Geoscience Reference
In-Depth Information
Debris flow hazard and structural damage
HUMAN IMPACT
Debris flows are among the most unpredictable, fast-moving forms of mass wasting process, capable of self-
regeneration in transit. For these reasons they pose particular hazards to human settlements and structures. They
are initiated by a wide range of stimuli - landslides, seismic activity, intense rainfall, rapid snow or glacier melt, water
eruption (e.g. a bursting pipe), volcanic eruption, etc. - depending on the critical condition of slope materials at the
time of potential failure. Their essential requirement is rapid fluidization of granular debris, hence the rapid water
delivery and/or high-porosity debris components listed. They are common on arctic and alpine slopes (see
Chapter
24).
Explosive eruptions on ice-capped stratovolcanoes generate some of the most spectacular, destructive debris
flows and provide their own granular debris in the form of ash falls. They are also, however, an increasing phenomenon
in steep terrain aggravated by more intense rainfall, as an impact of climate change, and through human propensity
to build on land unsuitable through its inherent instability and sensitive vegetation. In the Transverse Ranges north
of Los Angeles in California, for example, desirable residences above the urban smog and with long southern views
add to a potent cocktail of steep, seismically active slopes, climate desiccation and aromatic and oily Mediterranean
vegetation which is naturally rejuvenated by fire. Avoidable brush fires denude the slopes - and infrequent but intense
rainstorms wash away the exposed debris (
Plate 13.21a
)
.
Debris flows and lahars (volcanic mud flows) triggered by the Nevado del Ruiz eruption in Colombia (1985) swept
over tens of kilometres and claimed more than 25,000 lives. The 1980 Mount St Helens eruption in Washington
State (United States) claimed few lives but debris flows travelled over 20 km along the Toutle river valley. The typical
sequence of events started with an earthquake, triggering the eruption via a major landslide which opened up the
blast vent. Unconsolidated slope materials were disturbed by both shocks and fluidized by ice melt, surface drainage
disruption and lake burst.
Such catastrophic debris flows are rarely experienced in Britain, although the 1966 Aberfan disaster claimed 144
lives and was caused by the fluidization of dumped coal waste and debris avalanche/flow. Debris flows are a previously
underestimated and probably increasing hazard. Imagine driving along the A5 Euro-route through Snowdonia or around
the Great Orme Marine Drive, Llandudno - or any other upland road - and confronting a wall of boulders up to 2 m
high, bouncing and jostling along at 20-30 km hr
-1
. Intense summer rainfall exceeding 120 mm in three to five hours
in September 1983 and June 1996 triggered such debris flows in complex colluvial deposits of glacial, periglacial and
talus materials. Shallow initial slides were rapidly fluidized and transformed to debris flows, travelling downslope as
a series of turbulent pulses.
Each flow gouged a track 3-10 m deep, displacing debris thrown out to form parallel
levées
(banks) by violent boulder
collisions. The same collisions contribute to the buoyancy of the flow but large 'grains' move faster than others and
eventually form a boulder front through which water drains. This debris 'slug' then grinds to a halt but the water may
continue and repeat the process several times before draining away. The largest pulsed debris flow on the A5 travelled
approximately 600 m down slopes of 22-35 before cutting the carriageway, and over twenty flows devastated the
Marine Drive. One flow travelled over 900 m along the Drive, unable to drain through its impervious surface, leaving
a meandering trail of debris slugs
(
Plate 13.21b
)
. Debris punched several large holes in the Drive and retaining walls
and formed new flows before plunging into the sea. These are among several dozen similar events in north Wales
alone since 1980. The cost of immediate clearance, longer-term repair and remedial work and the loss of revenue
amounted to some £500,000. Over 50,000 km
2
of upland slopes in Britain may be susceptible to debris flow hazard,
and their sensitivity will increase with changing climate and land management.
