Why beaches lose their sand – and then suddenly reappear

A entire beach in Ireland has returned 33 years after being washed away

Dooagh beach, Ireland

In the spring of 1984, a whole beach disappeared. Over just two days, the golden sands of Dooagh on the island of Achill, off the west coast of Ireland, were stripped away to reveal bare rocks.

This occurred during a series of southwesterly (from the south west) storms and, crucially, during peak equinox – short periods in spring and autumn when tides are particularly strong due to the nature of the Earth’s movement around the sun. Jutting out into the Atlantic, Achill is exposed to the worst the ocean can throw at it, and southwesterly winds can generate substantive waves.

Thirty-three years later, in April 2017, the beach returned and the rocks were covered with sand once again.

First, let’s consider how Dooagh beach disappeared in the first place. The movement of pebbles, sand and sediment around the coast is caused by a mixture of waves, tides, and what is growing on the seafloor. Waves not only have the ability to stir up the sea bed and to wash beach sand into the ocean, but they can also create longshore drift as they strike a coast at an angle, gradually shifting sediment along the coast.

Rip currents are created in certain circumstances when water piles up close to the shore, and can only exit the beach in narrow and intense flows. These strong currents are best known to humans for causing many fatal swimming accidents, but they can also carry sand and pebbles far offshore. Seaweed can help stabilise those fragile sediments, but its growth is limited in the early months of the year.

Achill Island, County Mayo, Ireland (Shutterstock)

When a beach gets unlucky and all of these factors come together – strong tides, abnormally high sea levels, a lack of seaweed – it can suddenly disappear. This is what happened to Dooagh in spring 1984, and had previously happened in a similar incident there in 1890.

Enter sand

So, what brought the beach back in 2017 (and in 1927)? Almost the same processes that stole it in the first place. Some of the conditions were identical: April has the highest tides of the year and low algal growth. However, this time the beach replenishment occurred during northerly winds.

It’s hard to say for sure where the sand has been for the past 33 years, but likely it was simply sitting on the seafloor some miles offshore and in adjoining bays. The strong tides lifted the sand from the seafloor (and from the neighbouring bays that had benefited from the loan). Unhindered by algae, the northerly winds caused the waves to carry the sand onshore.

This didn’t happen overnight. Over the past two years, surfers had reported seeing sand offshore. By March 2017 (the second highest tide of the year), local fishermen reported the appearance of a sand bar offshore and a small amount of sand was deposited at the low water mark. The continued northerly winds followed by the exceptional April tides gave the final push and returned sand to Dooagh, depositing it well above the mean water level mark on the beach.

Changing coastlines

People in Achill shouldn’t get too complacent, as there’s no doubt Dooagh beach will disappear again at some point in future. It just needs to experience similar conditions to those of 1890 and 1984.

A beach disappearing so quickly is rare but not unheard of. In January 2015, for instance, the residents of Porthleven in Cornwall woke to a rocky shore after a strong southwesterly storm stripped the sand from their beach, again overnight. The bay has the same southwards orientation as Dooagh. It occurred during a spring tide, which is a fortnightly high tidal flow (not to be confused with the spring season). However, in this case a shift of the wind (and hence waves) to southeasterly returned the sand just one day later.

In 1995, I was involved in deploying some equipment to measure flows on a pebble beach near the town of Southsea on the English Channel. A strong storm hit overnight, and on returning the following day the beach and all of our scientific equipment had gone, leaving behind a soft, sandy beach with an entirely different shape. Great for tourism, not so for science.

In fact, the only science we achieved was through recovering some of our battered and heavy equipment – much of it ten miles to the east, which is presumably where most of the pebble beach had gone, although where the sand had come from is still a bit of a mystery. Perhaps it was beneath the pebbles all along.

These are just a few examples; I could cite many others. The coastline of the British Isles is constantly in flux for good reason. Each cubic metre of seawater weighs a tonne and Britain and Ireland are exposed to the pounding of Atlantic storms as well as some of the strongest tides on the planet. Predicting where and when the coast will erode is far from simple.

There is also the issue that one coastal area’s loss is another’s gain. If beaches are overprotected without careful planning, retention of the beach-line in one location could mean that there is no longer replenishment sand further along the coast, causing a new problem.

The sea has shaped and re-shaped our coastlines since the Earth first had oceans, and will continue to do so for many more years yet. Ireland’s “reappearing beach” is just a tiny snapshot of this process.

Simon Boxall is a lecturer in ocean and earth science, University of Southampton. This article first appeared on The Conversation (theconversation.com)