Deformation of stream bed deposit and runoff process at debris flow initiation zone

Abstract

Generally, debris flow occurs in response to heavy rainfall. The way in which debris flow occurs may be influenced by the conditions of the stream bed deposit, such as its shape, gradient and deformation. In past studies, a hydraulic experiment conducted to analyze debris flow occurrence used a steep flume and uniform gradient, and then some water was supplied to saturate the stream bed deposit before the debris flow was occurred. But in a natural torrent, the riverbed gradient becomes gentler downstream, the stream bed deposit is not uniform thickness, and also the deposit is sometimes unsaturated. We observed and analyzed the condition and deformation of a stream bed deposit and the runoff process of debris flow in the zone where debris flow is initiated through hydraulic experiments. The experiments were carried out on an experimental flume whose bed slope was 30° at the upstream end and 12° at the downstream end. We supplied a small amount of water, nearly as much as the infiltration flow at first, and rising as the surface water level rose. The experimental results clarified the following: 1) In the steep section with a gradient of 27°-30° and a small amount of surface water, a moving layer was formed that flowed downhill slowly. This moving layer started to move when the layer was saturated by the in ltration flow. When the moving layer flowed down in this section, no surface water or infiltration flow was observed at the downstream end. 2) As the gradient decreased to 21°-24°, the moving layer started to deposit and form a dune. The upper part of the dune had no surface water but the lower part did. The dune flowed down gradually, nally reaching the downstream end with a constant discharge. This dune started to move or deform when the surface water reached and owed over the shoulder of the dune, forming a conspicuous peak. 3) As the amount of supplied water discharge (qin) was small and the moving layer flowed down, the runoff discharge containing sediment at the downstream end (qout) was constant and the ratio of qout / qin was around 0.8. Over time, qin increased and the dune moved dy- namically, forming a conspicuous peak. Values for qout were variable and the ratio of qout / qin was around 2.0.