OREGON'S DEBRIS FLOW WARNING SYSTEM

Severe winter storms impacted Oregon in 1996. These storms produced record rainfall and triggered landslides and debris flows, mostly in western Oregon. Two debris flows killed five people during the November storm in southwest Oregon. To better protect its citizens in the future, the State of Oregon developed a landslide risk reduction plan that included a debris flow warning system. This warning system is designed to inform Oregonians when and where shallow landslide induced debris flows can be expected. This system is modeled after a USGS developed warning system for the Bay area of California. The system is based on forecasted and measured precipitation intensity and duration. "Advisories" are issued when forecasts indicate threshold precipitation is reasonably possible. "Warnings" are issued when threshold precipitation has been measured near debris flow prone terrain, or if such precipitation is likely during periods of darkness. Five advisories and no warnings have been issued since the program began in 1997. Rainfall thresholds are based on information from past debris flow producing storms. A lower warning threshold is used for the Tyee Core area, which has a higher susceptibility to rapidly moving landslides. The system relies mostly on Department of Forestry meteorologists and geotechnical specialists. There is limited radar and real-time precipitation information in the locations most vulnerable to debris flows. A landslides and public safety project team has recommended obtaining additional radar site and real-time rain gages so that the current warning debris flow warning system is significantly improved.