GeoJournal 49: 173–183, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 173 Lahar hazard micro-zonation and risk assessment in Yogyakarta city, Indonesia Franck Lavigne Universit´ e Paris 1 Panth´ eon-Sorbonne, Institut de G´ eographie, 191 rue St.-Jacques, 75005 Paris, France and Laboratoire de G´ eographie Physique, CNRS URA 141, 1 place Aristide Briand, 92190 Meudon, France, e-mail: lavigne@univ-paris1.fr) Received 18 January 1999; accepted in revised form 6 March 1999 Key words: hazard-zone mapping, Indonesia, Java, lahar, Merapi, risk assessment, vulnerability, Yogyakarta Abstract 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. Introduction Terminology The term lahar, of Javanese origin, is a rapidly flowing mix- ture of rock debris and water (other than normal streamflow) from a volcano (Smith and Fritz, 1989). The flow behaviour exhibited by lahars may be complex, and includes a debris flow phase, where sediment concentration is in excess of 60% by volume. Additionally, there are also precursor and waning stage hyperconcentrated-streamflow phases, where sediment concentration ranges from 20 to 60% by volume (Beverage and Culbertson, 1964). According to the terminology adopted by United Nations Disaster Relief Organization (UNDRO), a natural hazard is the probability of occurrence, within a specific period of time in a given area, of a potentially damaging natural phenomenon. Hazard appraisal is obtained by the following equation: Hazard = extension × frequency× × magnitude of the events. The risk is the expected number of lives lost, persons injured, damage to property and disruption of economic ac- tivity due to a natural phenomenon. It can be defined as the product of hazard by vulnerability (Slaymaker, 1996; Blaikie et al., 1997), whereas some authors add the elements at risk (UNDRO, 1979) or georesources (Nossin and Javelosa, 1996) as a third component. Vulnerability is a complex concept, commonly appreci- ated from a quantitative approach. It is defined as the degree of loss to a given element at risk, expressed on a scale from 0 (no damage) to 1 (total loss) (UNDRO, 1979; Smith, 1992). However, a new approach, developed more recently, aims to appreciate the social and cultural factors which reduce or amplify the effects of a natural phenomenon (Drabek, 1986; Thouret and D’Ercole, 1996). Yogyakarta city Yogyakarta urban area is located on the highly populated, (>1000/km 2 ), fluvio-volcanic plain beside Merapi volcano (2961 m) in Central Java (Figure 1). The city is the political, economic, social and cultural center of the special Province of Yogyakarta. For 50 years, this city of 500,000 inhabi- tants has attracted people from the surrounding overcrowded rural areas. In order to preserve the productive tilled lands and irrigation networks around the city, the government has attempted to control the urban growth. The guidelines in the present master plan (1985–2005), limit northern exten- sion of the city between Magelang street and Gadjahwong river, and western and southern extension is allowed only along five main roads, leading to Wates, Godean, Bantul, Parangtritis and Imogiri (Figure 2). However for the last 20 years, thousands of migrants have settled within areas prone to floods and lahars along the Code river. Thus, vulnerability has increased greatly in Yogyakarta. Lahars at Mt Merapi Lahar generation is complex, resulting from a combination of volcanic and climatic processes. At Mt Merapi, lahar is triggered by two main processes (Lavigne et al., 1998b): (1) eruption-induced lahars or primary lahars from the ad- mixing of pyroclastic flows, or less frequently, from debris avalanches, with running water; (2) rain-triggered lahars or secondary lahars from heavy rainfall upon recently erupted volcaniclastics, usually during the rainy season (from No- vember to April). Rain-triggered lahars can be occasionnaly