Atmospheric rivers and debris flows in California

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Young, Allison M.
Atmospheric rivers (ARs) are characterized by long, narrow bands of enhanced integrated water vapor (IWV) and integrated vapor transport (IVT) that are associated with precipitation extremes and high impact events (e.g., floods and flash floods) in California. California also experiences debris flows when extreme rainfall events mobilize debris flow source regions of particles >2 mm. The first part of this thesis illustrates the relationship between landfalling ARs and debris flows in California. A climatology illustrates that days with DFs (e.g., DF days) occurred more frequently in northern California, in the winter months, and with AR conditions. Composite analyses of IWV, IVT, and sea-level pressure on days with debris flow reports indicate that debris flows are likely related to intense precipitation in conjunction with landfalling ARs. The composite analyses also illustrate that along-AR IVT direction is important in fostering orographic processes that lead to the heavy precipitation necessary for debris flows. 48-h, areal averaged precipitation analyses show that DF days that occur on the day of or day after a landfalling AR contain significantly more precipitation in comparison to DF days with non-AR conditions. This result suggests an important role of ARs as the main atmospheric phenomenon that can lead to debris flows. A landfalling AR and its associated heavy precipitation, however, do not guarantee the occurrence of debris flow. xviii The second part of this thesis analyzes the other two necessary ingredients (slope and abundance of debris) and other factors (curvature of land, lack of plant root system, soil characteristics, post-fire locations, etc.) that produce debris flows using a GIS framework. Results indicate that debris flow source regions are most sensitive to slope and planform curvature, moderately sensitive to Normalized Difference Vegetation Index and Kw-Factor erodibility indices, and are least sensitive to hydrologic group classifications. The results are limited due to a low sample size in locations and times of debris flow. Additional data is needed in order to train a model for debris flow source region analyses. A future assembly of the two products of this thesis, precipitation thresholds for debris flow formation and debris flow potential source regions, can be mobilized using the Flow-R program and overlaid with a debris flow impact map (in progress) to produce a set of debris flow risk analyses.