Wednesday, January 4, 2012

Landslides and Screening Tools

Update: I made a few edits to this based on some comments I received as well as my own self editing. Thanks! Ought not to have called screening tools models.

I've been working on some landslide evaluation and forestry. A very large storm in December 2007 that brought sustained heavy rain to northwest Oregon and southwest Washington provided an opportunity to evaluate the performance of a couple of slope stability screening tools that the Washington State Department of Natural Resources has developed. Turns out that both tools performed very well.

One DNR tool was developed using digital elevation models (DEMs) and assigning high, medium and low instability dependent primarily on landform characteristics and broad assumptions regarding soil properties and other stability factors in one tool (SLPSTAB) and one tool (HAZONE) relies primarily on historic slides and projects to the known areas of geology instability. The three slides in the approximate middle of the above image all correspond to areas the SLPSTAB designated as high instability. There are are highly unstable areas designated both north and southwest of the three slides that are tree covered and they did not fail. Likely the water interception combined with root strength held the soils in place. The clear cut harvest area to the west of the slides across the creek was designated as a medium instability area and did not fail - that slope is a little less steep. 

The December 2007 storm and over one thousand landslides associated with that storm event has been keeping a few geologists busy as well as policy setters and lobbyists.

Washington’s Forest Practices Rules include site-specific prescriptions intended to prevent the increase in landsliding caused by forest practices beyond natural background rates and thus to reduce the impacts of landslides on aquatic species and public resources. That policy does not appear to have been met from the December 2007 storm. Big storms will cause landslides; however, the recurrence interval and magnitude of slides from the events like the December 2007 storm are higher than what would otherwise be expected.

The impacts to the fish bearing stream can be seen by comparing two Google Earth images from another area Kara Whittaker the lead author on the study evaluated.

Note that in the post slide image (3 years after the slides) the entire river valley is filled with new sediment and trees in the valley were removed by the debris flows that traveled down the river. The 1996 black and white image shows the river tree lined its entire length. The sediment loads into the river system from extensive landslides will have both short term and long term impacts on the river morphology and hence on fish species that live in this system.

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