Geoscience Reference
In-Depth Information
16.2
Groundwater processes in valley and
scarp development
(Dunne, 1980). There has been considerable confusion
over the terminology used to describe these processes,
with terms such as piping, pipe formation, tunnel ero-
sion (Bennett, 1939), sapping, spring sapping (Bates and
Jackson, 1980), spring erosion, artesian sapping (Milton,
1973), basal sapping and seepage erosion (Hutchinson,
1968) often used imprecisely and interchangeably. In the
following discussion the terminology suggested by Dunne
(1990) is adopted.
The formation of subsurface pipes may result from both
tunnel scour or seepage erosion and, if such pipes col-
lapse, may lead to channel initiation and ultimately valley
development (Dunne, 1980). Both tunnel scour and seep-
age erosion may also lead to sapping, in its simplest sense
'the undermining of the base of a cliff, with the subsequent
failure of the cliff face' (Bates and Jackson, 1980, p. 556).
If groundwater flow is sufficiently focused to emerge as
a spring then spring sapping may occur, while seepage
erosion may lead to weakening and collapse along a more
diffuse seepage zone. The processes of tunnel scour in dry-
land drainage development have been discussed in Chap-
ter 11 in the context of badland development. As such the
following section focuses predominantly upon the role of
seepage erosion in scarp and valley evolution.
There are two main ways in which groundwater can act as
a factor in dryland valley and scarp development. First, the
processes of tunnel scour and seepage erosion associated
with subsurface water emerging at a free face or along a
hillside or scarp or valley floor can generate surface chan-
nels. Second, the operation of in situ deep-weathering
processes (principally chemical and biochemical corro-
sion) associated with the lateral and vertical movement of
groundwater along preferential subsurface flowpaths can
progressively lower land surfaces. Neither of these pro-
cesses are unique to arid environments, but some of the
best-documented resultant landforms occur within dry-
lands.
16.2.1
Erosion by exfiltrating water: definitions
and mechanisms
In general, subsurface flow will discharge from the ground
surface either (a) where a water table in an unconfined
aquifer intersects the landscape or (b) where the land
lies below the piezometric surface of a confined aquifer
that is linked to the surface by a fracture or fault in the
aquiclude (an impermeable rock stratum that prevents the
upward passage of groundwater). Erosion by exfiltrating
(i.e. emerging) subsurface water can operate in three ways.
First, near-surface groundwater flow may apply stress to
the walls of a pre-existing macropore, commonly within
a partially or fully consolidated material, which may have
originated by a variety of means (e.g. as a result of subsur-
face flowing water, as a shrinkage crack or from burrow-
ing animals or plant roots). Second, sufficient drag force
may be generated as water seeps through and exfiltrates
from a porous, usually semi- or unconsolidated material,
to entrain particles, cause failure or liquify the material
(Dunne, 1990). Third, groundwater outflow may, through
the operation of biological, chemical and physical weath-
ering processes, exert stress on the walls of pores, weaken
the material and ultimately lead to mass wasting (e.g. by
providing a moist microenvironment for algal growth or
through the precipitation of salts in pore spaces; see Laity,
1983).
The first of these three processes is termed
tunnel scour
while the second and third both contribute to the process
of
seepage erosion
(Dunne, 1990). Of the two, seepage
erosion appears to be the most significant in terms of
scarp and valley formation (Uchupi and Oldale, 1994),
16.2.2
Seepage erosion and valley formation
The earliest reference to the role of seepage erosion in
dryland valley formation can be traced to Peel (1941),
arising from observations made as part of Major R.A.
Bagnold's expedition to the Gilf Kebir plateau of Libya
in 1938. In this region, Peel identified wadis with flat
floors and steep sides that terminated in a headward cliff,
with little or no evidence of fluvial activity in the plateau
region surrounding the valley head. This led him to sug-
gest that the wadis appeared to have been 'cut out from
below
rather than 'let down from above' (Peel, 1941,
p. 13, italics author's original). This description neatly
summarises the main difference between scarp and valley
development by groundwater seepage erosion processes
as opposed to surface incision by rivers - seepage erosion
and sapping effectively undermine valley heads and sides
due to enhanced weathering and erosion within a zone of
groundwater emergence, while erosion by flowing water
operates from the surface downwards (Laity and Malin,
1985). For further details of the role of seepage erosion
in the formation of various landforms, see Higgins (1984,
1990), Howard, Kochel and Holt, (1988), Baker (1990)
and Howard and Selby (2009).
Since Peel's observations, seepage erosion has been
