area suffered highest death toll from mass movements in recent times in
December 1999 when 30,000 died in towns along Caribbean coast. actual number
is unknown as many people swept out to sea. mass movements fuelled down steep
valleys on the north side of the sierra de Avila mountain range which runs parallel
to the coast onto the towns below.
Factors influencing mass movement hazards were both natural and human: coastal
area had densely populated and mainly urban, despite being frequently affected by
slides and flows. no one survived in the many shanty towns where flimsy shacks
were easily destroyed. many coastal towns were built on old debris flows. very
steep mountains behind narrow coastal strip have many slopes steeper than the
angle of repose for loose materials. metamorphic rocks have planes of weakness
along which landslides will occur where they are inclined towards lower areas.
December 1999 was extremely wet, as 911mm fell 52hours equivalent to years
average rainfall. The slopes had lost protection and stability because trees had
been cleared by local people.
Effect on lives and property: 1000’s of landslides triggered by rain affected a 60km
stretch of coast. as rock and soil slid down, water turned them into debris flow with
high densities of rock including large boulders and mud. moved very fast and were
destructive. after caraballeda suffered similar experience in 1951, the channel had
been followed by the debris flow then was lined with concrete in an attempt to
guide any future flow safely to sea. the 1999 flow left the channel at a bend and
ploughed through the town, destroying houses and first two floors of apartments.
as further landslides were possible 100,000 survivors were evacuated immediately.
all public services were lost from many areas and rebuilding has been slow.
thousands were still homeless nine years after the event and value of surviving
property had fallen by 70%.
Impacts of mass movements on lives and property
many of the impacts on lives and property are similar of tectonic hazards but
usually on smaller scale, so they don’t usually attract overseas aid.
Prediction and monitoring
warning signs include cracks resulting from tension in buildings and ground, tilted
structures, bulging walls and steep slopes, especially with deposited material
above.
arcuate shaped cracks on ground above cliffs indicate size of future slumps. once
noticed can be monitored.
large moving masses monitored by GPS and laser surveys to determine amount of
movement each year of a number of fixed markers on landslip surface. GPS
measures distances and direction accurately. arrows drawn on map to show
direction and length of annual movements. compare movements with rainfall you
can determine how much water the slope can hold without moving.
rainfall and changes in soil moisture are monitored. boreholes measurements
indicate changes in groundwater content and pressure it exerts; weight of
additional water can trigger mass movements.
in areas of active faults; tiltmeters record amount of tilt occurring and strain
meters record changes in crustal strain to high degree of precision.
landslide hazard mapping: drawn up using factors that affect slope stability and
knowledge of previous movements. rocks are classed in their susceptibility to landslides and degree to which they’ve been affected in past. maps of routes taken
by rock falls in mountain areas reduce change of building in their paths.
Perception of risk
varies according to distance from site of hazard, amount of knowledge and length
of time since hazard last occurred. insurance companies have clearly defined
assessment of risk. government agencies are well informed and take responsibility
of educating public of risk. private landowners have to do the same. in USA
warnings are issued during national and local weather forecasts.
