First, The Seattle Times obtained data on clear-cuts and landslides from the Washington State Department of Natural Resources (DNR). Using mapping software, we...
First, The Seattle Times obtained data on clear-cuts and landslides from the Washington State Department of Natural Resources (DNR). Using mapping software, we overlaid the clear-cut sites with the landslides from the December 2007 storm. In the Upper Chehalis River and Stillman Creek watersheds, 732 landslides were identified from the storm.
We analyzed where each of those slides occurred in the two watersheds, using landslide inventory data gathered in DNR aerial surveys. Nearly three-quarters (73 percent) of the landslides appear to have started near logging roads or in areas clear-cut in the last 15 or so years.
(There are limitations to DNR’s surveys. Aerial surveys likely missed some landslides, especially in more heavily forested areas where landslides are harder to spot.)
We then looked at some of the steepest clear-cuts in the watersheds by overlaying maps of “hazard zones,” which were drawn up during an analysis of the watersheds in the 1990s by scientists from Weyerhaeuser, the state and elsewhere.
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That analysis assigned each zone a high, moderate or low rating for landslide risk. We limited our analysis to 87 clear-cuts that had at least half of their acreage in a moderate- to high-hazard zone. Nearly half those sites had landslides during the storm.
Despite making up only 8 percent of the total acreage in the two watersheds, these 87 sites accounted for 30 percent of the total landslides.