Controlling factors of hazardous debris flow in Taiwan

Three unanticipated large-scale debris flows occurred in various rock type areas during typhoons that passed through Taiwan. The present study identifies possible hazard factors and the triggering mechanism. Several investigative methods relevant to these ends were selected, including aerial photo analysis, discontinuity patterns and geomaterial characteristics. In affected gullies, conjoined discontinuities protruded to the point of material slumping and producing large volumes of debris. In heavy rainfall, such debris tends to flow downward into the gullies. Discontinuity distribution produces geometric models of potential failure in plane, wedge and toppling forms. These slumping materials had been introduced and had accumulated along the gullies under study. Rapidly increasing water pressure caused by heavy precipitation provided a powerful force that triggered the debris flow movement. Large masses of slurry changed into an effective cutting force to erode the sidewalls, resulting in more masses of slumping materials converging in the gully and quickly funnelling down into the lower gullies.