LANDSLIDE DENSITY AND ITS ASSOCIATION WITH RAINFALL, FOREST STAND AGE, AND TOPOGRAPHY, DECEMBER 2007 STORM, WILLAPA HILLS, SOUTHWEST WASHINGTON

An unusual and powerful storm in early December, 2007, caused record flooding and triggered thousands of landslides across southwest Washington and northwest Oregon. The objective of this study was to develop estimates of landslide density and use them to establish associations with precipitation, topography, and forest stand age for 152,000 ha of Weyerhaeuser land in the Willapa Hills. Landslide inventory data were collected from both aerial photos and systematic field surveys. We estimated the probability of detecting landslides on aerial photos for 6 strata defined by stand age and rainfall intensity, expressed as percent of the 100-year, 24-hour, maximum rainfall. We found that landslide detection probability decreased with increasing stand age, but was similar across rainfall intensities. The fraction of field–detected landslides 0-20 meters wide at the inferred initiation point (90% of total field inventory) that were not detected on 1:12,000-scale aerial photos was 40%. Very few landslides occurred in the 0-100% of 100-year rainfall category, regardless of stand age or slope gradient class. At higher rainfall intensities, significantly higher landslide densities occurred on steep slopes (>70% gradient) compared to lower gradient slopes, as expected. Above ~150% of 100-year rainfall, the density of landslides was ~2-3 times larger in the 0-5 and 6-10 year stand age categories than in the 11-20, 21-30, 31-40, and 41+ categories. The effect of stand age was strongest at the highest rainfall intensities. Our results demonstrate the importance of collecting ground–based landslide inventory data to correct for detection bias from aerial photos and to develop reasonable estimates of landslide density across environmental gradients, such as rainfall magnitude and topography, before interpretations regarding forest management effects can be made.