Lahars at Merapi volcano, Central Java: an overview

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Abstract

Merapi volcano, in Central Java, is one of the most active volcanoes in the world. At least 23 of the 61 reported eruptions since the mid-1500s have produced source deposits for lahars. The combined lahar deposits cover about 286 km2 on the flanks and the surrounding piedmonts of the volcano. At Merapi, lahars are commonly rain-triggered by rainfalls having an average intensity of about 40 mm in 2 h. Most occur during the rainy season from November to April, and have average velocities of 5–7 m/s at 1000 m in elevation. A wide range of facies may be generated from a single flow, which may transform downvalley from debris flow to hyperconcentrated streamflow.

Because of the high frequency and magnitude of the lahar events, lahar-related hazards are high below about 450–600 m elevation in each of the 13 rivers which drain the volcano. Hazard-zone maps for lahar were produced by Pardyanto et al. (Volcanic hazard map, Merapi volcano, Central Java (1/100,000). Geol. Surv. of Indonesia, Bandung, II, 4, 1978) and the Japanese–Indonesian Cooperation Agency (Master plan for land conservation and volcanic debris control in the area of Mt Merapi, Jakarta, 1980), but these maps are of a very small scale to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000- and 1/2000-scale) and risk assessments have been completed for a few critical river systems.

Introduction

Merapi stratovolcano, Central Java (2965 m), is located 30 km north of Yogyakarta (Fig. 1). Its >61 historical eruptions (VSI, Volcanological Survey of Indonesia, 1990) make it one of the most active and hazardous volcanoes in the world. Although it has erupted many times on a significant scale (VEI≥3) in recent years (1872, 1930, 1961), 1.1 million people are still living on its flanks (VSI, 1995). About 200,000 people live at risk in areas prone mainly to pyroclastic flows and heavy tephra fallout (respectively the forbidden zone, and the first danger zone), and 120,000 more live along the 13 rivers draining lowlands prone to lahars (Fig. 1).

The term lahar, of Javanese origin, was introduced by Scrivenor (1929) in a report of diamict flows produced by ejection of crater lake water at Kelut volcano in East Java. He translated lahar as “mudstream”. Later, Van Bemmelen (1949) expanded the definition; “a mudflow, containing debris and angular blocks of chiefly volcanic origin”, but also added, “…volcanic breccias, transported by water”. Some authors now prefer restricting usage of the term to the flow, rather than the deposit. This usage is consistent with the consensus definition agreed upon at an international conference of volcaniclastic sedimentologists; “…a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano” (Smith and Fritz, 1989). We accept this definition, and recognize that the flow behaviour exhibited by lahars may be complex. Thus lahars can involve a debris flow phase, and also precursor and warning stage hyperconcentrated-streamflow phases. Either flow type can erode or deposit along any reach of its channel. Debris flows are mixtures of solid and fluids, with sediment concentration generally in excess of 60% by volume and 80% by weight. Sediment concentration in hyperconcentrated streamflows ranges from 20 to 60% by volume and 40 to 80% per weight (Beverage and Culbertson, 1964).

At least 23 major eruptions from the mid-1500s to 1990 (VSI, 1990, Voight et al., 2000) produced source deposits for lahars (Table 1). The lahar deposits cover an area of 286 km2 (JICA, 1980). Especially during the last decade, much progress has been achieved in understanding lahars at Merapi. This paper summarizes this new knowledge, reports on the historical lahar events and related damage (Table 2) and discusses lahar generation, lahar behaviour, and lahar hazards at Merapi.

Section snippets

Extent of deposits, and channel characteristics

Because recent historical lahar deposits are hard to distinguish from older lahar deposits (and sometimes even from pyroclastic flow deposits), we have delineated lahar-prone channels by study of aerial photographs, publications and local newspaper accounts. Thirteen rivers surrounding Merapi have experienced lahars, from the Apu River on the northwest, to the Woro River on the southeast (Fig. 1). In general, the western rivers and southern rivers can be grouped separately. On the west, the

Lahar triggering mechanism

In Indonesia, the most hazardous lahars are produced by the outburst of a crater lake by a violent eruption, such as at Kelut volcano, East Java, in 1901, 1919, 1951 and 1966 (Sudradjat, 1991). The mechanism does not apply to Merapi. Likewise, “primary lahars” caused by pyroclastic flows that enter a river are scarce, because only a few of the rivers that drain the volcano slopes are permanent and contain significant water flows. Thus, lahars at Merapi are almost invariably initiated by heavy

Lahar generation

The factors that determine if a given rainfall triggers a lahar include the rainfall distribution, intensity and duration, the morphology of the upper drainage and sedimentological characteristics of the source deposits. These topics are discussed below.

Lahar dynamics

Much has been learnt of the dynamics of lahars at Merapi since the early-1980s, due to a wide range of lahar-detection devices that were installed on the slopes on the volcano (Lavigne et al., 2000, this volume). Important studies include collaborative research between the Sabo Technical Center and Japanese colleagues in the 1980s and 1990s (Suwa and Sumaryono, 1995, Jitousono et al., 1995). These studies have included novel instrumentation, including video and cinematography with wire or

Lahar-related hazards

In the following, we consider hazard as the product of the natural event and its frequency.

Risk assessment and mitigation

Risk is the expected number of lives lost, persons injured, damage to property and disruption of economic activity from a particular natural phenomenon. Therefore, risk is the product of hazard, value, and vulnerability (Dibble et al., 1985, Scott et al., 1995). At Merapi, investigation of risk assessment within lahar-hazard zones was started in 1995 by Lavigne (1998), in collaboration with VSI staff. Assessment is based on six types of “tools” (Fig. 28), including 38 enquiries investigated to

Conclusion

Lahar deposition encompasses more than 280 km2 of the slopes of Merapi volcano and the surrounding lowlands. At least 23 of the 61 reported eruptions since the mid-1500s have triggered lahars, almost all of which have been rain-triggered. Intense rainfall is required to trigger lahars, >25 mm/h. Actual triggering-rainfall intensity can vary widely, due to such factors as rainfall duration, and permeability of pyroclastic deposits. Most large lahars are generated within 4 years of the last

Acknowledgements

We are indebted to the French Ministry of Foreign Affairs, the French Embassy in Jakarta, and the French Delegation aux Risques Majeurs (DRM) for financial support during a yearlong fieldwork and some other missions of Lavigne. Voight's work was supported by National Science Foundation and USGS. We are also grateful for the cooperation and help extended to us by VSI and MVO, and particularly Wimpy Tjetjep, K. Sukhyar and Mas Atje Purbawinata. We also thank the staff of Sabo Technical Center

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