Dolos

A dolos (plural dolosse- livestock knuckle bones used as toy oxen; Afrikaans with approximate pronunciation "dawl-awe-suh") is a concrete block in a complex geometric shape weighing up to 20 tons, used in great numbers to protect harbour walls from the erosive force of ocean waves. They were developed in East London, a port city in South Africa, in 1963.

Dolosse are normally made from un-reinforced concrete, poured into a steel mould. The concrete will sometimes be mixed with small steel fibers, to strengthen it in the absence of reinforcing. Construction is done as close as possible to the site of installation because the dolosse are very heavy.

They are used to protect harbour walls, breakwaters and shore earthworks. In Dania Beach, Florida, dolosse are used as an artificial reef known as the Dania Beach Erojacks. They are also used to trap sea-sand to prevent erosion. An order of 10,000 dolosse are required for a kilometre of coastline.

They work by dissipating, rather than blocking, 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, over time they tend to get further entangled as the waves shift them. Their design ensures that they form an interlocking but porous wall. However, they are not indestructible. Under extreme storm conditions they will hammer one another and be pounded into rubble.

The individual units are often numbered so that their movements can be tracked. This helps engineers gauge whether they need to add more dolosse to the pile.

Dolosse are also being used in rivers in the Pacific Northwest of the United States of America, to control erosion, prevent channel migration and to create and restore salmon habitat. Examples are engineered log jams, or ELJs, that may aid in efforts to save stocks of salmon. Local, county, state, federal and private industry experts in engineering design, fluvial geomorphology and fisheries conservation are working together to protect important public infrastructure such as roads and commercial and residential developments, while maintaining, improving, restoring, or creating aquatic habitat. The sheer mass of the dolosse provides ballast for logs and slash ("wrack" or "rack" organic debris) to create a stable, complex habitat structure, all the while precluding the need for excessive, environmentally-invasive and costly excavation for their placement into substrate. Such ELJs are of a new, innovative design that are on the leading edge of engineering and science technologies in aiding the recovery of Pacific Northwest (particularly Endangered Species Act-listed) salmonids, while protecting important public and private works and thus helping to balance the needs of Man and Nature.

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