December 2007 Storms

In December 2007, a series of snow, wind and rainstorms battered western Oregon and Washington, causing severe flooding and wind damage. Landslides occurred, roads and bridges were washed out, and homes were flooded.

The storm renewed interest in whether harvesting on steep slopes exacerbates landslides and flooding, and whether existing regulations that govern such harvesting are adequate.

A small portion of Weyerhaeuser timberlands received extraordinarily high rainfall and suffered hundreds of landslides. We responded with a comprehensive scientific study designed to inform and improve forest management, and evaluate the relationship between our harvesting practices and landslides. The study was published in 2010 in Forest Ecology and Management.

Key findings include:

• Aerial photos alone are not a reliable source of data on landslides. Ground-based landslide inventory data are required to correct for detection bias, develop reasonable estimates of landslide density across environmental gradients such as rainfall magnitude and topography, and make unbiased interpretations of relationships between forest management and landslide occurrence. For example, 39 percent of field-detected landslides were not detected on 1:12,000-scale aerial photos.
• Very few landslides occurred at rainfall levels up to a "100-year return interval," regardless of stand age or slope gradient class. A "100-year return interval" means that in a given year the likelihood of a storm that size or larger is one percent, and that over a long period of time a storm of that size or larger is likely to occur on average once every 100 years. This interval is meaningful because it is the modern design standard for culverts under forest roads in Washington — that is, culverts should be sized to accommodate a "100-year" storm.
• At higher rainfall intensities, significantly higher landslide densities occurred on steep slopes (greater than 70 percent gradient) compared to lower gradient slopes, as expected.
• At extreme rainfall levels — above about 150 percent of 100-year rainfall, the density of landslides was about two to three 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.

On March 25, 2010, based in part on this research, the Washington Department of Natural Resources and Weyerhaeuser reached a voluntarily agreement to apply additional protections to the two watersheds affected by flooding and landslides during the December 2007 storm. As a result, we will apply more tools to predict and avoid landslide-prone slopes supplementing existing assessment requirements. This agreement may result in additional environmental protection beyond what is currently required by state law.