwash.

Swimmers trying to get out of the sea suddenly feel that their legs are being sucked seawards by the strong backwash of such waves. These waves, coupled by king tides, which heighten the plane of wave attack, are responsible for much destruction and erosion of beaches, particularly scouring sandy and pebble beaches, and are associated with the creation of offshore rip currents.

On shorelines with a steep profile, surging waves produce huge swells and travel at high speeds and with no crests. They seem harmless as they do not break like other waves, but their sucking effect caused by the strong backwash can prove dangerous.

Shallow water waves originate, as their name implies, in less-deep water in depths less than one twentieth of the wave’s length. Their speed is related to the depth of seawater and can be equal to the square root of depth and their acceleration due to gravity.

They are also known as long waves.

Paris 2024 Olympic surfing championships

The spectacular TV coverage of this event from Teohupo’o beach in Tahiti enthralled many viewers with the tremendous height of the waves as they broke on an offshore coral reef. These waves start life in the ‘Roaring Forties’ with its westerly winds without lands in between to break their force and thus, gradually gain height and develop into a large sea swell.

These waves, with heights of two to five metres at intervals of 15 to 20 seconds, react with the seabed 200 metres offshore.

As the waves hit the steep ramp on the reef, they were shot upwards; thus, creating huge and powerful breakers. The combined effect of the shallow depth of the reef and the force of the Pacific Ocean created hollow tubular and high waves forming a deep trough at the front under the water.

The incredible achievements of those surfers riding through a tube or scroll before the wave finally ‘wiped them out’ amazed me.

Tsunamis

Eighty per cent of all tsunamis are caused by underwater earthquakes, volcanic eruptions and underwater landslides.

Travelling on the shock waves emitted at very high velocities, they are very dangerous and cause widespread devastation and destruction when these gigantic waves reach land.

They average a wavelength of several hundred kilometres and in fact, are considered as shallow water waves or ‘harbour waves’ from the direct translation of the Japanese ‘tsu’ meaning harbour, and ‘nami’, meaning wave.

Rogue or freak waves

These open ocean waves are frequently recalled by seafarers as ‘walls of water appearing out of the blue’. They are preceded by smaller waves, but the crests of these waves are longer and broader, making them impossible for a ship of whatever size to steer around.

They have been known to sink even the largest ships and offshore oil rigs.

Waves should always be respected, as I have learned often the hard way, for they are frequently generated by day-old offshore cyclones in stormy conditions and may appear on sunny days on what seem idyllic beaches for swimmers.

Beach profiles change daily and seasonally through tidal and wave scouring, and what seemed a safe beach a year ago for swimming may have changed its profile on which the waves break.

Today, I only swim on beaches with lifeguards and observe their warning flags unlike my 74-year-old sister, who swims three times a week with friends, all year round in all weathers in what I consider now to be cold British and French seas.

Do enjoy the tropical beaches of Sarawak and Sabah, but consider the weather forecasts for a few days before venturing in for a swim!