Flash floods and landslides have struck several areas across Sumatra again in recent weeks. Solok Regency, West Sumatra, was among the affected areas after a flash flood, locally known as galodo, struck the Kayu Aro-Sukarami (Arosuka) area on Monday (Sept. 21). Several days later, a similar disaster struck Central Tapanuli Regency, North Sumatra, when the overflowing Sibuluan River cut off road access and inundated several areas. This occurred while several affected areas were still recovering from the major disasters in November 2025.
UGM forest hydrology and watershed management expert Dr. Ir. Hatma Suryatmojo, S.Hut., M.Si., IPU, ASEAN Eng., fondly known as Dr. Mayong, explained that recurring flash floods in Sumatra are inseparable from incomplete post-disaster ecological processes. Mayong said that in 2025, landslide material was left in upstream areas that remains trapped and could be carried downstream again when heavy rainfall occurs.
“The flash floods that occurred in November 2025 were actually part of a natural process that had not yet been completed,” he said on Friday (Oct. 2).
According to Mayong, several areas in Sumatra have landscapes characterized by steep slopes and narrow river valleys. He explained that these conditions can cause landslide material from hilly areas to block river channels directly.
“The trapped landslide material then forms a natural dam, and when rain falls again and the volume of water increases, the natural dam can break, resulting in a flash flood,” he explained.
In addition to landslide material, another factor that increases disaster impacts is the declining capacity of watershed ecosystems to control or retain rainfall runoff. Forest areas play an important role in the hydrological cycle, from capturing rainwater through the tree canopy to helping store water in the soil.
“When the watershed was still in good condition, and the vegetation was healthy, when it rained, the water would not immediately flow downstream (into the river), but could still be retained by the canopy and stored in the soil,” Mayong explained.
However, following the major disaster in November 2025, the watershed’s ability to regulate water has declined significantly.
Furthermore, Mayong likened forests to natural machines that regulate water flow. In his forest hydrology research, Mayong found that around 25–35 percent of rainfall in forested areas is retained by the tree canopy and therefore does not immediately reach the ground surface. When forest cover decreases, the water retained by the canopy, around 25–35 percent, immediately becomes surface runoff and increases river discharge.
“If a river suddenly has a high discharge and overflows, that means there is something upstream that needs to be examined. It means there is a machine that is not working upstream. That machine is the forest,” he said.
To address this challenge, Mayong recommended that the government ensure disaster-prone areas are not converted back into residential areas. Post-disaster recovery should involve more than simply clearing flood debris or rebuilding damaged infrastructure.
“Areas affected by flash floods should no longer be inhabited and should be kept clear. If there used to be settlements there, people should not return and build houses there again,” he said.
Therefore, Mayong urged the government to evaluate spatial planning with disaster vulnerability in mind. In his view, high-risk areas should be restored as protected areas so their ecological functions can be restored.
“Under the current conditions, spatial planning needs to be reset, not merely revised. Considerations such as disaster vulnerability must be prioritized,” he concluded.
Finally, he reminded the public that people also need to change how they view current changes in weather patterns. The increasing frequency of extreme weather events can no longer simply be regarded as anomalies.
“We must begin to realize and accept that this is the new normal for our climate and weather,” he said.
He explained that the increasing frequency of extreme rainfall is one impact of climate change that must be anticipated through a paradigm shift in environmental management and disaster mitigation. Accordingly, weather conditions previously considered unusual need to become the new basis for shaping national policy.
“If we regard it as an anomaly, we will find ways to excuse it. But if we call it the new normal, then we have no choice but to adapt,” he concluded.
Author: Agito Sitepu
Editor: Gusti Grehenson
Post-editor: Jasmine Ferdian
Photo: Antara