Geology topics

Dianne Brien

Publications and source records attributed to Dianne Brien.

3 recordsLinked to original sources

USGS2016

Forecasting inundation from debris flows that grow during travel, with application to the Oregon Coast Range, USA

Many debris flows increase in volume as they travel downstream, enhancing their mobility and hazard. Volumetric growth can result from diverse physical processes, such as channel sediment entrainment, stream bank collapse, adjacent landsliding, hillslope erosion and rilling, and coalescence of multiple debris flows; incorporating these varied phenomena into physics-based debris-flow models is challenging. As an alternative, we embedded effects of debris-flow growth into an empirical/statistical approach to forecast potential inundation areas within digital landscapes in a GIS framework. Our approach used an empirical debris-growth function to account for the effects of growth phenomena. We applied this methodology to a debris-flow-prone area in the Oregon Coast Range, USA, where detailed mapping revealed areas of erosion and deposition along paths of debris flows that occurred during a large storm in 1996. Erosion was predominant in stream channels with slopes > 5°. Using pre- and post-event aerial photography, we derived upslope contributing area and channel-length growth factors. Our method reproduced the observed inundation patterns produced by individual debris flows; it also generated reproducible, objective potential inundation maps for entire drainage networks. These maps better matched observations than those using previous methods that focus on proximal or distal regions of a drainage network.

Geomorphology↗

USGS2010

Volcano collapse promoted by progressive strength reduction: New data from Mount St. Helens

Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced &sim;20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices.

Washington↗

USGS2006

Assessing massive flank collapse at stratovolcanoes using 3-D slope stability analysis

Massive rock failures pose one of the greatest hazards at stratovolcanoes; more than 20,000 fatalities have resulted worldwide from historical volcano edifice collapses. Although numerous processes can destabilize an edifice, gravitational instability is strongly influenced by the interplay of topography, variable potential failure surfaces, and the three-dimensional (3-D) distributions of rock strength and pore-fluid pressure. We have developed a 3-D slope stability analysis that can search digital topography and determine the locations of minimum stability and the volumes of potential failures. We use this 3-D method to conduct preliminary stability analyses of three stratovolcanoes that have had large rock failures: Mount St. Helens and Mount Rainier in the USA and Volcan Casita in Nicaragua. For the relatively uniform Mount St. Helens edifice, a 3-D analysis using topography alone provides a good predictor of the location and volume of the catastrophic 1980 collapse. At Mount Rainier, both topography and a 3-D distribution of weaker, hydrothermally altered rocks are needed to adequately characterize future hazard. For Casita, the location of the smaller, yet devastating, 1998 failure is predicted using topography and a reconnaissance interpretation of strength based on the distribution of fumarolic activity.

NATO Science Series↗