JAKARTA - The Center for Analysis and Applying Geospatial Information (CENAGO) of the Bandung Institute of Technology (ITB) has completed a forensic study based on extreme precipitation analysis, hydrological characteristics of three river basins (Badiri, Garoga, and Batang Toru), land cover changes, and hydrological-hydraulic simulations.
The results of the study showed that the incident was triggered by very extreme rainfall intensity due to the Senyar Tropical Cyclone, so that the existing flood control system capacity was not designed to withstand the scale of the event.
CENAGO stated that this finding shows the importance of data-based deepening and comprehensive analysis in drawing conclusions about the causes and responsibilities of a disaster.
The results of the research show that the contribution of land cover changes by a number of corporations such as PT NSHE, PT AGR, and PT TBS analyzed is relatively small compared to the scale of the extreme weather factors that occur.
CENAGO Research Team Coordinator, Heri Andreas, said the analysis of land cover changes showed the proportion of land function change of the three corporations to the DAS area was relatively small. PT AR around 1.6 percent, PT TBS 0.4 percent, and PT NSHE 0.02 percent.
"If quantitatively the contribution of land cover change is relatively small, then the determination of absolute liability (strict liability) against certain parties needs to be reviewed for objectivity," said Heri, Friday, February 20.
Not only relying on the identification and digitization of satellite images to classify land cover, CENAGO also combines BMKG and NOAA precipitation data, DAS and sub-DAS definitions, digital elevation models (DEM), and standard data of hydrological hydrological parameters.
Heri revealed that the results of CENAGO's analysis of high-resolution satellite images showed that the flood was triggered by the Senyar Tropical Cyclone phenomenon with very rare precipitation anomalies.
Rainfall in late November 2025 reached extreme levels (150-300 millimeters per day) to very extreme (more than 300 millimeters per day).
"Our probability model shows that this falls into the R700 to R1000 category, which means a cycle of events once every 700 to 1,000 years. This figure exceeds the standard flood mitigation which is generally designed to R50," said Heri.
With the intensity of rain exceeding the national mitigation standard, CENAGO assessed that the disaster was at a level that was beyond the existing flood control capacity in terms of planning.
This discussion emerged in a Focus Group Discussion entitled "Understanding the Root Cause of the 2025 Sumatra Floods for Reconciling Science-Based Conclusions" which was held on February 18, 2026 in Jakarta. The FGD was attended by representatives of ministries and agencies as well as professional organizations.
During the forum, BMKG representatives explained the phenomenon of the Senyar Tropical Cyclone in late November 2025 which triggered extreme rain in several areas of West Sumatra, North Sumatra, and Aceh was a very rare occurrence.
The combination of extreme rain and landslides triggered flash floods, including in Garoga Village, South Tapanuli.
ITB academician from the Geology Expert Group (KK), Dr. Ahmad Imam Sadisun, added that the landslide area affected by massive extreme rain that occurred in the Toba Tuff zone with very steep slopes was found upstream of the Garoga Watershed and protected forests.
Geomorphologically, the PT AR mine area, which is far from the Garoga Village and different sub-DAS, is confirmed not to contribute to flooding around the Garoga Village. In principle, water is unlikely to flow from lower elevations to higher areas. Meanwhile, PT TBS is outside the Garoga DAS.
Hydrological-Hydraulic Simulation
CENAGO also carried out a flood simulation of hydrological and hydraulic models with various scenarios, including scenarios for the entire watershed area in the form of forests and scenarios for the existence of mines and others.
The simulation results show that PT AR only contributed 0.32 percent to flooding or an increase in runoff (rainfall that does not seep into the ground) of 0.71 percent. Meanwhile, PT NSHE was recorded at only 0.05 percent and 0.01 percent, and PT TBS contributed to flooding 1.7 percent or an increase in runoff of around 0.06 percent.
CENAGO emphasized the importance of utilizing high-resolution geospatial data in disaster decision-making. A science-based approach is considered crucial to ensure that any conclusions regarding the causes of disasters are objectively, quantitatively, and proportionally formulated, including in conducting flood disaster forensics.
"There is still a very large amount of homework, namely the use of data and information, such as geoscience data, for reviewing and making decisions on various problems," he said.
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