2001 Volume 54 Issue 1 Pages 59-65
A pyroclastic flow occurred on the south-west flank of Mt. Merapi in Indonesia on July 19, 1998. As a result, a huge amount of pyroclastic deposit filled the gorge of Sat River. In order to understand the process of gully erosion on the deposit, we conducted cross-section survey and took photographs at fixed points on the site, and calculated the sediment yield by gully erosion by using the data of cross-section survey. The results are summarized as follows: 1) Rainfall after the occurrence of the pyroclastic flow caused gully erosion on the doposit. Networks of gullies developed and the cross-sectional size of gullies rapidly increased during the first rainy season.
2) The total eroded sediment volume by gully erosion was estimated 680, 000m3 from the occurrence of the pyroclastic flow until February 2000. Most of the sediment yield by gully erosion occurred during the first half of the rainy season. Sediment yield by gully erosion did not occur during the dry season.
3) The annual specific sediment yield from the pyroclastic flow deposit of the first year was estimated 1.4×106m3/km2/year. This value is almost similar to that of the pyroclastic flow deposit on Mt. Pinatubo, and is 1 to 2 orders of magnitude larger than that of several study examples on other volcanoes.
