Geology Reference
In-Depth Information
ephemeral gully development in cropland is
attributed to different states of the soil surface
(roughness, degree of sealing) as affected by tillage
operations and antecedent rains. Nachtergaele
(2001) analysed 38 ephemeral gully erosion events
that occurred over a 15-year period in central
Belgium and found critical
P
values of 15 mm
in (late) winter (
n
might develop is presented by the threshold con-
cept, first applied to geomorphological systems
by Patton and Schumm (1975). This concept is
based on the assumption that in a landscape with
a given climate and land use, there exists for a
given slope gradient of the soil surface (
S
) a criti-
cal drainage area (
A
) necessary to produce suffi-
cient runoff to cause gully incision. As slope
steepens, this critical drainage area decreases and
vice versa. For different environmental condi-
tions and different gully initiating processes
(hydraulic erosion by Hortonian overland flow,
saturation overland flow and seepage erosion,
landslides), different topographic thresholds apply
(Montgomery & Dietrich, 1994). Threshold lines
for gully development by hydraulic erosion can
be represented by a power-type equation (Begin &
Schumm, 1979; Vandaele
et al
., 1996):
S
=
21) and of 18 mm in (early)
summer (
n
17), which is attributed to a differ-
ence in soil moisture content between the two
seasons. Threshold rains for gully development in
land under forestry operations in Australia are
significantly larger (
P
=
80-100 mm) than those
for ephemeral gully development on seedbeds.
Sudden snowmelt on frozen /thawing soil presents
a special case of a meteorological threshold con-
dition at higher latitudes, higher altitudes or areas
with a continental climate, which can lead to the
rapid development of ephemeral gullies. Øygarden
(2003) documented how the combination of fro-
zen subsoils, saturated topsoils with low strength
and intense rainfall led to the development of
ephemeral gullies in Norway, even in areas with
gentle slope gradients. These observations point
to the fact that a gradual climate change to more
unstable winter conditions (i.e. freezing and
thawing combined with intense rain) is likely to
increase the risk of (ephemeral) gully erosion.
One of the difficulties encountered when assess-
ing critical rain depths for gully initiation is the
lack of representative rain data for the sites where
gully erosion processes have been observed
(Vandekerckhove
et al
., 2000).
=
aA
b
with
a
and
b
coefficients depending on the environmen-
tal characteristics. Table 19.1 presents an over-
view of studies reporting topographic thresholds
for incipient gullying in a range of environments
and reveals that
b
ranges in value from −0.10
to −0.80. Kirkby
et al
. (2003) have shown that
power law equations describing sediment trans-
port for water erosion occurring on runoff plots
are consistent with
S-A
relations describing the
location of ephemeral and permanent gully chan-
nel heads in the landscape.
The topographic threshold concept for gully
initiation permits one to predict, for a given land
use, the location in the landscape where gully
channels may develop by providing a physical
basis for the initiation of gullies. Poesen
et al
.
(1998) compared ten published critical
S
-
A
data-
sets for ephemeral gullies and permanent gullies
in different environments and found that not only
the environmental characteristics, but also the
methodology used to assess critical
S
and
A
, affect
the reported topographic threshold for incipient
gullying. Poesen
et al
. (2003) demonstrated that
topographic threshold conditions for gully initia-
tion in non-cultivated land (i.e. sagebrush and
scattered trees, open oak woodland and grass-
lands, coastal prairie, logged forest and swampy,
reed-covered valley floors) plot well above
those needed to initiate ephemeral gullies in
=
(iii) Topographic thresholds
Where do gullies
develop in the landscape? Most models predict-
ing soil erosion by water do not predict the loca-
tion of gullies. Yet this issue is important for
land managers and for predicting possible impacts
of climatic or land-use changes on the spatial
distribution and density of gullies. This ques-
tion can be reformulated as follows: where do
gully channels start and where do they end in the
landscape?
Where do (ephemeral) gully channels start?
An
approach to predict locations where gully heads
