The purpose of this study is to estimate the critical flood discharge and flash flood trigger rainfall for alarm system providing for a flash flood. The flash flood need is nonlinear approaching method, because rainfall-runoff is nonlinear and it is difficult to explain the existing linear rainfall-runoff.
Hydrological characteristics would be utilized to apply such as hydrologic modeling or basin management. this study was effectively estimated a topographic characteristics factor of basin using the GIS(Geographic Information System, GIS). Especially, decided stream order using the GIS at stream order decision that was important for input variable of GIUH(Geomorphologic Instantaneous Unit Hydrograph, GIUH).
A flash flood defined as a flood which follows shortly after a heavy or excessive rainfall event, with a few hours. In this study, we gave a definition that a critical flood for alarm(Warning I) is the flood when stream depth is over 0.5m from the channel bottom.
Result that calculate threshold discharge to use GIUH, at the Machun basin, flash flood trigger rainfall(Warning I) was 16.33mm in the first 10minutes when the threshold discharge was 49.12m³/sec.