Geomorphological investigation on gully erosion in the Rift Valley and the northern highlands of...

Geomorphological investigation on gully erosion

in the Rift Valley and the northern

highlands of Ethiopia

P. Billia,*, F. Dramisb

aDipartimento di Scienze della Terra, Universita di Ferrara, Ferrara, ItalybDipartimento di Geologia, Universita di Roma 3, Roma, Italy

Received 28 March 2000; received in revised form 19 February 2001; accepted 30 March 2001

Abstract

Gully erosion phenomena are very common in Ethiopia. They affect large areas with different

morphological, pedological and climatic characteristics. The amount of soil loss due to gullying has

become a very serious problem in the recent decades as it was associated to remarkable depletion of

cultivated land. Field investigations on gully morphology and its genetic processes were carried out

in two study areas of Ethiopia, representative of different geo-environmental conditions: the Lakes

Region in the Rift portion north of Shashamene and the area surrounding the town of Mekele in

Tigray.

Two main types of gullies were identified on the basis of their morphological and hydraulic

geometry characteristics: (1) discontinuous gullies which generally develop on low gradient slopes

(1–5j on average) and the hydraulic radius of which increases from an upstream minimum to a

maximum, at approximately their mid length, and decreases again to a relative minimum at their

downstream end; and (2) stream gullies, formed by deep erosion processes typically migrating

upslope. In order to investigate the main causes originating the different types of gullies, shear stress

data were collected in the field from their hydraulic geometry. Hypotheses on the mechanisms

responsible for both discontinuous and stream gullies development and for their different

characteristics are discussed considering the pattern of shear stress variation in the downstream

direction.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Gully erosion; Hydraulic geometry; Shear stress; Ethiopia

0341-8162/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

PII: S0341 -8162 (02 )00131 -5

* Corresponding author.

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Catena 50 (2003) 353–368

1. Introduction

Gullying is one of the most important erosion processes which largely contribute to the

sculpturing of the earth surface. The development of gullies has many negative impacts as

it normally involves the loss and (in some cases) the deposition of a great amount of soil.

For many countries, the loss of large soil masses by gully erosion often stands for the

depletion of a basic natural resource. Moreover, the formation of gullies implies an

alteration of overland flow, a shortening of runoff lag time and an increase in runoff

volume. Once a gully is formed, it tends to develop further and this process is seldom

inverted or halted naturally. This results in a substantial damage to the economy and may

represent a relevant constrain to the development of poor countries. In spite of the many

efforts made to understand the main factors and processes originating gullies, they are not

yet well understood (Boardman, 1998).

The first authors facing the problem of gully formation postulated a development from

rills (e.g. Ireland et al., 1934), but gullies have different geomorphic and hydraulic features

as reported by Oostwoud Wijdenes and Gerits (1994) and partly in this paper. Gullies can

also develop from headward stream retreat affecting unincised, upstream slopes (De

Oliveira, 1989). Soil tunnelling, too, is considered a common factor in triggering gully

formation (Oostwoud Wijdenes and Gerits, 1994).

A very important role is also played by man through disturbance to vegetation. Its

removal by logging or cropland expansion, in humid areas, or by overgrazing, in semiarid

zones, favours the development of gullies (Trimble, 1974; De Ploey, 1990). In fact, a

sparse vegetation cover results in a diminished boundary roughness opposing a reduced

resistance to overland flow. The erosive capability of runoff is therefore increased, while a

deficit of organic matter in the soil decreases its aggregate stability.

Unless the causes of gully formation and development can be discerned, any efforts in

designing countermeasures to gullying will be frustrated. Gullies are probably initiated and

develop through different, concurrent factors, though, we agree with Oostwoud Wijdenes

and Gerits’ (1994) statement that in all cases runoff response and sediment transport on

slopes play a crucial role. According to Parsons et al. (1990) interrill flow tends to

concentrate downslope into pathways leading to a more efficient flow and to a decrease in

flow resistance. It follows that the interaction between overland flow and ground surface

roughness is a process relevant to gully initiation (Poesen and Govers, 1990; Grissinger,

1996). As a gully forms, its development keeps on being controlled by different factors

(Heede, 1976) including the relationship between flow and flow resistance which, in the

end, are responsible for its morphological features. The analysis of their hydraulic

geometry can therefore contribute to the understanding of the processes acting on the

development of gullies.

In order to investigate the effectiveness of hydraulic factors in gully development and

shaping, the hydraulic geometry of 16 gullies has been measured and expressed in terms of

spatial distribution of flow energy (i.e. downstream variation of boundary shear stress).

These measurements were coupled by field observations on the local geomorphological

setting and by direct inspection during intense rainstorms in the rainy season. The gullies

studied are located in Ethiopia, in areas with different geo-environmental characteristics.

The study sites are included in the Rift Valley bottom (Langano Lake), in its northern

P. Billi, F. Dramis / Catena 50 (2003) 353–368354

margin, west of Ziway lake (Tora-Butajira area), and in the northern highlands, in a small

area south of Mekele.

2. Description of the study areas

The study area in the Rift Valley is located southeast of Butajira (Fig. 1) and comprises

a few tens of kilometres wide belt stretching from the northern Rift margin to the great

lakes in the Valley bottom. The physiography is given by a sequence of a few fault scarp

benches, placed at different altitudes and connected by gentle slopes giving the area a large

scale stepped morphology. Almost the entire area is underlain by Quaternary volcanic

rocks (namely tuffs, volcanic ash, ignimbrites and lava flows) with the exception of a

narrow belt around the lakes on the valley bottom where Quaternary lacustrine deposits

outcrop. Extensive slope deposits are also very common on foothills. Below the elevation

Fig. 1. Location map.

P. Billi, F. Dramis / Catena 50 (2003) 353–368 355

of 1800 m a.s.l., the natural vegetation consists of scanty grass and acacia trees, reflecting

the lack of sufficient rainfall characterising the Rift Valley floor (Fig. 2a). The land is

cultivated mainly around the lakes while on the surrounding hills animal husbandry

prevails. At higher elevations the larger quantity of rainfall (Fig. 2b) favours a much

denser vegetation cover. Almost all the land is cropped, though animal husbandry is

carried out as well.

The study sites in the northern highlands are located between the village of Adi Gudum

and the town of Mekele. Here Mesozoic (mainly Jurassic) sedimentary rocks outcrop.

They consist of clays, marls, limestones and sandstones, while scattered dolerite domes

punctuate the area. Slope deposits are common as well. Faults, pertaining to the main

rifting system, have dislocated the area into block structures bounded by steep fault scarps.

The very sparse vegetation reflects the semi-arid conditions affecting the area (Fig. 2c). In

the study site animal husbandry is common and the cultivated land consists of rainfed

crops of grains. The water scarcity is as marked as in the Rift Valley bottom. In both these

areas, decades of overgrazing and uncontrolled exploitation of forestry resources seem to

have influenced the water resources depletion and the erosion processes intensification

(Ethiopian Mapping Authority, 1988).

In the Rift Valley study area, a neat differentiation can be observed in both the annual

and monthly rainfall distribution between the ‘‘lowland’’ of the valley floor and the upland

of the rift margin. The rainfall data measured at Ziway (1650 m a.s.l.) and Butajira (2000 m

a.s.l.) gauging stations were considered respectively (Fig. 2a and b). The annual precip-

itation of Ziway amounts to 769 mm, while at Butajira it is 1194 mm. In Ethiopia,

traditionally, the rainfall distribution is subdivided into three periods: the dry season (from

October to February), with almost no rain; the short rains, from March to May (locally

known as Belg—Ethiopian Mapping Authority, 1988) and the summer long rains, from

June to September (known as Keremt—Ethiopian Mapping Authority, 1988). At Butajira,

two relative monthly peaks are recorded: the first is in March with 135 mm, while the

second, corresponding to the yearly maximum, occurs in July with 171 mm. The driest

month is November with only 10 mm. At Ziway, Belg and Keremt are not so well distinct as

the monthly values gradually increase from the lowest value of November (1.7 mm) to the

annual maximum recorded in July with 145 mm. In Mekele (2070 m a.s.l.) the precipitation

is concentrated in July and August since 70% of the annual rainfall occurs during these two

months, while the Belg is characterised by very small rain amounts (Fig. 2c).

The rapid expansion of gullied areas that seems to have occurred in Ethiopia during the

recent decades has been associated to deforestation and overgrazing (Sagri,1998), but also

a change in the rainfall regime was postulated. Data records referred to the last 3–4

decades seem to indicate that Belg rain amounts tend to increase, while those of Keremt

show a decreasing pattern for several Ethiopian stations (Billi and Dramis, 1999, 2000).

An increase in Belg rains may have some relevance in favouring gully development since

this precipitation occurs after the long dry period, when soil dissection cracks are at their

maximum extension (tunnelling prone soils) and vegetation cover is at its minimum.

Moreover, though Belg precipitation is smaller than Keremt, their maximum intensities for

hourly values (i.e. the shortest time interval available for the study areas from the

Ethiopian Meteorological Agency) are of the same order of magnitude, i.e. around 40

and 43 mm h� 1 at Ziway and Mekele, respectively.

P. Billi, F. Dramis / Catena 50 (2003) 353–368356

Fig. 2. Monthly rainfall distribution for the study areas. Ziway is on the shore of the homonymous lake on the Rift

Valley floor. Butajira is located near the northern Rift margin, while Mekele is on the northern Ethiopian

highlands.

P. Billi, F. Dramis / Catena 50 (2003) 353–368 357

3. Field observations on gully morphology

In Ethiopia, gullies can be found everywhere, no matter the climate, the top soil

characteristics, the physiography or the lithology of the substratum. Normally, they

develop on low gradient slopes that seldom exceed 10j (1–5j on average).

Two main types of gullies were recognised. They have some similarities with those

described by De Oliveira (1989). The first type resembles the discontinuous gullies of

Leopold et al. (1964), but commonly were found as individual, isolated gullies that

entirely develop within a single slope stretch. They originate at a highly variable distance

from the divide (as observed by Heede, 1976) and reach their maximum depth at a variable

distance from their downslope end. Width is not uniform, as assumed by Poesen and

Govers (1990), since it may either constantly increase in a downstream direction or, less

commonly, reach a maximum around its mid length to decrease again to a relative

minimum at the downstream end. The 10 discontinuous gullies considered in this study are

from a few tens to more than a hundred metres long (Table 1). Their upstream edge may be

very narrow (less than 1 m) and not deeper than a couple of metres. They have a width/

depth ratio typically comprised between 1 and 3 and the distinction based on this

parameter and high-intensity low-frequency and low-intensity high-frequency rainstorms

proposed by Poesen and Govers (1990) does not seem to apply here. In their upstream

half, these gullies become deeper and wider downstream and rectangular in cross-section,

with almost vertical banks, but in places where collapsed bank material locally gives rise

to trapezoidal channel geometry. In the downstream half width/depth ratio may either

increase or be constant around an average value, though bank failure is not so extensive as

Table 1

Main morphological features of the study gullies

Gully name Ltot J H H Sdev W W Sdev

Discontinuous gullies

Gully 1 113 0.0026 1.35 0.23 7.9 2.14

Gully 3 36 0.1843 1.29 0.7 1.67 0.97

Gully 5 19 0.0682 0.59 0.15 1.22 0.39

Gully 9 20 0.1117 0.75 0.33 1.33 0.56

Agoddo1 55 0.0570 0.98 0.36 5.43 1.23

Agoddo2 54 0.0550 1.94 0.49 5.88 0.96

Langano North 33 0.0488 0.64 0.22 0.97 0.33

Mt. Lanfuru 90 0.5022 1.43 0.69 0.83 0.37

Lamgano South 35 0.0194 0.83 0.33 1.81 0.52

Kile 30 0.0330 0.44 0.1 0.88 0.20

Stream gullies

Maikei 127 0.0291 1.79 0.26 5.30 1.36

Tora 2 60 0.0393 2.32 0.46 1.76 0.67

Ejersa Lele 76 0.0145 0.62 0.21 2.20 1.31

Kedida 38 0.1104 0.88 0.30 0.94 0.20

Kedida North 37 0.0366 1.23 0.50 1.12 0.41

Hondolesa 28 0.0734 0.47 0.10 0.69 0.14

Ltot = gully total length (m); J= average bed gradient; H = average depth (m); H Sdev = standard deviation of gully

depth (m); W= average width (m); W Sdev = standard deviation of gully width (m).

P. Billi, F. Dramis / Catena 50 (2003) 353–368358

in the upstream half. The slope gradient of the discontinuous gullies studied varies

between 0.046 and 0.093, while their average bed gradient is commonly lower (0.003–

0.184).

The second type of gully investigated in this study are named stream gullies. Their

characteristics (Table 1) are very similar to those observed for the upstream portion of

discontinuous gullies but for the size and the fact that they pertain to a well-established

river system. Like slope gullies, river gullies originate as a small incision at a variable

distance from divide and grow downstream with depth and width increasing at an almost

constant rate. Their width/depth ratios range between 1 and 4, but there is no evidence of a

clear downstream trend. Bank failure is common and the collapsed material is removed by

the flow that normally has energy in excess to entrain and transport all the supplied

sediment. The stream gullies studied developed on slopes with almost the same gradient

(range 0.003–0.084) of discontinuous gullies, but the bed gradient of stream gullies (range

0.014–0.123) is commonly higher than slope gradient. This suggests that the stream

gullies studied were nothing but former slope gullies captured by upslope growing of river

network headwaters (i.e. continuous gullies in the sense of Leopold et al., 1964). Both

slope and stream gullies enlarge by headward expansion and widening and in places they

may coalesce to form wide belts of severely degraded land.

Field observations indicate that, in the study areas, gullies develop on different settings

and under different conditions: lithological substratum, geomorphology, soils with variable

characteristics and depth, annual precipitation, vegetation cover, land use, etc. This led to

consider the hydraulic processes (i.e. the flow capacity to entrain and transport sediment)

as the main factors in providing both discontinuous and stream gullies with their actual

geomorphological characteristics.

4. Field methods

The slope gradient, the bed gradient and the hydraulic geometry of 16 gullies were

measured in the field. The hydraulic geometry was obtained by cross-section surveying at

1- to 4-m interval along the entire length of discontinuous gullies and down to stretches

where bankfull condition could be clearly identified for stream gullies. Tributary junction

sectors and reaches extensively affected by recent bank failure were skipped. The bed

gradient was measured by levelling at each cross section or, more often, for individual

reaches with uniform gradient within the same gully. By these data, at each cross-section it

was possible to calculate the hydraulic radius and the shear stress, assuming a condition of

bankfull flow. This approach was justified by the fact that both the discontinuous and the

stream gullies studied experienced bankfull discharge during high intensity rainfalls. This

was ascertained by direct inspection during a summer field campaign and confirmed by

local people. Shear stress was calculated by the classical Chezy formula:

s ¼ cRS

where S is the energy slope (assumed equal to bed gradient); R is the hydraulic radius and

c is the specific weight of water. The specific weight of water was kept constant and

P. Billi, F. Dramis / Catena 50 (2003) 353–368 359

assumed equal to 1000 kg m� 3 as no data on its variation is available. Moreover, since

the relative variation of shear stress was considered (see next section), keeping the

specific weight of water constant does not imply a relevant bias to the analysis proposed

here.

In order to compare gullies with different size, dimensionless expressions for shear

stress and downstream distance were used. Dimensionless shear stress is defined as

T ¼ s0=smax

where s0 is the actual shear stress calculated at a given cross-section and smax is the highest

value of shear stress obtained for the same gully.

The dimensionless downstream distance was instead defined as

L ¼ Lp=Ltot

where Lp is the distance from the gully headcut and Ltot is the total length of the gully.

5. Data analysis

The downstream variation of shear stress for the gullies studied is reported in Figs. 3

and 4. For discontinuous gullies (Fig. 3), shear stress tends to increase up to a maximum,

typically coinciding with the gully midpoint, beyond which it again decreases to a

minimum at the gully mouth. Although the diagram of Fig. 3 shows some scatter, all

Fig. 3. Downstream variation of shear stress for 10 discontinuous gullies. T and L are dimensionless shear stress

and distance, respectively (see text for their definition). In the diagram, the best fit curve (solid line) and the

boundary curves (dashed line) are reported as well.

P. Billi, F. Dramis / Catena 50 (2003) 353–368360

the data are distributed within a curved belt and also the data of individual gullies follow a

parabolic function whose general, best fit equation is

T ¼ 1:36L2 þ 1:24Lþ C

with the constant C ranging from 0.2 to 0.8.

Gully morphology and, consequently, shear stress distribution reflects the erosion and

deposition conditions observed in the field. In fact, in the upstream half of slope gullies

erosion processes prevail as excess flow energy is dissipated through gully upslope

migration, bottom scouring and sediment transport. Flow energy is therefore decreased in

the downstream portion and in-channel deposition of part of the material supplied from

upstream takes place. The resulting decrease in bed gradient and the relative increase in

flow resistance causes the flow to spread laterally and, in places, induces the widening of

the downstream end of discontinuous gullies. Bank height is normally smaller at the gully

mouth and the material coming from bank failure is relatively limited compared to that

entrained in the gully upstream and middle portions. The latter can be considered the main

sediment source in a discontinuous gully. Beyond its downstream end, the lack of flow

confinement, the increase in flow resistance and the decrease of sediment transport

capacity lead to further deposition with sediment accumulating in elongated lobes or,

more commonly, in fan-shaped splays ahead of the gully mouth.

Part of the discontinuous gullies studied did not show significant variation of bed

gradient, while others are composed by different reaches with distinctive gradients.

Schematically, three main reaches can be distinguished: (1) from the upstream end to

about the gully midpoint we have the highest bed gradient that can be associated to

scouring processes; (2) the following reach is commonly very short and characterised by

intermediate values of bed gradient. Here, boundary material entrainment and deposition

are more or less in equilibrium (transport reach); (3) the downstream reach is normally

Fig. 4. Downstream variation of shear stress for six stream gullies. T and L are dimensionless shear stress and

distance, respectively (see text for their definition). R is the correlation coefficient.

P. Billi, F. Dramis / Catena 50 (2003) 353–368 361

longer than the intermediate (but shorter than the upstream one), has the lower bed

gradient and depositional processes prevail. As a result, the downstream portion of a

discontinuous gully is generally more stable than the upstream and middle reaches. In

these latter reaches, in fact, bank failure is rather common since bottom scouring results in

high, unstable banks that can be also undermined by fast, shallow flows during the

receding phases of the gully flooding.

Fig. 4 reports the downstream changes of dimensionless shear stress for six stream

gullies. Its increase with distance is linear, with a relatively high coefficient of determi-

nation (R = 0.70), for both whole and individual gully data sets. In stream gullies, as well

as in the upstream part of discontinuous gullies, flow energy is partially consumed by

gully upslope migration, channel scouring, boundary material entrainment and transport.

The relative uniformity of bed gradient (commonly higher than slope gradient) and the

downstream addition of flow from tributary gullies and runoff contribute to maintain an

excess of flow transport capacity and, therefore, the conditions for erosion and transport of

the material supplied from upstream.

The width/depth ratios of stream gullies are a little lower than those of discontinuous

gullies, indicating some more hydraulic efficiency of the former. Stream gullies may

largely contribute to the sediment yield of a river system, while in discontinuous gullies

scouring is partially counterbalanced by deposition, with both these processes mainly

restricted to an individual slope.

6. Gully formation

In the study areas, gullies seem to be initiated by soil piping and the formation of soil

hollows such as the headcuts reported by Leopold et al. (1964) and by Bull (1997). The

Fig. 5. Converging flow to headcuts during a high-intensity rainstorm near Butajira.

P. Billi, F. Dramis / Catena 50 (2003) 353–368362

analysis of soil characteristics was beyond the aim of this study and no soil investigation

was carried out. Nevertheless, soil piping was found very common in areas where multiple

slope gullies developed.

Headcutting is a poorly known processes though rather common in the study areas. It

consists of an upslope migrating terracette whose cuspate morphology is given by

contiguous arcuate niches that are very similar in shape to the headcuts of discontinuous

ephemeral streams reported by Bull (1997) (Fig. 5). The front height is normally

controlled by harder soil horizons (e.g. calcrete) typically occurring a few tens of

centimetres below the ground surface. No satisfactory explanation of the main factors

originating the initial soil step was found and further investigation is needed. However,

once an incipient soil step is formed, sheetflow converges to it (Bull 1997, his Fig. 12c)

Fig. 6. South Langano discontinuous gully. This gully formed during a short (2–3 h) high-intensity rainstorm.

Notice the gully sinuosity, the aggradation in the downstream reach and the high rate of bank failure in the middle

reach.

P. Billi, F. Dramis / Catena 50 (2003) 353–368 363

(Fig. 5) and the headcut is undermined by the tumbling down flow and migrates upslope.

The sediment supplied by failure of front portions is redistributed downslope on very low-

angle, washout splays. In the study areas, headcut development and migration give rise to

three different ground morphologies: (1) individual fronts may advance upslope and merge

together forming a badland or (2) leave back a flat, gently inclined surface, that is

punctuated by mounds or pinnacles of uneroded soil, and its general morphology recalls,

though at a much smaller scale, that of a peneplain; (3) during heavy rainfall, the larger

concentration of flow in a cuspate headcut can break the resistant bottom soil layers

resulting in a fast, deep ground scouring which leads to the formation of a new gully

(Leopold et al., 1964; Bull, 1997).

Oostwoud Wijdenes and Gerits (1994) rejected the hypothesis of Ireland et al. (1934)

that gullies may develop from rills. In fact, they say rills are not always the dimensional

equivalent of gullies, since in the latter bankfull discharges are rarely achieved. During our

field surveys in the Keremt rainy season the formation of rills was observed, but none of

them was found to develop into a gully. By contrast discontinuous gullies were seen to

fully develop during a single, short (2–3 h) rainstorm (Fig. 6), in agreement with the

assumption of Kirkby and Bull (2000) for the formulation of their gully simulation model.

Field evidence and discussions with local people revealed that small stream and

discontinuous gullies may experience bankfull discharge during their formation (like the

one depicted in Fig. 6) and at least once in a year.

7. Discussion

In spite of the rich literature on the subject, the main factors responsible for the

formation of gullies are not yet well known (Boardman, 1998). In the study areas,

discontinuous gullies are originated by any process and/or small scale, ground surface

feature leading to the concentration of overland flow. Other factors such as slope gradient

and soil characteristics seem to play a secondary role since discontinuous gullies were

observed to occur on a wide range of slopes and soils. Once the gully is formed, its

morphological development is mainly governed by the flow capacity to entrain and

transport boundary particles (Kirkby and Bull, 2000) and by the soil characteristics and

stratigraphy that exert some control on the rate of gully widening and deepening.

In the study discontinuous gullies, field observations and the downstream pattern of

shear stress (Fig. 3) indicate that in the upstream portion scouring processes prevail and the

sediment flux is less than a capacity load as postulated by Howard (1994) for detachment-

limited steep channels. The flow reaches transport capacity around the gully midpoint,

while, in the downstream portion, the system becomes transport-limited, as assumed by

Willgoose et al. (1991) in their simulations of erosional processes and drainage network

development on slopes. In fact, in the downstream reach, the supply of sediment, derived

from the eroding processes upstream, exceeds transport capacity and deposition takes

place (Fig. 6). Previous theoretical and field studies (e.g. Meyer-Peter and Muller, 1948;

Gomez and Church, 1989; Talling and Sowter, 1998) have shown that shear stress and

sediment transport are related in rivers; since this relationships seems to occur also in

discontinuous gullies, they can be regarded as micro fluvial systems in which basic

P. Billi, F. Dramis / Catena 50 (2003) 353–368364

hydraulic processes, similar to those of larger systems, act, though at different rate and

scale. According to Knighton (1999), total stream power, defined as the rate of flow

energy expenditure per unit channel length, X = cQs (where c is the specific weight of

water, Q the discharge, s the energy slope, assumed parallel to bed gradient), is predicted

to be maximum at an intermediate location, whose position is determined by the ratio b/b,where b and b are the downstream rates of change of discharge and slope, respectively.

The same author also pointed out that b is a conservative quantity having a relatively

narrow range of values, hence the position of maximum stream power depends mainly on

the shape of the longitudinal profile and its degree of concavity. For different river

systems, as b decreases, the position of maximum stream power moves downstream

(Knighton, 1999, his Fig. 2). Discontinuous gullies are relatively short and the rate of

discharge increase in a downstream direction is expected to be very little. The bed profile

of the study discontinuous gullies is commonly flat or has a low degree of concavity,

therefore, also b is expected to assume low values. In this situation, the maximum of total

stream power should be found near the mouth of discontinuous gullies. Knighton’s (1999)

model also predicts that specific stream power, x =X/w = sv (where w is channel width, saverage shear stress and v average flow velocity), is maximum halfway between the source

of the river and the location of the total stream power maximum. Following the previous

considerations, if we assume that total stream power maximum occurs at the mouth of

discontinuous gullies, specific stream power is expected to peak around the gully mid

length. Obviously, further studies are necessary to verify this hypothesis; however, it is

worth noticing here that specific stream power and shear stress are strictly related (x = sv)and that the longitudinal variation of shear stress in discontinuous gullies follows a

parabolic function whose apex abscissa (maximum shear stress) coincides with the gully

mid length (L= 0.46) (Fig. 3). This implies that shear stress increases and decreases at the

same rate in both the upstream and downstream portion of the study discontinuous gullies,

irrespective of their size and slope gradient. The pivoting point where stream power/

resisting power ratio converges to one is not located at the intersection with the slope

profile, as postulated by Bull (1997), but just around the gully midpoint. As the gully

grows in size, this point may migrate upstream, following the gully headhcut in the

process of upslope advancing, or keeps approximately the same position if the gully is

simultaneously expanding in a downstream direction. By additional field monitoring it will

be possible to ascertain the development trends of discontinuous gullies (Billi, in

preparation).

Stream gullies are characterised by a constant, downstream increase of shear stress. In

fact, these systems are dominated by erosion processes, are detachment-limited since

sediment supply is commonly less than transport capacity and deposition does not take

place. Their hydraulic geometry is similar to that observed in the upstream portion of

discontinuous gullies and characterised by small width/depth ratios. In stream gullies, the

bed gradient is almost constant, while discharge increases noticeably in a downstream

direction. Specific stream power, and by analogy shear stress, maxima are therefore

expected further downstream of the study reaches as indicated by the constant increase of

shear stress depicted in Fig. 4.

In the study areas, stream gullies can be seen as the ultimate configuration of

discontinuous gullies development. As the latter expand, they can merge into each other

P. Billi, F. Dramis / Catena 50 (2003) 353–368 365

forming a larger gully system. The detachment-limited to transport-limited transition

progressively moves far downstream from the gully head and deposition occurs in the

lower gradient river reaches (depending on the grain size of the sediment supplied, Kirkby

and Bull, 2000) and on flood plains.

Theoretically, any factor capable to tackle the migration of the gully midpoint, where

shears stress reaches its maximum, would result in a reduced rate of gully enlargement.

This aim could be achieved by introducing erosion control measures designed to increase

the bed roughness in the upper portion of the gully.

8. Conclusions

The field survey of 16 gullies, located in areas with different climatic, physiographic

and geomorphological characteristics of Ethiopia, led to group them in two distinct

categories: discontinuous and stream gullies. The former are individual slope incisions

originating at a variable distance from the slope divide and showing downstream

variations in top-width/depth ratio and hydraulic radius. In the upstream portion of

discontinuous gullies erosion and sediment transport are the dominant processes, while

in their downstream part, deposition prevails. In stream gullies, both depth and width tend

to increase downstream, flow energy is in excess and able to entrain and transport all the

sediment derived by bank failure, bottom scouring and supply by overland flow processes.

Plots of shear stress versus distance show a different pattern for discontinuous and stream

gullies. The predominance of erosion processes in the upstream part, followed by

deposition in the second half of discontinuous gullies, is well expressed by the down-

stream variation of shear stress. Its pattern follows a parabolic function reaching a

maximum value around the gully midpoint, while, in the downstream portion, it decreases

again to a relative minimum value near the gully mouth. In stream gullies, instead, shear

stress increases linearly in association with downstream increase in discharge and excess

transport capacity.

Gullies severely affect large areas of Ethiopia and reliable solutions to this problem are

still far from being found. The geomorphic analysis presented in this study indicates that

an increase in flow resistance in the upstream portion of active discontinuous gullies could

improve their stability, preventing them from developing into the larger stream gully

systems.

Further studies are necessary in order to define improved models of gully expansion

and to design valid countermeasures, but the hydraulic processes involved in gully

formation and growth should be taken into more consideration than in the past.

Acknowledgements

This research was supported by the EU STD3 Programme, contract no. STD3-TS3-

CT92-0076: ‘‘Land resources inventory, environmental changes analysis and their

applications to agriculture in the Abaya lake region’’ and by the Italian Ministry of

Foreign Affairs, Department for the Co-operation with Developing Countries. The authors

P. Billi, F. Dramis / Catena 50 (2003) 353–368366

are grateful to F. Vannacci, S. Accetta and the students of the 1996 Master Course of

Geomorphology of the University of Addis Ababa for their help in the field data

collection. A.J. Parsons and three unknown referees are acknowledged for constructive

comments and suggestion on the paper contents and language.

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