Recent experimental and theoretical work on the dynamics of submarine debris
flows is summarized. Hydroplaning was first discovered in laboratory flows
and later shown to likely occur in natural debris flows as well. It is a
prime mechanism for explaining the extremely long runout distances observed
in some natural debris flows even of over-consolidated clay materials.
Moreover, the accelerations and high velocities reached by the flow head in a
short time appear to fit well with the required initial conditions of
observed tsunamis as obtained from back-calculations. Investigations of
high-speed video recordings of laboratory debris flows were combined with
measurements of total and pore pressure. The results are pointing towards yet
another important role of ambient water: Water that intrudes from the water
cushion underneath the hydroplaning head and through cracks in the upper
surface of the debris flow may drastically soften initially stiff clayey
material in the "neck" of the flow, where significant stretching occurs due
to the reduced friction at the bottom of the hydroplaning head. This
self-reinforcing process may lead to the head separating from the main body
and becoming an "outrunner" block as clearly observed in several natural
debris flows. Comparison of laboratory flows with different material
composition indicates a gradual transition from hydroplaning plug flows of
stiff clay-rich material, with a very low suspension rate, to the strongly
agitated flow of sandy materials that develop a pronounced turbidity current.
Statistical analysis of the great number of distinguishable lobes in the
Storegga slide complex reveals power-law scaling behavior of the runout
distance with the release mass over many orders of magnitude. Mathematical
flow models based on viscoplastic material behavior (e.g. BING) successfully
reproduce the observed scaling behavior only for relatively small clay-rich
debris flows while granular (frictional) models fail at all scales. For very
large release masses, hydroplaning or significant softening of the shear
layer due to water incorporation must be invoked to recover the observed
scaling behavior; a combination of both effects likely will give the most
realistic description of the phenomenon. Detailed studies of the neck
behavior and the compositional dependence of the material properties are
needed to arrive at a quantitative model. Other related and important open
questions concern the rheological model appropriate for sandy debris flows
and the suspension rate from the dense body into the associated turbidity
current. |