Sediment-Driven Expansion of Tropical Mangroves in the Bengawan Solo Delta Revealed by Multi-Decadal Google Earth Engine Analysis

Despite the global crisis of mangrove deforestation, certain sediment-dominated estuaries exhibit remarkable morphodynamic resilience. This study investigates the spatiotemporal trajectory of the Ujung Pangkah estuary (1995–2025) to quantify natural progradation against anthropogenic pressures. Utilizing Google Earth Engine (GEE), Landsat archives, and a Random Forest classifier enhanced with advanced spectral indices (NDVI, mNDWI, EVI, MVI), we rigorously mapped the estuarine landscape. Accuracy assessment using independent historical validation yielded an exceptional overall accuracy of 97.30% for 2025. The change dynamics analysis revealed an explosive natural recovery. While the ecosystem suffered a gross historical loss of 574.02 ha primarily due to aquaculture conversion, this was vastly offset by a massive seaward gain of 1,245.15 ha on accreted mudflats. The total mangrove extent expanded from 613.88 ha in 1995 to 1,285.01 ha in 2025. Non-parametric statistical evaluation confirmed a highly significant, continuous median expansion rate of 27.65 ha/year. Crucially, hydrodynamic driver analysis using a red-band proxy for Total Suspended Solids (TSS) empirically validated that this progradation is intrinsically sediment-driven, fueled by the hyper-concentrated monsoonal discharge of the Bengawan Solo River. Safeguarding these dynamic frontiers requires urgent policy frameworks to legally protect newly accreted estuarine zones from future land-use conversion.