WITH A HUGE STORM HITTING CALIFORNIA, THE MUDSLIDES ARE ALSO
COMING. LEARN HOW TO PROTECT YOURSELF
AND YOUR PROPERTY
The first significant rainfall in drought-stricken California
in months was threatening more mudslides on Monday and Tuesday after motorists
were stranded on the iconic Pacific Coast Highway over the weekend. Los Angeles
County was battered by 1.4 inches of rain Sunday, causing mudslides and
rockslides on sections of Route 1, officials told NBC Los Angeles. No one was
injured but drivers were forced to abandon at least a dozen vehicles and a
stretch of the highway was closed west of Malibu.
With 80 percent of the state experiencing extreme
drought, the soaking came as a welcome reprieve in many areas. But with more
rain on the way Tuesday through Wednesday morning, the downpours were expected
to come with an increased risk of further mudslides. The National Weather
Service issued a flash flood watch for mountain areas in Southern California
through Wednesday morning, and Chad Burke of The Weather Channel said,
"There is always the risk there since a lot of structures are built on top
of or at the bottom of steep hills."
California's historic drought has played a role in
the more than 5,500 wildfires in 2014 — about 1,000 more than the yearly
average. The Weather Channel said the resulting burn-scar areas were at
particular risk of mudslides and flash floods.
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 Californians have been killed by debris flows
during the past 25 years. Most of these deaths occurred when debris flows
buried people sleeping in lower-floor bedrooms 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 Californians have been
killed by debris flows during the past 25 years. Most of these deaths
occurred when debris flows buried people sleeping in lower-floor bedrooms
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.
