THE 2006 FERGUSON ROCKSLIDE INCIDENT, MARIPOSA COUNTY, CALIFORNIA: COMMUNICATING SCIENCE INFORMATION DURING THE ONSET OF A GEOHAZARD

In late April 2006, re-activation occurred on a pre-historic rock slide in the Merced River Canyon and caused the complete blocking of California Highway 140. Highway 140 is one of three year-round routes to Yosemite National Park and the only one that is not significantly affected by winter snow. What initially appeared to be a problem of a few rocks rolling onto the highway from the adjacent slope quickly developed into a nearly 2.0 million cubic meter rock slide on Ferguson Ridge having serious implications for public safety. Significant public impacts from the Ferguson rock slide include potential losses of over $4 million in business revenue for the gateway community of Mariposa, increased commuting time for Park Service employees living in Mariposa, longer school bus rides for children from the canyon community of El Portal and added traffic congestion on one of the alternative Park access routes, California Highway 41. Potential impacts included landslide damming of the river with associated upstream inundation of infrastructure, homes and businesses and downstream flooding of campgrounds, roads, and other facilities after dam breaching.

With significant media coverage and community involvement, this event required geologists and technical professionals to address many public concerns. Major topics included explaining our understanding of the rockslide, how potential flood effects are modeled, the limits on engineering solutions, and uncertainty in forecasting natural events. Sharing this information with the public required thoughtful preparation for public meetings, patient interaction with the media and during public forums, and regular consultation with community leaders to ensure accurate and timely information was part of the public's education about this event. In this situation, it is important to communicate improved understanding from investigation and monitoring results. Real-time monitoring providing advance warning of additional rockslide movement and potential impacts is particularly critical to public safety and well being.