Rate of sediment yield following small‐scale volcanic eruptions: a quantitative assessment at the Merapi and Semeru stratovolcanoes, Java, Indonesia

Abstract Sediment yields were calculated on the flanks of Merapi and Semeru volcanoes in Java, Indonesia, using two different methods. During the first year following the 22 November 1994 eruption of Merapi, a sediment yield in excess of 1·5 × 10 5 m 3 km −2 yr −1 was calculated in the Boyong River drainage basin, based on the volumes of sediment that were trapped by five check dams. At Semeru, sediment discharges were assessed in the Curah Lengkong River from direct measurements on the lahars in motion and on the most significant streamflows. The calculated rate of sediment yield during one year of data in 2000 was 2·7 × 10 5 m 3 km −2 yr −1 . Sediment yields are dominated by rain‐triggered lahars, which occur every rainy season in several drainage basins of Merapi and Semeru volcanoes, mostly during the rainy season extending from October to April. The return period of lahars carrying sediment in excess of 5 × 10 5 m 3 is about one year in the Curah Lengkong River at Semeru. At Merapi, the volume of sediments transported by a lahar did not exceed 2·8 × 10 5 m 3 in the Boyong River during the rainy season 1994–95. On both volcanoes, the sediments are derived from similar sources: pyroclastic‐flow/surges deposits, rockfalls from the lava domes, and old material from the riverbed and banks. However, daily explosions of vulcanian type at Semeru provide a more continuous sediment supply than at Merapi. Therefore, sediment yields are larger at Semeru. Copyright © 2004 John Wiley & Sons, Ltd.