TY - JOUR
T1 - Thresholds for post-wildfire debris flows
T2 - Insights from the Pinal Fire, Arizona, USA
AU - Raymond, Carissa A.
AU - McGuire, Luke A.
AU - Youberg, Ann M.
AU - Staley, Dennis M.
AU - Kean, Jason W.
N1 - Funding Information:
This material is based on work supported by the Science Foundation Arizona under Grant No. BSP 0819‐17. Time series of rainfall and pressure data collected during the study period can be found in the Supporting Information. The numerical model used to simulate runoff will be made available through the Community Surface Dynamics Modeling System (CSDMS) model repository. We thank Carlo Gregoretti, John Moody, and Francis Rengers for constructive comments that helped to improve this manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
Funding Information:
This material is based on work supported by the Science Foundation Arizona under Grant No. BSP 0819-17. Time series of rainfall and pressure data collected during the study period can be found in the Supporting Information. The numerical model used to simulate runoff will be made available through the Community Surface Dynamics Modeling System (CSDMS) model repository. We thank Carlo Gregoretti, John Moody, and Francis Rengers for constructive comments that helped to improve this manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
Publisher Copyright:
© 2019 John Wiley & Sons, Ltd.
PY - 2020/5/1
Y1 - 2020/5/1
N2 - Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h−1 lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils.
AB - Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h−1 lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils.
KW - critical discharge threshold
KW - dimensionless discharge
KW - rainfall thresholds
KW - receiver operating characteristic analysis
KW - soil hydraulic properties
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U2 - 10.1002/esp.4805
DO - 10.1002/esp.4805
M3 - Article
AN - SCOPUS:85077847429
SN - 0197-9337
VL - 45
SP - 1349
EP - 1360
JO - Earth Surface Processes and Landforms
JF - Earth Surface Processes and Landforms
IS - 6
ER -