The Challenge of Improving Identification of Rockfall Hazard after Wildfires

Varnes (1978) described rockfall as a situation in which rock “… of any size is detached from a steep slope or cliff … descends mostly through the air by free fall, leaping, bounding, or rolling. Movements are very rapid to extremely rapid ….” There are many well-documented rockfalls throughout the world (Evans and Hungr, 1993; Wieczorek et al., 1999; McSaveney, 2002; Strouth et al., 2006; and Ravanel et al., 2010). Most published literature documents relatively large rockfall occurrences. Power-law distributions demonstrate that the more frequent threat involves smaller volume rockfalls (Malamud, 2004; Malamud et al., 2004). Even falls consisting of one or more large rocks can produce significant damage or even fatalities. This is exemplified by case studies from Colorado and the tragic 1999 Sacred Falls landslide in Hawaii (Jibson and Baun, 1999; Colorado Geological Survey, 2008). Rockfall hazard during wildfire suppression activities is a recognized safety concern for firefighters. Individual rocks, and occasionally small clusters of rock, bound or roll down steep slopes from the burning fire above (Swanson, 1981). Commonly rocks range in size from pebbles to boulders or even larger rocks. These rocks come to rest where the slope gradient decreases to a gentler angle. In some instances, this gradient change results from a road crossing the slope. If the slope is concave or has a natural swale leading down to the gentler slope gradient, rockfall may produce a talus cone or add to an existing one. An example of this process was witnessed by Ivan Erskine, a Forest Service Fire Management Officer, while he was serving on the team directing fire suppression activities for the July 2008 Ukonom Complex wildfire in northern California (Figure 1). A talus cone was forming where rock and rock fragments falling down a steep …