Having spent 16 years employed with the state’s Department of Natural Resources assessing landslide risk on forestlands throughout Washington, I am all too familiar with the public perception surrounding logging and landslides. So I wasn’t surprised when media reports surfaced a few days following the Oso landslide that identified timber harvesting as a factor that may have contributed to the hillside collapse and loss of dozens of lives.
However, what nearly all the media reports failed to acknowledge is that while science supports the idea that logging can contribute to the occurrence of some kinds of landslides, research has not demonstrated the same cause-and-effect relationship for other landslide types. Like many things in the natural world, landslides are extremely complex and no two are exactly alike. Therefore, it’s no surprise that some landslides are more susceptible to the effects of logging than others.
One way geologists characterize landslides is based on “depth.” When a landslide occurs, the mass of soil, sediment, and rock that moves downslope breaks away from the underlying earth along what is known as a “failure plane.” It is the depth of this failure plane below the ground surface that is often used to classify landslides. “Shallow” landslides are those where the failure plane is within the rooting zone of vegetation — generally about 10 to 15 feet below the ground surface. “Deep-seated” landslides, such as the Oso landslide, are those where the failure plane is several tens or even hundreds of feet below the ground surface.
Research into the effects of logging on landslides over the past four decades has shown that timber harvesting on steep, concave (or bowl-shaped) slopes increases the incidence of shallow landslides. Studies have found a link between timber harvesting and shallow landslides by comparing the rate of landslides in harvested areas with the rate of landslides in mature forests over long periods of time. The primary logging-related factor that contributes to more frequent landslides on steep, concave slopes is thought to be the loss of tree-root strength. Tree roots help bind soil particles together and reinforce the slope. When trees are harvested, the root systems of the cut trees decay over a period of several years. At the same time, newly planted trees haven’t had time to develop extensive root systems. The slow root decay of the harvested trees combined with the limited root systems of new trees results in an overall reduction in total root strength, making the slope more susceptible to shallow landslides.
Unlike shallow landslides, deep-seated landslides like the one that occurred near Oso are not affected by root strength since the failure plane lies well below the rooting zone of trees. Then how, you might ask, could logging trigger a deep landslide? Good question. For those who believe logging influences deep-seated landslides, their rationale is based on the principle that timber harvesting increases the amount of water entering the ground and more water entering the ground can destabilize the slope. Seems reasonable right? The problem is, we don’t know if or to what extent increases in groundwater attributable to timber harvesting affect water tables that are tens or even hundreds of feet below the ground surface. Groundwater flow within deep-seated landslides is often highly variable, so to say that increases in groundwater that result from timber harvesting can trigger deep slope movement is to oversimplify a very complex issue. Furthermore, and perhaps more importantly, there are many examples of deep-seated landslides throughout western Washington where early logging practices removed large portions of the trees covering both the landslide and the slopes above the landslide, yet there was no detectable deep slope movement.
If timber harvesting clearly influenced deep-seated landslides, researchers would have used these historic “experiments” to discern a cause-and-effect relationship just as they’ve done with shallow landslides. Instead, the historic record seems to support the notion that deep-seated landslides are generally indifferent to the effects of timber harvesting.
What happened near Oso the morning of March 22 is an incredible tragedy. Forty-three lives were lost and thousands more were seriously affected. As a society, we will learn from what happened and those lessons may very well lead to changes in public policy. Let’s hope those policy changes are informed by sound science and a clear understanding of what we know and don’t know about landslides, the factors that affect them, and the threats they pose to public safety and the natural resources we value.
Jeff Grizzel is a licensed engineering geologist in the state of Washington and a “qualified expert” under Washington’s Forest Practices rules governing landslide risk assessment. He worked for the Washington Department of Natural Resources from 1994 through 2010 and is currently employed as the natural resources director with the Grant County Public Utility District in Ephrata, Washington.