Debris aggregates in debris flows and implications for debris-flow dynamics

Sediments related to debris flows consist usually of single clasts of various sizes. A kind of composite debris, i.e., elongate debris aggregates, occurring in the Jiangjia Gully debris flows in China, is reported herein. They can occur as isolated individuals, and more commonly as clusters of a few to several hundred individuals. 2,225 measured aggregates vary in maximum and minimum dimensions from 2 to 187 cm (average 17 cm) and from 2 to 107 cm (mean 12 cm), respectively. The aggregates with an elongation ratio of ≤ 0.8 make up 63%, fat ones having a flatness ratio of ≤ 0.8 represents only 37%, and at least 53% of the aggregates are prolate spheroidal. The average convexity of 0.92 illustrates that the aggregate surfaces are smooth. Massive structure (sometimes with cobble cores), concentrically banded structure, and deformation structure are developed in aggregate interiors. Slate clasts ranging in size from clay to cobbles, which originate from landslide debris in the debris-flow source areas and moving debris-flow slurries, comprise the aggregates. Field observations of more than 30 surges show that most of the aggregates belong to the objects transported by the debris flows, and rolling process is essential to their development. Fibrous palygorskite, which acts as the rock-forming mineral of the slate bedrock and is released by slate detritus, is responsible for the widespread occurrence of the aggregates. The marked presence of the elongate aggregates indicates that the debris flows can be approximated as one-dimensional, laminar flow of homogeneous single-phase yield stress fluids. Debris aggregates in debris flows merit attention because of their significant implications for debris-flow dynamics and of facilitating sediment transfer.