Uncertainty Associated with Evaluating Rockfall Hazard to Roads in Burned Areas

During and following wildfires affecting steep mountain slopes, there can be an increase in rockfall activity usually taking the form of individual rocks, and occasionally, groups of rocks rolling, sliding or bouncing downslope. This increase results from removal of stabilizing vegetation, downed wood, and organics within the soil matrix as well as increase in erosional processes such as dry ravel. The hazard posed to vehicles is difficult to assess because of uncertainty manifested in several ways. First, there is uncertainty in defining the road segments that will be impacted by increased rockfall activity. Second, it is difficult to quantify the size, number, and/or travel behavior of rocks which may impact a given road segment. Finally, there is uncertainty as to how long increased rockfall activity may persist after a wildfire. Between 2007 and 2013, some insight into the first two uncertainty issues was provided by observed rockfall on roads within eight different wildfires in California and Idaho. This insight provided an efficient and effective means to prioritize rapid assessment for rockfall hazard for a large number of roads within the 2013 Rim Fire in the central Sierra Nevada, California. Data on the third rockfall uncertainty issue, persistence, was developed for a road on the Olympic National Forest in Washington. Monitoring of rocks accumulating on the road at sixteen sites between July 2006 and April 2007 recorded 3,463 rocks with the number of rocks found to decrease over time.