Fishing for diamonds and dolosse

by Diana Studer

- gardening for biodiversity

in Cape Town, South Africa

The only dirt toll road in South Africa is on the service road for the Sishen Saldanha railway. From Eland’s Bay to Lambert’s Bay. You can look at spring flowers without a fence excluding you from the farmer’s fields. No traffic, so you can stop for pictures.

Sishen-Saldanha railway

Iron ore comes from Sishen in the Northern Cape 861 km to the port in Saldanha Bay. The longest trains in the world – from Sishen-Saldanha railway line (with a train for my Ungardener). In 1989 a world record was set for the longest and heaviest train ever assembled. In the Guinness Book of Records:

Length of train - 7.303km
No. of loaded trucks - 660
No. of locomotives - 9 electrical and 7 diesel
Average speed - 38,04km/h
which carried 68,640 tonnes of ore

Sishen-Saldanha railway

In August 2010 we were going to Lambert’s Bay, one of the West Coast fishing villages. The air smells of chips (French fries). 6 out 10 South African potatoes grow here in the Sandveld. Remember the threat of the Moutonshoek tungsten mine – as you order your fish and chips!

Some little boats fish for diamonds. Divers are sent down to find diamond containing gravel in depressions on the sea-bed. The diver vacuums up the diamond/gravel. Gravel is sorted on board, then brought ashore in Lambert’s Bay. Divers work only six days a month – at depths up to 20 metres, in cold water, with strong waves, and in poor visibility.

Fishing for diamonds

Diamonds occur in marine placers on wave-cut platforms, along the coast in the Vredendal District and continue northward into the Namaqualand District (Northern Cape Province) - from geoscience.org.za

'Since diamonds are heavier than most minerals found in sand and gravel, the continual re-distribution also led to diamond concentration. Because of the weight difference, the diamonds accumulated in low lying depressions while the lighter sand was moved onwards' 

Diamond-fishing boat at Lambert's Bay

One of South Africa’s claims to fame is the invention of dolosse. Concrete blocks to build breakwaters and hold back the sea. I thought dolosse was a French word! But it is an Afrikaans word for the knuckle bones of sheep or cattle, which once children played with as toy animals. Or for sangomas to throw the bones to foretell the future.

Dolosse for concrete breakwaters

Invented in 1963 by M E Merrifield, a harbour engineer in East London, and Aubrey Kruger. Cape cormorants like to nest on dolosse. 18 ton dolos are cheap and stable compared to 37 ton concrete blocks swept away by 8 metre waves in a storm. The interlocked dolos did not even move.

Dolosse with a skein of cormorants

They dissipate the energy of waves. Their design deflects most wave action energy to the side, making them more difficult to dislodge than objects of a similar weight presenting a flat surface. Though they are placed into position on top of each other by cranes, they tend to get further entangled as the waves shift them. Their design ensures that they form an interlocking, but porous, wall. They are often numbered so that satellites can track their movement. This helps engineers gauge whether they need to add dolosse.

Cormorants on a rock at Lambert's Bay

On the road from Porterville we saw a truck with ONE dolos, and a second on its own trailer. Imagine how many /truck miles to build a breakwater!

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