Subaqueous debris flows were studied through a set of laboratory experiments examining the behaviour within the flow and the effects of varying the clay-sand mass fraction at a fixed water content. They confirm earlier experiments where mobility is enhanced by a lubricating layer and run-out is largely influenced by other mechanisms than the rheological properties of the debris flow itself for high clay content flows. Through particle tracking by high-speed video, a better understanding of the mobility of subaqueous debris flows has been established. For highly coherent flows, the heads of the flows were hydroplaning. In many cases, hydroplaning stretched, and eventually ruptured, the flowing mass. Fitting the observed velocity profiles to Bingham rheology and by comparing with standard rheological measurements of the pre-flow slurries demonstrates very substantial weakening and softening in the bottom shear layer, but not in the overlying plug layer. This effect is attributed to mixing with ambient water. Weakly coherent flows show an intensively fluidized front; the entire head breaks up into a turbulent flow. Behind the head, one finds a dense flow layer where sand grains drop out during the flow and form a depositional layer. The highest mobility is found for slurries with a low yield stress combined with sufficient competence to allow a moderate settlement of sand during flow. Similar mechanisms are likely to operate in natural subaqueous debris flows and may thus provide an additional explanation for the long run-out distances of those flows.
Bibliographical noteFunding Information:
This research was carried out with financial support from The Research Council of Norway (project 133975/431, “Submarine mass wasting–rheology, flow behaviour and deposition geometry”), the EU project COSTA (“Continental slopes and stability”, contract EVK3-CT-1999-00006), the International Centre for Geohazards, Norway and the U.S. Office of Naval Research. This is publication number 58 of the International Centre for Geohazards. The experiments could not have been performed without help from the laboratory staff and students at the St. Anthony Falls Laboratory, University of Minnesota. In particular, G. Parker is acknowledged for his valuable contributions during planning as well as execution of the experiments. Thanks are due to David W. Giles at the Department of Chemical Engineering and Material Science, University of Minnesota, for help with the rheological testing of the slurries, and to A. Moe of the Norwegian University of Technology and Science for help with the particle tracking. Help from C. B. Harbitz, Norwegian Geotechnical Institute, for his useful comments on the article was greatly appreciated. The authors also wish to thank the staff at the Geotechnical Department of the Norwegian University of Science and Technology, and Prof. J.P. Nystuen, University of Oslo, for enjoyable discussions at different stages of the work. The manuscript benefited from careful reviews by K. Høeg and B.D. Bornhold.
- debris flows
- flow behaviour
- gravity flows
- laboratory experiment
- particle tracking
- sand/clay ratio