Geology Reference
In-Depth Information
combination of human-induced land-cover
changes and extreme rainfalls has been docu-
mented for various parts of the world, for exam-
ple: induced by intensive land use in central
Belgium during Roman times (Vanwalleghem
et al
., 2006); caused by a change in catchment
hydrology in response to human-induced vegeta-
tion change in the UK in the 9th and 10th cen-
turies AD (Harvey, 1996); due to high land-use
pressure and extreme rains in central Europe
in the 14th century in Germany (Bork
et al
.,
1998); during the Little Ice Age in Slovakia
(Stankoviansky, 2003); caused by the introduc-
tion of cattle (leading to overgrazing) and a cli-
matic shift in the southwestern US (Webb &
Hereford, 2001); and in eastern Australia since
European settlement 200 years ago (Prosser &
Winchester, 1996).
Several recent case studies have documented
the significant impacts of a gradual or sudden
shift in land use on the triggering of gullying or
the increase in gully erosion rates. For instance,
field observations in central Belgium indicate
that the increase in area under maize over the
last two decades has resulted in an increased
ephemeral gully erosion risk (Nachtergaele,
2001). Faulkner (1995) reported on the trigger-
ing of gully erosion associated with the expan-
sion of unterraced almond cultivation after
hasty clearance of native Mediterranean
mator-
ral
in southern Spain. This land-use change also
caused the development or reactivation of bank
gullies along ephemeral streams in southeast-
ern Spain (Oostwoud Wijdenes
et al
., 2000).
Bork
et al
. (2001) documented the effect of agri-
cultural intensification in the second half of the
20th century in the Upper Yangtze river basin
(southwest China) on rapid gully development
and the subsequent gully stabilization as a con-
sequence of reforestation by air-seeding. Several
studies conducted in a range of environments
have documented the impact of road construc-
tion on the increased gully erosion risk on steep
slopes (e.g. Moeyersons, 1991; Montgomery,
1994; Wemple
et al
., 1996; Croke & Mockler,
2001; Nyssen
et al
., 2002; Takken
et al
., 2008).
Gully incision is significantly more likely below
culverts on steep slopes with longer than aver-
age contributing ditch length (Wemple
et al
.,
1996; Nyssen
et al
., 2002). Montgomery (1994)
showed that for a given slope gradient, the
drainage area required to support a gully head
is smaller for road-related runoff than for
undisturbed slopes. Contributing road length
and the gradient of the discharge hillslope have
been successfully used to separate gullied and
non-gullied flow pathways within catchments
(Croke & Mockler, 2001). Burkard and
Kostaschuk (1997) attributed the increased
growth rates of bank gullies along the shoreline
of Lake Huron to increased snowfall and extreme
flow events, but also to the extension of munic-
ipal drains and the use of subsurface drainage.
Vanacker
et al
. (2003) have documented the
impact of collapsing irrigation canals and the
mismanagement of excess irrigation water on
the extension of the rill and gully network in a
semi-arid region of Ecuador. Several studies
have reported the strong impact of urbanization
through vegetation removal, the drastic increase
in runoff response and the concentration of
runoff on gully erosion risk, particularly in
Third World countries (e.g. Plate 19).
Many more detailed case studies are needed if
we want full understanding of the impact of vari-
ous types of land-use change and its interaction
with extreme weather conditions on gully devel-
opment. Also, more research is needed on the
(socio-economic) drivers of land-use changes
causing increased or decreased gully erosion risk.
19.3.2
Gully erosion models
This section reviews models to predict gully loca-
tion in landscapes, the soil losses caused by
ephemeral and permanent gully erosion, and the
rate of retreat of (bank) gully headcuts.
(i) Modelling the location of ephemeral gullies
and permanent gullies
As discussed earlier, gully
location and morphology depend upon topogra-
phical, geological, land cover and hydrological
characteristics. The exact position of gullies in the
landscape depends on the specific processes that
