Zacharias Wuragil, Jakarta – The landslide disaster in Cisarua and Lembang, West Bandung, Indonesia, early Saturday morning was no ordinary landslide. It was triggered by the collapse of a natural dam upstream, and there is still the potential for a recurrence.
Imam Achmad Sadisun, a landslide geology expert from the Bandung Institute of Technology (ITB), stated this in Bandung on Sunday, January 25, 2026. According to him, the landslides that struck Pasirkuning and Pasirkuda hamlets in Pasirlangu Village, Cisarua District, and Sukadami hamlet in Sukajaya Village, Lembang District, were caused by a mudflow mechanism, which is far more destructive than local landslides.
"The residents' houses did not actually slide off the slopes where they stood, but were affected by landslide material sent from upstream through the river channel," Imam said, as reported by Antara. This type of landslide, which turned into a mudflow, aligns with a previous explanation by Adrin Tohari, Head of the Geological Disaster Research Center at the National Research and Innovation Agency (BRIN).
Imam further emphasized that the primary cause of the incident was the formation of a blockage or landslide dam on the upstream southern slope of Mount Burangrang. There, landslide material blocked the river channel, holding back the water until it was saturated, then suddenly burst, carrying a load of mud, sand, and boulders.
Imam stated that the interaction of natural factors, such as old volcanic eruptions with thick layers of weathering upstream, was more likely the cause of the disaster than simply land conversion. When prolonged rainfall saturates the soil pores, the slope's strength decreases drastically, causing the material to slide downstream, blocking the river flow.
"The boundary between the weathered soil and the relatively more impermeable bedrock often becomes the slip surface," Imam said, as quoted on the ITB website.
He warned that the threat was not over, as the team of experts still found indications of similar blockages in the upstream part of the river. If heavy rainfall occurs again, the accumulated water behind the blockage is at risk of bursting again, sending mud downstream.
Mitigation methods
Imam revealed that one of the most sought-after, yet rarely known, pieces of information is visual signs before a disaster occurs. Therefore, Imam asked residents along riverbanks to monitor water levels during rainfall.
If a normally flowing river suddenly recedes or disappears during heavy rain, people should be alert. This indicates the presence of damming upstream. "Immediately move away from the river channel because water can suddenly surge in the form of mudflows," he said.
Having observed similar disasters in various regions, Imam emphasized that living along riverbanks carries a high risk of being affected by debris flows. This means, he believes, that future mitigation efforts should not only focus on reforestation but also include structural measures such as building debris flow barriers. They should also include flow tracking systems using geophones or vibration sensors.
"The most damaging thing is not the water, but the sediment carried by the flow," he said, adding, "Therefore, mitigation systems need to focus on sediment control."