After the Landslide, Flash Floods are Now Lurking in Banjarnegara

Over the past month, hydrometeorological disasters have successively struck various regions in Indonesia. Extreme weather factors such as heavy rainfall accompanied by lightning and strong winds, along with a lack of preparedness efforts in facing disasters, have led to building damage and loss of life.

One of the hydrometeorological disasters, namely a landslide that resulted in fatalities, occurred in the Majenang District, Cilacap Regency, Central Java, on Thursday (13/11/2025). During more than a week of search and rescue efforts, the joint team reported that the number of fatalities from this landslide reached 20 people, with 3 others still missing.

Three days after the incident, a landslide occurred again in the southern region of Java, specifically in Pandanarum Village, Banjarnegara Regency, on Sunday (16/11/2025). According to the latest data from the National Disaster Management Agency (BNPB) as of Saturday (23/11/2025), a total of 12 people have been declared dead and 16 others are still missing.

The landslide in Banjarnegara has garnered attention as it could trigger a domino effect of other disasters. The Geological Disaster Emergency Response Unit from Gadjah Mada University (DERU UGM) has even highlighted the threat of flash floods due to the formation of a soil dam following the landslide incident in Pandanarum Village.

Warnings regarding the threat of flash floods were issued after the Deru UGM team, along with the Kodim team and the Regional Disaster Management Agency (BPBD), discovered several new fissures that continue to shift. Additionally, a large spring with a flow rate of up to 0.2 meters per second was found at the top of the hill in the landslide area in Pandanarum Village.

These two findings indicate that the mid-slope body is experiencing extreme water saturation, which can trigger sudden mass movement.

UGM geology expert Dwikorita Karnawati stated that the water flow from the new spring source has the potential to accumulate behind the material of the old landslide, forming a natural pool that could burst at any time. The threat of subsequent landslides still looms over the residents of Pandanarum Village, and the risk of flash floods has now increased significantly.

New cracks that have emerged accelerate the infiltration of rainwater into the soil mass on the slope. When pore water pressure increases, previously stable materials can be pushed down and block the small river channels beneath them.

This condition represents a classic scenario for the formation of an earthen dam that is prone to failure. If the walls of the dam are unable to withstand the increasing volume of water, a sudden release could result in a high-speed flash flood heading towards the settlements in the lower zone.

"If the earthen dam breaks, the flow will not only carry mud but also all the debris from the previous landslide," said Dwikorita in his statement, Sunday (23/11/2025).

Dwikorita estimates that the impact of subsequent disasters could be much broader and stronger compared to the initial landslide if such conditions occur. This is because the situation is also triggered by the pressure of water from a pool the size of a football field with a depth of approximately 1.5 meters.

This danger is further exacerbated by the presence of a blue clay layer that slopes outward, found beneath the fracture zone. This layer contains clay minerals such as montmorillonite, smectite, and illite, which are highly sensitive to swelling when exposed to water.

When dry, the layer can become as hard as stone. However, when saturated with water, it can transform into a material resembling very slippery toothpaste.

"This blue clay, in geological terminology, is referred to as shale, making the soil piles above it prone to movement and accelerating the occurrence of repeated landslides. Once saturated with water, the strength of the clay diminishes drastically," said Dwikorita.

The presence of the blue clay layer is the reason why the land movement in Pandanarum does not stop and continues to increase even after a major landslide has occurred. The combination of new cracks, large discharge springs, and the blue clay layer also makes the slope conditions very unstable.

In light of this situation, the UGM Geology Team assesses that the main priority at this time is to prevent the increase in water volume behind the landslide material. Emergency measures such as creating a diversion in the landslide deposit pile as temporary drainage are crucial to reduce water pressure and prevent the risk of soil dam failure.

Dwikorita also urged the public not to engage in activities at the base of cliffs, along the banks of small rivers, in narrow valleys, or in water flow paths that have the potential to become flash flood routes. With the high rainfall in recent days and the increased flow rate of springs on the slopes of landslides, the pore water pressure within the slopes is expected to continue to rise.

"Safety must be prioritized. Avoid areas at the foot of the slope and immediately report any new cracks or unusual water flows. The dangers in Pandanarum are not only landslides but also the possibility of flash floods that can occur suddenly," he stated.

The Head of the Center for Volcanology and Geological Disaster Mitigation (PVMBG), Priatin Hadi Wijaya, stated that the landslide in Banjarnegara was exacerbated by the presence of a napal-lepmuing layer from the halang formation. Historically, this condition has been recognized as a sliding plane for several major landslides in the region.

"Therefore, the characteristics of the local geology play a significant role, so even though the locations are different, the recurring landslide patterns continue to occur year after year. We have emphasized the importance of very intensive mitigation in Banjarnegara and Cilacap because areas with a high category of land movement are widespread in these two districts," he said.

Reflecting on the landslide incidents in Banjarnegara and Cilacap, Hadi emphasized the importance of implementing short-term solutions for emergencies and long-term measures as mitigation efforts. For the short term, the actions taken include immediate evacuation from the vulnerable zones due to the high potential for subsequent landslides in those two locations.

Subsequently, the search for victims must be conducted with consideration of the weather conditions, given that the ground remains very unstable. On the other hand, it is also necessary to temporarily close high-risk areas around steep slopes to prevent access from the public.