DEBRIS FLOWS AND PUBLIC SAFETY IN OREGON

In order to develop landslides and public safety rules for Oregon, it was necessary to assess the risk of debris flow fatalities in Oregon. The number of people killed by debris flows historically in Oregon was determined and a fatality rate was calculated. According to historical records, there have been at least 25 fatalities attributed to debris flows in Oregon since 1890. However, prior to 1950 the historical records appear to be incomplete and it is suspected that a number of debris flow fatalities were misreported or went unreported. Thus, analysis has focused on recorded events that occurred from 1950 to the present. At least 21 deaths in Oregon have been attributed to debris flows since 1950. The debris flow fatality rate averages about one fatality every five years for the entire population of Oregon. The risk of being killed by a debris flow in Oregon for the average citizen appears to be relatively low, about 0.02 fatalities per 100,000 people per year. However, the risk appears to be significant and several orders of magnitude greater, up to 70 fatalities per 100,000 people per year, for small segments of the population known to be living, working, or traveling in identified areas of high or extreme debris flow hazard. The risk to any individual depends on their level of exposure, which is determined by their location in relation to the paths of potential debris flows and by how much time they spend in these locations. Field investigation of 18 different debris flows with varying degrees of impacts to roads and structures identified the following factors associated with the most severe debris flow impacts: 1) Locations within 110 feet of the channel at the loss of confinement and within 12 degrees of the channel alignment; 2) Steeper gradients in the last 300 feet of channel above structures or roads; and 3) Debris flows initiated by failures of large road fills.