Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses and cars, thus blocking bridges and tributaries causing flooding along its path.
Debris
flow is often mistaken for flash flood,
but they are entirely different processes.
Muddy-debris
flows in alpine areas cause severe damage to
structures and infrastructure and often claim human lives. Muddy-debris flows
can start as a result of slope-related factors and shallow landslides can dam stream beds,
resulting in temporary water blockage. As the impoundments fail, a "domino effect" may be created, with a
remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The
solid-liquid mixture can reach densities of up to 2 tons/m³ and velocities
of up to 14 m/s (Chiarle and Luino, 1998; Arattano, 2003). These processes
normally cause the first severe road interruptions, due not only to deposits
accumulated on the road (from several cubic metres to hundreds of cubic
metres), but in some cases to the complete removal of bridges or roadways or
railways crossing the stream channel. Damage usually derives from a common
underestimation of mud-debris flows: in the alpine valleys, for example,
bridges are frequently destroyed by the impact force of the flow because their
span is usually calculated only for a water discharge. For a small basin in the
Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and
Luino (1998) estimated a peak discharge of 750 m3/s
for a section located in the middle stretch of the main channel. At the same
cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s,
a value about 40 times lower than that calculated for the debris flow that
occurred.
Earthflows are downslope, viscous flows of
saturated, fine-grained materials, which move at any speed from slow to fast.
Typically, they can move at speeds from 0.17 to 20 km/h. Though these are
a lot like mudflows, overall they are slower moving and
are covered with solid material carried along by flow from within. They are
different from fluid flows in that they are more rapid. Clay, fine sand and
silt, and fine-grained, pyroclastic material are all susceptible to earthflows.
The velocity of the earthflow is all dependent on how much water content is in
the flow itself: if there is more water content in the flow, the higher the
velocity will be.
These flows usually begin when the pore pressures
in a fine-grained mass increase until enough of the weight of the material is
supported by pore water to significantly decrease the internal shearing
strength of the material. This thereby creates a bulging lobe which advances
with a slow, rolling motion. As these lobes spread out, drainage of the mass
increases and the margins dry out, thereby lowering the overall velocity of the
flow. This process causes the flow to thicken. The bulbous variety of
earthflows are not that spectacular, but they are much more common than their
rapid counterparts. They develop a sag at their heads and are usually derived
from the slumping at the source.
Earthflows occur much more during periods of
high precipitation, which saturates the ground and adds water to the slope
content. Fissures develop during the movement of clay-like material creates the
intrusion of water into the earthflows. Water then increases the pore-water
pressure and reduces the shearing strength of the material.