Just Another Clearcut Landslide

May 4, 2009

Harvest related landslides have been in the public eye for awhile. Hal Bernton and Justin Mayo of the Seattle Times (and photographer Steve Ringman) have really brought landslides and logging into the minds of people in Washington, if not many places in the nation. Science backs that logging can cause landslides with a lot of research focused on rooting strength and the 3-15 year (or so, see Loughlin and Ziemer, 1982) window of weakening roots (variable with climate, soil, tree species, among other things). Perhaps less studied are canopy coverage and its impacts to landslides. A study in Canada (Horel, 2006) has shown that in their study area, landslides failed in the years after harvest and reduced in landslides when rooting strength was at its lowest. So, perhaps we have a myriad of factors for how trees can influence landslides. That is just an introduction to the following area. I have been working on digitizing landslides from the January 7-8th storm with help from Trevor Contreras and Kelsay Davis-Stanton. A lot of the landslides I digitize in are in a mixture of timber ages, from clear cut areas (0-5 years), young stands (5-15 years), and submature timber (15-50 years). However, there are areas where we see a higher incident of landslides in a single age of trees or specific landuse.
This area is located along the White River (and Highway 410) at about Twin Creek, an area that was hit hard by landslides during the January storm. This was one of the areas where we had reports of landslides during the storm, but was blocked from investigating these landslides due to flooding and debris flows across the road. On Saturday morning, I was digitizing the landslides in from our oblique photos and ran across this:

Area map of landslides

Area map of landslides

Area map of historical landslides

Area map of historical landslides

Historically, this area has had many debris flows and is covered in deep-seated landslides. The red stars indicate where landslides have initiated from the January 7-8th, 2009 storm.

These maps are made with a 2006 NAIP Orthophoto and as you can see on the photos, harvest has occurred between 2006 and now:

Photo of area covering the western side of the map

Photo of area covering the western side of the map

Photo of area covering mid-western section of map

Photo of area covering mid-western section of map

Photo of area covering the mid-eastern side of map

Photo of area covering the mid-eastern side of map

Photo of area covering the eastern side of the map

Photo of area covering the eastern side of the map

So, is this clearly a case of clearcuts causing landslides? Lets look at some of the data of this area (this data is for example use only and is very much in draft form).

Graph of landslides verse landuse

Graph of landslides verse landuse

Eight landslides initiating in clearcuts, three in submature timber and eight in roads, is that clearly a clearcut answer? Not really, first we would need to normalize the data, that is quantify the size of different landuses and then divide that with the landslides. That way, if there is, say, 16 acres of clearcuts and 6 acres of submature timber, then the data would not suggest that landslides were more frequent in either landuse. In this area, roughly speaking, there are 11,200 acres of clearcut and 6,400 acres of submature timber. Normalized that would come out to about 7 landslides/1,000 acres for clearcuts, 5 landslides/1,000 acres for submature timber. Not that this sample is statistically large enough, but the difference between the two landuses is not all that significant.

This data doesn’t mean that clearcuts aren’t causing landslides, but the rate of landslides is similar to that of submature timber. Landslides are a naturally occurring event, but can sometimes be aggravated by various landuses. The real question is; is the rate of landslides greater than that of natural rates, that is, is our management of the forests creating a greater number of landslides. With that, we would have to look at historical records to determine a pre-harvest landslide rate and compare that to the various landuses today. Unfortunately, such data would be very difficult to create and the accuracy for a specific area would almost be impossible. Some data exists for site specific areas, but no statewide data exists. Feel like arguing in a circle for awhile? We are faced with a question that cannot be clearly answered with our given knowledge of landslides in Washington, but we can continue to collect data and make inferences as to how landuse might be impacting our land.

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