Developing and Testing Physically Based Triggering Thresholds for Runoff‐Generated Debris Flows

Hui Tang,Luke A Mcguire,Dennis M Staley,Joel B Smith,Francis K Rengers,Jason W Kean

doi:10.1029/2019gl083623

Hui Tang, Luke A Mcguire + Show 4 more

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https://doi.org/10.1029/2019gl083623

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Abstract

AbstractRunoff in steep channels is capable of transitioning into debris flows with hazardous implications for downstream communities and infrastructure, particularly in alpine landscapes with minimal vegetation and areas recently disturbed by wildfire. Here, we derive thresholds for the initiation of runoff‐generated debris flows based on critical values of dimensionless discharge and Shields stress. These thresholds are derived by using a numerical model to estimate the hydrodynamic conditions coinciding with the timing of debris flow activity in a recently burned basin. A benefit of hydrodynamic thresholds is that they can be used to assess debris flow likelihood based on measurable hydrologic and geomorphic parameters and therefore provide more universal criteria for quantifying the runoff‐to‐debris flow transition in landscape evolution studies and hazard assessments. We then demonstrate how hydrodynamic thresholds can be used to estimate rainfall intensity‐duration thresholds for runoff‐generated debris flows without the need for historic debris flow observations.

Highlights

Runoff-generated debris flows are common in recently burned areas due to wildfire-induced effects on the soil and vegetation that substantially increase runoff and erosion relative to similar unburned areas (Moody et al, 2013)
We hypothesize that the hydrodynamic forces needed to initiate runoff-generated debris flows are universal, but rainfall ID thresholds are variable in time and space because the transfer of rainfall to overland flow is largely regulated by regional differences in topography, vegetation, and soil infiltration properties
Physical Thresholds Simulations of runoff and sediment transport at the Las Lomas watershed demonstrate that q* is greater during debris flow producing storms relative to storms that only produce water-dominated flows (Figure 2)

Summary

Introduction

Runoff-generated debris flows are common in recently burned areas due to wildfire-induced effects on the soil and vegetation that substantially increase runoff and erosion relative to similar unburned areas (Moody et al, 2013). Gregoretti and Fontana (2008) suggest that runoff-generated debris flows initiate once a slope-dependent dimensionless discharge has been exceeded. Laboratory experiments indicate that the initiation of runoff-generated debris flows can be related to hydrodynamic limits of dimensionless discharge (Gregoretti, 2000; Gregoretti & Fontana, 2008) and Shields stress (Prancevic et al, 2014). We test the applicability of these two metrics, dimensionless discharge and Sheilds stress, for predicting debris flow activity at the basin scale using a postfire debris flow dataset

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