Geology Reference
In-Depth Information
From this review we conclude that detailed
information on the impact of various land uses
on topographic thresholds needed to initiate gul-
lies under a range of climatic conditions is rather
scarce. Yet, such information is crucial for pre-
dicting where in the landscape gully development
might be expected under different environmental
conditions.
present, concentrated flow may erode ephemeral
gullies several metres deep (Poesen, 1993;
Vanwalleghem
et al
., 2005b). Erosion of Bt-horizons
caused by various processes (i.e. water erosion,
tillage erosion, removal of soil during root and
tuber crop harvesting, land levelling) therefore
largely increases the risk of deep gully develop-
ment. Other soil horizons observed to be resistant
to gully erosion are plough pans, fragipans, petro-
calcic horizons, ironstone hardpan (petroplinthite)
or unweathered bedrock. On the other hand, less
permeable soil horizons can induce positive pore-
water pressures in the overlying soil layers, which
in turn lowers the erosion resistance of these soil
horizons, particularly when seepage conditions
(return flow) occur (e.g. Moore
et al
., 1988; Huang
& Laflen, 1996; Poesen
et al
., 2003). This in turn
may alter the topographic threshold for gully head
initiation (e.g. Montgomery & Dietrich, 1994;
Vandekerckhove
et al
., 2000; Poesen
et al
., 2002).
(iv) Pedological and lithological controls
Soil type
Many studies have investigated the
susceptibility of soils (soil erodibility) to inter-rill
and rill erosion (see Bryan, 2000, for a review).
Knapen
et al
. (2007) recently reviewed published
data on the effects of soil properties on the res-
istance of soils to concentrated flow erosion
(expressed in terms of critical flow shear stress and
channel erodibility) and proposed a general soil
erosion resistance ranking according to soil tex-
ture and tillage practices. Comparatively few stud-
ies have investigated the susceptibility of soils to
gully erosion. Soil type and in particular the verti-
cal distribution of the erosion resistance of various
soil horizons largely controls the size and more
specifically the depth and cross-sectional morphol-
ogy of gullies. Ireland
et al
. (1939) were the first to
point to the important role of the resistant
Bt-horizons in controlling gully depth and gully
head shape in the southeastern United States. Other
studies conducted on gully development in duplex
soils in Australia (e.g. Sneddon
et al
., 1988) and on
loess-derived soils in Europe (Poesen, 1993) also
came to the same conclusion. Poesen (1993) found
that soil shear strength at saturation of the various
loess-derived soil horizons is a good indicator of
their resistance to concentrated flow erosion. For
loess-derived soils, Nachtergaele and Poesen (2002)
showed that: (1)
Lithology
Many field observations clearly reveal
that lithology significantly controls the size and
density of gullies that can develop in a given
landscape. For instance, the maximum depth of
the permanent gully shown in Plate 16 (b) is con-
trolled by the presence of hard unweathered bed-
rock at a depth of ca. 8 m. The occurrence of
landscapes heavily dissected by gullies in the
Mediterranean (i.e. badlands) is strongly controlled
by the presence of particular lithological condi-
tions: unconsolidated or poorly sorted materials
such as shales, gypsiferous and salty-silt marls and
silt-clay deposits of Tertiary and Quaternary age
(Poesen & Hooke, 1997; Gallart
et al
., 2002).
Faulkner
et al
. (2003) reported on the role of site
geochemistry in morphological development of
badlands. In tropical environments, the develop-
ment of large gully systems typically occurs on
unconsolidated loose sandy deposits (Plate 19). In
contrast with sheet and rill erosion, relatively lit-
tle is known about the properties of soils or parent
materials and the associated processes that control
the dynamics of their resistance to gully erosion.
τ
c
and channel erodibility (related to
concentrated flow erosion) for a Bt-horizon were sig-
nificantly larger compared with
τ
c
and erodibility for
an Ap or a C horizon; and (2) an increasing anteced-
ent moisture content of each horizon had a negative
effect on their erodibility. In landscape positions
where Bt-horizons are still present, ephemeral gully
depth is limited to a maximum of 0.50 m. However,
for landscape positions where no Bt-horizon is
(v) Land use thresholds
Gully development in
prehistoric and historical times triggered by a
