THE REACTIVATED FERGUSON ROCKSLIDE: IMMEDIATE AND SHORT-TERM IMPACTS OF A RURAL LANDSLIDE

A large landslide in an urban environment, such as the 2005 Bluebird Canyon landslide, has many obvious immediate and short-term impacts. Large landslides in rural settings also have immediate and short-term impacts; some similar to urban landslides and some unique. An example of a large landslide in a rural setting is the Ferguson rockslide on the Sierra National Forest in Mariposa County, CA. This reactivated rock-block landslide occurred in the Merced River Canyon within 10 miles of Yosemite National Park (YNP). Most of its recent movement took place between late April and early June 2006. Landslides in urban and rural settings often impact both roads and utilities. The Ferguson Rockslide buried and blocked California Highway 140, one of the main routes to YNP, and prompted pole re-setting of the Exchequer-Yosemite 75-kv power line serving both the Park and the community of El Portal. Traffic detoured to other roads increased one-way driving between Mariposa and El Portal from 45 min. to nearly 2 hrs. Public safety is a common concern with both urban and rural landslides, although the number of people affected can differ greatly. Urban landslides typically displace several families to whole neighborhoods. The administrative closure of the National Forest System land near the Ferguson rockslide affected a limited number of hikers, sightseers, and fisherman. However, the potential blockage of the Merced River by additional rockslide movement would have required evacuation of residents within a mile upstream. Unlike most urban landslides, the Ferguson rockslide did not affect any structures. While it did not directly affect homes, potential blockage of the river would have threaten homes upstream. Another difference is the ability to easily monitor landslide movement during future triggering events. Increased travel time and limited communication options can make monitoring landslide activity in a rural setting difficult. At the Ferguson rockslide, near real-time monitoring of water levels in the river upstream and downstream of the landslide together with Global Positioning System (GPS)-based stations on the landslide utilized satellite, radio telemetry, and internet-based connections to reach observers. Alerts could be quickly issued to both emergency response agencies and the general public based on this system.