Wildfire/debris flow in Southern California

Katherine Thompson

Every year, some residential areas of Southern California are hit by debris flows: thick, fast-moving slurries of mud and rocks that are loosened from hillsides by heavy rains, usually following a wildfire. Because such events are very difficult to predict but can be extremely dangerous, local agencies often issue evacuation orders only to have no debris flows materialize. These “false alarms” appear, from anecdotal evidence, to discourage residents of debris flow-prone areas from evacuating when later warnings are issued. Before creating an interactive game that will simulate the information and decisions people are faced with during real-life debris flow events, we need additional, empirical information about what Southern California residents know about debris flows and their relationship to wildfires, and what types of information they typically rely on in deciding whether to evacuate from their homes. This study will survey the residents of neighborhoods that have recently had wildfires nearby, in order to gather such information while their memories are fresh. This event-dependent sampling allows us to measure what people really did and felt during a time when evacuation may have been necessary. This information will augment other surveys we have, which measure what people think they will do in the future, or imperfectly recall having done in the more distant past.


Major Findings

  • Results of the first baseline debris flow knowledge survey indicate that misconceptions about debris flows are pervasive but not as severe as thought. Fifty-four percent of respondents (Southern California residents) knew that a wildfire in the foothills makes debris flows more likely; only 6% thought wildfires make debris flows less likely. However, 67% believe that a lack of debris flows in their area means that future ones are unlikely, which is not the case.
  • Knowledge about and experience with wildfires is more prevalent than that for debris flows, but the majority of respondents (71%) did know what debris flows are. Although anecdotal evidence from first responders indicates that people do not tend to evacuate at high rates when debris flow warnings are issued, people overestimate the probability of being affected by a debris flow (on average, 11%).
  • As respondents in this survey reported different frequencies of media use as those in previous hurricane simulation studies, we plan to tailor the new simulation to the greater use of Internet and in-person newsgathering strategies that appear to be used by Southern California residents when faced with debris flow warnings.


Broader Impacts

  • This work feeds into planned projects to create virtual simulation games that will both study evacuation behavior with respect to debris flows and wildfires in California, and also serve as a teaching tool for CA first responders and emergency managers.


CRED2 Award (2010-2015): Funding was provided under the cooperative agreement NSF SES-0951516 awarded to the Center for Research on Environmental Decisions.