MAGELANG – Geological researcher of Japan, Dr. Yoshitada Mito, proposed that dredging process of cold lava in Kali Putih river is conducted not only by restoring the flow and function of the river, but also elevating the levee at each bend of the river flow. This needs to be done in order that runoff lava does not threaten the existing houses in Kadingo, Kemburan, Duwakan, and Seloiring hamlets in Jumoyo Village, Salam Sub-district, Magelang. "The excavated material placed at every bend will put a halt to the flow of lava and normalize the river," said Mito after observing lahars in Kali Putih, Saturday (12/3).
According to the researcher in rock engineering from Kyoto University, the dredging of material in Kali Putih is very important to anticipate the threat of lava flood. Besides deepening the river, he proposed that the material dredged should be stacked at every bend of the river by adding chunks of stone for strengthening levee construction to withstand the flood. From Mito’s observation, the elevation only used sand sediment from the dredged material. "I’m afraid this has caused erosion along the river walls," he added.
Head of Jumoyo village, Sungkono, said that sand dredging at Kali Putih was fully conducted by Center of Serayu Opak (BBSO). By using 7 heavy equipments, the process has been carried out continuously. "But, after two floods this week alone, Kali Putih is already loaded with material," he said.
Mito deplored a letter from Magelang regency government prohibiting sand dredging at Kali Putih by heavy equipment. According to Mito, this condition is actually very dangerous because the sand being dredged is not removed, but only replaced on the riverbank. Therefore, during floods, the sand is carried by the river flow again. "We asked for a recommendation from UGM about the best model to dredge the sand. I am worried residents in the four hamlets in Jumoyo are threatened by lava flood," he said.
Building Suspension Bridge
On that occasion, Mito also observed the condition of Tlatar Bridge on Pabelan River which has been broken by lava. This bridge connects Dukun and Sawangan Sub-districts in Magelang Regency.
To anticipate the collapse of similar bridges in the future, Mito proposed to construct a suspension bridge using steel wires. He is concerned if using a pillar, the pillar that is hit by boulders will actually cause the bridge to collapse. "Ideally, the bridge uses no pillars, but steel wires," he said.
From his observation of the condition of some of sabo dams, Mito admitted the sabo dams are very important and they have been carrying out their functions to resist, accommodate, and control the flow of cold lava. Interestingly, the result of Mito’s notes, the sabo dam is able to keep the river wall from erosion. Mito proposed in future sabo dam construction, it is very important to elevate the edges in order to withstand the lahars from overflowing.