Slamet Susanto and Bambang Muryanto, The Jakarta Post, Yogyakarta | Archipelago | Wed, October 24 2012, 10:34 AM
Due to the huge amount of volcanic debris spewed in 2010 by Mt. Merapi in Yogyakarta, which reached about 77 million cubic meters, residents living on the slopes of the mountain have again been warned about the danger of lahar floods, a hot or cold mixture of water and rock fragments that can flow down the slopes of a volcano and into rivers.
“Lahar floods continue to threaten, thereby forcing us to undertake a mapping exercise,” spokesman of the National Disaster Mitigation Agency (BNPB) Sutopo P. Nugroho said in Yogyakarta on Tuesday.
“We spent Rp 9 billion [US$936,765] on examining the peak of Merapi using optical remote sensor technology called Lidar,” Sutopo said.
With the application of the technology, he said, they had been able to map areas prone to lahar floods.
“Aside from mapping [through the application of Lidar technology], we’re also continuing to construct infrastructure like dams and other supporting facilities,” he said.
“The Lidar method, with the use of high resolution, has produced more valid results than the application of manual mapping,” said Subrandrio, head of the Yogyakarta Volcano Technical Investigation and Development Agency.
Subandrio explained that out of the 140 million cubic meters of eruption material from Merapi in 2010, 77 million cubic meters remained at the peak of the mount. Approximately 30 cubic meters are located on the west side of the mount and the other 40 million cubic meters are located on the southern side, he said.
“The largest volume of the volcanic material is concentrated at the headwaters of Gendol River, reaching 27 million cubic meters,” Subandrio said.
Other areas where residents have to be cautious include Boyong River. There are only about 2.7 million cubic meters of volcanic material located there, but it is also dangerous because the river runs close to heavily-inhabited areas along the Code River where over 12,000 people live.
Lahar floods destroyed hundreds of houses and hectares of agricultural fields during the rainy season in 2010 and also plagued those who lived along the banks of Code River, which passes through downtown Yogyakarta.
The floods are formed by intense rainfall during or after an eruption, with rainwater eroding loose volcanic rock and soil on hillsides and in rivers.
However, the worst scenario would not occur soon because major flooding would only take place if there were heavy rains for a duration of at least two hours, Subandrio said.
“People must be alert before we enter December, January and February when the intensity of rain is projected to be high,” he said.
The results of the mapping have been given to the local administration to be used to inform policy that would anticipate the threat, he added.
Before entering the rainy season last year, residents living along the banks of Gendol River rushed to finish guide canals to direct the flow of lahar away from their homes.
Rainy seasons will give rise to the threat of lahar floods for the next three to four years for residents near Merapi.