Reading the many newspaper accounts of the tragic Oso-area mudslide in recent days prompts me to offer a few observations on landslides based on my own 50 years experience as an engineering geologist.
From my experience, landslide control has involved three basic tenants: 1) buttress and protect the toe (bottom) of the slide; 2) unloading the top areas of the slide when feasible; 3) controlling soil moisture throughout the slide zone, and water erosion within the total slide area. When water enters the upper and lower portions of potential slide areas, it decreases the shear strength (water softening), and adds to the overall weight of the soil slide mass, thereby increasing the driving force within the slide.
Applying some of these generalized concepts to the Oso-area slide, it would appear that river erosion at the toe of the slide may well have critically removed the toe buttress which was holding the potential slide mass in place.
In addition, removing a substantial area of the forest directly above the slide could have allowed more water to saturate and lubricate potential failure zones within this deep-seated slide. Each pound of wood which is added as a tree grows requires 1,000 pounds of water. Therefore when trees are removed, more water may be available to the slide zone.
While these observations may seem like hindsight, it is hoped that they may also help improve public understanding of this type of natural hazard.