Initiation of the Tungmen Debris Flow, Eastern Taiwan

A large-scale debris flow caused at least 39 deaths and covered nearly half a village during typhoon Ofelia on the 23rd of June, 1990. No precautions had been taken. This debris flow consisted of rock fragments, silt and clay, and tree and wood material that flowed quickly through a long, narrow gully to the end of the gully within approximately 2 hours. This paper investigates the distribution of rock discontinuities, geo-morphological changes, and geo-material characteristics to highlight the possible hazardous factors and to identify the triggering mechanism. These results demonstrated that the rock discontinuities were distinctly developed on both sides of the gully. In particular, they were a major factor in forming the deposited materials and creating the hazard. These deposited materials were triggered and moved to the lower part of the gully by high intensity rainfall. Erosion and undercutting of the sidewalls by a rapidly enlarging debris flow resulted in massive amounts of material converging in the gully. Experimental simulation shows that the water pressure in the gully deposits appeared to increase more from the channel bottom than from direct surface precipitation. This rapidly increasing water pressure evidently contributed a sizable dynamic force to initiate movement of the debris flow.