George Taylor, Christopher Daly, Wayne Gibson and Jennifer Sibul-Weisburg
Oregon State University, Corvallis, Oregon
Several intense rain storms during 1996 caused widespread flooding in western Oregon and throughout the Pacific Northwest. Moist subtropical air masses brought record-setting rainfall on several occasions. The rain, coupled with significant snow melt and substantial runoff from saturated soil, pushed some stream levels above all-time crests. In addition, numerous landslides and mudslides occurred; these destroyed homes and roads, caused significant property damage, inundated stream channels, and killed several people. Many of the slides were in forest areas which had earlier been harvested, and this has led to demands by some environmental groups for a moratorium on clear-cutting.
The best way to minimize damage from landslides would be by curtailing or modifying certain land use activities in areas prone to slides. Unfortunately, no map or guideline regarding landslide potential currently exists. Oregon State University and the Oregon Department of Forestry were authorized to investigate landslide/mudslide potential, and to produce a series of maps which specify the relative risk throughout western Oregon.
The primary criteria which influence landslide potential are precipitation intensity and slope; vegetation and geology are important as well. A detailed slope data set was constructed using high-resolution topographic information. This was combined with precipitation intensity coverage developed using the PRISM model. Initially, the latter was for 24-hour totals for a 25-year return period; later, the coverage was expanded to include shorter averaging periods (1-, 3-, and 6-hours) and other return periods (5- and 10-years).
The landslide potential maps were not meant to be site-specific decision-making tools, but rather to provide planners wit h overview information. Areas identified as "high potential for slides" would be scrutinized more thoroughly from the ground than would those with "low potential." Ultimate decisions regarding site land use will continue to be made only after a site inspection. But the slide potential information represents a significant and useful new tool for officials who make those decisions