Geoscience Reference
In-Depth Information
Flows
As the name implies, flows are mass-wasting events involving
sediments that are very wet. Such conditions occur when rainfall is
high for a period of time, or when humans have stripped hillslopes
of protective vegetation. These processes are catastrophic forms
of mass wasting and are a serious environmental hazard in many
parts of the world. Flows are common in Southern California, for
example, and are especially dangerous in suburban areas where
homes are built on steep hillslopes. Every two or three years some
form of major flow occurs in this area, causing extensive property
loss and even the occasional human death. Three basic kinds of
flow occur: (1) earthflows, (2) mudflows, and (3) debris flows.
An
earthlow
is a slow-to-rapid type of mass movement
that involves soil and other loose sediments, some of which may
be coarse. This kind of flow may move downhill across a fairly
broad surface. Although a
mudlow
is very similar to an earth-
flow, it differs because it consists of fine-textured sediments and
is extremely fluid; thus, it moves more quickly. Mudflows are
most frequently associated with steep canyons in mountainous
regions and are initiated in humid zones when heavy rains fall on
already saturated slopes. In arid zones, they occur when heavy
rains fall on hillslopes that are unprotected by vegetation. Re-
gardless of the environment, mudflows start when large volumes
of water begin to run directly off the surface down the canyon. As
the water flows, it picks up increasing amounts of mud from the
underlying soil, resulting in a thick, viscous flow of water-laden
sediment that has tremendous power. In large mudflows, the
power is so great that large boulders are frequently transported
great distances and human structures such as homes and bridges
can be destroyed. Once the mudflow reaches the valley, it often
spreads out across the landscape (Figures 14.16 and 14.22).
down the slope along a concave plane relative to the surface.
Slumps typically occur when impermeable clay-rich deposits
such as shale underlie more porous rock or sediment. Under
such circumstances, water will flow through the upper deposits
and then parallel to the clay surface upon encountering it. This
water lubricates the contact between the rock layers and thus
reduces frictional force, causing the rock and sediment mass to
rotate as it slides down the hill (Figure 14.21a).
At the top of the slump, a wall-like escarpment is produced
where the slump block breaks away, leaving a steep face. The
sediment in the middle of the slump usually moves more quickly
than near the scarp, resulting in a series of step-like features called
slump terraces
. At the base of the slump, the sediment flows more
slowly and bulges out from the middle section of the flow as a
distinctive
toe
. Although slumps happen virtually everywhere, they
are by far more common in humid regions because the sediments
are more frequently saturated there. They usually occur in isolated
places, such as in Figure 14.21b, with very little impact on humans.
Occasionally, however, a large slump occurs in a residential zone
and causes severe damage to personal property.
Scarp
Slump terraces
Toe
(a)
Earthflow
A slow-to-rapid type of mass movement that involves
soil and other loose sediments, some of which may be coarse.
Mudflow
A well-saturated and highly fluid mass of fine-
textured sediment.
(b)
Figure 14.21 Slump processes.
(a) A typical slump includes a
scarp, slump terraces, and a toe. (b) Slumps like this frequently
occur in north-central Kansas because a thin shale (and asso-
ciated regolith) at the base of hills becomes lubricated during
heavy spring rains. Repeated slumps like this one have gradually
caused the hillslopes to retreat throughout the area, forming these
small hills.
Figure 14.22 Mudflow deposits.
Mudflows occur when a signif-
icant amount of saturated fine sediments flows quickly. This mud-
flow deposit is located in the Never-Summer Range in Colorado.
Note the lack of coarse fragments such as rocks and boulders.
