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 gul ly to the end of the gully within approximately 2 hours. This paper investigates the distribution of rock discontinuities, geomorpho logical changes, and geomaterial characteristics to highlight the possible hazardous factors and to identify the triggering mechanism. These results d emonstrated 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 wer e triggered and moved to the lower part of the gully by high intensity rain fall. Erosion and undercutting of the sidewalls by a rapidly enlarging debr is flow resulted in massive amounts of material converging in the gully, Ex perimental simulation shows that the water pressure in the gully deposits a ppeared to increase more from the channel bottom than from direct surface p recipitation. This rapidly increasing water pressure evidently contributed a sizable dynamic force to initiate movement of the debris flow.