Ocean waves – their beauty and dangers

Frenchman Kauli Vaast receives a barrel in the men’s gold medal final during the Paris 2024 Olympic Games in Teahupo’o, on the French Polynesian island of Tahiti, on August 5, 2024. — AFP photo

As a young boy, I experienced the tremendous power of breaking waves when my parents took my sister and me to the beach at Sennen Cove in West Cornwall, England, during our summer holidays.

On a wide sandy beach with sand dunes we shared a prototype bodyboard made of plywood with a curled front. It was pure joy to ‘catch’ a breaking wave that would take us right up the beach.

A few years later, during a summer break from university, a former school friend took me to the same beach to experience kayaking in his brother’s wooden kayak.

The waves breaking offshore were quite high and I watched as my friend Colin quickly turned the kayak around to catch a large wave, which carried him ashore.

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

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

The second time, now more confident, I slowly turned the kayak around and a huge breaking wave knocked me over into deep water. I had learned to use my body like a corkscrew to automatically release the spray deck from the kayak and I did so immediately, but to my surprise my feet touched the sandy seabed at a depth of four meters and I could see daylight above the sea surface.

I thought I was about to meet ‘my maker’ and swam desperately upwards until I broke the surface of the water and reached land.

The force of nature had outwitted my brain, because I was lucky to be alive and survived so I could remember this story!

Understanding Waves

Waves are basically the means by which mass energy from the oceans and seas reach their final landing place. They are usually driven by the wind or propagated by sudden movements of the earth.

The latter resemble the action of a stone thrown into a pond causing ripples, and are merely incidental disturbances. It was Aristotle, in the 4th century BC, who first recorded wind-wave connections.

The general profile or shape of waves can be described 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 over which the wave travels). The waves that capsized my kayak had a fetch of 5,060 km from the east coast of North America.

The force of the wind combined with the length of time the wind blows leads to the gradual build-up of a wave. Large waves of over five meters high only occur in stormy seas with winds of over 17 meters per second.

These occur in the cyclonic storm zones of the North Atlantic and North Pacific and especially in the ‘Roaring Forties’ regions of the Southern Hemisphere, where the Olympic surfing championships were recently 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 transferred by the seawater. As waves approach the shore, their profiles are modified by the resistance offered by the sloping coastline, with the wave energy being lost to friction.

The wave’s movement is impeded by the seabed, but the rest of the wave continues at its normal speed. This causes the wave to lean forward as it gradually approaches the shore.

When the wave steepness ratio reaches 1:7, the crest exceeds the moving wave trough and the entire wave collapses, forming a breaking wave.

Splashy or ‘mushy’ waves form on gently sloping beaches, where the energy of the waves is lost through friction, causing the crest to slowly sink and creating soft waves.

These waves take longer to break and with much less force than other types of waves. Diving breakers move over a steeply sloping ocean floor and the top of the wave curls; trapping an air pocket underneath.

Once they reach the steepest slope of the coast, they ‘explode’ and their energy dissipates over a short distance.

These waves are driven by the wind from the sea and have a high energy and move very quickly. They pose a danger to unsuspecting beachgoers, because they wash up the beach and then cause a powerful backwash.

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

On steep-profile coastlines, incoming waves produce huge waves and travel at high speeds and without crests. They seem harmless because they do not break like other waves, but their suction effect caused by the strong backwash can prove dangerous.

Shallow water waves, as their name suggests, are formed in shallower water at depths less than one twentieth of the wave length. Their speed is related to the depth of the seawater and can be equal to the square root of the depth and their acceleration due to gravity.

They are also called long waves.

Waves are the way that mass energy from the oceans and seas eventually reaches land. — Pixabay photo

Paris 2024 Olympic Surfing Championships

The spectacular TV coverage of this event from Teohupo’o Beach in Tahiti captivated many viewers with the sheer height of the waves as they broke on a coral reef offshore. These waves begin their lives in the ‘Roaring Forties’ with their westerly winds with no land between them to break their force and gradually gain height and develop into a large sea swell.

These waves, measuring two to five metres in height with intervals of 15 to 20 seconds, interact with the seabed 200 metres offshore.

As the waves hit the steep slope of the reef, they were shot upwards, creating huge and powerful breakers. The combined effect of the shallowness of the reef and the force of the Pacific Ocean created hollow, tubular, high waves that formed a deep trough at the underwater front.

I was amazed at the incredible feats of the surfers who surfed through a tube or a roller before the wave finally ‘wiped’ them away.

Tsunamis

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

They travel on the shock waves that are sent out at very high speeds. They are very dangerous and cause great destruction and devastation when these gigantic waves reach the land.

They have an average wavelength of several hundred kilometers and are therefore considered shallow water waves or ‘harbor waves’, derived from the Japanese word ‘tsu’ meaning harbor and ‘nami’ meaning wave.

Rogue or strange waves

These open ocean waves are often remembered by sailors as ‘walls of water that appear out of nowhere’. They are preceded by smaller waves, but the crests of these waves are longer and wider, making them impossible to navigate for a ship of any size.

They are known to sink even the largest ships and oil platforms at sea.

Waves should always be respected, as I have often learned the hard way, as they are often caused by day-old cyclones at sea during stormy conditions and can occur on sunny days on beaches that seem idyllic for swimmers.

The profile of a beach changes daily and seasonally due to the tides and waves. What seemed like a safe beach for swimming a year ago may have changed its profile by now as the waves break on it.

Nowadays I only swim at beaches with lifeguards and I heed 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 now consider cold British and French seas.

Enjoy the tropical beaches of Sarawak and Sabah, but be sure to check the weather forecast for the coming days before going swimming!