Monitoring Pilot Study of Temporary Permeable Structures for Mangrove Restoration

Abstract

Temporary permeable structures of bamboo and brushwood have been implemented for mangrove restoration along retreating coastlines worldwide. However, deriving lessons from previous studies is inhibited by their lack of morphodynamic context, with missing bathymetric data or control comparisons. In this paper, we present a low-tech, low-cost, data collection methodology to support morphodynamic system understanding and modeling of mangrove coastlines. This method was applied to monitor a mangrove restoration project featuring temporary permeable structures of bamboo and PVC, installed in late 2021 on the subsiding muddy coast of Demak, Indonesia. Seabed level changes were regularly tracked with bathymetric surveys and monitoring poles across structures and at a nearby control site. Structures were positioned landward of a chenier, at −0.7 m to −0.9 m relative to mean sea level (MSL), and 30–70 m seaward of the mangrove fringe. Measurements from August 2021 to December 2022 revealed seabed erosion (−0.33 m to −0.4 m) seaward of the structures, with mixed responses landward: two sections eroded (−0.04 m to −0.05 m), one remained stable, and a creek-adjacent section eroded by −0.43 m. At the nearby control site, chenier migration and vertical growth promoted landward accretion, though elevations remained below MSL and thus unsuitable for mangrove colonization. The bathymetric and monitoring pole measurements presented in this study constitute valuable datasets for modeling studies aiming to unravel the dominant processes driving morphodynamic changes. Such models could also inform integrated approaches to mangrove restoration in subsiding coastlines, considering sediment supply, subsidence management, and structure integrity.