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.