Coastal protection of the Bạc Liêu province by rehabilitation of the mangrove forest

Abstract

In the southern part of Vietnam, where the lower Mekong Delta is located, coastal erosion problems occur. In the Bạc Liêu province, one of the twelve provinces in the Mekong Delta, the width of the mangrove belt between the sea dyke and the sea is reduced by 100 m in the last four years. By rehabilitation of the mangrove forest, the hinterland can be protected against flooding. Mangrove rehabilitation is the action of improving the health of the forest to a previous condition. The primary goal of this project is to design a coastal protection system, including mangrove rehabilitation, to provide the Bạc Liêu province with a sufficient safety level against flooding till at least the year 2050. For this project a Systems Engineering (SE) approach is used. Currently, the coastal area of the Bạc Liêu Province consists of a sea dyke and an integrated-shrimp system, which means that there are mangrove plants inside and in front of the shrimp ponds. The current characteristics of the mangrove ecosystem is described by a four spheres approach, the hydrosphere, lithosphere, biosphere and the atmosphere. Several laws have been adopted on the management of mangrove forest. The obligation to assure a 500 m width of mangrove belt in front of the sea dyke and the rule of 60% mangrove coverage in the shrimp ponds are the most important requirements for this project. In the ideal situation, the coast is protected by a healthy mangrove ecosystem which has a variety of flora and fauna (Stieglitz, Ridd, & Müller, 2000). Older trees have more salinity resistance and can handle wave disturbances better. Therefore, a zonation is highly recommended for the ideal situation to create good living conditions for every species. Mangrove ecosystems need sediment to sustain the elevation of the trees in the tidal range and to counteract erosion. In Bạc Liêu a mangrove belt width of at least 500 m is required for flood protection. At the moment, the ideal situation and current situation deviate. A wider mangrove forest creates better conditions for a zonation with a better distribution of different mangrove species. Sea level rise causes a backward movement of species within the mangrove forest. This backward movement is hindered by the integrated shrimp ponds and the existing sea dyke. If the mangrove forest cannot expand backwards, it is likely that mangrove trees will drown and erode at the seaside, whereby the width of the mangrove forest will decrease further. When erosion occurs, the erosion pattern will likely create a concave-up mud flat profile which leads to stronger wave attack enhancing even more erosion. The government has great power to prevent this, since they influence the land use by means of policies. After the stakeholder analysis, two dilemmas were identified. First, at locations where the width of the mangrove belt is less than 500 metre it might be more convenient to restrict the land use and make space to plant mangrove at the landside instead of taking expensive measures to improve the conditions for mangrove plants to grow at the seaside. Second, the 60% mangrove coverage rule for the shrimp farms does not guaranty 500 m of mangrove belt everywhere. The system requirement specification, which is an overview of all the requirements for this coastal protection system, is partly based on the customer requirements, and partly on a function analysis based on the Function Analysis System Technique (FAST) method. However, the specification process is an iterative process, which is developed further during the research of different alternatives and the final design. After the FAST method, different alternatives like groynes, soft measures and breakwaters are investigated. There are two scenarios’ possible to guarantee a 500 metre mangrove belt for the flood protection system. The first scenario (scenario 1) is focussed on optimised land use behind the mangrove belt. By rearranging the shrimp ponds, a significantly smaller area of mangrove forest has to be rehabilitated in the seaward side. In the second scenario (scenario 2), the shrimp ponds will not be adjusted and mangrove rehabilitation fully takes place in front of the ponds, which results in a large area that needs to be elevated and protected for mangrove planting. A multi criteria analysis (MCA) is executed to evaluate which alternative is considered to be most suitable for this project location. The top results from the MCA are permeable fences, Geohooks, floating breakwaters, permeable groynes and longshore sandbars (cheniers). The final design for scenario 1 is chosen to be permeable bamboo T-fences. Calculations on the strength of the structure are made together with a mangrove-zonation, a cost estimation, a risk analysis and an implementation plan. For the second scenario, the choice is made to combine the permeable fences with a chenier, which provides a higher safety level and a faster accretion for the larger area that has to be elevated in the second scenario. Again, the structure is tested for the wave loads and risks are analysed, an implementation plan and cost estimation are made. These analyses are done for comparison with scenario 1. Thereafter, the uncertainties, limitations of calculations and assumptions are discussed and recommendations on further research are given. The biggest limitation during this project was the limited available data, most values are based on data from other locations. Lastly, the conclusion about the best way to protect the hinterland of Bạc Liêu against flooding is given.