By Sri Hidayati, Surono and Subandriyo
Center for Volcanology and Geological Hazard Mitigation (CVGHM), Indonesia
Merapi, an andesite stratovolcano 2968 m-high, is one of the most active and dangerous volcanoes in Indonesia. The latest eruption took place in October 26 – November 12, 2010 and generated pyroclastic flows that traveled down slope mostly to southeast, south and southwest. The largest one occurred on November 5, 2010 (Fig. 1), it travelled 15 km southward and inundated Gendol River. The eruption has directly affected the rural area and claimed 367 human fatalities, 277 injured, 410,388 evacuated as well as the damaged of infrastructures, houses, live stocks and agriculture areas. The volcanic ash spewed few kilometers above the summit and wind blows the ash endangered the cities and aviation. During the peak of crisis the Airport of Yogyakarta was closed.
Figure 1 - Merapi eruption on November 5, 2010
At the beginning of December 2010 the eruption of Merapi volcano subsided. Nevertheless, approximately 130 million cubic meters of volcanic materials were produced and deposited around the slope of the volcano. It caused morphologically changes at the summit area and the aperture of the crater toward the south-southeast (Fig. 2). This phenomenon implies that the threats and risks for the future eruptions have increased in the southern slopes. However, the immediate threat is the pile of eruption’s materials accumulated at the upper slopes that can turn to dangerous lahars when the rainy season comes. These lahars flow down along the rivers and could sweep inhabitants living next to the riverbanks.
Figure 2 - Photos of Merapi summit. The left picture was taken in September 2010 (before eruption) while the right one was taken in June 2011 (after eruption). The formation of the crater that opened toward south – southeast after eruption indicates that the threat of future eruptions would be dominant to the southern part (Photo courtesy of CVGHM)
The threat of lahars occurs every rainy season to the inhabitants living around Merapi volcano, mostly in the southern and western slopes .About the 35% of 130 million m3 of volcanic deposit generated during the 2010 eruption inundated mainly the Gendol River in SE slope and other rivers that disgorge on the slopes of Merapi, such as Woro (at SE), Boyong (SW), Putih (W), Pabelan (W), Senowo (W) and Krasak (SW). The most dangerous lahars occurred in March 2011. On March 19, 2011, in fact, the heavy rain has turned volcanic deposit into lahars that hit settlements along Putih River and caused the damaged of two bridges and the evacuation of 118 people. On March 21, 2011 a powerful lahar occurred and swept the settlements of Sleman’s villages, burying 21 houses close to Gendol River. This lahar ran down as far as 20 km and forced about 200 people to be evacuated.
After the rainy season of November 2010 - March 2011, there are still about 60% of the 130 million m3 on the volcano slopes and those materials potentially would turn to lahars in the next rainy seasons. The area that potentially could be hit by lahars covers a distance of around 300 meters from the riverbanks, mostly in the southern and western slopes. To anticipate the future threats of lahars, BNPB (National Agency for Disaster Management) in collaboration with Gajah Mada University and The Center for Volcanology and Geological Hazard Mitigation (CVGHM) installed some lahars sensors and Closed-circuit Television (CCTV) at some rivers (Fig. 3). By knowing the potential occurrence of the lahars, it is expected to be able to provide early warning to the various interested parties and assist the stakeholders to take action and policies to minimize the impact of lahars.
Figure 3 - Lahar monitoring using camera at Senowo (left) and Gendol (right) rivers
Government mitigation measures
The 2010 eruption caused the devastation of many residential areas and some zones on the slopes of Merapi are still covered by volcanic material, burned trees and half buried houses. As a consequence, new spatial planning was necessary to be performed. The government of Indonesia through the collaboration of five ministries and two provinces (Ministry of Public Works, Ministry of Energy and Mineral Resources, Ministry of Forestry, National Agency for Disaster Management, National Development Planning Board, Yogyakarta and Central Java Provinces) initiated to redesign residence area, especially around the high risk slopes of Merapi. As it is mandated by Law No. 24/2007 (Disaster Management) and Law No. 26/2007 (Spatial Planning) the spatial planning should be based on hazard mitigation and, as a consequence, the Merapi Hazard Map published by CVGHM (Fig. 4) is used as reference. The risk analysis carried out a year after the eruption shows that the southern slope of Merapi is the area with the highest risk (Fig. 5).
Figure 4 - Merapi Hazard Zone. The dark pink, pink and yellow indicate Hazard Zone III, II, and I, respectively.
The Hazard Zone III is mostly affected by pyroclastic flow, lava, heavy ash fall and direct blast. Hazard Zone II is affected by pyroclastic flow and ash fall whereas lahar hazard is concentrated in Hazard Zone I
Figure 5 - Risk map of Merapi volcano
The government has encouraged the villagers living in southern slope and those hit by lahars along Gendol River to be relocated into safe zones. In particular, the local government of Sleman regency discouraged 9 villages on the southern slope not to be inhabited anymore, proposing an incentive and a small plot of land elsewhere. Nonetheless, some inhabitants of Srunen village, one of the most devastated villages by pyroclastic flow in Cangkringan District, preferred to return and rebuild their old homes though they know well that the next eruption probably will threaten their village again. The strong bond with their origins had conquered the fear of the next eruption and the trauma of lethal one that they have just experienced.
Although the eruption of Merapi ended a year ago, about two thousands villagers are still living in temporary shelters (Fig. 6). The government set the temporary shelters in the villages of Gondang, Kuwang, Plosokerep, Dongkelsari and Kentingan which are located in safe zones in order to be easily transformed into permanent residential areas. The temporary shelters that are still in the dangerous area will be moved to other safer locations.
Figure 6 - Temporary shelters
In addition, the government has accelerated the process of cleaning the rivers from pile of volcanic material encouraging the local companies to transport the sand deposit out of the rivers. This action brings advantages for both the government and the inhabitants: the government gets revenue from retribution and has accelerated the cleaning process, whereas the inhabitants can make a profit from the same volcanic material that created severe damage. Moreover, the people living next to the riverbanks have taken advantages for the eruption building/reconstructing their own houses with the volcanic material that has inundated the rivers and some entire communities have worked together building public facilities such as small mosques, village roads etc with that material.