The geomorphological comparison of two debris flows and their triggering mechanisms

 The paper reports on debris flows in the Tungmen and Hoser areas of Taiwan, triggered by Typhoon Ofelia on 23 June 1990 and Typhoon Herb at the end of July 1996 respectively. The intense rainfall resulted in large volumes of debris being eroded from the valleys. The village of Tungmen was almost totally destroyed by in excess of 100,000 m³ of material which was moved by the 1990 debris flow while the 1996 typhoon was responsible for the movement of almost 1.5 million m³ of material, 80% of which extended to the confluence area of the two side gullies and the Hoser River. Attention is drawn to the change in shape of the valley sides from generally V-shaped prior to the debris flow to U-shaped following the storms. A comparison of aerial photographs as well as field data was used to define the main topographical effects of the 1990 storm (Tungmen) and 1996 event (Hoser). A study of the discontinuity pattern within the valleys emphasised the significance of toppling and gravity slumping of material in the sides of the valleys in producing large quantities of debris which, in heavy storm conditions, can flow down the gullies. Laboratory tests were used to simulate the effect of saturation on the materials. These indicated that in their saturated state they would easily become unstable. Received: 1 December 1999 · Accepted: 11 January 2000