These GIS materials are provided to help people understand why this landslide happened and to help the public understand how the use of available geologic resources prior to property development activities can help prevent or minimize future tragedies.
Soft clay, sand, silt and gravel (glacial lacustrine and glacial) deposits can cause potential geohazards associated with erosion and/or landslides that impact property, development and transportation.
What can we learn from this and How can we prevent this loss of life and property from happening again?
Oso Landslide Discussion
The northwestern portion of the 2014 Oso landslide appears to be in Qgo - Glacial Outwash, while the majority of the up-gradient landslide is within Qls-mass wasting deposits (formerly activated landslides) and down gradient are Qa- Quaternary alluvium deposits. This indicates the scale of the available geologic map may not reflect the locations of the Qls deposits precisely enough for this evaluation or more probable, it indicates that land within Qgo deposits may be as equally susceptible to landslides as Qls. Rather than indicate that all Qgo and Qa deposits be designated landslide hazards in addition to the known Qls deposits, it would be prudent to consider both elevation and the existing geologic deposit. A GIS spatial analysis can help designate all of the Qgo and similar deposits greater than a specified slope that can be additionally considered landslide hazards besides Qls and the associated down-gradient Qa deposits.
The above August 1, 1954 aerial photograph suggests the perimeter of the 1952 landslide activity. The 1952 motion may have been initiated along an historic dirt Road. Laying out roads on steep slopes introduces erosion to previously vegetated and stabilized slopes. Dense tree growth is a critical factor in promoting infiltration and reducing runoff and erosion on the landscape. Removing vegetation, even in a narrow swath can have devasting effects on a steep slope. The 2014 motion involves failure from the top and the bottom of the slope. At this time, adequate records are unavailable to indicate whether or not deforestation played a factor in the failure of the upper slope. Factors clearly playing critical roles are the amount of rainfall and saturation of the subsurface, the sedimentology and the steep gradient. The slope failed precisely where the topographic contours suggest failure may occur. In the future it is critical to keep the terraces between the slopes as well as the slopes themselves densely forested.
Future Planning Discussion
Dave Petley, PhD (April, 2, 2014) at http://blogs.agu.org/landslideblog/ indicated "the disaster represents a failure of hazard management". While this is true, it is also a failure of practical landuse planning and it may have been a failure of adequate land cover on landslide prone terraces. We as members of this planet must all shoulder the blame and responsibility for tragedies that happen when earth processes meet civilization. As the responsibility is multifaceted, so must the solution be. Landowners, citizens, public entities, research scientists and consultants must all be a part of the solution. If one entity is omitted, a variable may be missed or inadequately addressed and the solution may fail to be objective. It is necessary for the Town of Oso to again become a safe, vibrant and sustainable community to live in and this can and will be achieved. It is our responsibility to make a concerted effort to achieve this for the Town of Oso as well as the rest of the earth.
The colors of the indicated geologic deposits do not necessarily indicate that the up-gradient deposits are more of a hazard than the down gradient deposits. As we can see, several types of the deposits were involved in the 2014 landslide motion. The up-gradient slopes that failed can be considered more of a hazard than some of the surrounding slopes because they have a steeper slope and that is indicated by elevation contours that are closer together. The slopes with the steeper slopes and the associated down-gradient deposits can be considered greater landslide hazards than those with less steep slopes (where the contour lines are further apart from each other on the slopes). The selection of the indicated deposits is based on the area selected for analysis by Haugerud, R. A., 2014, Preliminary interpretation of pre-2014 landslide deposits in the vicinity of Oso, Washington: U.S. Geological Survey Open-File Report 2014-1065, 4 p., http://dx.doi.org/10.3133/ofr20141065.