Abstract
The runoff analysis advocated by HINO and HASEGAWA was adopted the outflow through drain boring at Irahara landslide. The unit-hydrograph of surface runoff and interflow of No.1 carried out at 1989 changed after the drain boring executed at 1990. The unit-hydrograph of ground-water runoff did not change. The distance between No.1 and the drain boring at 1990 was about 100 m, therefore the sphere of drain boring influence was the same. The time constant of runoff became longer from about 9 hours at Aug. 1989 from about 30 or 40 hours at 1990, 1991 and 1992.
It is clear that a small landslide is observed by extensometer and borehole-inclinometer. The unithydrograph of grount-water runoff does not change but the unit-hydrograph of surface runoff and interflow change. Therefore it seems to be the most probable that the ground-water gives rise to a small landslide.
