Lahar hazard micro-zonation and risk assessment in Yogyakarta city, Indonesia

Yogyakarta urban area (500,000 inhab.) is located in Central Java on the fluvio-volcanic plain beside Merapi volcano, one of the most active of the world. Since the last eruption of Merapi in November 1994, the Code river, which goes across this city, is particularly threatened by lahars (volcanic debris flows). Until now, no accurate hazard map exists and no risk assessment has been done. Therefore, we drew a detailed hazard map (1/2,000 scale), based on morphometric surveys of the Code channel and on four scenarios of discharge. An additional risk assessment revealed that about 13,000 people live at risk along this river, and that the approximate value of likely loss is US $ 52 millions. However, the risk level varies between the urban suburbs.