Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 8 3 2008 10.5194/nhess-8-421-2008 http://www.nat-hazards-earth-syst-sci.net/8/421/2008/ http://www.nat-hazards-earth-syst-sci.net/8/421/2008/nhess-8-421-2008.html http://www.nat-hazards-earth-syst-sci.net/8/421/2008/nhess-8-421-2008.pdf 421 432 2008-05-06 Investigation and hazard assessment of the 2003 and 2007 Staircase Falls rock falls, Yosemite National Park, California, USA G. F. Wieczorek G. M. Stock greg_stock@nps.gov P. Reichenbach J. B. Snyder J. W. Borchers J. W. Godt US Geological Survey, MS926A, Reston, Virginia, USA Division of Resources Management and Science, Yosemite National Park, El Portal, California, USA CNR-IRPI, via della Madonna Alta, 126, 06128 Perugia, Italy Yosemite National Park (retired), Davis, California, USA US Geological Survey (retired), Sacramento, California, USA US Geological Survey, MS966, Denver, Colorado, USA Since 1857 more than 600 rock falls, rock slides, debris slides, and debris flows have been documented in Yosemite National Park, with rock falls in Yosemite Valley representing the majority of the events. On 26 December 2003, a rock fall originating from west of Glacier Point sent approximately 200 m³ of rock debris down a series of joint-controlled ledges to the floor of Yosemite Valley. The debris impacted talus near the base of Staircase Falls, producing fragments of flying rock that struck occupied cabins in Curry Village. Several years later on 9 June 2007, and again on 26 July 2007, smaller rock falls originated from the same source area. The 26 December 2003 event coincided with a severe winter storm and was likely triggered by precipitation and/or frost wedging, but the 9 June and 26 July 2007 events lack recognizable triggering mechanisms. We investigated the geologic and hydrologic factors contributing to the Staircase Falls rock falls, including bedrock lithology, weathering, joint spacing and orientations, and hydrologic processes affecting slope stability. We improved upon previous geomorphic assessment of rock-fall hazards, based on a shadow angle approach, by using STONE, a three-dimensional rock-fall simulation computer program. STONE produced simulated rock-fall runout patterns similar to the mapped extent of the 2003 and 2007 events, allowing us to simulate potential future rock falls from the Staircase Falls detachment area. 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