EGU21-7389, updated on 29 Jan 2024
EGU General Assembly 2021
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Downstream evolution of grain size in extremely mobile post-earthquake debris flows, China

Erin L. Harvey1, Tristram C. Hales1,2, Daniel E. J. Hobley1, Xuanmei Fan3, Jie Liu3, Qiang Xu3, and Runqiu Huang3
Erin L. Harvey et al.
  • 1Cardiff University, Cardiff University, School of Earth and Ocean Sciences, United Kingdom of Great Britain – England, Scotland, Wales (
  • 2Cardiff University, Cardiff University, Sustainable Places Research Institute, United Kingdom of Great Britain – England, Scotland, Wales
  • 3The State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, Sichuan, China

Large, catchment transitioning debris flows are an important mechanism for transporting sediment from hillslopes into higher order channels. Extremely large flows can exceed volumes of 109 m3, however even flows with volumes of  ~103 m3 can lead to fatalities and extensive damage. Few processes transport a wider range of grain sizes than debris flows, which can transport grains from clays to 10 m boulders. While the structure of debris flows can often be inferred by their deposits, the range of grain sizes presents a challenge for their interpretation. Debris flow grain size distributions can be used to constrain debris flow runout due to their effect on excess pore pressure dissipation. Currently, there is limited data available for the entire grain size distribution of debris flow deposits in the field.

We constrained the entire grain size distribution for two extremely large (>1 km in length) post-earthquake debris flows in Sichuan Province, China. These debris flows were triggered in August 2019 after an extreme rainfall event occurred close to the epicentre of the 2008 Wenchuan earthquake. We sampled the debris flows in November 2019 at intervals of 200 m and 500 m, respectively. At each site, we used a combination of field and laboratory sieving to obtain the coarse and fine fraction for both the surface and subsurface. We dug 1 m x 1 m x 0.5 m pits, excavating each layer at 10 cm depth increments. We sieved these increments into five size fractions in the field, including < 1 cm. We sieved 1 kg of the <1 cm fraction in the laboratory to estimate the distribution of the finest grains. The coarse surface fraction was then independently constrained using photogrammetry. Preliminary results for one debris flow show that the distribution of fine grains (~<4 mm) is consistent both laterally and vertically across the runout. This suggests that the processes occurring vertically and laterally during deposition result in the consistent distribution of fines.

How to cite: Harvey, E. L., Hales, T. C., Hobley, D. E. J., Fan, X., Liu, J., Xu, Q., and Huang, R.: Downstream evolution of grain size in extremely mobile post-earthquake debris flows, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7389,, 2021.


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