Environmental Engineering Reference
In-Depth Information
Plate 1
One end point of desertification is severe soil erosion. These
'organ pipe' badlands in central Spain are formed by soil wash and
gullying. The calcareous silty clays (marls) are coherent when dry,
thus giving vertical faces.
Photo: Ken Atkinson.
therefore disperse when exposed to any surface wetting, and form a seal on drying.
They are often colonized by crusts of lichens which appear dead when dried but swell
back into life when wetted, forming a biological crust. Even light rain does not infiltrate,
but runs off to erode the sides and bottoms of gullies.
Secondary factors and processes are also important in explaining the distribution of
badlands. Gullying often goes hand in hand with
piping
, an underground network of
pipes formed from cracks, joints and minor faults. Pipes enlarge into tunnels, some large
enough to walk up, then into gullies. Marls often contain salts and gypsum, both of which
are quite soluble, or 'cracking' clays such as montmorillonite which shrink and crack,
and so are especially disposed to piping. Networks of pipes develop where large
hydraulic gradients develop at the base of steep slopes. Outflow of seepage water carries
fine particles which start to excavate a tunnel from the downflow end. Steep steps in the
initial slope profile help to drive high hydraulic gradients; thus piping is stimulated by
human actions such as road building agricultural terracing and the construction of earth
