Hazards From
"Mudslides"...Debris Avalanches and
Debris Flows in Hillside and Wildfire
Areas
The single most important
action that should be taken by
residents on rainy nights is NOT
to sleep in lower-floor bedrooms
on the sides of houses that face
hazardous slopes.
More than 100 Californias have
been killed by debris flows during
the past 25 years. Most of these
100 deaths occurred when debris
flows buried persons who were
sleeping in lower-floor bedrooms
that were adjacent to hazardous
slopes. |
Sudden "mudslides" gushing down
rain-sodden slopes and gullies are
widely recognized by geologists as a
hazard to human life and property. Most
"mudslides" are localized in small
gullies, threatening only those
buildings in their direct path. They can
burst out of the soil on almost any
rain-saturated hill when rainfall is
heavy enough. Often they occur without
warning in localities where they have
never been seen before.
The ashy slopes left denuded by
wildfires in California are especially
susceptible to "mudslides" during and
immediately after major rainstorms.
Those who live downslope of a
wildfire area should be aware of this
potential for slope failure that is
present until new vegetation rebinds the
soil.
What Are Debris
Avalanches and Debris Flows?
Debris
avalanches and debris flows
(both popularly called
"mudslides") are shallow
landslides, saturated with
water, that travel rapidly
downslope as muddy slurries. The
flowing mud carries rocks,
bushes, and other debris as it
pours down the slopes.
A debris avalanche
(Figure 1) is a fast-moving debris flow
that travels faster than about 10 mph or
approximately 25 yards in about
5 seconds. Speeds in excess of 20 mph
are not uncommon, and speeds in excess
of 100 mph, although rare, do occur
locally.
Figure 1. Sketch of a typical debris
avalanche scar and track. Although this
figure shows the "zone of deposition" as
quite near the source, debris avalanches
can travel thousands of feet or, in
exceptional cases, miles from the point
of origin. Original drawing by Janet K.
Smith.
What Dangers Are
Posed by Debris Avalanches?
Debris avalanches
pose hazards that are often overlooked.
Houses in the path of debris avalanches
can be severely damaged or demolished.
Persons in these structures can be
severely injured or killed.
Most rainstorms are
of such low intensity that they do not
trigger debris avalanches. Some intense
storms may trigger only a few debris
avalanches. However, when the ground is
already saturated from previous rain,
even relatively short high-intensity
rainstorms may trigger debris
avalanches. For example, in January
1982, an intense rainstorm triggered
literally tens of thousands of debris
avalanches in the San Francisco Bay
Area. These 1982 debris avalanches
caught people unaware and caused 14
deaths and many injuries and destroyed
or damaged several hundred homes and
other structures.
What Causes Debris
Avalanches and Debris Flows?
The most common cause
of debris avalanches and debris flows is
the combination of heavy rainfall, steep
slopes, and loose soil. Most fairly
steep slopes have enough soil and loose
rock for potential landslides. Although
"stable" when dry, such slopes can
produce local debris flows, often
without warning.
Normally the source
of the excess water is intense rainfall,
although broken water pipes or
misdirected runoff concentrated by
roads, roofs, or large paved areas may
trigger, or help to trigger, debris
avalanches and debris flows. In
California, most debris flows occur
during wet winters.
Where Do Debris Flows
and Debris
Avalanches Occur?
Debris avalanches
occur all over the world. They are
particularly common in mountainous areas
underlain by rocks that produce sandy
soils. Debris avalanches have been noted
in southern California during at least
nine rainy seasons since 1915. They have
occurred in northern California during
at least 14 rainy seasons since 1905.
Debris flows are
known to start on slopes as low as
15 degrees, but the more dangerous,
faster moving flows (debris avalanches)
are more likely to develop on steeper
slopes. About two-thirds of all debris
avalanches start in hollows or troughs
at the heads of small drainage courses.
Typically, a debris avalanche bursts out
of a hillside and flows quickly
downslope, inundating anything in its
path. Because the path of a debris flow
is controlled by the local topography
just like flowing water, debris
avalanches and debris flows generally
follow stream courses.
Slopes burned by
range and forest fires are especially
susceptible to debris avalanches and
debris flows because of the absence of
vegetation and roots to bind the soil.
The areas directly downslope are
especially subject to damage from debris
flows.
What Can Be Done to
Avoid or Reduce the Hazard Posed by Debris
Avalanches?
To be safe, assume
that all drainages in steep, hilly, or
mountainous areas are capable of
carrying debris flows, especially if
relatively loose, sandy soils are
present in the watershed. Areas that
have been burned by regional fires are
especially vulnerable.
Avoid building sites
at the bottoms and mouths of steep
ravines and drainage courses. These
areas are the most likely to be
inundated by debris flows. The outer
"banks" of bends along such ravines also
should be avoided because swiftly
flowing debris avalanches can "ride up"
out of the bottom of the stream channel
where it bends.
Avoid building on or
below steep slopes. In general, the
steeper the slope the greater the risk.
If these areas must be used, consult
with a soils engineer and an engineering
geologist. These specialists will be
able to evaluate the potential for
mudslide problems and give advice on the
best way to minimize the risk to life
and property.
The hazard from
debris flows that occurs in modified
slope cuts can be decreased by
1) limiting the height and slope of cuts
and fills, 2) properly compacting fills
and keying them into bedrock, and
3) properly controlling the flow of
water onto slopes. If steep cuts or
fills occur below the discharge points
of runoff water from streets,
downspouts, or similar drainage
facilities onto a slope, it may be wise
to obtain advice from an engineering
geologist or erosion control specialist.
In some cases, walls
can be built to deflect potential
mudflows away from or around structures
(Figure 2). To be effective, diversion
walls must be properly designed and
regularly maintained.
Figure 2. Methods to reduce the hazard
from debris avalanches include
construction of a) deflection walls and
b) debris fences. Because of the extreme
force of impact associated with debris
flows, these and similar structures
should be carefully engineered and
constructed. The specifics of these
designs will vary from site to site.
After Hollingsworth and Kovacs, 1981.
"Mud Floods" and
"Debris Floods" Pose Hazards, Too
Residents living
directly downslope of mountainous
wildfire areas should be aware that, in
addition to life-threatening potential
debris flows and other forms of mass
movement, there is another, perhaps
deadlier hazard-- debris flooding or mud
flooding at and near the mouths of
channels that drain burned-over, ashy
slopes. Studies have shown that, in the
first year following a wildfire,
sediment yields and peak discharges or
such streams can increase up to 35-fold.
Thus occupants of dwellings near such
drainage channels could be endangered by
floods that incorporate enormous amounts
of debris and mud washed off the burned
hillsides.
Tips and Clues That
May Save Your Life...
-
Mitigation of
hazards from debris flows and debris
avalanches through construction of
permanent engineering measures takes
considerable time and money. In the
meantime, preparation for rapid
evacuations should be made.
-
Before and during
rains, frequent inspection of the
slopes (above vulnerable sites) for
extension cracks and other symptoms of
downslope movements of slope materials
can be a guide to impending failure
and a warning to evacuate. In
particular, watch for new springs or
seeps on slopes; cracks in snow, ice,
soil, or rock; bulges at the base of
slopes; the appearance of holes or
bare spots on hillsides; tilting
trees; or increased muddiness of
streams. Any sudden increase in runoff
is cause for concern.
-
Listen for unusual
rumbling sounds or noises that may
indicate shifting bedrock or breaking
vegetation or structures.
-
Stay alert to the
amount of rain falling locally during
intense rainstorms. Buy a rain gauge
(an inexpensive plastic one will
suffice) and install it where it can
be checked frequently.
-
Whenever rainfall
has exceeded 3 or 4 inches per day or
¼ inch per hour, the soil may be
waterlogged and more rain can trigger
mudflows.
-
Again, the single
most important action that should be
taken by residents on rainy nights is
NOT to sleep in lower-floor bedrooms
on the sides of houses that face
hazardous slopes. More than 100
Californias have been killed by debris
flows during the past 25 years. Most
of these 100 deaths occurred when
debris flows buried persons who were
sleeping in lower-floor bedrooms that
were adjacent to hazardous slopes.
Where Can More
Information Be Obtained?
For general
information about debris avalanches and
other kinds of landslides, contact your
city or county geologist, or any office
of the Division of Mines and Geology.
For an assessment of
the landslide risk to an individual
property or homesite, obtain the
services of a state-licensed engineering
geologist (see the Yellow Pages of the
telephone directory). The Division of
Mines and Geology does not perform
individual site assessments or recommend
particular consultants.
For more information
about the design and construction of
debris basins, debris fences, deflection
walls, or other protective works,
consult your city or county engineer,
local flood control agency, or the U.S.
Department of Agriculture, Natural
Resources Conservation Service.
REFERENCES
California Department
of Conservation, Division of Mines and
Geology Staff, 1979, Landslides in the
Los Angeles region, California— Effects
of the February-March 1978 rains:
Division of Mines and Geology Open-File
Report 79-4LA. This report summarizes
the effects of debris flows resulting
from some storms in southern California.
Campbell, R.H., 1975,
Soil slips, debris flows, and rainstorms
in the Santa Monica Mountains and
vicinity, southern California: U.S.
Geological Survey Professional Paper
851, 51 p. This paper describes the
causes and effects of debris flows and
avalanches.
Cannon, S.H. and
Ellen, S.D., 1985, Rainfall conditions
for abundant debris avalanches, San
Francisco Bay region, California:
CALIFORNIA GEOLOGY, v. 38, no. 12, p.
267-272. The authors describe how to use
a rain gauge to determine the threshold
of risk for a debris avalanche.
Ellen, S.D., and
Wieczorek, G.F., editors, 1988,
Landsldies. floods, and marine effects
of the storm of January 3-5, 1982, in
the San Francisco Bay region,
California: U.S. Geological Survey
Professional Paper 1434, 310 p. Most
relevant are the six chapters on debris
flows and other landslides.
Hollingsworth, R. and
Kovacs, G.S., 1981, Soil slips and
debris flows, prediction and protection:
Bulletin of the Association of
Engineering Geologists, v. 18, no. 1, p.
17-28. This paper provides information
about deflection walls and similar
structures.
Smith, T.C. and Hart,
E.W., 1982, Landslides and related storm
damage, January 1982, San Francisco Bay
region: CALIFORNIA GEOLOGY, v. 35, no.
7, p. 139-152. This article summarizes
the effects of debris avalanches
triggered by a storm in northern
California.
Weber, F.H., Jr. and
Treiman, J.A., 1979, Slope instability
and debris flows, Los Angeles area:
CALIFORNIA GEOLOGY, v. 32, no. 1, p.
3-5. This article describes the effects
of debris flows in southern California.
DMG NOTE 33
was written by Alan Barrows and Ted
Smith
|