Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States

Staley, Dennis M.; Negri, Jacquelyn A.; Kean, Jason W.; Laber, J. L.; Tillery, Anne C.; Youberg, A.

doi:10.1016/j.geomorph.2016.10.019

Cited by 184 publications

(301 citation statements)

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“…Estimates of hillslope erosion derived here from repeat TLS add to the growing database of studies that utilize high‐resolution topographic data (e.g., DeLong et al, ; Orem & Pelletier, ; Rengers et al, ; Staley et al, ; Wester et al, ) to quantify the spatial patterns of erosion within a recently burned landscape. Likewise, the rainfall records and channel monitoring data provide additional constraints on the precise timing of debris flows within rainstorms, which can be combined with larger data sets of postwildfire debris‐flow activity throughout the western United States to improve empirical models used to estimate debris‐flow likelihood (Staley et al, ). While individual postwildfire studies may frequently be limited in scope (e.g., one hillslope or one drainage basin), when viewed together as a community data set, these studies will help elucidate common trends related to the impact of wildfire on surface processes in different geologic and climate settings and provide additional data for developing and validating empirical models (e.g., Gartner et al, ; Wagenbrenner & Robichaud, ).…”

Section: Discussionmentioning

confidence: 99%

“…It is not possible to attribute debris‐flow initiation to one of these two mechanisms based on available field observations alone. However, identifying the prevalence of particular debris‐flow initiation mechanisms in different settings could help refine methods for estimating thresholds associated with debris‐flow initiation and potentially aid in explaining observed differences in rainfall intensity‐duration thresholds for debris flows that are known to exist among different geographic regions (Staley et al, ). By designing a series of numerical model experiments (e.g., McGuire et al, ), we were able to determine that bed failure processes within the channel were responsible for generating some, but not all, of the observed debris‐flow surges (Figure ).…”

Section: Discussionmentioning

confidence: 99%

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Evolution of Debris‐Flow Initiation Mechanisms and Sediment Sources During a Sequence of Postwildfire Rainstorms

et al. 2019

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Wildfire alters vegetation cover and soil hydrologic properties, substantially increasing the likelihood of debris flows in steep watersheds. Our understanding of initiation mechanisms of postwildfire debris flows is limited, in part, by a lack of direct observations and measurements. In particular, there is a need to understand temporal variations in debris‐flow likelihood following wildfire and how those variations relate to wildfire‐induced hydrologic and geomorphic changes. In this study, we use a combination of in situ measurements, hydrologic monitoring equipment, and numerical modeling to assess the impact of wildfire‐induced hydrologic and geomorphic changes on debris‐flow initiation during seven postwildfire rainstorms. We predict the impact of hillslope erosion on debris‐flow initiation by combining terrestrial laser scanning surveys of a hillslope burned during the 2016 Fish Fire with numerical modeling of sediment transport throughout a 0.12‐km2 basin in southern California. We use measurements of sediment thickness within the channel to constrain numerical experiments and to assess the role of channel sediment supply on debris‐flow initiation. Results demonstrate that debris flows initiated during rainstorms where hillslopes contributed minimally to the event sediment yield and suggest that large inputs of sediment from rill and gully networks are not essential for runoff‐generated debris flows. Simulations suggest that both the gradual entrainment of sediment and the mass failure of channel bed sediment can increase sediment concentration to levels associated with debris flows. Finally, postwildfire debris‐flow initiation appears closely linked to the same rainfall intensity‐duration threshold despite temporal changes in the sediment source, initiation processes, and hydraulic roughness.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Evolution of Debris‐Flow Initiation Mechanisms and Sediment Sources During a Sequence of Postwildfire Rainstorms

et al. 2019

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“…A broader approach would be to look at all precipitation events over a particular threshold in the region. However, PFDF thresholds have been noted to vary in space (Staley et al 2016) thus choosing one representative threshold may not suffice. Focusing on events known to produce impactful PFDFs ensures precipitation was indeed sufficient.…”

Section: Limitations and Future Workmentioning

confidence: 99%

Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California

et al. 2017

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The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property situated on alluvial fans. The combination of steep topography, highly erodible rock and soil, and wildfire, coupled with intense rainfall, can initiate PFDFs even in cases of relatively small storm rainfall totals. This study identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November-March. A compilation of 93 PFDF events during 1980-2014 triggered by 19 precipitation events is compared against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times. Each event was analyzed to determine common atmospheric features and their range of values present at and preceding the trigger time. Results show atmospheric rivers are a dominant feature, observed in 13 of the 19 events. Other common features include low-level winds orthogonal to the Transverse Ranges and other conditions favorable for orographic forcing, a strong upper level jet south of the region, and moist-neutral static stability. Several events included closed low-pressure systems or narrow cold frontal rain bands. These findings can help forecasters identify more precisely the synoptic-scale atmospheric conditions required to produce PFDF-triggering rainfall and thus reduce uncertainty when issuing warnings.

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“…Debris flows pose a serious threat because they move rapidly, travel significant distances from their point of origin, and exert destructive force along their flow path and within their area of deposition (Hungr, 2005;Giraud and McDonald, 2007;Jordan and Covert, 2009) Post-wildfire debris flows clearly pose a hazard when burned watersheds are adjacent to populated areas (Cannon and DeGraff, 2009) and can even be a hazard in a less populated rural settings (DeGraff et al, 2011). Consequently, post-fire evaluation needs tools to effectively identify, within a burned area, those drainage basins having a greater likelihood of generating debris flows in order to undertake timely and effective mitigation (DeGraff et al 2007;Cannon et al 2011;DeGraff et al, 2013;DeGraff, 2014;Staley et al, 2017).…”

Section: Post-fire Debris-flow Hazard Potentialmentioning

confidence: 99%

A rationale for effective post-fire debris flow mitigation within forested terrain

Graff

2018

Geoenviron Disasters

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Watersheds recently burned by wildfires are recognized as having an increased susceptibility to debris flow occurrence. The great majority occur within the first 2 years following wildfires. These debris flows are generated primarily through the process of progressive entrainment of material eroded from hillslopes and channels by surface runoff and appears independent of the vegetative community burned. The decreased likelihood of debris flows over time is linked to the restoration of hydrologic function as vegetative cover and soil infiltration functioning return to pre-fire conditions. An exception to this pattern of post-wildfire debris flow susceptibility occurs in burned drainage basins with forest cover. A second, later period of increased debris flow susceptibility due to infiltration-triggered landslides can occur in burned forested basins. This later period of debris flow susceptibility is largely attributable to the fire-induced tree mortality and subsequent decay of tree root networks decreasing soil strength on steep hillslopes which produces an increased likelihood of debris flow occurrence 3 to 10 or more years after the wildfire. Consequently, post-fire mitigation measures in forested terrain must address the risk posed by debris flows caused by progressive entrainment during the 2 years following the wildfire and debris flows due to infiltration-induced debris slides three or more years later. Mitigation for the later debris flows in forested terrain involves identification of areas with infiltration-induced debris slides coincident with concentrations of fire-killed trees. Timely reforestation of these areas after a wildfire limits the loss of soil strength from decaying roots.

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Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States

Cited by 184 publications

References 65 publications

Evolution of Debris‐Flow Initiation Mechanisms and Sediment Sources During a Sequence of Postwildfire Rainstorms

Evolution of Debris‐Flow Initiation Mechanisms and Sediment Sources During a Sequence of Postwildfire Rainstorms

Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California

A rationale for effective post-fire debris flow mitigation within forested terrain

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