Floods caused by rainwater mixing with volcanic rocks and sands have forced hundreds from their homes, cut off roads and endangered six villages in Magelang District, 26km from the peak of the still-active volcano, Mt Merapi.
Arief Setyohadi, of the Magelang District resources staff of the International Federation of Red Cross, estimated that more than 3,000 people were living in flooded areas but that no formal assessment had been conducted.
The floods caused the Blongkeng River to overflow and damage houses in the Ngempak village, as well as one primary healthcare centre and elementary schools in Ngluwar sub-district.
Cold lava and mud from the slopes of Mt Merapi began inundating houses and farms in Magelang District on 3 January.
The flooding resumed with more devastating effects on 9 January, when the Red Cross evacuated dozens of people trapped in their homes in Sirihan village.
Some people sought refuge in government offices and schools, Setyohadi said. "The local government had a contingency plan by creating an evacuation route and declaring an area 300m from the [Blongkeng] river banks as dangerous, but the flooding was worse than expected."
The Red Cross and local government have set up a shelter in Ngrajek village hall where they have delivered biscuits and water to 600 people.
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While the Red Cross is calling the latest disturbance a "cold lava flood", leading government volcanologist Suwono called it a "lahar flood", which is common when it rains after a volcanic eruption.
Lahar floods carry sand, volcanic ash and rocks at an accelerated rate down a mountain slope into rivers. "Lahar floods are a natural phenomenon that [is] very dangerous for inhabitants and infrastructure," Suwono said.
The flooding has resulted in one reported death and another injury to date.
Volcanic eruptions at Mt Merapi starting in late October have resulted in 386 reported deaths and 131 injuries. While more than 300,000 people have been able to return home, another 11,000 remain displaced, living with family or in camps, according to the government's National Disaster Management Agency.