Construction of Coastline Breakwater Fences
GIZ 2014
Planning & Construction of Coastline Breakwater Fences
To stop ongoing erosion along the coastline breakwater fences were installed. This building block addresses the coastline protection in the Mekong Delta area of South Vietnam. In particular the T-shaped bamboo fences are designed and placed at severe erosion sites. Through the transformation of waves and the loss of their force the fences cause the settlement of transported sand and mud particles. The result is enhanced sedimentation alongside the coastline. On these newly created mudflats, mangroves can naturally grow within months. On an overall length of 12 km breakwater fences were set up by the programme. In the provinces of Bac Lieu and Soc Trang 99% of the coasts are now no longer directly threatened by water at the dyke. The breakwater fences have not only stopped the loss of land to the sea of up to 50m per year, but have completely reversed this trend by winning back land. This success has been achieved due to a vertical sedimentation of up to 120 cm. This higher floodplain has created a natural protection of the dyke toe and for recruiting mangroves. Additionally the natural biodiversity has been recovered by 70% after 4 years. In one province the diversity of species was comparable to a natural forest after 18 months.
Successful design and construction depends on results from: • Numerical modelling • Documented observations from coast line changes. • Records (e.g. rapid shoreline video-assessment, comparison of satellite images) of loss of floodplains through erosion. • Sound execution of T-fence construction and construction supervision.
Involvement of technical departments, local authorities and people living in the planning, design and construction process was important to ensure acceptance, support, long-term commitment and ownership. The use of natural and locally available construction materials keeps construction costs low, compared to the traditional response of building concrete dykes and seawalls. Concrete structures often fail because the subsoil of mangrove-mud coasts and foreshores is too soft and prone to consolidation and base failure. Construction of the actual breakwater fences must follow quality standards and must be well supervised. The most effective way to restore eroded foreshores is to use permeable t-shaped breakwater fences which create calm water conditions for sediment deposition and reduce erosion. This also provides an immediate solution of an acute threat in areas where the foreshore erosion has progressed all the way to the dyke.