Unusual July 10, 1996, rock fall at Happy Isles, Yosemite National Park, California

Effects of the July 10, 1996, rock fall at Happy Isles in Yosemite National Park, California, were unusual compared to most rock falls. Two main rock masses fell about 14 s apart from a 665-m-high cliff southeast of Glacier P oint onto a talus slope above Happy Isles in the eastern part of Yosemite V alley. The two impacts were recorded by seismographs as much as 200 km away . Although the impact area of the rock falls was not particularly large, th e falls generated an airblast and an abrasive dense sandy cloud that devast ated a larger area downslope of the impact sites toward the Happy Isles Nat ure Center. Immediately downslope of the impacts, the airblast had velociti es exceeding 110 m/s and toppled or snapped about 1000 trees. Even at dista nces of 0.5 km from impact, mind velocities snapped or toppled large trees, causing one fatality and several serious injuries beyond the Happy Isles N ature Center. A dense sandy cloud trailed the airblast and abraded fallen t rucks and trees left standing. The Happy Isles rock fall is one of the few known worldwide to have generated an airblast and abrasive dense sandy clou d. The relatively high velocity of the rock fall at impact, estimated to be 110-120 m/s, influenced the severity and areal extent of the airblast at H appy Isles. Specific geologic and topographic conditions, typical of steep glaciated valleys and mountainous terrain, contributed to the rock-fall rel ease and determined its travel path, resulting in a high velocity at impact that generated the devastating airblast and sandy cloud, The unusual effec ts of this rock fall emphasize the importance of considering collateral geo logic hazards, such as airblasts from rock falls, in hazard assessment and planning development of mountainous areas.