Contrasts between debris flows, hyperconcentrated flows and stream flows at a channel of Mount Semeru, East Java, Indonesia

Abstract 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.