Water at the Heart of Science
21
When Water
Becomes a Threat
Water is crucial to life
1
but can also generate extreme events with sometimes disastrous consequences, such as
floods, inundations and landslides. Research is done on these phenomena in order to predict
them more accurately and limit their effects.
Understanding
Predicting
Prevention
Devastating floods are a result
of random natural phenomena
(for instance exceptional rains)
converging with a vulnerable
environment (steep-sided
valleys, urbanisation in flood
plains, etc.). By making use
of rainfall and weather data,
measurements of river flow
and environmental studies
(land relief, geology, soil,
vegetation…), researchers are
able to simulate floods through
mathematical computer
models.
To study the frequency of
floods, researchers collect
historical, often fragmented
data on floods in the past.
Based on a statistical analysis
of these data, the probability
of recurring rain or floods can
be estimated depending on
their intensity. This information
is invaluable to decision-makers
when defining risk-free housing
areas, sizing a bridge, dam or
dyke, or determining stages of
alert for evacuation plans.
Due to cost, specific areas
cannot be safeguarded against
floods that occur rarely, for
instance at a recurrence
interval of more than 100 or
1,000 years. It is therefore vital
to be able to anticipate such
events in order to move people
and goods to safety.
1 In urban areas, soil sealing tends
to increase the amount of water
that flows towards rivers, resulting
in bigger floods. North Dakota, USA.
4 Landslides are caused by heavy
rains that make the ground surface
unstable.
2 Rivers in developing countries
5 The human toll of landslides
tends to increase with uncontrolled
urbanisation in high-risk areas.
are rarely monitored, which usually
implies heavy human losses and
material damage during floods.
Iquitos, Peru.
2
3 Floods in September 2009 in
Ouagadougou (Burkina Faso) left
150,000 disaster victims, of which
50,000 lost their homes.
1
7 Weather radars locate and assess
the intensity of rainfalls within a
radius of 80 km. These data allow
researchers to predict possible
floods.
6 Solutions exist to protect large
towns against floods. Plans have
been made in Vietnam to direct
the flow of the Red River towards
another river. Excessive floods
can inundate an area of 70,000
hectares.
4
5
3
6
7
Water at the Heart of Science
22
Flash Flood
in the Mediterranean
In 2002, torrential rains
1
hit the Languedoc region in France, causing the Gardons (the Gard River and its tributaries)
to burst out of their seams. Considerable material damage and 22 casualties were reported.
Researchers have intensified their efforts in order to better understand and predict such phenomena.
The flooding of the Gard
River and its tributaries is a
typical phenomenon in the
Mediterranean region. It is
characterised by short but
heavy rains, causing water
levels to rise by several metres
for a few hours. Researchers
are working on improving rain
and river-flow measurements,
identifying the factors that
determine floods, and putting
effective protective measures
in place.
Despite the great absorption
capacity of the soil in
the Cévennes region, the
heavy rains in 2002 caused a
significant run of water towards
the Gardons. The typography
of the area also played a major
part in triggering the flood:
the bottleneck entrance to
the gorges of the Gard was
unable to absorb all of the flow,
causing the plain at the foot of
the hills to become inundated.
Research has been undertaken at the CévennesVivarais Hydro-meteorological Observatory
(OHM-CV), which includes about ten French
research laboratories under the management
of the research laboratory for hydrological and
environmental transfers (LTHE, UMR CNRS-IRDUJF). The HydroSciences Montpellier laboratory
(HSM, UMR CNRS-IRD-UM) participates in the
OHM-CV and has been working together with
SCHAPI since 2005 to develop flood-forecasting
aide tools.
Such phenomena can be
anticipated through research,
which also aims to improve
the spreading of information
to civil protection services,
raise inhabitants’ awareness
of the risk, and provide realtime flood management. Since
the 2002 floods occurred,
a multitude of communitybased emergency plans have
sprung up around France,
identifying actions to be taken
according to the stage of
alert: reducing traffic, alerting
schools, evacuating exposed
neighbourhoods, etc.
1 Reflecting the irregularity of
rainfall, most Mediterranean rivers
have a strongly intermittent flow
pattern. Heavy and sometimes
destructive rains often occur from
spring to autumn, but become rare
or virtually inexistent for many long
months, particularly in summer.
2 Flood levels of the Gardon in
1958 (top) and 2002 (bottom),
indicated on the wall of a house
in Anduze.
Gardons
3 The Gardon at Anduze at the
start of autumn.
4 The Gard department was hit
particularly hard by the Gardon
rivers in flood.
5 Floods comparable to those in
2002 are likely to occur throughout
the Mediterranean region, which
justifies this kind of research that
is also undertaken in developing
countries. Floods in the Tafilalet
valley in Morocco.
3
1
2
4
5
Water at the Heart of Science
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