Geology Reference
In-Depth Information
to 90% depending upon environmental controls
(climate and weather, soil type, topography, land
use, gully type) as well as on the spatial and tem-
poral scales considered (Poesen et al ., 2003).
Moreover, once gullies develop, the eroded chan-
nels increase the connectivity for runoff and sedi-
ment within a catchment significantly, leading to
a rapid transfer of eroded soil from the uplands to
the lowlands, hence contributing significantly to
muddy floods and to pond and reservoir siltation
(Verstraeten et al ., 2006). It is through (ephem-
eral) gully channels that a large fraction of soil
eroded within a field or catchment is redistrib-
uted and delivered to watercourses.
From this discussion it becomes clear that
gully erosion cannot be neglected when assessing
the impacts of climatic and, in particular, land-
use changes on the rates of soil erosion by water
and on soil quality. However, as indicated above,
relatively few data on gully erosion rates are
available from the literature. Hence, modelling
gully erosion remains an alternative approach.
Therefore, this chapter provides background on
gully types and factors controlling gully erosion,
reviews strategies to model gully erosion and dis-
cusses the interactions between gully erosion,
hydrological and other erosion processes.
(Foster, 1986; Grissinger, 1996a,b). Even though in
the literature ephemeral gullies are recorded on
many photographs of erosion, it is only during the
last three decades that these erosion phenomena
have been recognised as being a major part of the
erosional systems on cropland (Evans, 1993).
According to the Soil Science Society of America
(2001), ephemeral gullies (Plate 17) are small
channels eroded by concentrated overland flow
that can be easily filled by normal tillage, only to
reform again in the same location by additional
runoff events. Poesen (1993) observed ephemeral
gullies to form in concentrated flow zones, located
not only in natural drainage lines (thalwegs or
hollows) but also along (or in) linear landscape
elements, such as drill lines, dead furrows, head-
lands, parcel borders and access roads. Channel
incisions in linear landscape elements are usually
classified as rills according to the traditional defi-
nitions that associate rill formation with the
micro-relief generated by tillage or land forming
operations (Haan et al ., 1994). However, such
incisions may also become very large, so this clas-
sification seems unsuitable. In order to account
for any type of concentrated flow channels that
would never develop in a conventional runoff plot
used to measure rates of soil loss by inter-rill and
rill erosion, Poesen (1993) distinguished rills from
(ephemeral) gullies by a critical cross-sectional
area of 929 cm 2 (i.e. the 1 square foot criterion first
proposed by Hauge (1977) ). Other criteria include
a minimum width of 0.3 m and a minimum depth
of about 0.6 m (Brice, 1966), or a minimum depth
of 0.5 m (Imeson & Kwaad, 1980). As to the upper
limit of gullies, no clear-cut definition exists. For
instance, Derose et al . (1998) studied sediment
production by a large gully (i.e. 500 m wide and
300 m deep). In other words, the boundary between
a large gully and a(n) (ephemeral) river channel is
very vague. Nevertheless, it must be acknowl-
edged that the transition from rill erosion to
ephemeral gully erosion (Plate 17) to classical
gully erosion (Plate 18) and to river channel ero-
sion represents a continuum, and that any classi-
fication of hydraulically related erosion forms
into separate classes, such as microrills, rills,
megarills, ephemeral gullies and gullies, is, to
19.2
Gully Erosion and Gully Types
Gully erosion is defined as the erosion process
whereby runoff water accumulates and often
recurs in narrow channels and, over short peri-
ods, removes the soil from this narrow area to
considerable depths. Permanent gullies (Plate 16)
are often defined for agricultural land in terms of
channels too deep to ameliorate easily with ordi-
nary farm tillage equipment; typically they range
from 0.5 m to as much as 25 to 30 m depth (Soil
Science Society of America, 2001).
In the 1980s, the term ephemeral gully erosion
was introduced to include concentrated flow ero-
sion greater than rill erosion but less than classi-
cal gully erosion, as a consequence of the growing
concern that this sediment source used to be over-
looked in traditional soil erosion assessments
Search WWH ::




Custom Search