Mangrove-Sediment Connectivity in the Presence of Structures Used to Aid Restoration

Abstract

Mangrove forests are important wetland ecosystems that inhabit intertidal zones within tropical and sub-tropical latitudes. They offer a diverse array of ecosystem services, including coastal protection and carbon sequestration. Despite their significance, global mangrove cover has declined in recent decades, prompting various restoration initiatives, most of which often fail due to a lack of system understanding of the local mangrove habitat and associated physical and biological processes. A method gaining prominence involves the use of permeable dams to restore the sediment balance along eroding coastlines, thereby facilitating the restoration of mangroves in the area. These dams help dissipate the approaching waves and reduce current magnitudes, thus creating a low energy environment behind them, ideal for sediment deposition. However, a comprehensive understanding of the system-wide implications of utilisation of permeable dams remains largely unexplored.

The study is focused on examining the response of a coastal system to structural presence, based on currents, waves and sediment and propagule pathways, to optimise restoration strategies. The coastline of Demak in Indonesia is chosen as the system to be modelled due to the persistent erosion problem in the region and the ongoing use of permeable and impermeable structures to mitigate the problem. A nested model was set-up in Delft3D-4 to estimate the flow hydrodynamics resulting from river discharge, tide and wind forcing in the area. Wave propagation in the region was also modelled using Delft3D-4 with a standalone nested wave model. Output from the hydrodynamic models served as input for the Lagrangian particle tracking model, SedTRAILS, to compute the sediment and propagule pathways.

The hydrodynamic models demonstrate a decrease in both current magnitude and wave activity behind the structures, with a greater reduction observed in the case of impermeable structures. However, the radius of influence is constrained to less than a kilometre from the structures. The sediment pathways reveal reduced sediment movement behind the structures. However, the sediment imported due to the structures are sourced from within the intertidal basin which could trigger coastline retreat in the region. Additional tests uncover impermeable structures with larger length to opening size ratio, placed closer to the mangrove fringe to be able to retain sediment with comparatively higher efficiency. In case of propagules, the trapping behaviour of the structures is less apparent, with dependencies on the relative location of the structure to the sources and the direction of wind. In Demak, when structures are located relatively offshore to the propagule sources, they are able to trap the propagules, thereby creating a possibility for mangrove restoration. The study concludes that coastal structures enhance sediment and propagule retention, with varying efficacy depending on the type, location and length of the structure, suggesting their potential utility as a valuable tool for mangrove restoration.