Re-evaluating Urban Flood Causality: Integrating InSAR Time-Series and Multi-Sensor Satellite Observations for the 2024 Makassar Flood Agustan Agustan1,2*, Mukhsan Putra Hatta3, Ilham Alimuddin3, Takeo ITO1
1* Earthquake and Volcano Research Center, Nagoya University, JAPAN
2 National Research and Innovation Agency (BRIN) - Jakarta, INDONESIA
3 Universitas Hasanuddin, INDONESIA
Abstract
Urban flooding in coastal Southeast Asia is often attributed to land subsidence driven by excessive groundwater extraction and natural compaction. However, such assumptions require critical evaluation using robust geospatial evidence. This study investigates the causes of a major flood event that occurred between 8 and 20 December 2024 in Makassar, South Sulawesi, Indonesia. While local narratives linked the flood to accelerated land subsidence, our multi-sensor satellite analysis suggests a more complex hydrological context. We integrated time-series InSAR data processed using MintPy with Sentinel-1 radar composites and Sentinel-2 optical indices (NDWI, NDVI, moisture index, and SWIR) to evaluate the spatial dynamics of land deformation and surface moisture. The InSAR analysis shows that while some areas in southeastern Makassar exhibit long-term subsidence trends of up to -20 cm over six years, no significant acceleration or abrupt displacement was observed prior to or during the flood period. This undermines the hypothesis that sudden subsidence triggered the inundation. Conversely, Sentinel-1 pre- and post-event imagery reveals extensive backscatter anomalies in low-lying urban and peri-urban areas, indicating widespread surface water presence consistent with flood patterns. The Sentinel-2 NDWI and moisture index maps highlight pre-existing hydrologically vulnerable zones, especially near poorly connected irrigation and drainage networks. These findings point to systemic water mismanagement, rather than tectonic or anthropogenic ground failure, as the primary flood driver. Our results emphasize the importance of multi-sensor remote sensing approaches in disentangling overlapping geohazards in rapidly urbanizing deltaic cities. The study calls for a rethinking of flood attribution in policymaking and advocates for integrated spatial diagnostics in hydrological infrastructure planning.
