Geology Reference
In-Depth Information
are responsible for gully formation, but basically,
their location is controlled by the way water is
concentrated and where the capacity of the water
is sufficiently high to cut a channel. Three proc-
esses have been identified that are important in
gully formation: overland flow, seepage and shal-
low landsliding. Any overland flow, whether it
results from infiltration or saturation excess, will
exert a shear stress on the underlying soil surface
which, if it exceeds a critical or threshold value,
will result in channel initiation and development.
Seepage erosion involves the entrainment of mate-
rial as a result of water flowing through and eme-
rging from the soil (Dietrich & Dunne, 1993),
lowering its erodibility through seepage forces
(Gabbard
et al
., 1998). This erosion process is
responsible for the initiation of subsurface pipes
and tunnels, which have been found to be impor-
tant for gully formation especially in semi-arid
areas (Faulkner
et al
., 2004). Finally, shallow land-
slides can concentrate water in the landscape and
act as headcuts for the development of gullies.
Montgomery and Dietrich (1994) defined a theo-
retical framework for delineating the range of top-
ographic conditions (local slope gradient and
drainage area) where each of these processes acts.
In practice, however, interactions are likely to
occur and gullies are formed by simultaneous or
subsequent action of various processes.
Although the topographic threshold concept,
as explained above, has some limitations (Chaplot
et al
., 2005), it is of great practical significance to
predict locations in the landscape where gully
heads might develop. For each pixel in the land-
scape, the upslope drainage area (
A
) and local
slope gradient (
S
) must be calculated and, using
an appropriate critical
S-A
relation for that envi-
ronment (see Table 19.1), one can then assess the
risk of having a gully head developing in that
pixel. Using such an approach, Prosser and
Abernethy (1996) predicted the extent of a stable
gully network successfully. Along the same lines
Desmet
et al
. (1999), Jetten
et al
. (2006) and
Knapen and Poesen (2010) successfully predicted
the location of ephemeral gullies in cropland.
Ephemeral gully channels end in a downslope
direction where massive sediment deposition and
fan-building occurs. This is where either surface
roughness increases suddenly (e.g. where a differ-
ent land use begins - land use-induced sediment
deposition) or where local slope gradient decreases
(i.e. slope-induced sediment deposition: Beuselinck
et al
., 2000). Here transport capacity of the concen-
trated flow will drop sharply leading to massive
(coarse) sediment deposition. Very few
S-A
rela-
tionships for sediment deposition exist. For several
European cropland conditions, Nachtergaele
et al
.
(2001a,b) reported data indicating that the topo-
graphic threshold (
S-A
relationship) for sediment
deposition at the bottom end of ephemeral gul-
lies was smaller than the corresponding
S-A
rela-
tionship for incipient ephemeral gullying. The
difference between the critical topographical con-
ditions for ephemeral gully initiation and those
for sedimentation vary among different environ-
ments, and depend, among other factors, on the
texture and rock fragment content of the topsoils
(Vandekerckhove
et al
., 2000; Poesen
et al
., 2002).
These few datasets allow one to locate the ini-
tiation point and the sediment deposition point
of an ephemeral gully, based on topographic
attributes (
S
and
A
) and on rock fragment content.
Consequently, ephemeral gully length can be
derived by routing concentrated flow from the
gully head towards the fan at the gully end. For
other environments, more data are needed to pre-
dict ephemeral gully length.
Desmet
et al
. (1999) investigated the possibil-
ity of predicting the location of ephemeral gullies
using an inverse relationship between local slope
gradient (
S
) and upslope contributing area per
unit length of contour (
As
). Predicted locations of
ephemeral gullies were confronted with the loca-
tions recorded in three intensively cultivated
catchments over a 5-year observation period. The
optimal relative area (
As
) exponent (relative to
the slope exponent) ranged from 0.7 to 1.5. A
striking discrepancy was found between the high
relative area exponent required to predict opti-
mally the entire trajectory of the ephemeral gul-
lies, and the low relative area exponent (0.2)
required to identify the spots in the landscape
where ephemeral gullies begin. This indicates
that zones in the landscape where ephemeral
