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International audience; Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity, and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results from the growth of two edifices: the Mahameru ‘old' Semeru and the Seloko ‘young' Semeru. On the SE flank of the summit cone, a N130-trending scar, branched on the active Jonggring- Seloko vent, is the current pathway for rockslides and pyroclastic flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5 to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941– 1942. Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m3 each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit, was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment yield of 2.7×105 m3 km−2 and a denudation rate of 4 105 t km−2 yr−1, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang can be affected by debris avalanches and debris flows if the steep-sided summit cone fails.

In order to differentiate between different types of sediment-laden-flows in volcanic channels, we carried out observation of debris flows, hyperconcentrated flows, and stream flows in the Curah Lengkong river on the southeast slope of Mount Semeru in East Java, Indonesia. The aims of this study are: (1) to provide quantitative data for these flows in motion; (2) to compare the data for different types of flow that occur in the same river; (3) to assess the influence of rainfall on debris flows, hyperconcentrated flows, and streamflow generation. The Curah Lengkong river transports large volumes of sediment, in the range of 1×10 5 to 5×10 5 m 3 per debris flow, and 10 3 to 10 5 m 3 per hyperconcentrated flow and stream flow. Large sediment discharges result from the following factors: continuous and voluminous sediment supply of fine juvenile material by daily explosions of the Semeru volcano, pyroclastic source material emplaced on steep slopes, strong erosion of weathered river banks, and strong rainfall intensities. The occurrence of the flows focuses in the period from November through April, and the daily timing of it is the mid to late afternoon. Nearly all debris flows are triggered by stationary rainfall confined to the upper slopes of Mount Semeru, whereas hyperconcentrated flows and stream flows are mainly generated by migratory or regional rains driven upwards on the eastern slope. This slope receives its maximum of annual rainfall (3800 mm) at 800 m asl. The peak surface velocity of debris flows is always greater than the peak frontal velocity. The peak discharge of debris flows occurs several minutes after the passage of the flow front. Volumetric concentrations of sediment are high (48% to 69%) between the debris flow front and the point of peak discharge; after the peak discharge it usually decreases gradually. Contrary to the case of debris flows, high concentration of sediment appears in various portion of hyperconcentrated flows and stream flows: near the front, in the mid point or at the back of flows, although the concentration of sediments in them is much less than in of debris flows. Relationships between sediment discharge, total volume and peak discharge show a very strong positive correlation for debris flows and hyperconcentrated flows. Therefore, sediment discharge can be roughly estimated from direct visual observation.

More than 50 volcanic debris flows or lahars were generated around Mt Merapi during the first rainy season following the nuees ardentes of 22 November 1994. The rainfalls that triggered the lahars were analyzed, using such instruments as weather radar and telemetered rain gauges. Lahar dynamics were also monitored, using new non-contact detection instrumentation installed on the slopes of the volcano. These devices include real-time seismic amplitude measurement (RSAM), seismic spectral amplitude measurement (SSAM) and acoustic flow monitoring (AFM) systems. Calibration of the various systems was accomplished by field measurements of flow velocities and discharge, contemporaneously with instrumental monitoring. The 1994–1995 lahars were relatively short events, their duration in the Boyong river commonly ranging between 30 min and 1 h 30 min. The great majority (90%) of the lahars was recognized at Kaliurang village between 13:00 and 17:30 h, due to the predominance of afternoon rainfalls. The observed mean velocity of lahar fronts ranged between 1.1 and 3.4 m/s, whereas the peak velocity of the flows varied from 11 to 15 m/s, under the Gardu Pandang viewpoint location at Kaliurang, to 8–10 m/s at a section 500 m downstream from this site. River slopes vary from 28 to 22 m/km at the two sites. Peak discharges recorded in various events ranged from 33 to 360 m3/s, with the maximum value of peak discharge 360 m3/s, on 20 May 1995. To improve the lahar warning system along Boyong river, some instrumental thresholds were proposed: large and potentially hazardous lahars may be detected by RSAM units exceeding 400, SSAM units exceeding 80 on the highest frequency band, or AFM values greater than 1500 mV on the low-gain, broad-band setting.