The Role of Debris Flow Grain Size in the Post-Earthquake Sediment Cascade, Wenchuan, China

Abstract:

Co-seismic landslide sediment is evacuated from hillslopes at an unknown rate. This is partly due to the fact seismically-sourced sediment must transit low order catchments to enter the fluvial network. Hence large, catchment transiting debris flows are an important mechanism for transporting this sediment into higher order channels. The runout mechanisms of these flows, such as the dissipation of excess pore pressures, are partly governed by the grain size distribution (GSD) of the flow. Further, debris flow runout GSDs are likely to control the onward transport of this sediment, and subsequently the post-earthquake sediment cascade.

Here, we present preliminary GSDs for two post-earthquake debris flows which occurred in 2019 close to the epicentre of the 2008 Wenchuan earthquake. These large debris flows mobilised co-seismic landslide sediment stored in low order channels and traversed their catchments to deposit into higher order rivers within the mountain range. They are also representative of typical post-earthquake debris flows in the area. To capture the GSD of both the fine- and coarse-grained sediment we used two techniques. To estimate the coarser grains, we dug four to eight 1 m x 1 m x 0.5 m pits on each debris flow. We sieved the sediment excavated from these pits at 10 cm depth increments into five size fractions, > 8 cm, 4 – 8 cm, 2 – 4 cm, 1 – 2 cm and <1 cm. We then sieved ~1 kg of the <1 cm fraction in the laboratory to estimate the distribution of the finest grains. The coarse fraction was independently constrained using photogrammetry and coupled with drone imagery to ensure the coarsest fraction was measured (≥1 m). Preliminary results indicate GSDs of debris flows are primarily influenced by the coarsest boulders, with the distribution of fine grains similar both vertically and laterally across the runout. We also noticed an absence of inverse grading throughout the deposits. These findings help to further our understanding of sediment transport and deposition from debris flows in the years following an earthquake.