Imam Achmad Sadisun, from the Bandung Institute of Technology (ITB), stated that the pre-dawn disaster on Saturday, January 24, hit Pasirkuning and Pasirkuda hamlets in Cisarua Subdistrict and Sukadami hamlet in Lembang Subdistrict was caused by a mudflow mechanism rather than localized slope failures.
“The houses did not slide from the slopes where they stood, instead, they were hit by landslide material delivered from upstream through river channels,” Sadisun told reporters in Bandung, West Java, on Sunday.
He explained that the primary cause was a landslide dam on the southern slopes of Mount Burangrang, where collapsed material blocked upstream river channels.
The blockage trapped large volumes of water until saturation caused it to burst, sending mud, sand, and large rocks downstream at high speeds, greatly amplifying destructive force.
Sadisun noted that the disaster was not solely due to land-use change. He cited the interaction between old volcanic terrain with thick weathered layers and prolonged rainfall that saturated soil pores.
This rainfall sharply reduced slope shear strength, allowing material to slide and block river channels, creating sudden dam failure.
Imam warned that the threat remains, as expert teams have identified signs of similar blockages still present in upstream sections of the river system.
If heavy rainfall returns, water accumulation behind these blockages could collapse suddenly, sending fresh waves of debris toward downstream settlements.
He identified a critical, often-overlooked warning sign which are visual changes in river flow. He urged residents along riverbanks to monitor water levels closely.
“If a river that usually flows suddenly recedes or disappears during intense rain, it signals upstream damming,” he said, urging residents to move away from river channels immediately.
Imam concluded that mitigation should extend beyond reforestation to include debris-flow barriers and monitoring systems like geophones, emphasizing that sediment, not just water, causes the greatest damage.
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Translator: Ricky P, Rahmad Nasution
Editor: Arie Novarina
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