Dr. Langgeng Wahyu Santosa, M.Si., a researcher at Environment and Disaster Mitigation Clinic, Faculty of Geogrpahy UGM emphasized that morphological approach of pyroclastic material and lava distribution of Mount Merapi in disaster prone areas should be made the the main basis in arranging policy of spatial planning and settlement in disaster prone areas of Mount Merapi. It is to anticipate casualties and destruction due to future Merapi eruptions.
“We should consider pyroclastic morphology and lava distribution in arranging a policy of spatial planning and settlement,” Langgeng emphasized on Wednesday (26/1) in Seminar on Knowledge Partnership at Multimedia Room, UGM Main Office.
The history of Merapi eruptions during 1911 – 2011, Langgeng said, had shown distribution pattern of pyroclastic material and cold lava which are relatively regular and recurrent around cone morphology and slopes of the volcano. The flow of cold lava follows the river flow which is relatively stable.
Activities which happened in Merapi, he continued, is part of geomorphology process. The process will always happen in the future with certain time repetition, though with different intensity.
“Theoretically, the process will occur with similar pattern but in different intensity. The pyroclastic area and lava will follow the pattern that had been formed since the beginning of Merapi eruption,” he explained.
In its history, flow pattern of Merapi's lava and hot cloud is dominantly toward the West, South West and South passing Apu, Tlising and Senowo Rivers which run through Pabelan and Blongkeng Rivers. Furthermore, the patterns goes through Batang and Bedog Rivers which join Krasak, Boyong, Kuning, Gendol and Woro Rivers. “Activity of Merapi which is relatively consistent certainly can be strong basis in formulating policy of handling hot cloud and cold lava impact in disaster prone areas of Merapi,” this lecturer of Faculty of Geography explained.
Langgeng added that rivers which have upstream in the cone and flow in western and south slopes of Merapi are very potential to be hit by cold lava flood. Pabelan River, Putih River and Code River are three from many rivers which are very potential for this condition. “Seeing the fact, rivers which are potential to be exposed to cold lava flood should be determined as a river with “Awas” (watch out) status to Merapi eruptions,” the man born in Klaten, 10 September 1972 explained.
According to Langgeng, formulating actions to handle threats of Merapi cold lava flood should consider pattern and morphometry of riverflow which is potential to be exposed to Merapi secondary disaster. Langgeng explained that cold lava’s characteristic is different from waterflow. Cold lava flow is passive with relatively straight current that makes it very difficult to follow the riverflow which is winding. It is also difficult for the cold lava to flow through river with morphometry of valley which is narrow and craggy.
As it is known that most big rivers in western and southern of Merapi slopes have meander flow, not few of them such as Putih River have sharp curves with 900 angle and narrow and craggy morphometry. Morphometry aspect of meander river with craggy slope causes cold lava flood overflowing Jumoyo Village, Magelang.
To protect the possibility of cold lava overflowing, Langgeng mentioned that areas which had been exposed by hot cloud and cold lava flow should be buffered as far as 500 meters and determined as “overflowing zone”. The zone should not be directed as new settlement, but as savanna functioning as rainwater catchment that supports animal husbandry. Meanwhile, to anticipate cold lava flood threat, watershed as cold lava flood way should be buffered 350 meters up and 100 meters in the middle and bottom part which passes the city established as “secondary danger zone”. This area should be especially arranged related to land use which can be in the form of green area which is limitedly cultivated. This considers the fact that many areas in riverbanks are used as settlement,” he said.