Winter 2016

Coastal structures with geotextile sand elements can serve as an alternative to traditional riprap-based methods. Research into such aspects as breakwaters, environmental impact and cost-effectiveness indicates the advantages and disadvantages of both options.  

TenCate Geotube® enables dunes, dykes, banks and beaches to be sustainably rehabilitated, strengthened and raised quite quickly. Another potential application is breakwaters: either perpendicular to or parallel underwater to the coast. This technology has been used for years in the United States for protecting and strengthening shorelines and for building dunes. Applications in Europe can be found, for example, in the French coastal resorts of Cannes and Sète.

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TenCate Geotube® containers put into place and filled near Sète 

The Étang de Thau is a lagoon at Sète, also on the Mediterranean Sea. A narrow and relatively vulnerable strip of land with a motorway and a railway line separate the lagoon from the sea. The road has already been relocated to the other side of the railway line owing to progressive erosion. However, to protect it further, four potential solutions were originally considered for reinforcing the shoreline. Finally one solution remained: TenCate Geotube® systems as a breakwater. So here again a row of filled TenCate Geotube® containers was placed in front of the coast, parallel to the shoreline. This course of action has been taken not only to protect the coast but also to stimulate sand accretion on the beach. 

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The containers eventually are invisible from the beach but visible via Google Earth
 

Over a period of just a few years, the installation has led to the accretion of sand particles to such an extent that the beach has already become four to five metres wider. Already solutions based on geosynthetics are frequently applied abroad; in this respect, the Netherlands is lagging behind. The most important reasons are inexperience and uncertainty regarding the results.

‘Morphologists charting the Dutch coastal region prefer a soft solution for the coast,’ says Edwin Zengerink, technical manager geosystems of TenCate Geosynthetics EMEA. ‘They want to see small sand particles migrating, spreading out and shifting along the coast. We want to retain the sand particles, a “hard” solution that you also see in the United States.’

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Tom Stephens, commercial director Latin America of TenCate Geosynthetics USA, thinks there are four reasons for the success of TenCate Geotube® in breakwater projects in the United States. ‘We find many projects along coasts or at resorts where the owner prefers to have a dune core covered with sand because rock has a less natural appearance. What’s more, it’s expensive to apply rock armour offshore or close to the coast. In many cases, the alternative based on TenCate Geotube® is less than half the cost of a traditional solution. Using this technology to replace a complete structure or even just a part means the owner benefits from substantial cost reductions.’
A third factor is that in many situations small local contractors are brought in that are trained by TenCate and need less equipment. Moreover, should the breakwater be erected in the wrong place, or should developments require it, a TenCate Geotube® construction can be easily removed or relocated.

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La Croisette, Cannes (France) 

The French Mediterranean Sea coast regularly suffers damage caused by storms with waves several metres high and wind speeds of over 100 km per hour. To limit future environmental and economic damage, the sandy beaches of La Croisette in Cannes are protected by a 545m-long submerged breakwater. This ‘reef’ consists of two rows of TenCate Geotube® containers (diameter 4 metres, length 25 to 30 metres) that have been hydraulically filled with sand. They rest on a 20m-wide erosion-protective underlayer of geotextile. This is safeguarded on both sides by geotextile tubes (diameter 50 centimetres) filled with dredged sand. The structure is 20 metres wide, and lies approximately 1 metre below the water’s surface, some 80 to 100 metres from the shore. The breakwater is composed of four sections and between each section a channel is kept open for small craft.

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