(Conditions)
Perhaps the most common medium for landslides is colluvium -- basically particles
of weathered rock in the process of becoming soil. Mature soils settle out into
homogenous layers, called horizons. If colluvium stays still long enough, it
may form horizons too. But on unstable hillsides it remains young and restless,
a poorly sorted conglomeration of silt, sand, pebbles, and rocks.
Colluvium covers most of the ground surface in humid temperate zones and
semi-arid mountainous areas. It is ubiquitous in the western United States and
the Appalachian Range, from Maine to Alabama. Along the California and Oregon
coasts tectonic stress has shattered the sandstone-and-shale bedrock of the
Coast Range, and frequent ocean storms have accelerated the weathering of the
rock. The result is a mantle of loose, cruddy dirt -- pre-dirt, if you will -- that
produces some of the best forests on earth. Colluvium appears to be just
sitting there, growing trees, but over hundreds and thousands of years it is on
the move, and the direction of its movement is always downhill. It trickles
into narrow gulleys, or swales, which form distinctive tracks running straight
down the steep foothills. These natural sluices also catch water draining off
the hilltop ridges. Colluvium may accumulate for millennia in a swale, until it
is washed out by a debris flow into the valley below and the cycle starts anew.
The angle of repose for colluvium slopes averages 35 degrees but varies widely.
There are thickly forested 45-degree slopes in the Oregon Coast Range that
appear stable, and 26-degree hillsides in the San Francisco Bay area that have
failed catastrophically. The stability of the glacially compacted bluffs on
Washington's Puget Sound is sorely threatened beyond 33 degrees, especially
when conditions are unusually wet. The overall incline of the cliff at Rolling
Bay is just under 40 degrees.
The first axiom of debris flows is that where one has occurred, others will
inevitably follow. But nobody knows when. Water must enter the soil not only in
sufficient but also in correctly timed amounts. Typically what is required is a
long saturation period followed by intense bursts of rain concentrated in just
a few hours or days. Water passes rapidly through surface material until it
hits bedrock or clay, creating a saturated zone as it fills the pores between
solid particles. Rising pore pressure creates a buoyancy that effectively
reduces the stabilizing friction of the colluvium on the slope beneath it. At
some point gravity overcomes the natural inertia, and the soil mass breaks
loose, sliding down the less permeable surface below.
--> to the Aftermath
Or maybe not. The debris-flow scenario depends on numerous factors: soil depth
and composition, the kind of vegetation and the size of tree roots, subtle
variations in slope shape, road cuts, drainage pipes, incongruities in
underlying bedrock, even the presence of small animal burrows. Water can
collapse a slope after traveling beneath the surface from miles away.
Vibrations from trains are suspected of triggering debris flows. And sometimes
there seems to be no particular cause. The 1978 Bluebird Canyon landslide, near
Los Angeles, which caused $15 million in property damage, had no obvious
triggering event. In June of 1993, 75 acres of Ontario farmland suddenly
liquefied and clogged the South Nation River, creating the great Lemieux
landslide. In September of 1997 a man sitting at a bar in Port Angeles,
Washington, was crushed to death when a mudslide destroyed the building. The
area had had no appreciable rain for months.
There are basically two kinds of landslides: slow and fast. The latter are more
likely in loose, coarse material like sand or colluvium; most clay soils stick
together better and are less prone to rapid movement. Large, deep-seated
slides, in which the dislocation originates far below the face of a slope, tend
to occur more slowly than shallow debris flows, which are more deadly. Because
a few growing seasons will cover the tracks of shallow slides, their telltale
scars are harder to spot. Slide-prone slopes may remain hidden for generations
until revealed by the perfect storm. For instance, it took geologists most of
this century to grasp how vulnerable the San Francisco Bay area is to debris
flows. "We had always considered it a southern-California problem," Ray Wilson,
a landslide expert with the USGS, says. "Then we got this storm in 1982."
In January of that year a Pacific front stalled over the Bay area, dumping as
much as twenty-four inches of rain. The steady downpour was punctuated by
intense cloudbursts that reminded Wilson of his home town near Houston, where
tropical deluges are commonplace. In San Francisco they precipitated the
largest natural disaster in the region since the earthquake of 1906, causing
thirty-three deaths and nearly $300 million in property damage. When the
weather cleared, aerial photographs identified 18,000 debris flows in the ten
counties around San Francisco Bay.
For most of California's history the periodic slides in the Coast Range took
place far from populated areas. But when suburban development pushed out into
the foothills, in the latter half of this century, it spread into the
serpentine valleys beneath the swales. Seventy percent of the slides in 1982
took place in the swales.
The 1982 storm spawned volumes of research papers and forever altered
geologists' understanding of the northern-California landscape. But it was also
instructive for what it revealed about human nature. Jerry Weber, a consulting
geologist in Santa Cruz, says that our "collective disaster memory" goes back
only two years, even when lives have been lost. Even when those lives were next
door. "People cannot focus any longer than that unless they're directly
involved," Weber says. As time passed after the 1982 storm, the horror of the
slides faded from memory -- and so did the idea that they might recur. When Santa
Cruz County officials ordered two dozen houses removed from a dangerously
unstable area where ten people had perished in a slide, homeowners fought the
action and sued to reoccupy their dwellings. One turned away inspectors at
gunpoint.
The Oddstad Tragedy
NE of the most publicized debris flows of the 1982 storm occurred on Oddstad
Boulevard, in Pacifica, a town of 37,000 people south of San Francisco. Mud
with the consistency of wet concrete poured down a hillside in a 1970s
subdivision, smashing two houses and trapping three small children inside one
of them. Rescuers shoveled through the wreckage for thirty-six hours, hoping
the children had survived in a pocket of air. They hadn't. Cal Hinton, a
Pacifica city councilman who was the fire chief at the time, helped to dig out
the bodies. "I remember taking the blanket off them. They'd been in bed asleep.
They were still clean underneath, no blood or anything. I don't remember what
the autopsy said, whether they were crushed or smothered. I imagine it was a
combination."
The two lots where the houses had stood remained vacant for years. Ray Wilson
often passed by on his way to a hiking trail at the end of Oddstad. One day he
noticed a FOR SALE sign on one of the properties. By 1998 new houses had been
built on both lots and sold to new owners. This did not seem to be a noteworthy
event in Pacifica. The last newspaper story on the Oddstad tragedy was written
years ago. The parents of the dead children divorced and moved away. Deflection
walls have been built behind the new houses; dirt and vegetation have been
scraped from the hillside, and subsurface pipes are in place that are supposed
to collect water before it can cause more slides.
A legacy of the 1982 storm is Pacifica's policy requiring expert review of new
construction on problem sites like these. These sites are common in
Pacifica -- as might be expected in a town bounded by earthquake faults, a
mountain range, and cliffs that are sloughing into the sea. But despite the
precautions being taken, the San Francisco consulting geologist Robert Wright
is uneasy. "I spent about five years of my life studying that slide," he says,
"and I'm personally uncomfortable with rebuilding houses there. You just don't
put homes in the mouths of swales if you can help it. It's just too risky."
There are "hard" solutions to landslide risk -- debris dams, retaining
structures, graded slopes, elaborate drainage systems, even, in Japan, a $210
million highway overpass that coils improbably around a large slide area.
Geoscientists argue that "soft" solutions like zoning and building codes, which
rely on simple avoidance, are cheaper and safer in the long run. San Mateo
County, which permits only one living unit per forty acres in geologically
hazardous areas, probably has the strictest such ordinances in the country. But
they don't apply inside Pacifica and other San Mateo towns, where local
governments say "maybe" to developers far more often than "no." Saying no is
especially difficult because slides occur so erratically, over such long
intervals. New developments notwithstanding, the question persists: What should
be done about dangerous places where people already live?
A landslide-warning system would at least alert residents when slide conditions
were developing. "Even if you're in an area that in 1982 would have been a
deathtrap, you're really only in danger for a few days a decade -- usually at
night," Ray Wilson says. "You can broadcast a warning over the weather radio
that advises people when they need to leave. If they choose to stay -- well, God
bless 'em and keep 'em." In 1986 in the Bay area the USGS set up the nation's
first warning system, employing a two-step rainfall threshold that correlated
with high debris-flow activity. The first threshold was a seasonal accumulation
of eleven inches, normally reached a few weeks after the winter solstice. The
second threshold was any single storm that exceeded 30 percent of the local
mean average precipitation. By monitoring a network of seventy-five rain gauges
and several piezometers (which measure pore pressure in the water table), the
USGS knew when rainfall was approaching dangerous levels, and issued warnings
so that residents of risky areas could evacuate. There was talk of expanding
the system to Los Angeles County. But when the agency downsized, in 1995, the
program was abruptly canceled. "The whole thing cost peanuts -- maybe fifty
thousand dollars a year," Wilson says. "We got all kinds of good publicity, and
it really raised public consciousness. My worry is that people have forgotten
all about it now."
Torrential rains brought by 
HE earth comes unglued for a variety of reasons, but there is usually a single
triggering event. The largest, most catastrophic debris flows are caused by
earthquakes and volcanic eruptions -- hence the prevalence of landslides
throughout the Pacific Rim and the mountainous regions of China, the former
Soviet Union, and Central Europe. But most often the culprit is simply
water -- in the form of rainfall, snowmelt, or river flow. Thus there have also
been spectacular slides in Iowa and in the placid river valleys of Quebec.
Pittsburgh and Cincinnati, constructed on loose sediments cut by river systems,
incur some of the highest landslide costs in the United States.
