Defining eco-morphodynamic requirements for rehabilitating eroding mangrove-mud coasts

We present an eco-morphodynamic analysis on causes of erosion along degraded mangrove-mud coasts, and the causes of failure to rehabilitate these coasts. Our analyses are based on studies in Thailand, British Guyana and Suriname, and observations in Indonesia, the Philippines and southern China, where degradation is directly attributed to the erection of fish/shrimp ponds too close to the waterline. Ecological determinants of low success rates of rehabilitation efforts are well explained in literature, and are briefly summarized in this paper. Our analyses show that ongoing erosion of degraded mangrove-mud coasts are also the result of a disturbed balance in fine sediment dynamics, and the subsequent change in mudflat morphology. The initial drivers for this degradation are a decrease in on-shore sediment flux and a local increase in wave height near the fish/shrimp ponds. A positive feed-back loop is initiated by which stable, convex-up cross sectional profiles of mud flats evolve towards unstable, concave-up profiles. This loop is induced by an unfavorable feedback between tide-induced sedimentation and wave-induced erosion, deteriorating habitat conditions for mangrove recruitment. Based on the current analysis, we present ingredients for a more sustainable use of mangrove-mud coastal systems, and a more successful rehabilitation of eroding mangrove-mud coasts.