Reconciling field observations of pyroclastic density currents with conceptual and computational analogs using a GIS and a newly developed global database
Ogburn, Sarah Elizabeth
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This work explores the use of both a geographical information system (GIS) and a global database for reconciling pyroclastic density current field data with conceptual and computational analogs. This study has four parts: 1) The suitability and use of different PDC mobility metrics is investigated by developing FlowDat, a global database of mass flows. This work reexamines the use of mobility metrics for characterizing PDC mobility and the frequency-magnitude relationships of PDCs, and shows how such data might be used for producing event trees. The work also tests the use of measurable deposit parameters as inputs for geophysical mass flow models, and shows that ΔH/L (height-dropped/runout) values can be used as an estimate for the basal friction input parameter for TITAN2D; that the constant retarding stress parameter for VolcFlow can be estimated from field measurements, but not predicted; and that the calculation of the LAHARZ coefficients for the planimetric- and cross-sectional area-volume relationships using statistical analysis of data is problematic for forward modeling of PDCs. 2) The relative effectiveness of three popular computational models to simulate PDCs is explored by comparing both best-fit and FlowDat-derived model simulations to all mapped PDCs from the eruption of Soufrière Hills Volcano (SHV), Montserrat. This work expands upon single-deposit comparisons of models, and is instead able to compare models over a range of volumes and emplacement environments. Results show that TITAN2D and VolcFlow are both able to replicate smaller-volume flows better than larger-volume flows, probably due to the increasing complexity in source characteristics and unsteadiness and non-uniformity of PDCs of larger volumes. LAHARZ is capable of reproducing mapped deposits, but its use for forward modeling is questioned. 3) The role of topography in the detachment of ash-cloud surges is derived from both field and GIS measurements of SHV deposits and digital elevation models (DEMs). The cross-sectional area of the channel is identified as a trigger of ash-cloud surge detachment, and a critical volume-specific cross-sectional area is determined. The results indicate that surge mobility and detachment are a complex product of flow mass flux, energy or granular temperature, and topography and that future efforts to model dense-dilute coupled flows will need to account for and integrate several mechanisms acting on different parts of the flow. 4) The relationships between surge detachment and topography mean that careful attention must be paid to the ways in which topography changes through time. Rapid topographic changes during an ongoing eruptive crisis can have important hazard implications, as in-filled valleys are less able to contain subsequent flows and steeper average drainage slopes can increase flow mobility. This work explores how topography has changed through time at SHV, Montserrat, and how these changes may impact future hazards in the Belham Valley. The Belham Valley, which is of paramount importance for hazard assessment on Montserrat, has steepened, lost cross-sectional area due to deposition, and the main channel has shifted toward the northern banks. Together, these changes increase the risks to populated areas from PDCs and lahars in the future. Each of these components contributes to our understanding of the mobility and behavior of PDCs, especially in relation to topographical effects. An important contribution is highlighting the differences between small-volume flows, which are better characterized by models and metrics, and larger-volume flows which are more complex and less well-characterized by models and metrics, which essentially average out aspects of these highly unsteady flows in both space and time. This presents important limits to the scalability of small-scale experiments and models developed in the context of these experiments. This work also seeks to use digital representations of deposits and topography to extract added value from already completed field investigations using a GIS. Field campaigns can be expensive, impractical, and even dangerous, and the work presented herein devises new ways to leverage existing data for further investigation.