SA’s wave-breaking dolosse

10 November 2006

What started with the toss of bones in African child’s play culminated in one of the world’s most successful coastal engineering inventions.

Dolosse – branching concrete blocks weighing up to 30 tons – are used across the globe to break up wave action.

They were the invention of East London harbour engineer Eric Merrifield after a storm ripped into the Eastern Cape coast of South Africa in 1963 and tore off 60% of East London harbour’s armour.

Go to Eastern Cape Madiba Action Merrifield wondered if the breakwater would have been more resistant if it had not been solid and taking on the full force of the thundering surf on just one plane. He decided to reconstruct the breakwater using a “porous” design to dissipate the water’s energy.

Thus the dolos was born and a momentous novelty in harbour engineering took off. Free of patents, this Eastern Cape design is today used on breakwaters, harbours, jetties and coastlines across the globe.

‘Knucklebone’
According to Giving Good Weight author John McPhee, Merrifield came up with the idea to line the East London breakwater with concrete blocks of a branching shape – much like jacks that are tossed in child’s play.

At that time, dolosse was the name given to crude toys of bone used by the white children of South Africa since the 1830s.

Voortrekker children had acquired them from tribal children they encountered during the Great Trek, when their families moved by ox-wagons away from the Cape Colony to the Transvaal.

Dolosse were the knucklebones of goats or sheep similar in shape to the letter “H” with one leg turned 90 degrees. They were among the bones thrown into the dust by African witchdoctors, or sangomas, to access advice from the ancestors.

The game that was played with dolosse by Voortrekker children was called “knucklebone”. These bone toys were also often thought of as imaginary oxen.

Merrifield replicated these bones on a grand scale in concrete – making dolosse that weighed 20 tons apiece to armour his breakwater.

Absorbing the ocean’s power
When the high seas struck, there were no cracks of thunder or geyser-like spurts. Instead, the waves seemed to crumble and disappear into the crevices of the dolosse pile. A coastal engineering wonder was born.

According to US Army Corps of Engineers’ Coastal Research Centre, even when quarrystone is available for rubble mound breakwaters, concrete dolosse “may be the least costly alternative”.

“Dolosse are used on the Humboldt, California jetties; the Manasquan Inlet, New Jersey jetties, and on several Hawaiian breakwaters – to name just a few.

The dolosse invention is anchored by the historic casting of the biggest dolosse on the African continent for the deepwater Port of Ngqura outside Port Elizabeth.

The multi-user deepwater port forms part of the Coega Industrial Development Zone. Some 26 500 dolosse weighing 30 tons apiece will form the top layer of the main breakwater which, at 2.5 kilometres long, is the largest in Africa.

This article was first published in Eastern Cape Madiba Action, summer 2006/07 edition. Republished here with kind permission of the author.