THE CANYON VIEW LANDSLIDE, SPRINGDALE, UTAH, USED AS AN UNDERGRADUATE EXERCISE IN ASSESSING LANDSLIDE HAZARDS

In May, 2005, residents of the Canyon View neighborhood of Springdale, Utah, noticed ominous fissures in a river terrace high above their homes and the Virgin River. Residents became concerned that the “Canyon View landslide”, though slowly moving, could fail catastrophically, dam the river, and flood their neighborhood. This slow-moving slide provided an excellent opportunity for undergraduate students to evaluate the danger and to debate the civic responsibility of scientists assessing geologic hazards.

Upon initially seeing the slope failure from a distance and observing its proximity to the neighborhood and river, students gave dire prognoses. Our goal then became to examine the slide in a more scientific manner. What are the geometry and extent of the slope failure, the stratigraphic units involved, and the reasons the slide started to move? Can further slide movement be predicted?

The failed slope is the river-facing side of an alluvial terrace containing two stratigraphic units. The lowermost ten meters of the terrace consist of rounded river gravel. The upper unit is unconsolidated colluvium of clay and shale with sandstone boulders and cobbles. Gray and purple shale of the Chinle Formation, characterized by expansive clays, is recognizable within the upper unit. Only the upper unit has failed. The landslide can be described as a rotational slump with clearly visible arcuate head scarps. Vertical displacement along the head scarps was generally three meters or less during observations. A prominent collapsing bulge defines the base of the slide at the top of the river gravel unit. It was determined that landside growth correlates with periods of above normal precipitation.

After analysis of the Canyon View landslide we have a better understanding of the stratigraphic controls and geometry of the slope failure. We interpret that the hazard from both continued movement and growth in extent of the slide is directly related to precipitation. It can be argued that, with a rapid catastrophic failure, only a part of the slide has the potential of temporarily damming the river before being washed away. However, continued growth in size of the active slide enhances the danger. We recommend a systematic transit survey of locations peripheral to the active slide to monitor growth of the slope failure.