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Climate Change, Steep Slopes Main Culprits Of Sumatran floods

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The flash floods that struck Aceh, North Sumatra, and West Sumatra provinces, have once again sparked a public debate over their underlying causes. Many people highlighted the activities of forest exploitation in steeply sloping areas as the contributing factor, which can be worsened by extreme rainfalls as a result of global climate change.

Suwardi, Professor of the Department of Soil Science and Land Resources at Bogor University of Agriculture (IPB University), emphasized that the contribution of oil palm plantations to the flooding was relatively small. He believes that topography, particularly slope gradient, is more as a determining factor than the type of crop cultivated.

Steep Slopes Dominant Factor Causing Floods

Suwardi stated that it is not correct to  blame the oil palm plantations for the flooding. Oil palms are not well cultivated on steep slopes, while in reality it is the slope of the land that has a greater influence on the cause of flooding.

“It’s not correct to attribute the flooding merely on the oil palm plantations. The slope is far more dominant as the contributing factor than the commodity itself,” Suwardi said on Tuesday (09/12/2025).¹

Sumatra Land Prone to Erosion, especially if not covered by vegetation

Sumatra is known for its red soil with easily eroded layers. In Soil Taxonomy, this soil is classified as Inceptisol, Ultisol, and Oxisol, which generally have the following characteristics:

  • Shallow soil layers
  • Low capacity of water absorption
  • High susceptibility for erosion, especially during extreme rainfalls

When hit by heavy rainfalls, the soil that is unable to absorb water produces surface runoff that carries soil particles and triggers erosion. Such risk increases sharply in areas with steep slopes and minimal vegetation cover.

Conversely, if the land is covered by vegetation such as forests, oil palms, rubber, coffee, or other industrial crops, the soil condition is relatively more stable because the roots of those plants help increase water infiltration and reduce erosion.

The importance of Spatial Planning and Land management based on carrying capacity

Suwardi underlined the need for land use activities to comply with spatial plans in respective regions. Activities of land use should be based on the land’s capabilities, which include:

  • Maintaining upstream and riverbank areas as protected zones
  • Limiting land conversion in water catchment areas and watersheds
  • Optimizing spatial planning based on environmental carrying capacity
  • Strictly monitoring land use changes from upstream to downstream

Weak policies of spatial planning can raise the land vulnerability to flooding, especially in areas with steep slopes.

Response To Plan of Reforesting Palm Plantation areas

Commenting on the government’s plan to conduct a reforestation of oil palm plantation areas as a response to the recent flash floods, Suwardi said that such plan needs further thorough study.

He noted that oil palm plantations have already had vegetation cover and are managed by communities, generating an annual economic value of approximately IDR 6 million per hectare. It is more appropriate if the government’s reforestation plan targets open and unvegetated land areas, which are more susceptible to erosion.

WALHI’s View: Deforestation as Ecological Vulnerability Factors

Meanwhile, WALHI (Indonesian Forum for Environment) reported that ecological fragility in Aceh, North Sumatra, and West Sumatra provinces is caused by high level of deforestation. Ecological infrastructure, such as forests, is no longer able to withstand the impact of tropical cyclones and extreme rainfall.

Based on WALHI data, deforestation in those three provinces reached 1.4 million hectares during period of 2016 and 2025.² Many activities of natural resource exploitation took place in the Bukit Barisan Mountains, including:

  • Mining
  • Plantations
  • Energy
  • Infrastructure projects

It is estimated that more than 600 companies operate there, potentially exacerbating ecological vulnerability of those areas. According to WALHI, the economic policy which targets 8% annual growth is contributing to environmental pressures.

The public debate over the causes of the massive flooding in Aceh, North Sumatra, and West Sumatra demonstrates that the problem is multifactorial. Scientifically, the following factors:

  • Slope gradient
  • Soil type and condition
  • Vegetation cover
  • Climate change and extreme rainfall intensity
  • and compliance with spatial planning play a more significant role as the primary triggers. Oil palm plantations are not directly the cause, as long as they are not located on steep slopes and are managed according to land conservation principles. (*)

¹ Wawancara Guru Besar IPB University, Suwardi, kepada Kompas.com (9/12/2025).
² Data deforestasi WALHI 2016–2025 untuk Aceh, Sumut, dan Sumbar. Walhi.or.id

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