- You would start with an area with substantial terrain and steep slopes.
- You would make sure the area had loose and unconsolidated soils.
- You would make sure there was plenty of rain and that much of that rain would fall in a relatively short period..
- You would have a group that would cut down large water-intercepting vegetation and disturb the soil to promote slope failures. And this this group would leave lots of debris around that would dam up rivers and streams, releasing quickly to promote torrents of downward-moving material.
- You would add large numbers of people and buildings on top of or at the bottom of sloping terrain.
- To top it off you might add earthquakes that could contribute to the initiation of slides.
So let's go through the checklist why the Washington State environment fosters large, damaging slides and debris flows.
First, there is terrain. We have lots if it, withmany relatively steep slopes (see map).
Then we have the impacts of glacial sediments. Washington is far enough north to have experienced glaciation during the past ice ages, with the one that ended about 10,000 years ago pushing over the north Cascades and Puget Sound (see map)
As the glaciers retreated they left sand, silt, and loose material (see image) and all of these materials are porous and weak. Melting glaciers can produce lakes in which finer clay particles settle out, leaving a near waterproof barrier. If one of these clay layers are underneath the loose sediments, that can result in the sediments not draining and becoming saturated. Water within the pores of sediments act as ball-bearings allowing soil movement and failure.
And glaciers do something else that contribute to landslides: as the glaciers melt, enormous weight is taken off the terrain, which subsequently rises over thousands of year. Rising terrain contributes to increases slopes, particularly when river are cutting through them. Retreating glaciers have also left fairly steep cliffs and bluffs, particularly along margins of Puget Sound, that are prone to failure.
As a result of the movement of glaciers southward during past ice ages and their subsequent retreat, northern Washington has been left with weak sediments, underlying clay layers, rising terrain, and bluffs--all contributors to slope failures.
And then there is heavy precipitation, which can saturate the soils. As the pores fill with water the soils become heavier (pulling them down the hill) and the lubricating effects of the water promotes weakening of the soil structures.
The western side of the Northwest has some of the heaviest precipitation the country, with some slopes receiving 60-120 inches a year (see map)
But it is even worse than that. The Pacific Northwest often experiences atmospheric rivers (also known as pineapple expresses), with moisture streaming out of the subtropics that can bring intense rainfall (10-20 inches) to the mountains over a day or two. (see graphic). Many landslide events have been initiated by torrential downpours of strong atmospheric rivers. Washington and Oregon get more atmospheric rivers than any place in North America. And, by the way, the latest research suggests that such rivers will intensify under global warming.
And then there is the timber industry. Many landslides have been the byproducts of the activities of some of the less responsible members of this group. Trees intercept water from reaching the soil and draw moisture from the soil with their roots. Thus, clear cutting results in much more water reaching and staying in the ground. Furthermore, tree roots can also help to bind the soil together. And then their are the roads built on steep slopes by the forest industry. Improperly designed such roads can be the source of initiating of slope failures (see image).
Poor forest practices can lead to debris falling into rivers and streams causing damming. And when such dams finally release during heavy rain events, one can get highly damaging debris flows. One example of such a flow occurred in 1983 after heavy rains above Lake Whatcom (near Bellingham). The result was severe damage at the base of several streams descending into the Lake (see image)
And then there are the region's earthquakes. Earthquakes can trigger landslides in a number of ways. The shaking can accelerate materials downward, initiating a slide. The shaking can cause failure of soil structures that were maintaining soil integrity on the slopes. Or earthquakes can cause liquefaction, in which the acceleration by the earthquake can increase the pressure of water in the pores, resulting in the loss of adhesion of the soil particles so they act more like a liquid than a solid.
And finally there is the tendency for too many Washington State resident to live in landslide-prone locations. Huge numbers of people have built homes on the bluffs above Puget Sound or on the Sound below. Many houses have been destroyed and some lives lost as a result (see picture from a tragic case on Bainbridge Island)
Others live on rivers, which often are adjacent to steep slopes produced by the river itself. The Oso landslide is an example of this, but there are many others.
For all of the above reasons, and some I have not discussed, Washington State has large numbers of landslides, with many of them being quite severe and damaging. The recent landslide near Oso is one of the greatest environmental disasters in Washington State history and hopefully it will initiate a conversation about where people live in our state and being more able to respond to environmental disasters.