Abstract
The volume of a debris flow caused by a heavy rain has been investigated along with the frequency of debris flows for estimating sediment volume of a reservoir planned in a small catchment area with past debris flow disaster records. It is not enough to just discuss the specific sediment volume alone for estimating sediment volume, because proportionally the debris flow volume to sediment volume is very high in a small catchment area.
The authors are of the opinion that the sediment volume in a small catchment area can be calculated with the frequency of debris flows in a durable period, of a dam and with each debris flow volume.
Since a debris flow occurs at regular intervals, it is supposed that a debris flow has a period of immunity. The frequency of debris flows can be estimated by this period of immunity. The period of immunity consists of two periods: a field condition and an external force condition. The frequency of debris flows was obtained by computing the two periods with past disaster records.
The debris flow volume was estimated by surveying traces of a recent debris flow in an area. This surveying included deciphering aerial photos and scene measuring of the traces.
The specific sediment volume in the area was calculated at 320 m3/km2/year as a result of the surveying and computations. This value is larger than others calculated using other methods.
It shows that discussion about debris flow is necessary for estimation of sediment volume in a small catchment area and that design sediment volume is adequate to that area.
