GEOPHYSICAL SURVEYS OF DEBRIS-FLOW-SUSCEPTIBLE AREAS IN BOULDER COUNTY; IMPLICATIONS FOR RISK ASSESSMENT

It is unknown whether public perception of the risk associated with natural disasters changes with the level of exposure to relative scientific evidence and the manner in which it is communicated by experts. Debates have circled around climate change, natural hazard preparedness, and health risk perception. One situation that lends itself to addressing these critical issues is debris flow hazards in inhabited mountainous regions and its impact on human life, property and amenity value judgments. We have performed geophysical surveys along the Front Range in Boulder County in order to assess debris flows in light of September 2013 flooding which brought a State of Emergency to Boulder County. The idea of the surveys was to provide subsurface images containing geotechnical information that can be interpreted in terms of debris flow susceptibility along a given profile. We acquired 6 along-slope and down-slope electrical resistivity tomography (ERT) profiles and have made preliminary interpretations of the information contained in the profile in terms of potential slope failure. The preliminary results show a complex subsurface geomorphology and we expect shallow localized channels of high hydraulic conductivity constrained by areas of low hydraulic conductivity. Understanding these subsurface structures gives better insight into the apparent risk of future rainfall-induced-debris-flows in specific areas. This information can then be communicated to the public and local stakeholders for risk assessment and mitigation.