Natural coastal structures to restore eroding mangrove-mud coasts

Abstract

Degrading mangrove forests and coinciding erosive coastlines is an urgent global concern. Mangroves are essential for their interaction with hydrodynamic and sediment processes at mangrove-mud coasts. Earlier on, solutions were sought in building ’hard’ structures, such as concrete sea walls, to lower incoming wave energy and enhance sedimentation to restabilize erosive coastlines and bring back mangroves. As these approaches proved to be counterproductive and even accelerate destabilization of the coastline, nature-based solutions might provide a new path to a durable solution. A pilot project was initiated in Demak, Indonesia, where several permeable brushwood and bamboo dams were constructed near the receding coastline. The open structure of the dams allows sediment to flow through, while they decrease wave heights by dissipation. This creates calmer conditions behind the dams and allows mud to settle. The deposition of fine sediments enhances a rise in nearshore bedlevel and provides nursery ground for mangroves to reestablish, realizing a first step towards rehabilitation of the mangrove ecosystem and restabilizing the coastline. A conceptual 2DH process-based model, using Delft3D FM, was created to analyse the effect of permeable structures under changing boundary conditions at various shorelines. Three boundary conditions were varied to see the influence on dam performance: wave height, wave direction and suspended sediment concentration. A decrease or increase in wave height resulted in similar rise in bedlevel elevation behind the dams. Without the dams, an increase in wave height increased the tidal flat elevation and steepened the foreshore, inducing a concave profile shape. Placing the dams on the tidal flat induces extra rise in elevation, but won’t mitigate the erosion of the foreshore. Smaller wave heights tend to reshape the profile towards a more convex profile. When dams are implemented on, an increase in bedlevel behind the dams occurs. The waves stir up sediment at the tidal flat boundary which are redistributed by the tidal currents forming a more convex profile. The dams could be placed best close to the shoreline to allow for the natural redistribution of the sediment when at the same time induce a rise in bedlevel for mangroves to reestablish. Altering the nearshore sediment supply affects the dam performance significantly. The dam performance increases for lower sediment supplies. Dams reduce erosion due to decreased sediment input, but it is likely a short-term solution. For higher sediment supplies, the coastline can more effectively restore by itself and the dams reduce in efficacy. The model results show an increase in dam performance for increasing wave angles (shore-normal to shore-parallel). The bed shear stress is decreased more effectively for smaller wave angles, but also reduce the sediment concentration behind the dams. With the many distinctive boundary conditions at mangrove-mud coasts in mind, the simplified model setup is made easily adaptable to enable future use of the model. Based on the model setup for Demak and the model results in this research, an improvement on the computational grid is suggested before applying the model to other coastal sites.