Sediment trap effect of the slit-dams at Boyong River, Mt. Merapi, Indonesia

Abstract

2 concrete-type slit dams, BO-D 5, BO-D 7 were installed at 1997 in Boyong river, at south slope of Mt. Merapi, Indonesia. Lahar occurred in Boyong river at February, 1998, and those slit dams captured most of sediment of the lahar. Riverbed fluctuation measurement before and after those lahars, rainfall and lahar observation were carried out by Sabo Technical Centre, Indonesia. From those measured data and field measurements by authors, findings as follows are made clear.
(1) Runoff analysis for the lahars of Feb. 11, Feb. 20 were conducted and it was made clear that runoff coefficient for the Rational Formula of those lahars varies with the scale of rainfall. This fact doesn't coincide with the runoff coefficient=0.75 (constant) for Mt. Merapi volcanic sabo master plan. And the coefficient seems to be increasing in accordance with the rainfall amount.
(2) It was observed that much sediment deposition occurred behind BO-D 5, while only little sediment deposition occurred behind BO-D 7. As a result of the examination by 1-dimensional riverbed fluctuation model, the reason of the difference between the deposition of BO-D 5 and BO-D 7 was supposed to be caused by sediment hydrograph of the lahar and delay of sediment runoff from BO-D 7 due to sediment storage effect of BO-D 7. It shows the sediment runoff control effect of the combination of several slit dams.