Environmental Engineering Reference
In-Depth Information
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 1a). 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.
Debris flows are frequently triggered by earthquakes in active orogens, and are
particularly large at higher altitudes by virtue of the high relief, steep slopes and fast
weathering rates (Plates 1b, c).
