Sal Rei Harbour Breakwater Testing

Armour Stability

One of the most important components of a breakwater is the armour layer. This layer protects the breakwater from the waves and ensures that the structure itself can withstand the design storm. Often, the armour layer consists of a double layer of rock material, a double layer of concrete elements (often cubes) or a single layer of interlocking concrete elements. In general, the concrete elements are used if the required weight cannot be provided by rock sourced from a nearby quarry.

One of the main criteria for the selection of the type of concrete layer is cost-effectiveness. The cubes are easy to cast and easy to apply, although many cubes are needed and so large volumes of concrete are required. The single-layer interlocking elements use less concrete, but they are not as easy to cast and place because placement rules usually apply to obtain the proper interlocking.

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Research

In the past, Deltares has investigated the use of single-layer cubes, combining the ease of cubes with the effectiveness of a single-layer interlocking element. This proved to be a successful concept and it has been applied in temporary breakwaters. However, it had never been applied in the design of a permanent breakwater.

 

Physical modelling Sal Rei

The design conditions for the breakwater of Sal Rei harbour (on the island of Boa Vista, Cape Verde) are extreme: significant wave heights of 7 to 9 metres in combination with a wave period of more than 18 seconds may occur. During an extensive 2D testing programme, different types and sizes of armour layers were varied. Single layers with either cubes, Accropode® and Accropode II® units were tested. A double layer of cubes was also tested. During these tests, it emerged that the armour layer with a single layer of cubes performed better than the other alternatives.

After this extensive 2D testing, the breakwater was modelled in a 3D physical model in the Delta Basin to verify and optimise the breakwater head as well. Once again, it was concluded that a single layer of cubes at the roundhead was the best alternative in terms of stability and cost-efficiency.

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Construction Sal Rei

The breakwater protecting the harbour of Sal Rei was built in 2014 and it is the first breakwater with a single layer of cubes as a permanent solution.