boulder impact

Ireland and Great Britain are continuously affected by storms created in the Atlantic Ocean, and the waves whipped up by these storms can transport energy great distances. The movement of coastal boulders can help us to infer how this energy is transferred to land, helping us make decisions about our changing coastlines. In October 2021, the first work from HIGHWAVE’s Breaking Wave Loads work package was published in the Journal of Fluid Mechanics. In the published article, we describe our laboratory experiments in which we recorded the acceleration and pressure on a model boulder when it was struck and moved by breaking waves of various crest shapes.

Before the HIGHWAVE project started, a fieldwork investigation recorded the position of boulders before and after the winter of 2013-2014. The research found that boulders sitting on the cliffs of the Irish West coast and weighing more than 200 tonnes had been moved dozens of meters. Later, in a laboratory at Queen’s University Belfast, video was recorded of model boulders being moved as waves created in the wave tank came over the top of their platform and struck them. The scaled-down waves in the laboratory were representative of those created by a strong storm in the Atlantic. Along with a multitude of other publications, these two pieces of research conclusively showed that it’s possible for waves created by Atlantic storms to move boulders heavier than 200 tonnes, something that had previously been debatable. Now, our own research has made a further step forward in understanding exactly how the type of breaking wave affects boulder movement.

In our experiments, undertaken at École Centrale de Marseille with Dr. Kimmoun, we measured the acceleration, pressure, and movement of a model boulder as it was struck by a single breaking wave in the laboratory. The scaled-down boulder was 8 kg and sat on a 20 cm high platform which was representative of a 216 tonne boulder on a 7.5 m platform in real life. By changing the wave creation method between each test, we were able to change the shape of the wave crest while keeping its energy the same. We found that the boulder experienced both slow and low accelerations as well as very quick and high accelerations, depending on the shape of the wave. The boulder moved the most when the top of the breaking wave had nearly – but not quite – fallen forward to create a barrel shape.

In our next boulder-wave impact experiments, we hope to find out how important the shape of the wave is compared to the wave’s height when it hits the boulder. There are many environmental characteristics that can affect the way that real boulders move. Therefore, slow, steady, and scientific progress is the way forward to confidently answer which are the most important and why.

Read the full open access article here.