THE ROLE OF TOPOGRAPHY AND GRAIN SIZE IN CONTROLLING POST-EARTHQUAKE DEBRIS FLOW DEPOSITION

In the years following the 2008 M_w7.9 Wenchuan earthquake, extremely large (>10⁵ m³), fluidised debris flows have posed a significant hazard to local communities. These debris flows traverse catchments, covering many kilometres, and bulk to volumes orders of magnitude larger than the initiation volume. However, we lack an understanding of why these debris flows can reach such large volumes and what conditions cause them to eventually stop and deposit material. The largest debris flows in Wenchuan often deposit material at channel junctions suggesting abrupt changes in topography may be important. Additionally, internal debris flow properties, such as the width of the grain size distribution (GSD) and proportion of clay and silt entrained during transport may affect depositional processes. As such, we anticipate that topography and internal debris flow properties may play an important role in controlling how these debris flows deposit material. Here, we use field data and GIS to assess how channel topography and grain size may control where these extremely large, mobile flows deposit.

Firstly, we compared the GSD of two post-earthquake debris flows with bed topography in the catchment to establish if there is an empirical relationship between the two controls. Preliminary results suggest a relationship between negative curvature and the deposition of fine-grained material. The deposition of fine sediment may reduce the mobility of debris flows by allowing excess pore pressures to dissipate more slowly. We explored the relationship between depositional properties and curvature for all post-earthquake debris flows in the region using a 10-year inventory. Our results suggest that highly mobile flows deposit where there are large changes in topography, such as at tributary junctions where rapid deposition of fine material can occur.