Where did the mountain go?

In 2004, half of Bawakaraeng, an extinct volcano in South Sulawesi, fell off. So what does it look like after 200 million cubic metres of rock decide they don't want to be part of a mountain any more? On holiday in Malino, I went to find out.

Landsat image of Bawakaraeng, taken in 2006. The eroded material is still clearly visible as the brown area.






An hour's journey by motorbike, through the mountains and rice fields brought me to the sleepy mountain village of Lengkese, and another couple of hours trek up the valley to a point where I could see the site of the collapse. Most of the rock detached from the Caldera initially came to rest a few kilometres down in the valley, and is now being eroded by the river Jenebareng into a steep sided gorge of unstable rock and earth.


Site of the original collapse. The area is still unstable, with freshly exposed rock visible where recent collapses have occurred.









The river cuts a gorge through the debris, with the steep walls collapsing into the channel.











Up there I met a team of Indonesian surveyors working for a Japanese engineering company. I introduced myself as an engineer from England, which got me an invitation to lunch at one of the team's homes (he was from Lengkese), and a chat about the sedimentation problems caused by the collapse. Bili-bili reservoir, which supplies water to the city of Makassar, is now filling up with sediment from the collapse as it is eroded from the valley. They were surveying the latest collapse from the wall of the valley, which happened the week before.
Survey team at work.














A series of sabo dams, concrete structures design to hold back sediment from reaching the reservoir, have been built with Japanese funding, with more under construction. The surprisingly good road up to Lengkese was also funded by Japan.


Sabo construction on the Jenebareng river.
















Clearly the Japanese engineers know a lot about how to deal with sedimentation, and the Indonesian surveyors were knowledgable, enthusiastic and hard working. "The sediment traps are working now.", one said. "But in two years time they'll be broken. That's Indonesia."