Coastal protection in the Mekong Delta: Investigation of shore nourishment and mangroves as Building with Nature solutions

To help to provide a desperately needed coastal protection strategy for the Mekong Delta in Vietnam this research looks into the possibilities for natural and sustainable coastal defence systems, including mangroves and active sediment management, such as a nourishment. The coastline of the Mekong Delta suffers from severe erosion and mangrove degradation. Due to relative sea level rise and climate change it is crucial to design adaptable and proactive coastal protection measures. From a literature study into ongoing erosion processes, local coastal protection problems, and mangrove habitat requirements, boundary conditions were determined. As a basic concept of a Building with Nature alternative for the Decision Support Tool, a combination of a mangrove fringe with its accompanying wave attenuating foreshore and an earth dyke were considered. Two requirements with respect to a maximum wave height in front of the mangrove fringe and earth dyke, were translated into a conceptual profile of the foreshore: a convex-up bathymetric profile. This schematised profile was adjusted for two case study areas. The numerical model SWASH was used to translate the offshore boundary conditions into near shore conditions. Annual and heavy storm conditions were modelled for both case study areas, in order to test the performance of the convex-up profile. The model results led to designs of the foreshores of the case study areas. Possible dredging locations, equipment, transport of the dredged material, and different nourishment techniques were considered. The proposed Building with Nature designs and design alternatives were compared with current coastal protection measures by a cost-effectiveness and cost-benefit analysis. It was found that a convex-up profile improves the conditions for mangroves to develop, as this significantly decreases the hydrodynamic forces in front of the mangrove fringe and creates an additional zone above Mean Sea Level suitable for mangrove colonisation. The minimal required width for mangroves to serve as a self-sustaining coastal protection measurement is dependent on multiple factors including fringe density and local hydrology. The minimal required mangrove width was established to be 700 m for the case study site of Tam Giang Dong at the east coast and 350 m for An Minh at the west coast, where the hydraulic conditions are milder. The point at which full restoration of the convex-up profile can compete economically with a breakwater is dependent on the foreshore profile and erosion rate. For the east coast, in order to maintain the present situation, an erosion rate of 24 m/year or less was determined as the tipping point. For full restoration of the convex-up profile to be economically favourable the upper limit of the erosion rate is 20 m/year. For the west coast these points are 17 m/year and 10 m/year respectively. For the case study site of Tam Giang Dong, as well as for the case study site at the east coast, due to high erosion rates and high cliffs in front of the mangrove fringe, the proposed design is the option 'mud or sandy-mud nourishment in combination with a permeable breakwater'. In other parts of the Mekong Delta area with lower erosion rates Building with Nature alternatives without permeable breakwaters are viable as well.Civil Engineering and GeosciencesHydraulic Engineerin