Grain-size variability in debris flows of different runout lengths, Wenchuan, China

Harvey, Erin L., Hales, Tristram C. ORCID: https://orcid.org/0000-0002-3330-3302, Liu, Jie, Hobley, Daniel E.J. ORCID: https://orcid.org/0000-0003-2371-0534, Yang, Fan, Xia, Bing and Fan, Xuanmei 2023. Grain-size variability in debris flows of different runout lengths, Wenchuan, China. GSA Bulletin 10.1130/B37027.1

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Abstract

Debris-flow grain-size distributions (GSDs) control runout length and mobility. Wide, bimodal GSDs and those containing a higher proportion of silt and clay have been shown experimentally to increase runout length. However, the relationship between grain size and mobility has not been well established in field conditions. Here, we compared the grain-size characteristics of two debris flows with considerably different runout lengths (1.5 km vs. 8 km) to understand the role of grain size in governing runout. The two debris flows were triggered in the same rainfall event from coseismic landslide debris generated in the 2008 Wenchuan earthquake in catchments with similar lithology and topography. We compared the deposited GSDs and their spatial patterns using our rare, three-dimensional GSD datasets. Surprisingly, the proportions of each size fraction deposited by the two flows were statistically indistinguishable. The spatial pattern of grain size differed between the two flows, with evidence of inverse grading only preserved in the smaller deposit. From these observations, we can infer that the GSDs of both flows were determined by the coseismic landslide source material, and that there was little difference in the GSDs of material entrained as the flows bulked. The contrasting spatial distributions of grains indicated that different internal processes were dominant within the two flows. These findings demonstrate that where GSDs are dominated by coarse grains and are governed by similar source conditions, grain size plays a lesser role relative to sediment supply and hydrology in controlling the runout length of large catastrophic post-earthquake debris flows.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Earth and Environmental Sciences
Publisher:	Geological Society of America
ISSN:	0016-7606
Date of First Compliant Deposit:	10 November 2023
Date of Acceptance:	3 October 2023
Last Modified:	15 Nov 2023 10:01
URI:	https://orca.cardiff.ac.uk/id/eprint/163816

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