Mindert de Vries

Pilot Study on the Erosion and Rehabilitation of a Mangrove Mud Coast

Abstract This pilot study describes an analysis of the erosion processes of the Bang Khun Thien mangrove mud coast situated at the Upper Gulf of Thailand, and discusses measures to stop this erosion process and to rehabilitate the area. The rapid erosion observed is the result of the decimation in intertidal area by the dikes directly behind the coastline, constructed to protect the fish and shrimp ponds in the coastal area. The decrease in sediment yield from the Chao Phraya River as a result of the construction of the Bhumipol and Sirikit dams, and the local subsidence due to ground water withdrawal and natural settling will augment the observed coastal erosion, but at a much smaller rate. The latter effects are expected to become important on a time scale of about a hundred years only. The key to stop the erosion processes and rehabilitate the area is therefore the restoration of the intertidal area. This can be done either entirely within the current coastline, but by sacrificing part of the fish and shrimp ponds, or partly within the current coastline and partly in the coastal zone, i.e. in the coastal area recently lost. In the latter case, the coastal area has to be protected from lateral transport of sediment by permeable groynes perpendicular to the coast. It is estimated that an intertidal mangrove belt of about 300 to 500 m is required to re-initiate sedimentation processes, hence to restore a favourable habitat for mangrove forest.

Coastal and riverine ecosystems as adaptive flood defenses under a changing climate

Adaptation planning for flood risk forms a significant part of global climate change response. Engineering responses to higher water levels can be prohibitively costly. Several recent studies emphasize the potential role of ecosystems in flood protection as adaptive risk reduction measures while also contributing to carbon fixation. Here, we use a conceptual model study to illustrate the built-in adaptive capability of ecosystems to reduce a wide range of wave heights, occurring at different water levels, to a narrower range. Our model shows that wave height of waves running through a forested section is independent of initial height or of water level. Although the underlying phenomenon of non-linear wave attenuation within coastal vegetation is well studied, implications of reducing variability in wave heights for design of ecosystem and levee combinations have not yet been properly outlined. Narrowing the range of wave heights by a vegetation field generates an adaptive levee that is robust to a whole range of external conditions rather than only to a maximum wave height. This feature can substantially reduce costs for retrofitting of levees under changing future wave climates. Thereby, in wave prone areas, inclusion of ecosystems into flood defense schemes constitutes an adaptive and safe alternative to only hard engineered flood risk measures.

Nature-Based Approaches in Coastal Flood Risk Management: Physical Restrictions and Engineering Challenges

Ecosystem destruction not only incurs large costs for restoration but also increases hydraulic forces on existing flood defence infrastructure. This realisation has made the inclusion of ecosystems and their services into flood defence schemes a rapidly growing field. However, these new solutions require different design, construction and management methods. A close collaboration between engineers, ecologists and experts in public administration is essential for adequate designs. In addition, a mutual understanding of the basic principles of each other’s field of expertise is paramount. This chapter presents some simple approaches for the integration of ecosystem-based measures into coastal engineering projects, which may be of use to experts from a range of fields. Further, it stresses the importance of ecological processes which determine the persistence and health of coastal ecosystems, a point which is rarely emphasised in coastal engineering. The main aim of this chapter is to highlight the role of ecosystem properties for flood defence to stimulate the coastal engineering community in adopting an ecosystem view. In the near future the hope is that greater awareness of ecosystem processes will lead to more sustainable and climate-robust designs. For this, engineers, ecologists and social scientists involved in coastal defence projects need to develop a common language, share the same design concepts and be willing to share the responsibility for these innovative designs.