Rainfall Intensity-Duration Thresholds for Debris-Flow Initiation in Recovering Burned Areas in the Southwestern United States

Abstract:

Wildfires reduce vegetation cover and alter soil properties which may subsequently increase runoff and erosion. Debris flows are often observed in recently burned, steep watersheds when overland flow concentrates and entrains sediment. Rainfall intensity-duration (ID) thresholds of debris-flow initiation can be used to assess the potential for debris flows following wildfire. The threat of debris flows may persist for several years as the landscape recovers, but there are few constraints on how and why rainfall ID thresholds change throughout the period of geomorphic recovery. In this study, we quantify how rainfall ID thresholds change as a function of time since burning and provide mechanistic explanations for those changes. We monitored rainfall and debris-flow activity, and measured soil hydraulic properties and vegetation cover within 3 different burned areas throughout the southwestern United States for a period of 3-5 years following a fire. The initiation of runoff-generated debris flows has been linked to exceedance of a slope-dependent dimensionless discharge threshold. Therefore, we estimate temporal variations in rainfall ID thresholds by using a hydrologic model, constrained by field measurements as well as hydrologic monitoring data, to estimate the rainfall intensities required to exceed the dimensionless discharge threshold. Results suggest extremely low values of hydraulic roughness immediately after fire, followed by consistent increases in hydraulic roughness with time since burning. Changes in soil hydraulic properties tend to promote increases in infiltration capacity with time since burning, though we did document a decrease in field saturated hydraulic conductivity over time at one site. Modeling suggests that rainfall ID thresholds may increase by roughly 30% or more each year. Results improve our ability to assess how post-fire debris-flow potential changes with time and provide additional constraints on the timescale of geomorphic recovery following wildfire.