Abstract
Catastrophic debris flows are common after large earthquakes and pose a significant risk for recovering communities. The depositional volume of these large debris flows is often much greater than the initiation volume, suggesting that bulking of the flow plays an important role in determining their volume, speed, and runout distance. Observations from recent earthquakes have driven progress in understanding the relationship between triggering rainfall events and the timing of post-earthquake debris flows. However, we lack an adequate mechanism for quantifying bulking and applying it within a hazard context. Here we apply a 2D dynamic debris flow model (Massflow) that incorporates a process-based expression of basal entrainment to understand how debris flow bulking may occur within post-earthquake catchments and develop hazard maps. Focussing on catchments in the epicentral area of the 2008 Mw 7.9 Wenchuan Earthquake, we first parameterised the model based on a large debris flow that occurred within the Hongchun catchment, before applying the calibrated model to adjoining catchments. A model sensitivity analysis identified three main controls on debris flow bulking; the saturation level of entrainable material along the flow pathway, and the size and position of initial mass failures. The model demonstrates that the difference between small and very large debris flows occur across a narrow range of pore-water ratios (λ). Below λ = 0.65 flows falter at the base of hillslopes and come to rest in the valley bottom, above λ = 0.70 they build sufficient mass and momentum to sustain channelised flow and transport large volumes of material beyond the valley confines. Finally, we applied the model across different catchments to develop hazard maps that demonstrate the utility of Massflow in post-earthquake planning within the Wenchuan epicentral region.
Highlights
Debris flows are water-saturated masses of soil and rock that rush down hillsides, funnel into stream channels, and evacuate large volumes of material from valley catchments (Iverson and George, 2014)
To assess the sensitivity of debris flow evolution to each of these controls, we explore the requisites necessary to initiate and sustain the largest events using a 2D dynamic model of debris flow evolution (Massflow) that incorporates a process-based expression of basal entrainment (Ouyang et al, 2015a)
To ensure model consistency, we compared our model outputs against a prior application of Massflow at Hongchun Gully (Wenchuan, China) by Ouyang et al (2015a), and assessed the impact of a modification made to limit the depth of entrainment to reflect the limited co-seismic landslide deposits available within the catchment. We used this process-based expression of bed entrainment to explore the potential debris flow hazard by performing a sensitivity analysis to examine the relative importance of initiation location, volume, and bed hydrology in controlling the size and final run out extent of debris flows
Summary
Debris flows are water-saturated masses of soil and rock that rush down hillsides, funnel into stream channels, and evacuate large volumes of material from valley catchments (Iverson and George, 2014). Few dynamic run out models include processbased descriptions of entrainment, but rather employ calibrated bulking factors that increase the flow volume proportionally to either topographic variables (slope angle, morphology, distance travelled) (Cascini et al, 2011; Fannin and Wise, 2001), or velocity and depth of the flowing mass (Chen et al, 2006; Cuomo et al, 2016; McDougall and Hungr, 2005) These approaches disregard changing local conditions that can drastically alter the final volume and run out extent. To assess the sensitivity of debris flow evolution to each of these controls, we explore the requisites necessary to initiate and sustain the largest events using a 2D dynamic model of debris flow evolution (Massflow) that incorporates a process-based expression of basal entrainment (Ouyang et al, 2015a) We apply this to a post-earthquake context by assessing three catchments affected by the 2008 Mw7.9 Wenchuan earthquake
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