Geology Reference
In-Depth Information
some extent, subjective (Grissinger 1996a,b).
In fact, Nachtergaele
et al
. (2002a) demonstrated
that (ephemeral) gullies can be considered as
channels characterized by a mean width (
W
)
between that of rills and (small) rivers. For all
these channels,
W
seems to be essentially control-
led by peak flow discharge (
Q
), and the relation
between both parameters can be expressed by the
equation
W
For more detailed information on gully erosion
processes and controlling factors, the reader is
referred to review papers by Bocco (1991), Dietrich
and Dunne (1993), Bull and Kirkby (1997, 2002),
Poesen
et al
. (2002, 2003) and Valentin
et al
. (2005).
19.3
Prediction of Gully Erosion
a Q
b
, with
a
being a coefficient and
the exponent
b
varying from 0.3 for rills, over 0.4
for (ephemeral) gullies, to 0.5 for (small) rivers.
For gullies, the proposed
W
-
Q
relation only holds
for concentrated flow incising relatively homoge-
neous soil material in terms of erodibility (i.e. soil
erodibility remains constant with depth). If a
resistant soil horizon is present at shallow depth
(e.g. frozen layer, plough pan, Bt-horizon, fragipan,
petrocalcic horizon, ironstone hardpan or bed-
rock),
W
will be much larger than the value pre-
dicted by this equation. Also, if a more erodible
layer is present at shallow depth, this relation will
no longer hold (Nachtergaele
et al
., 2002a).
By definition,
bank gullies
or
edge-of-field
gullies
(Plate 18) develop wherever concentrated
runoff crosses an earth bank. Given that the local
slope gradient of the soil surface at the bank riser
is very steep (i.e. subvertical to vertical), bank gul-
lies can rapidly develop at or below the soil surface
by hydraulic erosion, piping, tunnelling and even-
tually mass movement processes, even though
catchment areas are rather small (Poesen &
Govers, 1990). Once initiated, bank gullies retreat
by headcut migration into the more gentle sloping
soil surface of the bank shoulder and further into
low-angled pediments, river or agricultural ter-
races (Poesen
et al
., 2002).
So far, no systematic compilation of morpho-
logical characteristics (e.g. length, width, depth) of
the different types of gullies and their controlling
factors (e.g. topography, soil type, land use, hydrol-
ogy) in a wide range of environments has been
made. Such quantitative data would be needed so
as to allow land managers to foresee the type of
gullies they might expect when land-use changes
are taking place. These data could also be used to
develop gully erosion models for the different
gully types.
=
Questions of major interest to those wanting to
model soil erosion in landscapes affected by gul-
lying are:
(1)
whether and where gullies may form in a
given landscape;
(2)
how fast gullies will erode (in terms of chan-
nel length, channel cross-section, gully headcut
retreat); and
(3)
how gully development will interact with
hydrological and other soil erosion processes.
19.3.1
When and where do gullies develop?
Gully development is a threshold phenomenon.
It occurs only when a threshold in terms of flow
hydraulics, rainfall (or snowmelt), topography,
pedology (or lithology) and land use has been
exceeded. Here we discuss and illustrate the type
and magnitude of these thresholds.
(i) Hydraulic thresholds
Gully channels form
only if concentrated (overland) flow intensity
during a rain event exceeds a threshold value.
Horton (1945) first proposed the concept of a
threshold force required for channel initiation.
This force of flow is often expressed in terms of
the boundary flow shear stress (
τ
b
=
rgds
where
r
=
density of runoff water,
g
=
acceleration due
to gravity,
d
sine of the soil
surface slope angle). The threshold force required
to cause channel incision into the soil surface of
the concentrated flow zone is termed the critical
flow shear stress (
=
depth of flow and
s
=
τ
c
). A key question is: how large
should
τ
c
be for (ephemeral) gullies to initiate?
Critical flow shear stress values for incipient
motion of individual soil particles have been well
studied. Entrainment of loose silt and fine to
medium sand grains occurs at
τ
c
values of less
than 1 Pa (as deduced from the Shields curve;
