Understanding post-wildfire debris-flow activity across climates: insights on initiation conditions and flow identification
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Authors
Cavagnaro, David Benjamin
Issue Date
2023
Type
Dissertation
Language
Keywords
debris , fire , flood , flow , post , postfire
Alternative Title
Abstract
The size, frequency, and geographic scope of severe wildfires are expanding in the western U.S. and globally, exposing an ever-larger population to fire-related hazards. Compared to unburned areas, recently burned steeplands have an increased likelihood of runoff-generated debris flows, which are triggered by heavy short-duration rainfall and pose hazards to downstream communities. As the geographic and climatic scope of severe wildfire expands, the degree to which the initiation conditions of these flows vary with local hydroclimate is unknown. This research aims to both investigate the relationship between postfire debris-flow initiation and climate at regional and local scales, and to develop tools for accurately identifying recent debris-flow events. I organize this research into three chapters. The first chapter presents a regional analysis of debris-flow initiation across the Western U.S. and use three independent methods to demonstrate that initiation thresholds vary systematically with local rainfall-intensity climatology. The second chapter takes advantage of this variation to develop and test a rainfall anomaly metric which delineates debris-flow locations that we mapped within individual burn perimeters. The third chapter develops and tests a quantitative flow-type diagnostic metric which accurately identifies the type of flow that occurred in a steep catchment, ensuring the reliability of future training datasets for predictive debris-flow models. Together, these chapters address existing knowledge gaps on how postfire debris-flow generation may inherently vary with local climate, how hydroclimate-based metrics may serve as an important predictors of debris-flow location and initiation conditions, and how debris-flow events can be accurately identified through quantitative methods. These findings mark a contribution to our understanding of postfire hydrologic hazards that is essential under modern fire regimes and will only increase in relevance as anthropogenic climate change continues to expose an ever-larger population to these unique hazards.