Secondary Eruptions and Rain Formed Rainfall Lava Floods in Besuk Kobokan

LUMAJANG, KOMPAS - Rain-induced lahar floods swept along Besuk Kobokan in Candipuro District, Lumajang Regency, East Java, on Friday (21/11/2025). The lahar floods occurred due to a secondary eruption accompanied by heavy rain on the slopes of Semeru from the afternoon until the evening.

Rainwater reacts with deposits of hot pyroclastic material that are still at high temperatures, resulting in the formation of thick, black-gray pressurized smoke. The sound of rumbling and heavy ash rain accompanies the movement of smoke and lahar rain in the vicinity of the location.

As a result, the body of the Gladak Perak bridge spanning over Besuk Kobokan became obscured, and at times was not visible due to the very short visibility distance. This caused traffic from Malang to Lumajang and vice versa to be temporarily disrupted. Vehicles were waiting on both the left and right sides of the bridge until visibility returned to normal.

"Vehicle access was temporarily closed in both directions. Since 1:00 PM, the conditions have become increasingly dark. It is dangerous to force through, as visibility may be impaired," said First Inspector Sultoni, a member of the Traffic Unit of the Lumajang Police who was on duty at the location.

Officials also urged residents who were still busy repairing or collecting remnants of the damage in Sumbersari and Gumukmas hamlets in Supiturang Village, Pronojiwo District, to immediately leave the area for a safer location.

The Head of the Emergency and Logistics Division of the East Java Regional Disaster Management Agency (BPBD), Satrio Nurseno, confirmed the occurrence of the rain lahar. The vibrations on the monitoring equipment reached a maximum acceleration of 43 millimeters.

Meanwhile, residents affected by the eruption hope that the massive accumulation of material in Besuk Kobokan, near their settlement, will be promptly cleared. The residents are concerned that this accumulation will obstruct water flow during rain-induced lahar floods.

One of the residents of Gumukmas, Usman (33), stated that he and his family do not dare to stay in their home even though it has not suffered any damage. Usman's house is only buried in sand up to the floor level. He has chosen to evacuate to his mother's house in the Piket Nol area, Sumberwuluh Village, Candipuro District.

He and his family chose to evacuate while the pile of materials had not been removed. "If it is not removed there (Besuk Kobokan), then the water will come here (to the house) if there is a flood," he said on Friday (21/11/2025).

Following the major eruption in 2021, according to him, the lahar flood from Besuk Kobokan did not reach the residential areas in Gumukmas because the height of the embankment above the riverbed is still significant. Currently, the riverbed is filled with a pile of materials.

Moreover, the hot ash cloud that struck part of Sumbersari Hamlet on Wednesday (19/11) not only brought a large amount of sand to the area but also large rocks to the residents' yards.

A similar sentiment was expressed by Titin (30), another resident of Gumukmas. Titin, who is taking refuge at SDN 04 Supiturang in Pronojiwo District, stated that she, along with her two children and husband, chose to evacuate due to concerns. Their home is located near Besuk Kobokan.

Titin's house has just been renovated and is in good condition. "However, its location is at the end, on the edge of Besuk Kobokan. In situations like this, danger can threaten at any moment, especially from rain-induced lahar floods," she stated.

Until the second day after the eruption, the process of cleaning up the material is still ongoing, both in the homes of affected residents and on access roads using heavy machinery.

The busy repair of the broken electricity network is currently visible in several corners of Sumbersari and Gumukmas hamlets. Previously, East Java Governor Khofifah Indar Parawansa, during her visit to the affected locations, stated that there are around 28 electricity poles impacted, including transformers. Currently, representatives from PLN are carrying out the repairs.

Khofifah also mentioned that her side has heard the aspirations of the residents regarding the request for the diversion of materials at Besuk Kobokan in question. They have coordinated with several relevant parties regarding this matter. Thus, in the future, the overflow of lahar flood materials will not pose a risk to the surrounding settlements.

"The community wants an alternative route to be opened; we have also coordinated with the Public Works and Highways team, the Road Agency, and the National Disaster Management Agency. We are integrating what can be done to protect the community," she said. According to Khofifah, anticipation and mitigation are carried out alongside public vigilance.

Meanwhile, Semeru's current status remains at Alert (level 4). According to magma.esdm.go.id, on November 21, 45 eruption earthquakes, 6 avalanche earthquakes, 8 eruption earthquakes, and 5 distant tectonic earthquakes were observed.

Visually, the body of Semeru is clearly visible, albeit shrouded in mist. No smoke from the crater was observed. The weather was overcast with rain, and the valley winds were blowing towards the southeast. Eruptive tremors were recorded, but the height of the eruption and the color of the ash were not observed. Additionally, regarding the lava flow, its distance and direction were not observed.

The Center for Volcanology and Geological Hazard Mitigation still recommends that residents refrain from activities in the southeast sector, along Besuk Kobokan, up to 20 kilometers (km) from the summit.

Outside of that distance, the community is not allowed to engage in activities within 500 meters from the riverbank along Besuk Kobokan due to the potential risk of being affected by the expansion of hot clouds and lahar flows. Residents are also advised not to engage in activities within a radius of 8 km from the Semeru crater due to the risk of hazardous rock ejections (pyroclastic).

Be vigilant of the potential for hot clouds, lava flows, and lahars along the river or valley streams originating from the summit of Semeru, particularly along Besuk Kobokan, Besuk Bang, Besuk Kembar, and Besuk Sat, as well as the potential for lahars in the small rivers that are tributaries of Besuk Kobokan.