Prehistoric and modern debris flows in semi-arid watersheds: Implications for hazard assessments in a changing climate

Persistent Link:
http://hdl.handle.net/10150/312570
Title:
Prehistoric and modern debris flows in semi-arid watersheds: Implications for hazard assessments in a changing climate
Author:
Youberg, Ann M.
Issue Date:
2013
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
In a series of three studies, we assess modern debris-flow hazards in Arizona from extreme precipitation events and following wildfires. In the first study, we use a combination of surficial geologic mapping, ¹⁰Be exposure age dating and modeling to assess prehistoric to modern debris-flow deposits on two alluvial fans in order to place debris-flow hazards in the context of both the modern environment and the last major period of climate change. Late Pleistocene to early Holocene debris flows were larger and likely initiated by larger landslides or other mass movement failures, unlike recent debris flows that typically initiate from shallow (~1 m) failures and scour channels, thus limiting total volumes. In the second study we assess the predictive strengths of existing post wildfire debris-flow probability and volume models for use in Arizona's varied physiographic regions, and define a new rainfall threshold valid for Arizona. We show that all of the models have adequate predictive strength throughout most of the state, and that the debris-flow volume model over-predicts in all of our study areas. Our analysis shows that the choice of a model for a hazard assessment depends strongly on location. The objectively defined rainfall intensity-duration thresholds of I₁₀ and I₁₅ (52 and 42 mm h⁻¹, respectively) have the strongest predictive strengths, although all five of the threshold models performed well. In the third study, we explore various basin physiographic and soil burn severity factors to identify patterns and criteria that can be used to discriminate between potential non-debris-flow (nD) and debris-flow (D) producing basins. Findings from this study show that a metric of percent basins area with high soil burn severity on slopes ≥30 degrees provides a stronger discrimination between nD and D basins than do basin metrics, such as mean basin gradient or relief. Mean basin elevation was also found to discriminate nD from D basins and is likely a proxy for forest type and density, which relates to soil thickness, root density and the magnitude of post-disturbance erosion. Finally, we found that post-fire channel heads formed at essentially the same slope range (~30-40 degrees) as saturation-induced hill slope failures.
Type:
text; Electronic Dissertation
Keywords:
debris flows; hazard assessments; semi-arid; surficial geology; wildfires; Hydrology; Arizona
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Hydrology
Degree Grantor:
University of Arizona
Advisor:
Baker, Victor R.; Webb, Robert H.

Full metadata record

DC FieldValue Language
dc.language.isoen_USen
dc.titlePrehistoric and modern debris flows in semi-arid watersheds: Implications for hazard assessments in a changing climateen_US
dc.creatorYouberg, Ann M.en_US
dc.contributor.authorYouberg, Ann M.en_US
dc.date.issued2013-
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractIn a series of three studies, we assess modern debris-flow hazards in Arizona from extreme precipitation events and following wildfires. In the first study, we use a combination of surficial geologic mapping, ¹⁰Be exposure age dating and modeling to assess prehistoric to modern debris-flow deposits on two alluvial fans in order to place debris-flow hazards in the context of both the modern environment and the last major period of climate change. Late Pleistocene to early Holocene debris flows were larger and likely initiated by larger landslides or other mass movement failures, unlike recent debris flows that typically initiate from shallow (~1 m) failures and scour channels, thus limiting total volumes. In the second study we assess the predictive strengths of existing post wildfire debris-flow probability and volume models for use in Arizona's varied physiographic regions, and define a new rainfall threshold valid for Arizona. We show that all of the models have adequate predictive strength throughout most of the state, and that the debris-flow volume model over-predicts in all of our study areas. Our analysis shows that the choice of a model for a hazard assessment depends strongly on location. The objectively defined rainfall intensity-duration thresholds of I₁₀ and I₁₅ (52 and 42 mm h⁻¹, respectively) have the strongest predictive strengths, although all five of the threshold models performed well. In the third study, we explore various basin physiographic and soil burn severity factors to identify patterns and criteria that can be used to discriminate between potential non-debris-flow (nD) and debris-flow (D) producing basins. Findings from this study show that a metric of percent basins area with high soil burn severity on slopes ≥30 degrees provides a stronger discrimination between nD and D basins than do basin metrics, such as mean basin gradient or relief. Mean basin elevation was also found to discriminate nD from D basins and is likely a proxy for forest type and density, which relates to soil thickness, root density and the magnitude of post-disturbance erosion. Finally, we found that post-fire channel heads formed at essentially the same slope range (~30-40 degrees) as saturation-induced hill slope failures.en_US
dc.typetexten
dc.typeElectronic Dissertationen
dc.subjectdebris flowsen_US
dc.subjecthazard assessmentsen_US
dc.subjectsemi-ariden_US
dc.subjectsurficial geologyen_US
dc.subjectwildfiresen_US
dc.subjectHydrologyen_US
dc.subjectArizonaen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineHydrologyen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorBaker, Victor R.en_US
dc.contributor.advisorWebb, Robert H.en_US
dc.contributor.committeememberBaker, Victor R.en_US
dc.contributor.committeememberWebb, Robert H.en_US
dc.contributor.committeememberTroch, Peter A.en_US
dc.contributor.committeememberPellitier, Jon D.en_US
dc.contributor.committeememberPearthree, Philip A.en_US
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