Geology Reference
In-Depth Information
bank gully retreats by a single headcut or by mul-
tiple headcuts is controlled by factors such as
topography, material type and land use. Oostwoud
Wijdenes
et al
. (1999) classified gully headcuts
into four types (i.e. gradual, transitional, abrupt
and rilled-abrupt) and observed in southeast Spain
that gully head morphology could be used as an
indicator for gully development stage, and hence
for sediment production.
Several studies report that, besides topography
(drainage area), land use (change) has a significant
impact on bank (gully) head erosion (e.g. Burkard &
Kostaschuk, 1997; Oostwoud Wijdenes
et al
.,
2000; Wang
et al
., 2008). For instance, Burkard
and Kostaschuk (1997) attributed the increased
growth rates of bank gullies along the shoreline of
Lake Huron partly to the increased extension of
municipal drains and the use of subsurface drain-
age. Land use changes involving the removal of
semi-natural vegetation (
matorral
) and the exten-
sion of almond cultivation intensified the bank
gully head activity in southeast Spain (Oostwoud
Wijdenes
et al
., 2000). Wang
et al
. (2008) observed
in Yunnan (China) mean gully headcut retreat
rates (over 4 years) to range between 0.2 and 0.4 m
y
−1
for 15-20% vegetation cover (grass, shrubs and
forest) in the gully catchment, 3.73 m y
−1
for crop-
land, and 4.69 m y
−1
for bare land without any
vegetation. Several studies show that lithology
also has a clear impact on (bank) gully headcut
activity: for the same land use type in Spain,
headcuts in marls, sandy loams and loams were
significantly more active compared with head-
cuts that developed in gravels and conglomerates
(Oostwoud Wijdenes
et al
., 2000; de Luna
Armenteros
et al
., 2004). Similar observations
were reported for Romania by Radoane
et al
.
(1995). These authors reported that the mean rate
of gully headcutting was over 1.5 m y
−1
for gullies
developing in sandy deposits, and under 1 m y
−1
for gullies cut in marls and clays.
Several studies have attempted to quantify
and predict gully area increase or gully headcut
retreat (
R
) in a range of environments, including
linear measurements (e.g. Thompson, 1964;
Seginer, 1966; Soil Conservation Service, 1966; De
Ploey, 1989; Burkard & Kostaschuk, 1995, 1997;
Radoane
et al
., 1995; Oostwoud Wijdenes & Bryan,
2001; Vandekerckhove
et al
., 2001, 2003), area
measures (e.g. Beer & Johnson, 1963; Burkard &
Kostaschuk, 1995, 1997), volumetric measures
(e.g. Stocking, 1980; Sneddon
et al
., 1988,
Vandekerckhove
et al
., 2001, 2003) and weight
measures (e.g. Piest & Spomer, 1968). According
to Stocking (1980), volumetric measures are the
best compromise, avoiding difficult considera-
tions of bulk density of soils no longer
in situ
.
The resulting equations typically link
R
(obtained
from detailed cross-sectional surveys of gully
channel cross-sections taken at periodic inter-
vals throughout the study period, or from aerial
photographs to estimate changes in channel
dimensions over time) with parameters such as
drainage area (
A
) above the gully head (an index
for surface or subsurface runoff volume), rainfall
depth, erodibility, height of the headcut, relief
energy of drainage basin, and runoff response of
the drainage area. Here we list these equations,
based on field measurements, as they help in
understanding the effect of environmental fac-
tors on
R
.
For the deep loess area of western Iowa
(US), Beer and Johnson (1963) expressed gully
growth as:
0.0982
-
0.0440
0.7954
-
0.2473
-
0.0014
dp
R
=
81.41
R
A
L
L
e
(19.8)
t
t
g
w
where
R
is the gully surface growth over the
observation period (m
2
),
R
t
is the index of surface
runoff (mm)
, A
t
is the terraced area of the catch-
ment (m
2
)
, L
g
is the length of the gully at the
beginning of period (m), L
w
is the length from end
of gully to catchment divide (m), e is the natural
logarithm, and
dp
is the deviation of precipita-
tion from normal (mm).
In the US, Thompson (1964) studied gully head
advancement in Minnesota, Iowa, Alabama,
Texas, Oklahoma and Colorado, and developed
an empirical equation:
-
5
0.49
0.14
0.74
R
=
(7 .1 3
´
1 0
)
A
S
P
E
(19.9)
where
R
is the gully head advancement for the
time period of interest (m)
, A
is the drainage area
