YOGYAKARTA - Material deposition of lava from the Merapi eruption around Kali Gendol and Opak in Sleman potentially present the danger of lava flood that threatens Prambanan Temple. Kali Opak indeed passes near the compound of the Temple, while the lava deposits in the form of sand, gravel, and boulder have already been located about 6 kilometers from the temple area. The amount of rainfall is feared to bring much greater volume of lava flow. These are the results of UGM geological team’s findings after conducting volume mapping of lava flow along the Kali Gendol and Opak, Sunday (6/2).
Prof. Dr. Dwikorita Karnawati, M.Sc. explained that for the current condition, Prambanan Temple is still relatively safe. However, after monitoring the volume flow of lava, she was concerned if in occurrence of a larger lava flood, the area of the temple will be damaged. "It is important to be anticipated and controlled because high rainfall can erode the elevation of the river that is carried away with lava," she said.
Dwikorita warned it is not impossible that larger lava floods will occur and reach the temple area. That's why Dwikorita hoped all involved to be aware. Not just on the slope, rivers that have their upper course at the tip of Merapi are potential to threats, especially that the volcanic material currently crammed the flow of Kali Gendol and Opak. "So, dredging and levee-making must be done immediately," she added.
Dwikorita assessed that Prambanan Temple area is very potential to be damaged due to the presence of a turn of Kali Opak around the temple. In her opinion, despite the depth and width of Kali Opak around the temple that are about 20 meters and 40 meters, if the lava brings larger chunks of material, it is feared to damage the levee, overflowing into the riverbanks in the region. "At least, in 2-3 floods, it is estimated (lava material) can overflow and brim over into the temple area," she said.
Volcano expert, Ir. Bambang Widjaja Hariadi, said the current cold lava deposits in Kali Gendol begin to form around the Brongkol sub-village, Cangkringan. The volume is growing, while along the Kali Opak, the climax of lava flow has narrowed after passing through the area around the Cangkringan police precinct. "The potential of lava in greater amount in fact is from Kali Gendol," he explained. Based on Bambang's observation, Merapi eruption material around the Kali Gendol is not fully in form of cold lava deposit because some materials are still in high temperature conditions of 80-200 degrees. Consequently, when exposed to rain, evaporation process will occur. Different with the case at Opak, volcanic material mostly has been in the form of cold lava. "Nearly 30-40 percent of the Merapi eruption flowing into Kali Gendol have not completely disintegrated because the temperature is still high," he said. He added that Merapi lava flow in Kali Gendol until recently was located in the Giyan sub-village, Banjarharjo, Sleman.
In the one day monitoring, UGM geology team reviewed several areas of Kali Opak and Gendol upstream in Kinahrejo Village. Next, the team visited two broken bridges and Cangkringan police station which had severely been damaged by the cold lava flood. The team also monitored the condition of lava overflow at the confluence of the two rivers, Kali Opak and Kali Gendol at Krebet sub-village, Bimomartani, Ngemplak, Sleman. As is known, the confluence of the two rivers will flow into Kali Opak that will pass the Prambanan Temple area.