An assessment of mangrove restoration contribution for compound flood risk reduction in built estuary area based on a hydrodynamic model

This paper discusses the effectiveness of mangrove restoration in reducing compound flood risk in the Langsa Estuary. The restoration is proposed by replanting mangroves at the relocated shrimp ponds within the developed estuarine area. The novelty of this study lies in its integrative framework, which simultaneously couples hydrodynamic modelling, sediment–shear stress analysis, and blue-carbon quantification to evaluate the multi-functional performance of Nature-based Solutions (NbS) in a tropical estuarine system. Additionally, it quantifies the concomitant increase in blue-carbon stocks generated by these interventions as a major climate-mitigation co-benefit. The methods employed include one- and two-dimensional hydrodynamic modelling using HEC-RAS, hazard-parameter weighting through the Analytic Hierarchy Combination (AHC) approach, and sediment-characteristics and critical-shear-stress analyses for vulnerability assessment. The results demonstrate that the implementation of NbS successfully reduced the average compound flood risk index from 0.963 (moderate) to 0.772 (low), with the most significant reduction observed in areas subjected to intensive mangrove restoration. Furthermore, NbS interventions also increased blue-carbon stocks from 1869.66 tons CO₂-eq (existing) to 2440.33 tons CO₂-eq (post-restoration), representing a 23.38 % increase. Using Indonesia’s regulated carbon-pricing scheme (Rp 30,000 per ton CO₂-eq), this corresponds to an economic increase from Rp 123.0 billion to Rp 164.1 billion (33.4 %). These findings demonstrate that the combined strategy of shrimp-pond relocation and mangrove restoration is an effective, scientifically robust, and economically viable NbS for compound-flood mitigation and climate-change adaptation, offering strong support for sustainable coastal spatial-planning policies in tropical regions.