ROUTE III
III
HOW MAN DEALS
WITH NATURAL RISK
ROUTE III
1
2
3
4
5
6
Dynamic barriers
Rockfall protection
The dejection cone in la Comella
Floods
Scree
The elves
Strong footwear recommended, take water.
Font de Ferrús:
elevation change 180 m, 45’ to ascend and descend.
III
HOW MAN DEALS
WITH NATURAL RISK
In 1940, there were 1600 people living in Andorra la Vella
and today the census total is 24,678 residents (2008). The
population of the Principality has therefore grown substantially in recent years. This has resulted in the need for more
infrastructures, housing and services. So it has been necessary to occupy land that was traditionally used for primary
activities such as farming and livestock. Some of the occupied land is prone to geological risks, such as landslides,
avalanches, floods, etc. In such cases, one has to live with
the risk. As it is not always avoidable, protective measures
are needed.
There are various protective measures. Some involve permanent works, such as snow bridges to prevent avalanches
from falling or rockfall nets. Others are of a temporary nature,
such as teaching the population how to act in the face of risk
or about evacuation and confinement procedures.
Rockfall on “El Bon
Repòs” building, 21
January 1997.
ANA. SFG G-6397
Dynamic barriers
Part of the rocks of Andorra la Vella and its surroundings are of magmatic
origin, called granodiorites. These were first buried, then, when they resurfaced, cooled and decompressed. During this process they were affected
by a whole series of fractures. The climatic conditions to which these rocks
are today subjected are alternate periods of moderate temperatures, when
the fractures may fill up with water, and much colder periods, when the
water freezes, increases in volume and finally splits the rock. These
processes are called freezing/defreezing cycles and are very common in
the whole of the Pyrenees. They cause blocks of varying size to break off
the rocky massif.
The “El Bon Repòs” rockfall episode
On 21 January 1997, a period of rains caused a huge granodiorite block
(25 m3) to fall off the mountain and hit the building called “El Bon Repòs”
in Santa Coloma. The block came through the wall of the fourth floor and
fell vertically into the basement of the building.
After this occurrence, a series of studies were carried out on the risk of
rockfalls which concluded that protective works had to be carried out over
the inhabited zone.
Nets holding a fallen
block in Santa
Coloma, 20 April 2008
Rockfall protection
The mountain is covered with barriers. There are two types of protection,
those that are fixed directly against the rock face, with the aim of avoiding
rockfalls, called nets or meshes, and others like those around you, called
dynamic barriers. These act by stopping the blocks as they fall, absorbing
the energy through different mechanisms, usually through friction or deformation of the structures. The ones in this zone can take up to 5000 kilojoules of energy, equivalent to ten cars thrown from 32 metres up.
Residual risk
As we cannot cover the whole mountain with nets and other safety elements, and as, moreover, these are no longer effective over a certain energy level, there will always be some zone that is more exposed to falling
blocks. This means that we cannot totally eliminate the risk, there will
always be a residual risk. We must remain aware that the risk exists and
learn to live with it.
The dejection cone in la Comella
This whole circular area before us is the dejection cone of La Comella.
A dejection cone is a deposit of loose material washed down by a torrent
or mountain stream.
The dejection cone is the lowest part of what we call a mountain basin.
Mountain basins like the one at La Comella are formed, at the top, by a
steep relief, called the catchment basin, crossed by streams that flow into
a central channel called the drainage channel. This comes out at the top of
the dejection cone. At this point, the sediments transported by the stream
start to be deposited and form the cone.
There are no recently documented cases of problems in the dejection cone
at La Comella, in fact the catchment basin appears to have plenty of vegetation. This vegetation impedes flooding to a large extent but not absolutely. For this reason the possibility of it happening in the future has not been
ruled out.
Dejection cone at La
Comella. Aerial
photograph July 1970.
Govern d’Andorra
1982 flood.
Aiguat de 1982.
ANA. Fons: FP
Autor Fèlix Peig
Floods
Rivers usually flow along their river beds, running across the valley bottom
which in this case is flat. Sometimes, however, the rivers overflow their
usual channel and fill what is called the flood plain.
A flood is when a river, brook or stream overflows its normal channel. This
is usually due to an increase in the amount of water flowing down after
heavy rainfall or sudden snow melt on the tops of the mountains. It may also
be caused by an obstruction in the river channel when the obstruction is
broken.
In Andorra, there are various factors that may result in episodes of extraordinary rainfall. Normally, these rains take the shape of localised storms, and
the river or stream into which they flow cannot channel all the water that
arrives, causing them to overflow. In Andorra, the seasons with most risk of
such torrential rain are spring and autumn.
On 6, 7 and 8 November 1982, there was an episode of extraordinarily
heavy rainfall over most of the central and eastern Pyrenees. In Andorra, it
rained for practically two days without stopping, the rivers overflowed and
caused extensive material damage and 14 deaths.
Scree
In cold climates like in Andorra, the processes that occur on the slopes vary
throughout the year. In winter, the predominant slope processes are avalanches, while when the snow melts, torrential processes predominate, i.e.
violent flows of water and rocks into the bottom of the valley.
At El Solà in Andorra la Vella, avalanches form in the Pedra Plana gully, and
it also becomes a stream when it rains. At the foot of the gully is the La Pica
scree, the result of the accumulation of all the material that comes down
the gully.
In the Principality of Andorra, there have been numerous avalanches. An
avalanche is a mass of snow that moves downhill due to gravity. There are
different types of avalanche. The reasons they are unleashed depends on
many different factors such as the quantity of snow, humidity, slope gradient, etc. An avalanche can be extremely destructive, as in the case of the
Arinsal avalanche in 1996.
Font de Ferrús:
Elevation change 180 m, 45’ to ascend and descend.
Strong footwear recommended, take water.
Solà de Nadal
Els menairons
Do you believe everything you have seen so far? Some do not, and think
that the rock in front of you is irrefutable proof of it.
As you can see, whoever moved the block left behind his lever, no doubt
because he left in a hurry. In fact, it was the menairons or elves who
brought this rock here and probably built all the screes in the Pyrenees.
The ‘menairons’ or ‘minairons’ are small beings who live in the Pyrenees.
Their masters come out to catch them on the night of Saint John and keep
them in a needle-case. When their master lets them out, they jump and run
around, asking “What shall we do, what shall we say?”, and so they are
ordered to carry out some task, which they do swiftly, however laborious it
may be. However, if they are freed by mistake and not given work or conversation, the elves may end up killing their master. It is told that sometimes,
a youth, bitten by curiosity, opened his master’s needle-case to see what
was inside. When the elves came out, the youth hurried to find work for
them, and this normally consisted of gathering all the stones from all around
and piling them up somewhere.
This is the real origin of the screes in Andorra and throughout the rest of
the Pyrenees - some nosy youth who had to find work for the elves so they
would not throttle him. Even now, if you go out in the woods on the night
of Saint John, you might see one....
cenma
Centre d’estudis de la neu i de la muntanya d’Andorra
The interpretation routes and theme zones are an essential part of the Green
Ring of Andorra la Vella and structure a complex of natural, cultural and
social values which enable us to get to know the resources which existed
and social organization throughout history and tell us how the parish has
evolved and how the urban space has taken shape.
Dipòsit legal: AND. 386-2009
The Green Ring is a natural continuum around the city, formed by a series
of enclaves of high ecological, landscape, historical and cultural value,
which enables one to evaluate the city outskirts in terms of environment and
social culture.
Information
Tourist Office
Rotonda Square
AD500 Andorra la Vella
Phone.: 00376 873 103
[email protected]
[email protected]
www.andorralavella.ad/anellaverda