The surge of water and debris rushing down from upstream areas of Sumatra last week once again revealed that the island’s landscape holds a long history of disasters. Torrential flows that swept through residential areas carried timber, mud, and large masses of soil that had accumulated for years on the steep slopes of the Bukit Barisan range. While the event appeared sudden, its root causes are layered within geological structures, climatic dynamics, and long-term ecological changes.
In a Pojok Bulaksumur discussion held on Thursday (Dec. 4), Dr. Hatma Suryatmojo, a lecturer and researcher at the Faculty of Forestry, Universitas Gadjah Mada, explained that Sumatra’s geomorphological structure inherently makes the region vulnerable to major overflows during heavy rainfall.
Steep slopes stretching from Aceh to Lampung channel water directly into lowland areas, while volcanic fans have increasingly become sites of human settlement. These natural pathways accelerate runoff and transport large volumes of material when rainfall intensity increases.
“With such a pattern, heavy rain will inevitably carry large amounts of material at high speed,” he said.
According to Dr. Suryatmojo, flash floods laden with logs and sediment cannot be separated from increasingly degraded ecological conditions. Land clearing in upstream areas, settlement expansion into the highlands, and changes in forest function have intensified surface runoff. When forests are lost, the soil’s capacity to retain water collapses, and peak discharge can no longer be controlled.
“Those contributing to this ecological wrongdoing must stop,” he asserted.
He emphasized that forests naturally have a strong capacity to retain rainwater. Under ideal conditions, up to one-third of rainfall can be intercepted by the canopy, and more than half can infiltrate the soil before reaching the surface. When forest cover declines, the entire volume of water moves simultaneously toward rivers, accelerating flood occurrence.
“The water balance inevitably shifts, and peak discharge rises dramatically,” Dr. Suryatmojo explained.
Former Head of the Meteorology, Climatology, and Geophysics Agency (BMKG), Professor Dwikorita Karnawati, added that climate change amplifies risks that are already naturally high.
A global temperature increase of 1.55°C has made extreme rainfall events more frequent, with projections indicating a potential rise of up to 3.5°C by the end of the century if left unchecked. With daily rainfall reaching hundreds of millimeters, Sumatra’s hydrological systems are no longer able to absorb the flow.
“If ecological mitigation is ignored, we could face destruction,” Professor Karnawati warned.
Professor Karnawati also noted that Sumatra’s geological structure renders the region highly unstable. Rocks formed from tectonic plate collisions rose from the deep seabed in a fractured state, making them prone to landslides even during minor earthquakes. These landslides can block river channels and form natural dams that may collapse at any time.
“These fractures make the region extremely vulnerable to ground movement,” she said.
In addition, increasingly frequent tropical cyclone anomalies have worsened conditions. Cyclones that previously did not penetrate tropical zones are now forming over Indonesia and moving across land, bringing intense rainfall for days. This phenomenon not only heightens the risk of flash floods but also shortens disaster return periods that once spanned decades.
“Cyclones are no longer following their usual paths, and this anomaly is becoming more frequent,” Professor Karnawati explained.
She further elaborated that this year’s cyclone anomalies did not occur in isolation but are part of a series of phenomena observed since the emergence of Cyclones Seroja and Cempaka several years ago.
These cyclones have shown unusual behavior, including crossing landmasses and persisting longer within tropical regions that once acted as barriers. Cyclone Senyar has reinforced this trend by forming in areas previously considered unlikely and traversing land to reach the Malay Peninsula.
“This anomaly indicates that climate change is increasingly influencing cyclone dynamics in the Indonesian region,” she concluded.
Author: Triya Andriyani
Post-editor: Rajendra Arya
Photographer: Donnie Trisfian