Applied geomorphology and arid regions
473
Formations with expansive clays or high exchangeable sodium percentages (ESP) are
potentially capable of causing piping (i.e. of making subsurface conduits). In detrital rocks
such as shales, the effects of piping look somewhat similar to those seen in soluble rocks,
with the production of tunnels, caves, dolines, natural bridges, and subterranean drainage
systems similar to karst systems (Parker et al., 1990). Piping is a serious problem for roads,
and can cause the undermining of bridge abutments, the breakage of lateral concrete
drainage systems, the undermining of concrete drains, and even the breakage of the road
itself (Fig. 19.12) (Parker and Jenne, 1967).
Many earthen dams are subject to piping, a process that has caused more than a few to
break. In Australia, fracturing in dams is due to the presence of montmorillonite group
clays and the high percentage of exchangeable sodium (Ingles and Aitchinson, 1970).
Studies performed in the USA and other countries report the same conclusions (Sherard
et al., 1977; Sherard and Decker, 1977).
In some cases, pipes can cause landslides and subsidence and vice versa. Pipes can
make slopes susceptible to these processes, which end up causing sheet and rotational
slides. The undercutting of piping helps to generate mass movements on slopes (see Jones,
1981).
Piping has important effects on agriculture (Fig. 19.13). In some parts of Australia, deep
ploughing is used to temporarily destroy existing pipes (Hudson, 1981). Generally, intense
piping occurs in gullied terrain of little agricultural value. Piping often occurs because of
inadequate deforestation practices or overgrazing. The soil conservation agencies of
several countries, therefore, try to ensure the rational use of agricultural and forestry
resources.
The prevention and control of piping is an important civil engineering and forestry
problem. The solution used in earthen dams is to install sand filters to seal the escape of
dispersive clays (Sherard et al., 1977), or to intensely compact the material to reduce
hydraulic conductivity. The remedies to alleviate piping in linear constructions are based
on surface drainage (Parker and Jenne, 1967). Some authors suggest the reforestation of
the affected areas as a long-term solution. Reforestation would diminish the rapid drying
that occurs during summer and prevent subsequent cracking. It would also increase the
organic matter of the soil (Newman and Philips, 1957). Unfortunately, it is difficult to get
vegetation to take root properly in arid areas. Another way to alleviate the effects of piping
is to add gypsum to the soil to reduce the ESP, and therefore reduce clay dispersivity
(Stocking, 1976).
4.2. Fluvial channels
The types of channel that exist in desert regions are straight, meandering and braided, but
their topography can change rapidly. Research into hydraulic geometry and the behaviour
of drainage channels in these areas has led to important advances in fluvial
geomorphology. Though the types of channel are the same as those occurring in wet
regions, some aspects of fluvial theory, such as plain flooding, cannot be extrapolated to
arid areas because the feed mechanisms are different (Graf, 1988). The instability of these
channels is quite obvious, and human activity commonly causes fluvial changes that entail
considerable risk.
473
Formations with expansive clays or high exchangeable sodium percentages (ESP) are
potentially capable of causing piping (i.e. of making subsurface conduits). In detrital rocks
such as shales, the effects of piping look somewhat similar to those seen in soluble rocks,
with the production of tunnels, caves, dolines, natural bridges, and subterranean drainage
systems similar to karst systems (Parker et al., 1990). Piping is a serious problem for roads,
and can cause the undermining of bridge abutments, the breakage of lateral concrete
drainage systems, the undermining of concrete drains, and even the breakage of the road
itself (Fig. 19.12) (Parker and Jenne, 1967).
Many earthen dams are subject to piping, a process that has caused more than a few to
break. In Australia, fracturing in dams is due to the presence of montmorillonite group
clays and the high percentage of exchangeable sodium (Ingles and Aitchinson, 1970).
Studies performed in the USA and other countries report the same conclusions (Sherard
et al., 1977; Sherard and Decker, 1977).
In some cases, pipes can cause landslides and subsidence and vice versa. Pipes can
make slopes susceptible to these processes, which end up causing sheet and rotational
slides. The undercutting of piping helps to generate mass movements on slopes (see Jones,
1981).
Piping has important effects on agriculture (Fig. 19.13). In some parts of Australia, deep
ploughing is used to temporarily destroy existing pipes (Hudson, 1981). Generally, intense
piping occurs in gullied terrain of little agricultural value. Piping often occurs because of
inadequate deforestation practices or overgrazing. The soil conservation agencies of
several countries, therefore, try to ensure the rational use of agricultural and forestry
resources.
The prevention and control of piping is an important civil engineering and forestry
problem. The solution used in earthen dams is to install sand filters to seal the escape of
dispersive clays (Sherard et al., 1977), or to intensely compact the material to reduce
hydraulic conductivity. The remedies to alleviate piping in linear constructions are based
on surface drainage (Parker and Jenne, 1967). Some authors suggest the reforestation of
the affected areas as a long-term solution. Reforestation would diminish the rapid drying
that occurs during summer and prevent subsequent cracking. It would also increase the
organic matter of the soil (Newman and Philips, 1957). Unfortunately, it is difficult to get
vegetation to take root properly in arid areas. Another way to alleviate the effects of piping
is to add gypsum to the soil to reduce the ESP, and therefore reduce clay dispersivity
(Stocking, 1976).
4.2. Fluvial channels
The types of channel that exist in desert regions are straight, meandering and braided, but
their topography can change rapidly. Research into hydraulic geometry and the behaviour
of drainage channels in these areas has led to important advances in fluvial
geomorphology. Though the types of channel are the same as those occurring in wet
regions, some aspects of fluvial theory, such as plain flooding, cannot be extrapolated to
arid areas because the feed mechanisms are different (Graf, 1988). The instability of these
channels is quite obvious, and human activity commonly causes fluvial changes that entail
considerable risk.