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Earth Surface Processes and LandformsEarth Surf. Process. Landforms 27, 1267–1283 (2002)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.404

IMPACT OF ROAD BUILDING ON GULLY EROSION RISK: A CASESTUDY FROM THE NORTHERN ETHIOPIAN HIGHLANDS

JAN NYSSEN,1,4* JEAN POESEN,1 JAN MOEYERSONS,2 EDITH LUYTEN,1 MAUDE VEYRET-PICOT,1 JOZEF DECKERS,3

MITIKU HAILE4 AND GERARD GOVERS1

1 Laboratory for Experimental Geomorphology, KU Leuven, Redingenstraat 16, B-3000 Leuven, Belgium2 Royal Museum for Central Africa, B-3080 Tervuren, Belgium

3 Institute for Land and Water Management, KU Leuven, Vital Decosterstraat 102, B-3000 Leuven, Belgium4 Makalle University, PO Box 231, Makalle, Ethiopia

Received 29 October 2001; Revised 18 March 2002; Accepted 29 April 2002

ABSTRACT

Although obvious in the field, the impact of road building on hydrology and gullying in Ethiopia has rarely been analysed.This study investigates how road building in the Ethiopian Highlands affects the gully erosion risk. The road betweenMakalle and Adwa in the highlands of Tigray (northern Ethiopia), built in 1993–1994, caused gullying at most of theculverts and other road drains. While damage by runoff to the road itself remains limited, off-site effects are very important.Since the building of the road, nine new gullies were created immediately downslope of the studied road segment (6Ð5 kmlong) and seven other gullies at a distance between 100 and 500 m more downslope. The road induces a concentrationof surface runoff, a diversion of concentrated runoff to other catchments, and an increase in catchment size, which arethe main causes for gully development after road building. Topographic thresholds for gully formation are determined interms of slope gradient of the soil surface at the gully head and catchment area. The influence of road building on both thevariation of these thresholds and the modification of the drainage pattern is analysed. The slope gradient of the soil surfaceat the gully heads which were induced by the road varies between 0Ð06 and 0Ð42 m m�1 (average 0Ð15 m m�1), whereasgully heads without influence of the road have slope gradients between 0Ð09 and 0Ð52 m m�1 (average 0Ð25 m m�1).Road building disturbed the equilibrium in the study area but the lowering of topographic threshold values for gullyingis not statistically significant. Increased gully erosion after road building has caused the loss of fertile soil and crop yield,a decrease of land holding size, and the creation of obstacles for tillage operations. Hence roads should be designed in away that keeps runoff interception, concentration and deviation minimal. Techniques must be used to spread concentratedrunoff in space and time and to increase its infiltration instead of directing it straight onto unprotected slopes. Copyright 2002 John Wiley & Sons, Ltd.

KEY WORDS: drainage; drainage area; Ethiopian Highlands; gully erosion; road building

INTRODUCTION

Many rural communities in the Ethiopian Highlands are located more than 25 km away from any road. Aspart of the development strategy, many feeder and rural access roads were built recently to increase theaccessibility of these areas. Although obvious in the field and a recognized problem (Chadhokar, 2000), theimpact of road building on hydrology and gullying in Ethiopia has rarely been analysed.

In the East African Highlands, Moeyersons (1991) monitored and analysed progressive gully formationafter road building in Rwanda. Ogbaghebriel and Brancaccio (1993) present examples of gullies induced byroads on pediments in the Ethiopian Highlands. Preference for road building in Ethiopia is given to pediments,in order to avoid both flat, poorly drained, and steep areas. Surface runoff on the pediment is concentratedby the roads and induces gullying on the lower part of the pediments. Along a 42 km long road in Kenya,incipient gullying was observed downslope of 54 per cent of the culverts compared to 22 per cent of thedrifts (P. Jungerius et al., unpublished work 1999). This is explained by the fact that drifts conform more

* Correspondence to: J. Nyssen, P.O. Box 271, Makalle, Ethiopia. E-mail: [email protected]

Copyright 2002 John Wiley & Sons, Ltd.

1268 J. NYSSEN ET AL.

to the natural hydrological conditions. Gullying started here only after the road had attracted settlers frommore remote areas who cleared the existing vegetation cover. Also in Kenya, measurements by Dunne andDietrich (1982) show that rural roads and footpaths in a densely populated area cover about 2 per cent of acatchment’s area, but provoke 25 to 50 per cent of total soil erosion.

Elsewhere, and especially in North America, research on water erosion caused by road building has focusedon forested areas (Megahan, 1977; Reid and Dunne, 1984; Sidle et al., 1986; Montgomery, 1994; Baisleyand Cameron, 1996; Gucinski et al., 2000; Luce and Wemple, 2001; Croke and Hairsine, 2001).

Montgomery (1994) found that the main causes for gullying, after road building, are overland flow con-centration by the establishment of artificial drains and increased catchment area. Harden (1992) studied theinfluence of footpaths on runoff concentration in rural areas in Ecuador and Tennessee and stressed thenecessity to incorporate roads and paths in hydrologic and soil erosion models at the catchment scale.

The objective of this study is to analyse how a road built in the highlands of Tigray (northern Ethiopia)in 1993–1994, caused gully erosion immediately downslope of most of the culverts and other road drains.Topographic thresholds for gullying are determined in terms of catchment area and slope gradient of the soilsurface at the gully head. Gully heads usually represent the most active part of the whole gully. The influenceof road building both on the variation of topographic thresholds and on the interception of overland flowis analysed.

MATERIALS AND METHODS

The Makalle–Adwa road

The studied road was built in 1993–1994 and links two towns in Ethiopia’s Tigray region: Makalle andAdwa. The analysed road segment is near Hagere Selam, the main town of the Dogua Tembien woreda(district), situated at an altitude of 2650 m (Figure 1). The geological formations outcropping in the regioncomprise subhorizontal layers of Antalo limestone and Amba Aradam sandstone, both of Mesozoic age, andTertiary basalt flows with interbedded silicified lake deposits. One also finds some Quaternary formations,consisting of alluvium, colluvium and travertine. Eastbound from Hagere Selam, the road follows the foot ofa steep basalt slope, then occupies a ridgetop position (Figure 1). Five kilometres east of Hagere Selam, theroad descends the Amba Aradam sandstone cliff with two hairpin bends, and then follows the foot of thiscliff (Guyeha ridge) over another 4 km.

Permanent fields are the dominant landuse in the study area. The agricultural system in the NorthernEthiopian Highlands has been characterized as ‘grain–plough complex’ (Westphal, 1975). The main cropsare barley (Hordeum vulgare L.), wheat (Triticum sp.) and tef (Eragrostis tef ), an endemic cereal crop.Various species of pulses are also an important part of the crop rotation. Tillage of the soil is carried outby ox-drawn ard plough. The steep slopes are mainly rangeland, parts of which have been recently set asideto allow recovery of the vegetation. In some flat areas, agriculture was abandoned some years before roadbuilding in order to increase the grazing area. In many cases, especially if there is risk of gullying, the thalwegis occupied by rangeland.

Average annual rainfall is 750 mm, concentrated in three months (mid-June to mid-September). Averagestorm duration is 68 minutes, with a rain depth of 6Ð9 mm. Eighty-eight per cent of rain falls with an intensity<30 mm h�1, but rain is highly erosive, due to large drop sizes (J. Nyssen et al., unpublished work, 2001).Rainfall and runoff on soils, which have lost most of their natural vegetation by century-long action ofhuman society, cause intense soil erosion. The predominance of moderate to steep slopes induces a naturalvulnerability of the study area to sheet and rill erosion, despite the high clay content and high rock fragmentcover that produce a generally low soil erodibility (Nyssen, 2001).

Gullying in the Tigray Highlands has been attributed to an overall lowering of the infiltration capacity of thesoil due to depletion of the vegetation (Virgo and Munro, 1978). Nyssen et al. (unpublished work, 2000) alsoobserved that gullying is sometimes initiated after the abandonment of fields, especially if these are convertedinto grazing land. The overgrazed surface has a higher runoff coefficient than regularly ploughed fields. Surfacetrampling causes soil structure decay, decreased hydraulic conductivity, and therefore decreased infiltrationrates and higher runoff volumes. Furthermore, soil and water conservation structures are no longer maintained

Copyright 2002 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 27, 1267–1283 (2002)

IMPACT OF ROAD BUILDING ON GULLY EROSION RISK 1269

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in such areas and gullying often starts at places where these structures have collapsed. In valley Vertisol areas,subsurface erosion and collapse of polygonal structures provokes rapid regressive gully formation (Nyssenet al., 2000a).

The situation deteriorated after the building of the road in 1993 to early 1994. During the first rainy season(1994), farmers observed important gully initiation on their lands just downslope of culverts and outletsof lateral road drains (Figures 2 and 3); 16 new gullies were counted along the studied 6Ð5 km long roadsegment. Their length totals 1081 m and their volume 10 034 m3. The cross-sections of pre-existing gullies,

Figure 2. The gully in Halah (5 km east of area B) developed in 1994, immediately after the building of the Makalle–Adwa road

Figure 3. The northern part of area A in September 1999. Lateral drains from the road (right edge of the photo) are sent towards therangeland along the road and downslope to the cropland at the foot of the basalt cliff. Gully heads 8, 6bis and 8bis were incised afterroad building (top right of the photo). Drainage pattern towards gully head 8 is represented on the figure. Distance from the road to

8bis is 240 m



which were located further downslope, have also increased and overwash sediment is deposited yearly inmany fields.

Most inhabitants of the study area depend on agriculture, and encounter important soil erosion problems(Nyssen, 2001). Increased gully erosion after road building results in loss of soil and yield, a decrease of thecropped area, obstruction of tillage operations and lowering of the water tables. Checkdams are built in thenew gullies; this takes time and energy. Some fields in the study area even had to be followed due to majorgullying or sediment deposition after road building.

The studied gully heads

The road segment that was studied in July–September 1999 extends eastwards from Hagere Selam fromkm 48Ð5 to km 42, outside of any urbanized area. It was selected because: (1) it is located in a region withstepped relief, typical for the highlands; (2) the road was built recently which enables use of interviews withlocal residents to distinguish gullies induced by road building from others; and (3) in many areas, gully headsare present without road influence but in a similar topographical position. First, the position of all gully headsat least 0Ð5 m deep and 1 m wide was mapped (1) along the selected road segment, (2) in a 500 m widestrip downslope of the road segment, (3) in the area between the road and the main water divide and (4) ina topographically similar area north of the ridge. In this study area, gully heads without influence of theroad (for instance z4 to z14 on Figure 1) are infrequent because the area is located in a high position in thetopography, and there is relatively little concentration of surface runoff. In two locations with road-relatedgully heads, it was possible to reconstruct the pre-road drainage area (i.e. H8 and 5 on Figure 1).

Other gully heads induced by road building around Hagere Selam were not taken into account in the studyas the age and characteristics of the involved roads are different from those of the analysed 1994 road; theyinclude gully heads near to recently built rural access roads, those along an old road built around 1964 andthe ones under influence of urbanization.

Survey of the drainage areas of the gully heads

Drainage areas of gully heads were delimited in July–September 1999 by marking the water divide of thecatchment draining to the gully head by flags. The boundaries of the catchment were then mapped using anautomatic theodolite. Putting the surveyed catchments upon a digital terrain model (DTM) of the study areaallowed the calculation of drainage areas and the preparation of maps, using MapInfo software.

The actual drainage areas upslope from gully heads are quite different from those derived from a topographicmap or a DTM. This discrepancy is caused by important modifications of the runoff pattern after road building,but also by linear landscape elements such as footpaths, furrows created by tillage, drainage ditches in thefields and stone bunds. This was also observed by Harden (1992). Most difficult was the identification ofwater divides in places where such linear landscape elements caused a divergent flow. The best results wouldbe obtained if the delimitation of the drainage area was done during or immediately after rainfall events.However, it is expected that errors in drainage area calculation are less than the 30 per cent proposed byMontgomery (1994) for field surveys.

Information on the exact position of road culverts in the study area collected during the 1999 field seasonwas then used to reduce errors when manually deriving the drainage area of an additional 31 nearby gullyheads from a digital terrain model (Veyret-Picot, 2001).

Determination of gully head position and slope gradient

The actual position of the gully head could not always be exactly determined. In the case of digitate gullyheads (Figure 4(1)), data from the most upslope branch were used in the analysis. Rills extending upslope fromthe headcut (Oostwoud et al., 1999) (Figure 4(2)), piping, sunken polygons and ‘blind heads’ (Nyssen et al.,2000a) upslope of the gully head (Figure 4(3)) were not considered. Three gully heads, which are artificiallystabilized at the outlet of a pipe culvert (Figure 4(4)), were not included in the quantitative analysis.

The slope gradient of the soil surface at the gully head is measured in different ways by different authors.Patton and Schumm (1975) as well as Montgomery and Dietrich (1988) worked with the steepest slopegradient of the soil surface along the gully, Vandaele et al. (1996) used the slope gradient of the soil surface



Figure 4. Possible forms of gully heads in the study area: (1) digitate; (2) rilled-abrupt (Oostwoud et al., 1999); (3) sunken soil upslopeof gully head (Nyssen et al., 2000a); and (4) gully head at outlet of pipe culvert

just above the gully head, whereas other authors did not specify. In this study, we chose to measure the slopegradient of the soil surface over a distance of 10 m, parallel to the gully, of which 5 m were upslope of thegully head and 5 m downslope (Rutherfurd et al., 1997) (Figure 5). This position is expected to represent theadjustment of the system to new conditions (Graf, 1977), five years after the building of the road. Accordingto local residents, in all cases, gully development was rapid during the first two to three years after roadbuilding, and in 1999–2000 they generally considered that gully head retreat was relatively slow.

Slope gradient was measured by automatic theodolite and clinometer. At each gully head and in eachcatchment, additional environmental parameters (landuse, soil texture and rock fragment content at the gullyhead, soil conservation measures) were assessed.

A

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Figure 5. Schematic representation of field survey techniques. Arrows indicate runoff direction; S, slope gradient of the soil surface atgully head as used in this study; S1, slope gradient used by Patton and Schumm (1975); S2, slope gradient used by Vandaele et al.

(1996); A, drainage area at gully head



Statistical analysis

The slope gradient (S) of the soil surface near the gully head was plotted against the drainage area (A),which is used as a proxy for total runoff volume. This relation is not linear, but the use of a double logarithmicscale allows a linear representation of the topographic thresholds, drawn through the lowest plotted S–A pointsfor gully heads:

S D aA�b �1�

where a and b are parameters which take different values under various environmental conditions (Patton andSchumm, 1975).

This threshold line represents the critical relationship between drainage area and slope gradient for gullying(Vandaele et al., 1996). Such thresholds, valid in areas with the same or similar environmental characteristics,indicate the minimum slope gradient of the soil surface needed to possibly result in gully incision for a givendrainage area. No gully formation should be observed in the study area for conditions that plot below thethreshold lines. However, it was analysed if gully heads created after road building plot below the topographicthreshold lines for the study area.

Besides this analysis of topographical minimum conditions, the average topographical conditions of gullyheads in catchments with and without road were compared. Taking into account Equation 1, it is expectedthat within the same population of gully heads:

SAb D a D constant �2�

where b was set at 0Ð5, given that this is the value found for thresholds in this study (see below). The averagevalue of a for the sample of gully heads in catchments with roads was then compared to the value of afor gully heads in catchments without road, and the difference in (unpaired) average values tested for itssignificance by t-test (Diem, 1963; Beguin, 1979).

RESULTS

Examples of analysed gullies

In order to allow the reader to perceive the nature of gullying in the study area, some descriptions of thestudied gullies are summarized here. More details and photographs of the analysed gully heads can be foundin Luyten (2000).

The head of the large gully 5 (9Ð45 m wide and 2Ð5 m deep) is located 30 m downslope of a pipe culverton a 0Ð42 m m�1 steep lynchet (‘daget’; see Nyssen et al., 2000b), corresponding to the transition betweenbasaltic colluvium and more erodible silicified limestone (light coloured area on Figure 6). The area drainingto this point was very small (0Ð09 ha) before road building, since it is not situated in a thalweg (see dotted lineon Figure 7). After road building the catchment size increased to 8Ð6 ha, consisting of very steep, overusedrangeland, and overland flow was drained along the upper side of the road towards a pipe culvert. The gullydeveloped during the first rainy season after road construction and growth has continued, destroying largetracts of arable land and preventing the farmers moving from one part of their field to the other. Monitoringshows that despite the building of stone bunds upslope, the gully head is retreating at a mean rate of 1Ð5 m a�1

(Nyssen, 2001). The sediment-loaded runoff flowing through gully 5 reaches a basalt cliff, 100 m downslopeof the gully head. The gully continues at the foot of the cliff (Figure 8), but because of a decreasing slopegradient, deposition of a debris fan occurs, which partially covers arable land. Some 100 m further downslope,the runoff concentrates again in a slight depression (slope gradient 0Ð11 m m�1) occupied by Vertisols (FAOet al., 1998), where a new gully head (5bis) is formed. Such discontinuous gullies (Bull, 1997), comprising agully segment near to the road, a colluvial fan at the foot of the hill and renewed incision further downslopein valley Vertisols, are common in the study area.

Gully head z4 (Figures 7 and 9) developed in a Vertisol with a slope gradient of 0Ð09 m m�1 underrangeland. According to interviewed farmers, it was four years old in 1999. The gully head is 4 m wide and



Figure 6. Aerial view of gully head 5, developed on a steep slope without thalweg after a road culvert was installed 30 m upslope. Notepeople on the road for scale

about 2Ð5 m deep, with its depth decreasing downslope. The 8Ð9 ha catchment, which is not crossed by theroad, is partially situated below a basalt trapp and partially on top of it. This gully head developed afterthe upper part of its catchment had been transformed from field to overgrazed rangeland in 1995. Absenceof ploughing increased the runoff coefficient, and runoff is already concentrated in the upper part of thecatchment at places where ‘daget’ (lynchets), which are no longer maintained, collapse. In the lower part ofthe catchment, the farmers try to protect their fields from this runoff by diverting and concentrating it.

Gully head z123 (slope gradient 0Ð28 m m�1) is situated near a mountain top (Figure 1) in rangeland onLuvisol (FAO et al., 1998). The gully head and the gully itself are well controlled by checkdams. Gullyingin this 0Ð57 ha catchment is explained by the fact that an army camp occupied this mountain top before1989. Infiltration was reduced by the presence of tents, roofs, and compacted soil. The present drainagepattern was probably created at that time. Remnants of trenches and stone defence walls also contribute tothe concentration of runoff. For these reasons, this catchment was considered ‘urbanized’ and not used forfurther analysis.

The dataset

Tables I and II list the characteristics of the gully heads surveyed in 1999 and their catchments. The drainageareas range between 1Ð5 ha (gully head 6) and 14Ð1 ha (gully head 5bis), for those gully heads influenced bythe presence of the road (Table I). The catchments of the surveyed gully heads without influence of the road(Table II) have areas between 0Ð3 ha (z14bis) and 12Ð8 ha (z14). The difference between (unpaired) averagedrainage areas (8Ð5 and 6Ð1 ha) is not statistically significant at the 0Ð05 level.

The slope gradient of the soil surface at gully heads with influence of the road (Table I) varies between0Ð06 and 0Ð42 m m�1 (average 0Ð15 m m�1), whereas gully heads without influence of the road (Table II)have slope gradients between 0Ð09 and 0Ð52 m m�1 (average 0Ð25 m m�1). Standard deviations are quite high,



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Figure 7. Drainage pattern for the gully heads of area A. Catchments were surveyed by automatic theodolite and superposed on a DTM

Figure 8. Where gully 5 reaches the base of a 30 m high basalt cliff a first incision (arrow) was created in the first year after roadbuilding (1994) (left). It increased to a depth of 9 m by 1999 (right). In front is the upper part of the debris fan

hence the difference between average slope gradients is not significant. Most gully heads are on rangeland;sometimes they incise grassed waterways.

Total volume of soil lost by road-induced gullying along the 6Ð5 km long surveyed road segment in six yearsis 10 034 m3. This figure corresponds to an average soil loss of 257Ð3 m3 a�1 per km of road; considering



Figure 9. Gully head z4 (August 1999) developed in Vertisol under rangeland after the upper part of its catchment was transformedfrom cropland to rangeland

the affected 500 m wide strip downslope of the road, the specific soil loss rate due to road-induced gullyingis 514Ð5 m3 km�2 a�1.

DISCUSSION

Influence of road building on drainage

Whereas damage to the road by runoff remains limited (some sinking culvert outlets and rilling on theembankment where runoff is crossing the road), off-site effects of road building are very important. Since thebuilding of the road, nine new gullies formed immediately downslope of the studied road segment (6Ð5 kmlong) and another seven gullies at a distance between 100 and 500 m more downslope.

The road induces surface runoff concentration, changes catchment sizes and shapes and diverts concentratedrunoff to nearby catchments. This is illustrated by detailed maps (Figures 7 and 10), representing the drainagepattern after road building. The catchments of the ‘bis’ gully heads include the catchment of the upslope gullyhead. This is indicated on the maps by a bold arrow, indicating concentrated runoff, which starts from theupper gully head.

Concentration of overland flow. Due to their high topographical position, the drainage areas in the studyarea are rather small, and gullies in the absence of the road are relatively rare. This is probably one of thereasons the road engineers located the road in the uplands. Runoff is not very concentrated, and in the past itwas only in lower parts of the landscape, where catchment size is larger, that gullying became important. Theroad, however, concentrates the dispersed runoff in long drains and transfers it to pipe culverts, downslopeof which the new gully heads developed (e.g. gully head H4, Figure 10). In order to protect the road fromwater erosion, runoff from the slope is sometimes collected in drains some metres upslope from the roadand diverted away from it. Two cases are present in the study area, and both result in gully developmentdownslope (gully heads 6 and 8, Figure 7).

Size and shape of drainage areas. We measured the size of two catchments as they were expected to bebefore road building. The very small area (0Ð09 ha) of the ‘natural’ catchment of gully head 5 (Figure 7,Table 1), compared to its present-day area (8Ð6 ha), indicates to what extent road building can increasecatchment size. Increase in size of catchment H8 (Figure 10), due to road construction, is less spectacular(i.e. from 2Ð4 to 3Ð5 ha), but it also resulted in incipient gullying.

Most ‘old’ drainage areas could not be reconstructed in the field because of changes in microtopography,but a rough estimation of their size can be made on the maps. The catchment area of H2, for instance, wasonly 2Ð3 ha before road building; its width increased greatly after road building and its area is now 6Ð6 ha


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Table II. Characteristics of gully heads east of Hagere Selam, without road in the catchment

Gully head Areaa Slopeb Soil texturec Rock fragment Land cover at Landuse (%)d

(ha) (m m�1) contentc (%) gully head

z4 8Ð9 0Ð09 clay NA Rangeland 20z8 7Ð8 0Ð17 clay NA Rangeland 50z11 9Ð2 0Ð52 clay/sandy loam 70 Rangeland 50z12 1Ð2 0Ð31 clay loam 29 Rangeland 80z14 bis 0Ð3 0Ð11 sandy clay loam 5 Arable land 80z14 12Ð8 0Ð20 sandy clay loam 5 Rangeland 80z9 7Ð7 0Ð32 silt loam 30 Rangeland 20z123 0Ð6 0Ð28 NA NA Rangeland 0!

average 6Ð1 0Ð25 28 48(std. dev) 4Ð7 0Ð14 27 32

Catchment where gullying might startz13 0Ð3 0Ð44 Rangeland 50

a Area draining towards the gully head.b Slope gradient of the surface at the gully head.c At the gully head (depth 50 cm).d Landuse in the catchment: approximate percentage of arable land, the complement being rangeland (0! D 0% arable land C thecatchment is overgrazed).NA, not available.

(Figure 10). Pre-road catchments, draining to the position of the present-day gully heads, were estimated frommaps (Table I). The average catchment area is now 8Ð5 ha, which is significantly different (P < 0Ð001) fromthe average pre-road catchment area of 5Ð8 ha (paired averages).

Transfer of runoff to other catchments. Due to road building, important redistributions of runoff tookplace. Although part of their catchment is located south of the water divide, gullies 6 and 8 drain to thenorthern slope of the mountain ridge. This concentrated runoff also creates the new gully heads 6bis and8bis farther downslope (Figures 3 and 7). Such transfer from one side of the topographical water divideto the other through a small pass also induced the creation of gully head A3, south of Hagere Selam(Figure 1).

On the other hand, such diversions can also intercept and divert runoff away from pre-existing gully heads,which then can be deactivated after road construction. This is the case for five small gully heads south of theroad (Figure 7), whose runoff is now diverted to gully heads 5, 6 and 8. It should be stressed, however, thatall these older gully heads were situated in a very sensitive position, on top of the basalt cliff. The lengthof these gullies never exceeded 10 m (Figure 11). Their total volume is estimated at 100 m3 as compared to10 034 m3 of gullies induced by the road in five years.

Importance of the road position. It is striking that there are two clusters of gullies (A and B, Figure 1)along the studied road segment, but that there are no gullies along the 2 km section where the road occupiesa ridgetop position (Figure 1). Since it is slightly sloped to the sides, the ridgetop road does not act as acatchment and insufficient runoff is concentrated to cause incipient gullying.

Gullies are created only in those places where the road intercepts and concentrates runoff from the slopes,i.e. where it crosses hillslopes and pediments (see also Ogbaghebriel and Brancaccio, 1993).

The road never occupies a toeslope position, probably because building costs are much higher there sincebox culverts or even bridges must be built. Risks of incipient gullying in this landscape position are lower,however, since most of the runoff is already concentrated in clearly defined channels.

Impact of road building on topographic thresholds for gullying

According to Graf (1977), natural drainage systems are close to a situation of equilibrium (steady state) inwhich erosion, transport and deposition are adapted to the existing geomorphologic and climatic conditions.



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This steady state may be disturbed by human impact (e.g. changes in landuse). The steady state in the case ofgullying can be represented by topographic threshold values. The threshold value corresponds to those placesin the landscape where gullying could occur. Vandaele et al. (1996), Poesen et al. (1998) and Vandekerckhoveet al. (2000), among others, report topographic threshold values for incipient gullying in a range of landuseconditions. This threshold is justified by the fact that the position and the size of an incision are controlledby the presence of concentrated surface runoff, with a flow and a duration that are sufficient to initiate andmaintain erosion (Vandaele et al., 1996). In the absence of data on surface runoff in the catchments, thedrainage area is used as a proxy for runoff (Patton and Schumm, 1975).

Drainage area and slope gradient of the soil surface at the gully heads were plotted both for gullies inducedby road building and for ‘natural’ gullies (Figure 12). The topographic threshold line for gullies not influencedby the road is represented in Figure 12. One outlier (z14bis) was not taken into account for two reasons:

Figure 11. Gully head at a cliff edge (in between the two people), partially deactivated after road building, due to decrease of catchmentsize (immediately south of gully head 8, looking westward, July 2000)

0.01

0.1

1

0.1 1 10 100

A (ha)

S (m

m-1

)

Without road (Luyten,2000) (n=7)Without road (Veyret-Picot, 2001) (n=24)With road (Luyten, 2000)(n=11)With road (Veyret-Picot,2001) (n=7)

S = 0.26 A-0.5

z14bis

Figure 12. Topographic threshold for gullying in catchments without road in the Tembien highlands, as a function of drainage area (A)and slope gradient (S). Gully heads induced by road building are represented by open symbols



0.001

0.01

0.1

1

0.001 0.01 0.1 1 10 100 1000 10000

A (ha)

Slop

e gr

adie

nt (m

m-1

)

Belgium (rills)

Belgium (photos)

Portugal

North France

Belgium (field)

SouthDowns(U.K.)

Colorado (U.S.)

Oregon(U.S.)

Ethiopia

Sierra Nevada (U.S.)California

Figure 13. Comparison between topographic thresholds for gullying in the Northern Ethiopian Highlands (this study) and in variousregions of the world (data compiled by Vandaele et al., 1996)

(1) during field measurements it appeared very difficult to identify the boundaries of this elongated catchment;and (2) the gully head is located in abandoned cropland and corresponds to a breach in a stone bund, which isatypical for the study area. From Figure 12 it can be concluded that five gully heads induced by road buildingplot below the topographic threshold for natural gully heads. The average value for a D SA0Ð5 is 0Ð65 š 0Ð48(n D 30) for gully heads without road in their catchment, as compared to 0Ð51 š 0Ð43 (n D 20) for gully headsin catchments with roads (difference not significant at 0Ð1 level). Hence, there are indications, but withoutstatistical evidence, that, for the same catchment size, the critical slope gradient for gullying is lowered if aroad crosses the catchment. Such a lowering was expected since the improved runoff concentration (mentionedearlier), especially in drainage ditches along the road, logically results in higher runoff volumes, larger peakflows and shorter concentration times. Gullying after road building in the degraded Ethiopian Highlands isthus thought to be mainly due to increase of catchment size, since a lowering of topographical thresholdscould not be demonstrated.

Compared to topographic thresholds for gullying in other environments, compiled by Vandaele et al. (1996)(Figure 13), the threshold for gullying in the study area is among the highest observed in the world. Degradedas it is, the landscape of Dogua Tembien still offers a good resistance to gullying. This is attributed: (1) tothe generally low soil erodibility (Nyssen, 2001); (2) to the high rock fragment content (on average 30 percent) of the soils which increases the resistance to concentrated flow erosion (Poesen et al., 1999); and (3) tothe dense root mat of the grassed waterways which often occupy the thalweg. The presence of many steepslopes, however, allows this topographic threshold to be exceeded.

Possible solutions

Road builders generally avoid toeslopes because of the cost of building bridges. Road building on hillslopesand pediments, however, involves hidden costs of gullying, such as soil loss, lowering of the water table,decreased cropping area, fields abandoned after overwash deposition, decreased agricultural output, increasedtime for agricultural operations and gully control works by the local population.

Measures to decrease gully risk are necessary and possible. This study shows that it is important not toincrease local drainage areas when building roads. The distance between drifts or culverts should be limited,



as also stated by Croke and Hairsine (2001). Threshold values obtained in the study area give an indicationof the maximum size that drainage areas may have after road building to avoid the risk of gullying for agiven soil surface slope gradient downslope of culverts.

Water concentrated by culverts can also be brought on contour and, for example, spread in areas witha good vegetation cover (exclosures), as the farmers sometimes do in the study area. Peak flows can betemporarily stored in small detention basins, to be situated downslope of the culvert outlets. In many placesthe topography allows the building of small earth dams. Possibly, a metal pipe in the lower part of the dam willallow a gradual outflow at a controlled flow discharge, during and immediately after storms, without provokinggullying. Grassed waterways should be laid out in places where runoff is directed onto unprotected slopes.

In short, interception, concentration and deviation of runoff by road construction should be kept minimal.Techniques must be used to spread concentrated runoff in space and time and to increase its infiltration insteadof directing it straight onto unprotected slopes. Future road building projects should incorporate (and budget)these and other appropriate measures to prevent gully erosion when designing roads.

CONCLUSIONS

Although the 6Ð5 km long new road segment caused five small gullies (with a total volume of 100 m3)to become inactive, 16 new gullies were formed (total volume of 10 034 m3). Mapping the drainage areasof gully heads allowed the consequences of road building on drainage area and shape to be demonstrated.Culverts are often far apart and places that received runoff from a relatively small drainage area before roadbuilding may receive important increases in runoff due to the increase in catchment area. This is the maincause of gullying.

Furthermore, a lowering of the topographic threshold values for gully head development due to road buildingseems to exist in the study area, but could not be proven statistically.

In order to avoid new gully heads developing after building mountain roads, appropriate works to spreadconcentrated runoff discharges in time and space should be undertaken during road building.

ACKNOWLEDGEMENTS

This study is part of a research programme funded by the Fund for Scientific Research – Flanders, Belgium.Financial support by the Flemish Interuniversity council (VLIR, Belgium) is acknowledged. Thanks go toBerhanu Gebremedhin Abay for translation and assistance with all the fieldwork, to Girmay Hailemariam andEls Lavrysen for their participation in surveying, to Teka Haftu and Atakilte Fitsum of the local REST (ReliefSociety of Tigray) branch. Numerous farmers, several unknown drivers who gave lifts to the research teamalong the road, and the authorities of the district facilitated the research. We also thank Annemie Goossens forthe moral and material support during the whole research period. Constructive comments on an earlier versionof this paper by two anonymous reviewers are gratefully acknowledged. This study is a contribution to theSoil Erosion Network of the Global Change and Terrestrial Ecosystems core research programme, which ispart of the International Geosphere-Biosphere Programme.

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