1991 Gully Erosion: Processes and Models
1991 Gully Erosion: Processes and Models
1991 Gully Erosion: Processes and Models
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Abstract: Confusing and sometimes contradictory results and reports have led to a sizeable body of
literature on, but unfortunately not to a clear understanding of, gully erosion processes. In the
following review, relevant concepts on gully erosion are summarized. Their implications for gully
erosion modelling are presented along with some recommendations for future research.
Despite nearly a century of gully erosion studies (see Rubey, 1928), gully erosion remains
a poorly understood process (Hadley et al., 1985; Harvey et al., 1985; Foster, 1988).
Confusing and sometimes contradictory results and reports have led to a sizeable body of
literature on, but unfortunately not to a clear understanding of gully erosion processes.
Careful examination of the literature indicates some of the reasons for this shortcoming. In
the following review, relevant concepts on gully erosion are summarized. Their implications
for gully erosion modelling are then presented, along with some recommendations for future
research.
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393
sequence of events once a geomorphic threshold has been exceeded (Harvey et al., 1985: 20).
II Gully erosion
1 Concepts and main characteristics
Gullies have been defined as stream channels whose width and depth do not allow normal
tillage (Food and Agriculture Organization (FAO), 1965: 26); in other words, they are
channels that cannot be crossed by farm implements (Hudson, 1985: 38). An arbitrary
minimum depth of 0.5 m has been established to differentiate gullies from rills, both
features resulting from concentrated flow processes. Rills, however, are clearly dependent
on water supplied from inter-rill areas and behave more as river channels than do gullies
(Imeson and Kwaad, 1980). Enlarged rills filled in annually by normal tillage have been
termed ephemeral (cropland) gullies (Thorne et al., 1984; Watson et al., 1986). Gullies can
develop as enlarged rills (FAO, 1965: 26) but their genesis may be much more complex
(Morgan, 1979: 11), and usually involves an inter-relationship between: 1) the volume,
speed and type of runoff; 2) the susceptibility of the materials to erosion, or gully erodibility;
and 3) changes in cover caused by land use and conservation practices.
A more landscape-based definition of gullies was given by Morgan (1979: 11) and Hudson
(1985: 211). They defined gullies as steep-sided eroding water courses that are subject to
ephemeral flash floods during rainstorms. Gullies are always associated with accelerated or
anthropogenic erosion processes and with landscape instability. They may form in existing
channels or where there was no previous channel drainage (Ireland et al., 1939: 39), later
termed, respectively, valley-floor and valley-side gullies.
According to Imeson and Kwaad (1980), gullies are landscape features which have at some
time experienced rapid growth. They postulated that there is no simple relationship between
the slopes above the gully and the gully itself. Gullies develop when a geomorphic threshold
is transgressed (Patton and Schumm, 1975) due to either a decrease in the resistance of the
materials or an increase in the erosivity of the runoff, or both. The thresholds may be
extrinsic (climatic, anthropogenic) or intrinsic (inherent to the gully system itself) (Schumm,
1979).
According to Hudson (1985: 211-16), gullying is a result of the breakdown of the
equilibrium between process and form in a water course caused by either: 1) an increase in
the amount of flood runoff which the channel has to carry; or 2) a decrease in the ability of
the channel to carry that flow. Increased flows are related to either a change in land use
(e.g., forest to agriculture) or an increase in the catchment area caused, for example, by a
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394
diversion of water by road construction. Decreased channel capacity may be related to either
a decrease of velocity, due to an increase in the channel roughness, or a decrease of the
hydraulic radius following deposition.
Once gullying starts, the gullied channel has a more angular and deep v-shape than the
original bed. In the Manning equation:
v - (R2/3S1/2) / n
where V is velocity, R is hydraulic radius or depth, S is slope gradient and n is the roughness
coefficient, R increases; the gullied channel is bare, so n probably decreases; for the velocity
to remain constant, the gradient must decrease. This explains the fact that the gully floors
tend to be flatter than the gradient of the original streams or slopes (Hudson, 1985).
On balance, the overall effect is most likely to be an increase in velocity, and that is why
gully erosion is not a self-correcting but, nearly always, self-perpetuating process (Hudson,
1985). Hudsons conclusion contrasts strongly with a proposed tendency to gully
stabilization with time.
According to the FAO (1965: 27), gully development is caused by several processes which
may occursingly simultaneously: 1) scouring in the bottom or on the sides of the gully
or
by flowing water plus an abrasive material (soil or debris); 2) waterfall erosion at the gully
head (plunge-pool effect) leading to a quick cut-back into the nongullied land. The rate and
extent of gully development is closely related to the amount and velocity of runoff. Gully
development requires relatively large quantities of water to supply energy for both detaching
and transporting the soil mass (FAO, 1965). The amount of water is closely related to the
size and runoff characteristics of the catchment area. The storm runoff component is the
principal mover of gully debris (Piest and Spomer, 1968).
Thus in the literature, gullying is considered as primarily a fluvial phenomenon, and the
same fluvial processes are believed to initiate gullying. This will be discussed further below.
gullies are basically channels forming a fluvial system that tends to become stable with time.
Crouch (1987; Crouch and Blong, 1989) classified gully sides according to morphology/
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395
processes and assessed erosion rates for each form in order to identify the major sediment
sources within the gully systems. These findings emphasize that gully growth may be also
the result of processes other than gully-head retreat by nickpoint migration (see, for
example, Seginer, 1966; Harvey et al., 1985: 43).
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396
1) Because of the spasmodic nature of gully development, the assumption that a gully system
resembles a fluvial one may be misleading. A rill system is more like a fluvial channel
than is a gully (Imeson and Kwaad, 1980). Piest et al. (1975) could establish a general
relationship between runoff and sediment yield after seven years of measurements and
only then recognized the importance of gully-bank processes in gully development and
sediment production.
2) The tendency to equilibrium in gully slope development is strongly dependent upon the
nature of the materials. If the materials are susceptible to slope instability, there is no
reason to suppose that a stable stage allowing plant colonization will be reached. In
addition, if there is sufficient stability to support some plant growth, this cover may be
efficient in preventing channel erosion but may not prevent further slope development.
Gully banks may go on retreating due to slope processes, especially mass movements (see
Nir and Klein, 1974; Hadley et al., 1985: 32; Hudson, 1985: 41; La Roca and Calvo-
Cases, 1988).
3) In many instances gully erosion leads to badland development, an advanced stage of
erosion (see, for example, Bryan and Yair, 1982).
4) Ungullied areas upslope may become gullied independent of the channel activity in the
main stream. Tributaries can develop as a consequence of hillslope processes, such as
mass movements, and not only by (fluvial) head retreat. The relationship between the
gully and the slopes is a complex one (Imeson and Kwaad, 1980).
Thus the cyclic model, which is still present in more recent publications (FAO, 1977: 126;
Harvey et al., 1985: 47-57; FAO, 1986: 8), combined or not with the discontinuous model,
does not seem a reliable gully growth predictor and therefore a suitable means for guiding
conservation measures.
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397
The role of seasonal soil water tables and that of the top soil saturation overland flow in
mass wasting has been pointed out by several workers (Carson, 1969; Kirkby, 1969; Carson
and Kirkby, 1972: 154; Nir and Klein, 1974; Graham, 1984; Ohmori et al., 1986). A full
understanding of upland erosion, however, is complicated by the poorly defined process of
subsurface flow and by the lack of quantitative data and field observations on subsurficial
erosion (Hadley et al., 1985: 33).
We need a better understanding of the particular type of runoff that generates gullies. In
the same way that some materials may be susceptible to inter-rill erosion, but not to gully
erosion or vice versa (Imeson and Kwaad, 1980), a given type of runoff may be nonerosive
in respect of inter-rill or rill erosion but may increase gully erosion hazard.
1978; Kirkby, 1978; Sidle et al., 1985). Research on incised channels in north-central
Mississippi (quoted in Harvey et al., 1985: 148-49) defined a line of critical stability for a
given bank section by establishing critical values of bank height and slope angle. It was
concluded that bank failure, and therefore channel widening, could be reduced by reducing
either bank height or slope angle.
In Australia there has been an increasing interest in gully-side morphology and gully-side
processes as tools for: 1) estimating gully erosion rates, sediment yields and gully evolution;
and 2) analysing the implications for soil conservation. Veness (1980) devised a method for
estimating the relative contribution of sidewall processes compared with linear incision in
gully development. Blong (Blong et al., 1982; Blong, 1985) found that more than half of
the sediments derived from three gully systems in New South Wales came from gully
sidewalls. In some areas, erosion of sidewalls subsequent to headcut retreat yielded more
sediment than did the initial linear incision. A surface area ratio (defined as the length of
the actual gully side divided by the length of the gully centre line) was used as a comparative
parameter. Hannan (1983) related changes in gully-side morphology to processes (gully
activity; erotion/deposition) and discussed the results in terms of practical considerations for
soil conservation. Crouch (1987; Crouch and Blong, 1989; Crouch, 1990) assessed the
relative rates of soil movement from different gully sides and related the rates to side
morphology in order to identify major sediment-producing sources. He suggested that more
research was needed regarding the role of seepage processes.
Thus the role of subsurface flow in gully stability is relevant. In general, however, gully-
side processes, which are closely linked to gully-side stability, are considered as subordinate
to channel processes (i.e., incision; see, for example, Bradford et al., 1978; Harvey et al.,
1985: 149).
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398
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1) Gully erosion, including badland development, may occur in different, even contrasting,
geologic/geomorphic environments. The occurrence of short-duration, high-intensity
rainfall as well as relatively gentle original slopes seem to be common denominators.
2) The rates of gully growth differ widely from case to case, and they are also different
within the same study area. One storm accounted for more than 40% of gully erosion in
a seven-year period (Piest et al., 1975). Thus temporal variation is also great.
3) The approaches used are based mainly on field observations and measurement of
processes. The assessment of susceptible materials is rare.
4) Sequential aerial photographs are useful provided that: a) enough time elapses from
image to image and sufficient gully growth occurs; and b) the photo scale is large enough
to allow a reasonable stereo threshold and accurate linear measurements. In fact, aerial
photographs are the main source in data collection for modelling.
Several conclusions can be drawn from the literature:
1) The cyclic model of gully development, combined or not with the discontinuous model,
does not seem to be a reliable gully growth predictor, and it may therefore not be a
suitable means for guiding conservation measures. A suggested natural tendency to gully
stabilization cannot be expected in all environments.
2) There should be a thorough understanding of the specific gully-producing type of runoff.
A given type of runoff may be nonerosive in respect of inter-rill or rill erosion but may
increase gully erosion hazard.
3) There is no simple relationship between the slopes above the gully and the gully itself.
Mass movements have been overlooked (compared with fluvial processes) in both gully
initiation and gully growth. The relative importance of mass movements would be a
function of the properties of the slope materials where gullies initiate or grow.
4) Mass movements are closely related to high soil water content (see, for example, Caine,
1980; Crozier, 1986). In this sense, it is the amount of rainfall (and not only the intensity)
and the antecedent moisture which may be the critical climatic variables to be included
in gully erosion research.
5) Protective land covers that reduce splash impact and surface erosion may, by stimulating
infiltration, encourage mass movements and gullying.
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401
1964). The SCS accepted that not enough was known about the relative importance of the
various causal factors and that precise quantitative values could not be given to all variables.
Only the relationships involving area and precipitation were therefore used in the final
equations.
This raises several relevant issues:
1) The rainfall variable is based on mean annual data; the effect of individual events is
obscured (Stocking, 1980a).
2) It is assumed that headcuts migrate with the same form and depth of incision at every
erosional event (Stocking, 1980a).
3) Gully-head retreat as a measure of gully growth does not take into account gully-side
erosion, which was included in an early model by Beer and Johnson (1963). This raises
questions of: a) the importance of the drainage area (or distance to divide); and b) the
tendency to self-regulation, explained in terms of decreasing catchment area (above
headcut) as a surrogate for contributing discharge. A proved positive correlation between
catchment area and gully-head retreat does not justify an assumed tendency to a final
stage of gully stability. Runoff from slopes draining toward gully sides (or tributaries)
may also contribute to gully growth.
4) An assumed increase of soil erodibility with the decrease of clay content (SCS, 1977) also
assumes: a) the dominance of surficial runoff as eroding agent; and b) that gully
erodibility and inter-rill erodibility are the same.
Stocking (1980a) used multiple regression analysis on data from 66 gullies in Zimbabwe
(semi-arid/subhumid) to predict gully-head retreat (in terms of volume of gully growth).
The analysis was subdivided according to: a) temporal scale, into short (storm basis),
medium (1-20 years) and long (>20 years) terms; b) gully type, into waterfall headcuts,
piping and a combination of these. Long-term data were gained from historical records,
medium-term data from sequential aerial photos dating from 1956, 1964, 1968 and 1971 (no
photo scales are mentioned) and field measurements (1972-1976); short-term data were
obtained from detailed field measurements after each erosional event. The variables tested
against volume of gully growth were: a) for catchment conditions: precipitation, antecedent
precipitation index (of the previous 10 days; only for short-term analysis), index of piping,
vegetation cover, rainfall interception, catchment area and population density; b) for gully
conditions: height of headcut and slope of approach channel.
According to Stocking (1980a) population density, vegetation cover, rainfall interception
and slope of the approach channel were questionable as to their suitability or efficiency for
prediction equations. Precipitation was extremely significant for waterfall headcuts and
occasionally significant for piping heads. Stockings conclusion was that gullies migrating
through piping are not dependent on individual increments of precipitation and that
migration can occur after a relatively small storm provided that conditions of stability at the
headcut are suitable. Antecedent precipitation proved to be far more important for piping
heads since piping itself is dependent on preceding events. Headcut height was the most
important variable for piping heads on the short term and was also important for waterfall
heads. Catchment area, as a single variable, was inferior to precipitation, but it was highly
significant for all heads that migrate at least partially through waterfall erosion.
Several comments are required:
1) Provisions were taken for: a) different timescales (avoiding the averaging effect of
consolidated rainfall data); b) different gully types (on the basis of hydrologic processes);
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402
and c) height of headcut. This embodies more natural complexity than previous models.
The results, regarding gully-side processes (approached here by piping), concur with
conclusions by Beer and Johnson (1963). The contribution of catchment area to gully
erosion retreat is consistently assessed in both approaches.
2) The difficulties offered by the cover/interception variables may indicate their conflicting
contribution (a dense cover is good protection against erosion effects of intense rainfall
but may promote infiltration, in turn increasing piping). This fact was recognized
recently by Stocking himself (1988c).
Kirkby (1978: 357-60) approached the situation with respect to shallow landslides simply
by:
tan A =
0.5 tan 0
where A = slope angle (), and 0 = effective friction angle (). Carson and Kirkbys
approach has a practical value considering that only one variable, directly related to slope
angle, has to be estimated from laboratory or field measurements.
De Ploey (1989) proposed a deterministic model of headcut retreat encompassing stability
of headcut walls or gully banks. The stability of headcuts or banks was expressed in terms
of their critical slope height (Hc). The volume of soil eroded by the headcut retreat was
approached as being proportional to 1/Hc and to the kinetic energy of the mass of water
striking the plunge pool of the headcut. For large gullies, the model predicts a linear
relationship between rate of gully recession and total discharge, and recession values which
are independent of the height of the headcut.
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403
slope was the most significant individual factor explaining the variation of gully length.
Williams and Morgan (1976) developed an index of soil erosion density (SED) based on
the product of the numbers and lengths of gullies per unit area. SED was measured in third-
order drainage basins, using aerial photographs (1:20 000 and 1:25 000 scales), in two
contrasting areas (disturbed savanna land in Zimbabwe and disturbed tropical rainforest in
Malaysia). Using multiple regression, the SED was related to drainage density, basin relief
and basin shape. In this approach, the scale of the aerial photographs is critical.
IV Conclusions
Acknowledgements
The research on which this paper is based has been funded by the University of Mexico and
ITC (The Netherlands). Thanks are due to Ann Stewart for reading the manuscript.
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404
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