YOGYAKARTA – Researchers from UGM and Kyoto University, Japan, have made another mapping of lahar flows from Mt. Merapi to Kali Opak and Gendol rivers on Friday (11/3). This is done to know how big the threat of lahars to Prambanan Temple is.
The observaton found out that lahar sediments in the form of sand, gravels and rocks are situated some 6 km away from the temple. Material sedimentation in the Jabang Bayi sabo dam in Ngaglik sub-district which is 2 km from Prambanan consists only of sand. “This is not alarming. It will be alarming if big chunks of rocks are carried away,” said UGM vulcanologist, Ir. Bambang Widjaja Hariadi.
As long as the rain precipitation is relatively similar, he predicted that 2-3 rainy seasons will be able to send lahars to the temple compound. Only with extreme precipitation, the threat will remain. He suggested to deepen the rivers. “It’s important to elevate the dam so that the lahars won’t overflow that may danger residential areas,” he said.
Meanwhile, Prof. Dr. Dwikorita said that the heaping sand in the sabo dam was the effect of lahars excavation in Kali Gendol in Bronggang area. The excavated lahars will open ways for the next water flow that carries sand with it. It was good that the large rocks stayed where they are and not washed down. “Whether or not the rocks come to the temple compound will depend on the precipitation,” she said.
Dwikorita calculated that the distance from the bend of Kali Opak to Prambanan is 153 meter. River depth currently reaches 20 kilometer. "The Kali Opak bend is still a threat because in Kali Putih (Magelang), lahar overflows reached to 500 meter," she said.
Japanese researcher, Dr. Yoshitada Mito, said that to know the threats of lahars against Prambanan Temple, we need to have quantitative data to know the topography of the river, volume and speed of lahars and rain volume. “These will help us know if the lahars will reach the temple or not and we know how high the risk is,” he said.