Constraining the relative importance of raindrop‐ and flow‐driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales

Abstract

Mountain watersheds recently burned by wildfire often experience greater amounts of runoff and increased rates of sediment transport relative to similar unburned areas. Given the sedimentation and debris flow threats caused by increases in erosion, more work is needed to better understand the physical mechanisms responsible for the observed increase in sediment transport in burned environments and the time scale over which a heightened geomorphic response can be expected. In this study, we quantified the relative importance of different hillslope erosion mechanisms during two postwildfire rainstorms at a drainage basin in Southern California by combining terrestrial laser scanner‐derived maps of topographic change, field measurements, and numerical modeling of overland flow and sediment transport. Numerous debris flows were initiated by runoff at our study area during a long‐duration storm of relatively modest intensity. Despite the presence of a well‐developed rill network, numerical model results suggest that the majority of eroded hillslope sediment during this long‐duration rainstorm was transported by raindrop‐induced sediment transport processes, highlighting the importance of raindrop‐driven processes in supplying channels with potential debris flow material. We also used the numerical model to explore relationships between postwildfire storm characteristics, vegetation cover, soil infiltration capacity, and the total volume of eroded sediment from a synthetic hillslope for different end‐member erosion regimes. This study adds to our understanding of sediment transport in steep, postwildfire landscapes and shows how data from field monitoring can be combined with numerical modeling of sediment transport to isolate the processes leading to increased erosion in burned areas.

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

“…Water flow and sediment transport within multiple particle‐size classes are modeled using the two‐dimensional, nonlinear shallow water equations coupled with a set of advection equations used to track the movement of sediment in each particle‐size class. Additional source terms related to debris‐flow resistance, following Iverson and Denlinger (), are included within the momentum equations (McGuire et al, ). Debris flows are identified within the model solution based on the volumetric sediment concentration ( c * ) and flow depth ( h ; McGuire et al, ).…”

“…Additional source terms related to debris‐flow resistance, following Iverson and Denlinger (), are included within the momentum equations (McGuire et al, ). Debris flows are identified within the model solution based on the volumetric sediment concentration ( c * ) and flow depth ( h ; McGuire et al, ). If the flow depth exceeds 10 cm, and the volumetric sediment concentration is greater than 40%, then the flow is considered as a debris flow (McGuire et al, ).…”

“…Debris flows are identified within the model solution based on the volumetric sediment concentration ( c * ) and flow depth ( h ; McGuire et al, ). If the flow depth exceeds 10 cm, and the volumetric sediment concentration is greater than 40%, then the flow is considered as a debris flow (McGuire et al, ). Infiltration is represented using the Green‐Ampt equation (Green & Ampt, ).…”

“…Letting h , u , and v denote flow depth and depth‐averaged velocities in the x and y directions, respectively, the Hairsine‐Rose equations for sediment size class k are where c k is the sediment concentration of sediment size class k , m k is the deposited sediment mass for sediment size class k per unit area, r k and r rk denote rates of entrainment and reentrainment due to runoff, and e k and e rk denote the rates of raindrop‐driven detachment and redetachment. The deposition rate, d k , for sediment size class k is a function of the clear water settling velocity and the total sediment concentration (McGuire et al, ). Letting a and a d denote raindrop detachment and redetachment coefficients, the rates of raindrop‐driven detachment ( e k ) and redetachment ( e rk ) are given by where is the degree to which deposited sediment shields the underlying soil from erosion, is the mass of deposited sediment needed to completely shield the orig...…”

“…The raindrop detachment and redetachment coefficients, a and a d , decrease with flow depth according to where h 0 = d r /3 is a critical depth of overland flow over which the influence of raindrop impact is assumed to decay, d r denotes the median raindrop diameter, b =1 is an empirical coefficient, is the flow depth measured vertically from the soil surface, and θ t is the topographic slope angle. The mass of deposited sediment required to shield the original soil completely also decreases with increasing flow depth (see McGuire et al, ).…”