Sediment yield from the 1984 pyroclastic flow deposit covered hillslopes in Merapi volcano, Indonesia

Abstract

Merapi volcano located in the central part of Java, is one of the most active volcanoes in Indonesia and has had repeated volcanic eruptions with avalanche type pyroclastic flows caused by collapsing lava mass. Recently, larger scale pyroclastic flows occurred in June 1984. The pyroclastic flow deposits widely and deeply covered the upper reaches of the Putih and Bebeng rivers and created a radical alteration of the hydrological and erosion regime of the basins. As a result of that, much sediment was produced by sheet-rill and gully erosion from the hillslopes and was transported by volcanic debris/mud flows and floods to the lower reaches of the Putih and Bebeng rivers.
In this paper, the total amount of sediment yield by sheet-rill and gully erosion on the hillslopes covered with pyroclastic flow deposits in 1984 was evaluated and the rates of the sediment yield were estimated, based on a field survey and interpretation of the two sets of aerial photographs taken in 1984 and 1991. The total amount of sediment yield was estimated to be approximately 2. 2 million m³ for the Putih river and 4. 0 million m³ for the Bebeng river between the eruption in 1984 to 1991. It seems that most of the sediment was produced in a 4-year period from November 1984 to October 1988. The rate of sediment yield rapidly increased to a peak between November 1984 (5 months after the 1984 eruptions) and October 1985, and then slowly declined with time keeping a high level until October 1988.