La pedra en sec. Materials, eines i tècniques tradicionals a les illes ...

***
* * *
** * ** 6 6.4 DRYSTONE MASONRY STRUCTURES
Surface water in Majorca only flows
through intermittent watercourses due
to the seasonal irregularity of rainfall,
the small size of the island's drainage
basins, and the highly permeable
nature of its rock.
Watercourses on the island, called
torrents, only have water flowing
through them during periods of heavy
rainfall. On occasions these torrents
can have extremely high temporary
rushes of water which cause very
destructive flooding The rainfall
responsible for these extreme conditions
often totals more than 100 mm in
a day or occasionally 400 mm in a
day. This phenomenon is especially
frequent in the mountainous areas.
Given these conditions, it was
necessary to build drystone masonry
structures basically designed to prevent
soil erosion by water runoff and
to regulate drainage in order to make
farming possible. The most common
practice was to channel the torrents
between drystone walls following their
natural course and simply placing
retaining walls on both sides of the
main riverbed to stop the flooding of
the farmland. In other cases, the natural
course of the torrent was made
more geometric so as not to impinge
on the shape of a farm and to enable
the bed of the watercourse to be farmed.
Occasionally some stretches of
the torrent were covered with new soil
to obtain larger fields.
In the most extreme cases an artificial
channel (raWa) was built to divert
the water to the fields. This diversion
could be towards one side of the valley
bottom, towards the exterior part
of the farmland, towards karst absorption
features (avencs or potholes), or
towards another basin going over a
relief feature. A large part of waterrelated
structures are to be found in
terraced fields and give rise to constructive
singularities produced by the
presence of the terrace walls. If the
channels run at right angles to the
walls and are built into their structure,
the bed has a stepped profile with a
series of small waterfalls.
Small bridges were built across
these channels, walls arranged as if
they were funnels to collect and direct
the water to the channel, stretches of
channels were put underground in
mines (underground passages), etc.
It is common practice on the island
to invade the natural course of a river'
in this way the river beds and valle~
bottoms can be completely given over
to farmland. The method used is to
interdict the streambed with a series
of walls standing at right angles to the
course of the river (parats), which
support flat cultivated areas. These
walls usually enjoy certain features to
increase their resistance to water
pressure: curved sections, a highly
reinforced bra6 (double wall) raised
above the level of the field, and
facings made with large stones and
quite open joints.
Other water structures include the
albellons, drainage systems in the
shape of underground galleries,
which collect infiltration water and
remove it from the field or to watercourses.
The land on the Majorcan
Pia (Plain) and a large part of the
island's terraced fields are systematically
drained using this system.