Multi‐Model Comparison of Computed Debris Flow Runout for the 9 January 2018 Montecito, California Post‐Wildfire Event

Hazard assessment for post‐wildfire debris flows, which are common in the steep terrain of the western United States, has focused on the susceptibility of upstream basins to generate debris flows. However, reducing public exposure to this hazard also requires an assessment of hazards in downstream a...

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Bibliographic Details
Published in:Journal of geophysical research. Earth surface 2021-12, Vol.126 (12), p.n/a
Main Authors: Barnhart, K. R., Jones, R. P., George, D. L., McArdell, B. W., Rengers, F. K., Staley, D. M., Kean, J. W.
Format: Article
Language:English
Subjects:
Back calculation
Calibration
Coefficients
Debris flow
Detritus
Domains
Downstream
Estimates
Fires
Hazard assessment
inundation
Material properties
model comparison
Montecito
Physics
Rain
Rainfall
Rainfall intensity
runout
Simulation
Soil erosion
Soil mixtures
Soil properties
Soil water movement
Soils
Topography
Uncertainty
Wildfires
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Summary:Hazard assessment for post‐wildfire debris flows, which are common in the steep terrain of the western United States, has focused on the susceptibility of upstream basins to generate debris flows. However, reducing public exposure to this hazard also requires an assessment of hazards in downstream areas that might be inundated during debris flow runout. Debris flow runout models are widely available, but their application to hazard assessment for post‐wildfire debris flows has not been extensively tested. Necessary inputs to these models include the total volume of the mobilized flow, flow properties (either inherent material properties or calibration coefficients), and site topography. Estimates of volume are possible in post‐event (“back calculation”) studies, yet before an event, volume is an uncertain quantity. We simulated debris flow runout for the well‐constrained 9 January 2018 Montecito event using three models (RAMMS, FLO2D, and D‐Claw) to determine the relative importance of volume and flow properties. We broke the impacted area into three domains, and for each model‐domain combination, we performed a numerical sampling study in which volume and flow properties varied within a wide, but plausible range. We assessed model performance based on inundation patterns and peak flow depths. We found all models could simulate the event with comparable results. Simulation performance was most sensitive to flow volume and less sensitive to flow properties. Our results emphasize the importance of reducing uncertainty in pre‐event estimates of flow volume for hazard assessment. Plain Language Summary Changes in soil properties after fire mean rain can more easily erode the surface and initiate debris flows—mixtures of water, soil, and rock that rapidly move from steep source areas to downstream regions. The combination of frequent fire, steep landscapes, dense population, and intense rain makes southern California prone to debris flows. The scientific community has established methods linking debris flow likelihood and volume to rainfall intensity and burned area characteristics. We presently lack a basis for understanding how flow volume, flow material properties, representation of flow physics, and site topography all combine to produce an inundation hazard. We begin to address this by using observations from the 9 January 2018 Montecito, California, debris flow event to test three candidate models for debris flow inundation. We found that all models can s
ISSN:2169-9003
2169-9011
DOI:10.1029/2021JF006245