Preliminary Study on Rock Avalanche in Taiwan

Rock avalanche is a kind of rapid (average >100 km/h) granule flow caused by crushing and pulverization of rock materials during catastrophic rock slide. Literature researches show that rock avalanches typically occur on steep, high-relief slopes underlain by hard rocks, and have volumes >10,000,000 m3. Rock avalanches also are characterized by long runout distances, which are 5 to 10 times the total fall heights. Some cases can run up the opposing valley wall. Rock avalanches generally occurred in active mountains (e.g., New Zealand) and were triggered by earthquakes or rainfall (snowmelt), but with exceptions. There were few rock avalanches in historical time in Taiwan. This could reflect: (1) intrinsic instability of hillslopes due to weak rock, frequent earthquakes/heavy rains, which resulted in landslides of high frequency/low magnitude; (2) limited runout space along deeply incised river-valley systems, which increased the likelihood of rock-slope failures to transform to debris flows. However, there are ancient rock-avalanche records, found at Shou-shan coast (SW Taiwan) and Shin-she, Chang-pin, Tu-lan along Hua-tung coast (E Taiwan), which is likely to have undergone coseismic uplift. These places, with steep slopes, underlain by hard rock, and free for materials to run, are most prone to rock avalanches in the future.