Morphodynamic optimisation study of the design of semi-permeable dams for rehabilitation of a mangrove-mud coast

Abstract

Due to the removal of mangrove forests, the coastal zone of Demak district has suffered from severe erosion. One of the proposed solutions to restore the sediment balance and encourage mangrove re-establishment, is the construction of semi-permeable dams. This study aims to optimise the design of such structures. A 2DH site-specific model was set up in Delft3D Flexible Mesh to simulate the large-scale morphodynamics of the area. As Demak coastal zone is a data poor environment, the model validation was largely based on qualitative observations and expert judgement. In order to capture sediment behind the dams, the system mainly depends on locally eroded sediment. Waves dominate the availability of sediment in the area. After waves have stirred up the sediment on the foreshore, the tidal current transports the suspended sediment towards shore. The model results show that the dams successfully attenuate waves and capture sediment. Important design aspects were studied, aiming to formulate more generic guidelines for the design of such structures. New dams were found to have a negative influence on the sedimentation rate behind surrounding dams that are located backward or sidewards. For comparable mangrove-mud coasts, single coast-parallel dams are expected to capture more sediment than closed off cells. It is recommended to use the model to test new dam configurations and to study other design aspects. In order to fully validate the model, more data should be gathered.