Laboratory Study of the Compositional Dependence of Subaqueous Debris Flow Behavior Using Particle Tracking A. Elverh°i*, F. V. De Blasio*, D. Issler**, T. Ilstad*, and C .Harbitz***, *Department of Geology, University of Oslo/International Centre for Geohazards (ICG), Norway ** NaDesCoR/ International Centre for Geohazards (ICG), Switzerland *** Norwegian Geotechnical Institute/International Centre for Geohazards (ICG), Oslo, Norway Subaqueous gravity mass flows were studied in a 10 m long, 3 m high flume channel at St Anthony Falls Laboratory, University of Minnesota. All slurries contained a fixed amount of water, 35 wt %, while the clay (kaolin) and sand (silica) contents were varied from 5 to 32.5 wt. % and 60 to 32.5 wt. %, respectively. The front velocity was determined from video recordings with a moving camera while a high-speed camera at a fixed location allowed to measure the flow depth and deposition rate and to track 0.5mm coal-slag tracer particles. In addition, two pairs of total load and pore pressure sensors were mounted at the location of the high-speed camera and 4 m further downstream. In all runs, high excess pore pressure was found; for high and medium clay content, the head of the flows was hydroplaning. Previous laboratory experiments and theoretical studies show that the presence of water at the interface between the debris flow and the sea bed may dramatically enhance the mobility of the mass. Due to the acceleration of the hydroplaning head relatively to the main slide body, the head may become partly detached from the remaining body. In some cases, particularly at high clay contents, the detachment may become complete ("auto-acephaliation") and such hydroplaning blocks are even more mobile, and may have a run-out far ahead of the main debris depositions. All velocity profiles except one show a highly sheared layer at the bottom of the debris flow, whose relative thickness decreases with increasing clay content. In the "plug" layer above it, the relative motion between particles is quite significant at low clay concentration. Fitting the observed velocity profiles to Bingham rheology and comparing with rheological measurements of the initial slurries properties shows that very substantial weakening occurred during the flow in the bottom shear layer. This effect is attributed to mixing with ambient water during intensive shearing. It is conjectured that water penetrates from the edge of the hydroplaning layer. Similar mechanisms are likely to operate in natural debris subaqueous debris flows and may thus provide an additional explanation for the long runouts of those flows.