iii

ASSESSMENT OF HYDROLOGICAL CONTROLS ON GULLY FORMATION

NEAR LAKE TANA, NORTHERN HIGHLANDS OF ETHIOPIA

A Thesis

Presented to the Faculty of the Graduate School

of Cornell University

In Partial Fulfillment of the Requirements for the Degree of

Master of Integrated watershed management and Hydrology

By

Tigist Yazie Tebebu

May 2009

  iv

© 2009 Tigist Yazie Tebebu

  v

ABSTRACT

Over the past five decades, gullying has been widespread and has

become more severe in the Ethiopian highlands. Besides negatively affecting

soil resources, lowering crop yields in areas between the gullies and reducing

grazing land available for livestock, gully erosion is one of the major causes of

silting of reservoirs. Assessing the rate of gully development and the

controlling factors of gullying will help to explain the causes for current land

degradation and to design reliable conservation measures for already existed

gullies and preventing strategies for those areas susceptible to further gullying.

The study was conducted in the 523 ha of Debre-Mewi watershed south of

Bahir Dar, Amhara region, Ethiopia. A comparison of the gully area estimated

from 0.58 m resolution quick bird image with current gully area walked with a

garmin GPS, indicated that the total eroded area of gully was increased by

43% and 60% from 0.65 ha in 2005 to 1.0 ha on 2007 and 1.43 ha on 2008.

Semi structured group interview and monitoring of gully development through

time was made with profile measurements of contemporary gully volumes.

Gullying started in the beginning of the 1980`s followed the clearance of

indigenous vegetation, leading to an increase of surface and subsurface runoff

from the hillside to the valley bottoms. Gully heads retreat into the hillslope

during the rainy season. The water levels of gully contributing area showed

that actively eroding sections the water table was in general closer to the

ground surface on the gully shoulder than in stabilized sections. Piping and

tunneling together with a high water table facilitate the slumping of the gully

wall and their retreat. Estimated long-term average soil loss rate by gully

erosion in the mid slope gully was 21 t ha-1 yr-1 and 27 t ha-1 yr-1 in the valley

bottom saturated gullies. The area specific short-term gully erosion rates

  vi

between 2007 and 2008 were approximately 128 t ha-1 yr-1 for the midslope

gully and contributes to 1.7 cm soil loss for the 16.5 ha watershed and 402 t

ha-1 yr-1 for the valley bottom gully (equivalent to 3 cm soil loss of the 17.4 ha

watershed)

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BIOGRAPHICAL SKETCH

Tigist Yazie Tebebu was born in 1984 and grew up in Motta, Ethiopia.

She received her Bachelor of Science in Land Resource Management and

Environmental Protection (LaRMEP) in 2006 from Mekelle University located

in the Tigray Region, Ethiopia. In 2007, Tigist served in the Bureau of

Municipality in Bahir Dar, Ethiopia and worked to meet the millennium goal of

Ethiopia by preparing seedlings and planting indigenous trees for the

rehabilitation of Abay Park at the source of the Blue Nile. In the end of 2007,

she began a Master’s program offered by Cornell University at Bahir Dar

University, Ethiopia in the department of Integrated Watershed Management

and Hydrology. Tigist prepared for her Master’s research that was to be

conducted in the Debre-Mewi watershed, near Lake Tana, the source of Blue

Nile in the northern highlands of Ethiopia. She worked with the local

community to asses the hydrological mechanisms influencing gully formation

and to investigate the erosion rate of the region in order to develop reliable

integrated and sustainable resource conservation and protection mechanisms.

  viii

ACKNOWLEDGMENTS

My deepest respect and special thanks go to Prof. Tammo S. Steenhuis

offered this chance and provided an excellent advice and his patience when I

worked through this. I am very grateful to Cornell University and International

Water Management Institute (IWMI) for funding this project. My special thanks

and many appreciations go to Helen E. Dahlke for all of her support and

advice in the field and during write up of this paper. Anteneh Z. Abiy, an

excellent colleague, thanks to his patience when we worked together. With out

Helen and Anteneh I would not complete this research and paper as it done

now. Dr. Amy S. Collick was a wonderful coordinator not only in the field and

in the lab but also facilitating things. She was a reason that I accomplished this

program. Many thanks to Tesfamaryam Marey and Eric D. White for helping

during instrumentation. Anteneh Wubet, in the Debre Mewi Agriculture

development center, Ato Muche Nigatu and his wife and the youngsters of

Debre-Mewi were very helpful in the field. In Adet, Amhara Regional

Agricultural Research Institute (Adet ARARI), especially Tadele Amare, was

very collaborative and provided a transition to work with the local community. I

appreciate and thank Dr Farzad Dadgari and Mekonnen for the excellent

advice and comments they have provided, in addition to their field visits. In

Bahir Dar, organizations, such as Bureau of water resource development,

Amhara water works, Environmental protection Land Administration and Use

Authority (EPLAUA), Amhara region Sustainable water Harvesting and

Institutional Strengthening (SWHISA) were very helpful. Many thanks to every

body in the civil engineering Department lab at Bahir Dar University and Dr.

Ayalew Wondie who was a facilitator, Amhara Regional Soil Testing

  ix

Laboratory, Dr. Matthew McCartney, Dr. Selamyihun Kidanu and the

graduating team of Cornell University at Bahir Dar. .

Last but not least, Tilahun Yazie; my father and mother, Yazie Tebebu

and Abeba Mekonnen and the rest of my family were always with me, I am

truly very grateful for them every single day.

  x

TABLE OF CONTENTS

BIOGRAPHICAL SKETCH .............................................................................. vii

ACKNOWLEDGMENTS ................................................................................. viii

TABLE OF CONTENTS ................................................................................... x

LIST OF FIGURES .......................................................................................... xii

LIST OF TABLES ........................................................................................... xiv

LIST OF ABBREVIATIONS ............................................................................ xv

1 INTRODUCTION ........................................................................................... 1

2 MATERIALS AND METHODS ....................................................................... 5

Study Area Description ................................................................................. 5

Landscape Description .............................................................................. 5

Land use History ........................................................................................ 6

Crops and Livestock .................................................................................. 7

Measurement Techniques ............................................................................. 8

Instrumentation and Data Recording Techniques ...................................... 8

Monitoring Gully through Community Interviews ...................................... 12

Gully Evolution and Erosion Rate ............................................................ 14

3 RESULTS AND DISCUSSION .................................................................... 17

The Long term Evolution of Gully in Debre-Mewi ........................................ 17

Short term Evolution of Gully in Debre-Mewi .............................................. 19

Gully Erosion Rate and Soil Loss ................................................................ 21

Precipitation and Observed Discharge ........................................................ 26

Gully Formation Mechanisms ...................................................................... 26

Gully Formation due to cracks and pipes ................................................. 27

Gully Formation due to High Water Table ................................................ 28

Erosion of Stabilized Gullies ....................................................................... 34

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4 CONCLUSIONS AND RECOMMENDATIONS ........................................... 36

5 REFERENCES ............................................................................................ 38

6 APPENDIX .................................................................................................. 42

Appendix 1: The relative positions, and measured parameters of gully

segments compared for two monitoring periods (2007 and 2008). ............. 42

Appendix 2: The relation ship of daily discharge including base flow and

rainfall recordings for twenty consecutive days (Sep. 7, 2008-Sep 27, 2008).

.................................................................................................................... 44

Appendix 3: The studied saturated valley bottom gully water table was near

or at the surface resulted the slumping of the gully walls and the surrounding

soil (SG=Gully Section and P=Piezometer). ............................................... 45

Appendix 4: At the southern gully branch piezometer P19 down slope of the

dyke, show a water table below the ditch bottom (SG 17 and P19) and the

gully section was semi stabilized through out the monitoring period. While

Piezometers P13 upslope of the dyke show a water table above the ditch

bottom near or at the soil surface resulted the slumping of cropland. ......... 46

Appendix 5: Head cut migration and undercut deepening was contributed for

the development of northern gully branch at a point where the water table

was below the ditch bottom (GS 8-10 and P21) and the active gully section

Piezometer P16 water table was above the ditch bottom. ........................... 47

Appendix 6: The existence of grayish colored clay at different depths of the

gully wall and at the pipe outlets indicates the level of the local long-term

water table. ................................................................................................. 48

Appendix 7: The springs are the splitting points of the studied gully system

at different locations. ................................................................................... 49

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LIST OF FIGURES

Figure 1: The studied gully system in Debre-Mewi watershed. ........................ 6

Figure 2: The dimensions of a rectangular weir constructed at the outlet of the

gully ................................................................................................................ 11

Figure 3: The location of piezometers, the constructed weir, and metal rods

installed in the head cut areas. ....................................................................... 12

Figure 4: The location of springs, dyke and major active gullying segments in

the Debre- Mewi gully system. Gully areas marked with red triangles

expanded in width and depth during the 2008 rainy season........................... 14

Figure 5: The evolution of the studied gully system in Debre-Mewi (2005-

2008). ............................................................................................................. 20

Figure 6: Actively forming gully at the most downstream end, the saturated

valley bottom gully walls are actively eroding up to 5m deep and 26 m wide. 21

Figure 7: The estimated soil loss from gully volume for the Valley bottom (VB)

and midslope (southern (S) and northern (N) gully branches) gully in the two

monitoring dates. ............................................................................................ 23

Figure 8: The relative positions, widths (top and bottom) and depths of gully

segments compared for two monitoring periods (2007 and 2008). (a) Northern

gully branch( starting from intersection point to the north), (b) Southern gully

branch (starting from intersection point to the south) and (c) Valley bottom

gully(from the bottom end to splitting point) at a distance of 159 -187m away

the valley bottom gully was not present in 2007. ............................................ 25

Figure 9: A single storm event observed discharge for Debre Mewi gully

system in Sep. 13 2008. ................................................................................. 26

Figure 10: The cracks, pipe and pipe out lets indicates the sub surface erosion

and future incision of gullies. .......................................................................... 28

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Figure 11: Water table in the active gully areas remained near the surface

during the period of observation. .................................................................... 29

Figure 12: Gully dimensions before and after the 2008 rainy season for the

valley bottom. a) Depths and average ground water table; b) change in top

and bottom width and depth of the gully. ........................................................ 31

Figure 13: Gully dimensions before and after the 2008 rainy season for the

northern gully branch. a) Depths and average ground water table; b) change in

top and bottom width and depth of the gully. .................................................. 32

Figure 14: Gully dimensions before and after the 2008 rainy season for the

southern gully branch. a) Depths and average ground water table; b) change

in top and bottom width and depth of the gully. .............................................. 34

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LIST OF TABLES

Table 1: The depth of the piezometers below the ground surface to the

bedrock. .......................................................................................................... 10

Table 2: Event oriented calendar used in the questions ................................. 13

Table 3: Long and short-term erosion rates in different gully locations of the

Debre-Mewi watershed................................................................................... 20

Table 4: summary of the measured parameter average values and change in

% for south, north intermediate and valley bottom gully branches. ................ 24

Table 5: The location of representative piezometers in the studied gully system

....................................................................................................................... 29

  xv

LIST OF ABBREVIATIONS

AGERTIM Assessment of Gully Erosion Rates through Interviews and

Measurements

ANRS Amhara National Regional State

ARARI Amhara Regional Agricultural Research Institute

EPLAUA Environmental Protection Land Administration and Use Authority

IWMI International Water Management Institute

SWHISA Amhara region Sustainable Water Harvesting and Institutional

Strengthening

  1

CHAPTER ONE

1 INTRODUCTION

Soil erosion has been recognized as one of the major problems

restricting agriculture worldwide. Its extent and impact on human welfare and

global environment are increasing in an alarming rate (Pathak et al., 2005).

Gully erosion is the worst stage of soil erosion (Daba et al., 2003) and a global

phenomenon occurring in a wide variety of soils that can cause serious land

degradation (Valentin et al., 2005). According to the Soil Science Society of

America (1996), a gully is a channel or miniature valley cut by concentrated

runoff, and the runoff in gullies commonly flows only during and immediately

after heavy rains. The incision of new channels may be initiated by surface

scour, land sliding or pipe failure (Bocco, 1991). At present most research and

hydrological models dealing with soil erosion concern sheet and rill erosion

and do not include erosion due to concentrated flow channels and gullies

(Capra et al., 2005). While gully erosion represents an important fraction in the

sediment source in river systems and can account for as much as 70 to 90%

of the overall sediment production of the catchment (Mathys and Poesen

2006). Gullies are common features of mountainous or hilly regions with steep

slopes. Studies in Moldavia and Romania showed the aerial distribution of

gullies are mainly dominated by hillslope orientations (Radoane et al.,

1995).The steep slopes enhance gully processes, accelerate sediment

transfer from uplands to bottoms, and generate natural hazards, such as

mudflows, overflowing of heavily loaded floods, and silting up of reservoirs. It

results in the loss of fertile soil from cultivation land and damage to agricultural

land and agricultural infrastructure (Parkner et al., 2006).

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Climate (Valentin et al., 2005), (Zhang et al., 2007), lithology, soils

(Vandekerckhove et al., 2001), relief and land use/cover characteristics (Mitiku

et al., 2006 and Martinez, 2003) mainly affect gully erosion. In most cases,

gullies are formed due to intensification of human activities resulting from rapid

population growth in developing countries and expansion of agriculture and

grazing lands into forest areas. (Pathak et al., 2005) reported that in semi-arid

tropics (SAT) watersheds of Africa, most migrants cut trees, burn litter and

grasses and cultivate annual crops on hillsides or marginal lands without using

appropriate conservation measures, which has been increasing gully erosion

processes.

Gullies also occur in forestlands. In Meerdaal forest of central Belgium,

gullies were formed and continued to erode through the local disturbance of

forest cover situated on plateaus (Vanwalleghem et al., 2003).

(Vandekerckhove et al., 2000) indicates that erosion processes, which

sharpen the bank gullies, are related to soil material characteristics and the

occurrence of piping in soil layers with higher silt content, lower sand content

and a higher electrical conductivity. The existence of several distinct

groundwater flow systems are also related to gullying although this has not

been fully studied yet. In the Grgonum-Newton region (Márquez et al., 2005)

the ground water would flow from several systems to lower elevation areas,

the location of most of the gullies, mainly oriented radially to the center of the

basin and radially distributed from high elevation sites.

In Ethiopia, gullies can be found everywhere in all climatic, soil,

physiographic, lithologic and substratum settings (Billi and Dramis, 2002). The

Soil Conservation Research Project (SCRP) research stations confirmed that

soil erosion increases with wetness, while as other factors, such as soil type,

  3

stoniness, vegetation, etc. remain constant (Mitiku et al., 2006). Overgrazing

(Valentin et al., 2005), road construction and building activities with inadequate

drainage systems and insufficient road-ditch capacity (Nyssen et al., 2002)

constitute major factors of gully formation. Increased runoff due to reduced

infiltration produces sheet and rill erosion and the formation of new gullies or

the enlargement of older ones (Pathak et al., 2005). Proceedings of

MoWR/EARO/IWMI/IRLI International (Cornick et al., 2003) reported that

traditional irrigation practices used by the farmers result in gully formation due

to deep canals. In addition, the lack of maintenance of soil conservation

structures like stone terraces, lead to severe localized erosion and

downstream gully formation (Esser et al., 2002).

Over the past five decades, gullying has been widespread and has

become more severe in the Ethiopian Highlands (Nyssen et al., 2006). It

threatens soil resources leading to lower crop yields in inter-gully areas due to

enhanced drainage and desiccation, aggravated flooding and reservoir

siltation (Nyssen et al., 2006), which in turn promotes ecosystem instability

(Daba, et al, 2003), the most serious threat to water harvesting schemes and

hydroelectric power dams (Tamene and Vlek 2007) . However, a greater

attention has been given in recent times (Billi and Dramis, 2002) in a few study

areas and has been limited to the rate of damage of gully erosion.

Preventing gully formation is much easier and more economical than

treating the already formed gullies (Pathak et al., 2005). Therefore, it is crucial

to identify the cause and extent of gully development in order to practice

selective integrated control and rehabilitation measures. For this study, an

active gully system in the Debre-Mewi watershed, a representation of the

highland areas of Ethiopia was selected to assess the rates of gully

  4

development, changes and the associated soil loss and to investigate the

hydrological processes of gully incision and retreat.

  5

CHAPTER TWO

2 MATERIALS AND METHODS

Study Area Description

Landscape Description

The 523-hectare Debre-Mewi watershed is located approximately 30

km away Bahir Dar town, the capital town of the Amhara National Regional

State (ANRS) in Northern Ethiopia. Ranging from 1950 to 2309 meters above

sea level (masl), the watershed lies between 11o20’13” and 11o21’58” North

and 37o24’07” and 37o25’55” East near Adet, the capital city of Yilmana Densa

woreda, in the West Gojjam Zone. According to the meteorological station at

same elevation 10 km away the Adet Research Center, one of the satellite

centers of the Amhara Region Agricultural Research Institute (ARARI), Debre-

Mewi watershed receives an average annual rainfall of 1240 mm mostly during

the main rainy months from June to September. The average minimum and

maximum daily temperature vary between 9 0C and 25 0C. The upper part of

the watershed is gently sloped while the middle part flat valley bottoms

towards the outlet are characterized by steeper slopes. Within the Debre-Mewi

watershed, a 17.4 ha sub watershed with a branched gully system was

selected in order to conduct the field experiments. This gully system was

chosen based on the consideration of its slope, agricultural activity, soil type,

land cover and livestock density.

The sub watershed was divided into three sub-classes: the steeper

upper slope (from the top gentle slope to the foot of the hill), the steeper

middle slope (starts from the end of the steeper slope and ends at the

beginning of the valley bottom), and the valley bottom (the flatter, saturated

zone) (Figure 1).

  6

Figure 1: The studied gully system in Debre-Mewi watershed.

Land use History

Indigenous trees have been cleared and converted to cropland and

pasture in the upper steeper slope since 1991. Currently, the steeper

landscape is composed of scattered indigenous tree species and shrubs,

including Cordia sp., Limich sp., Agam spp., Croton spp, and rocky outcrops.

These standing tree species provide an indication of the composition of the

natural forest, which once covered the watershed. In the last decade,

eucalyptus tree plantations have been expanding at an alarming rate in all

parts of the watershed due to its short-term economic return to the farmers

Soils in the steeper upper slopes are generally less deep (on average,

less than 1 m) and characterized by a degraded A-horizon, or saprolite,

outcropping in heavily eroded areas. Well-stabilized stone bunds, common soil

  7

and water conservation measures, were implemented generally along the

contour at some distance from the base of the hillslope.

On the middle slope and in the valley bottom landscapes, advanced

erosion features were commonly observed. An active gully system, which

splits into two main branches (north and south), dissects the cultivated land.

Beside the active gully systems, dispersed homesteads or residential areas

are situated in the middle and valley bottom landscapes. A combination of U-

and V- shaped gully segments were observed at the head and at the down

stream sections of the gully system. Vertisol is dominant soil type in the study

area, and the soil depth increases down slope. White shoulder stones were

concentrated in the croplands. In general, soils in the middle slope and the

valley bottom show a 50 - 80 cm deep organic rich A-horizon on top of a red

clay layer of several meter thickness and underlain by saprolite (oxidized

basalt).

The valley bottom landscape is dominated by active gully systems that

dissect the existing pastureland. Over the past years, these active gully

systems, which started in the saturated area of the pastureland, have

extended into the croplands. The outlet of the gully and the watershed drains

into the Shanko Bahir and Andasa Rivers, both tributaries of the Blue Nile with

heavy sediment loads.

Crops and Livestock

The major land use types of the study area are cultivated land, pasture,

wood lots, and settlements and residential areas. The agricultural system in

the watershed is mixed farming system involving both rain-fed crop production

and livestock production. The common crops grown in the watershed are teff,

  8

maize, barley, wheat, potato and oil seeds. Oxen are used for plowing, and the

land is tilled three to seven times before planting, based on the crop. Live

stock density is very high and animals commonly graze in the communal

grazing lands particularly in the upper slope and the valley-bottom gully areas.

After the harvest of rain-fed crops, cereal stubbles and crop residues on

private cultivated lands become available for free grazing.

Measurement Techniques

Different hydrological measurements, satellite image interpretations,

gully profile measurements combined with semi-structured interviews and

group discussions were performed in order to understand the gully

hydrological processes and to analyze the gully development and erosion

rates. A detailed description of the instrumentation and measurement

techniques are given below.

Instrumentation and Data Recording Techniques

Twenty-four piezometers were installed in the gully’s contributing area

and inside of the gully. Piezometers were constructed from PVC pipes (~ 5 cm

diameter) in which several 1 cm diameter holes were drilled in the bottom 30

cm of the pipe. To prevent intrusion of silt and sand, the section of pipe with

the holes were wrapped with permeable fabric sheeting. The bottom, or buried

end, of the piezometer was enclosed with a stationary plastic cap, while the

top exposed end of the piezometer was covered by a removable plastic cap to

prevent the entrapment of surface runoff, rainfall, sediment and damage by

livestock.

  9

By using a hand auger, each piezometer was buried to a maximum

depth of 4m. For some piezometers, the installation depth was limited by the

saprolytic layer (oxidized basalt). Maximum piezometer depths were reached

in the middle slope areas (Table 1) where piezometers sit on top of the

saprolytic layer (oxidized basalt). Piezometer depths in the upper slope area

range from 55 to 186 cm with an average depth of 115 cm. Piezometers

installed in the midslope area around the actively eroding gully have a depth of

186 to 401 cm with an average depth of 276 cm, while piezometers installed

around the stabilized gully system showed depths ranging from 196 to 420 cm

and 317 cm on average. Even if a soil depth of five and more meters was

assumed in the valley bottom, the maximum piezometer depth was 312 cm

because of the extent of the saturation area and the difficulty of drilling. The

maximum pipe height above the earth surface was 80 cm and considered to

prevent the entrapment of surface runoff, rainfall, sediment and possible

damage due to livestock. Water levels in each piezometer were recorded with

a measuring tape twice on normal days and three times a day when there was

a storm. An individual recording of all water levels in all the piezometers was

completed in one hour to minimize any discrepancy in water table depths due

to time variations.

A two-level, rectangular weir was constructed with cinderblocks and

cement-sand fillings in a narrow section of the gully. The weir has a basement

of 40 cm center blocks that were secured with metal rods to strengthen the

base of the weir and to minimize the damage during heavy storms and peak

flows. The first level base width and depth dimensions were 63 cm and 43 cm

respectively. The second level had a width of 115 cm and 40 cm depth

(Figure 2).

  10

Table 1: The depth of the piezometers below the ground surface to the bedrock.

Piezometer No Piezometer symbol Piezometer depth below the ground surface (cm)

1 P8 239 2 P4 54.5 3 P2 146.5 4 P1 148.5 5 P13 186 6 P14 315 7 P18 338 8 P19 196 9 P20 420

10 P17 247 11 P21 384 12 P22 262 13 P23* 335 14 P[16 155 15 P15 198 16 P12 196 17 P11 401 18 P10 358 19 P7 125 20 P9 186 21 P6 93.5 22 P24* 168 23 P25* 312 24 P26 253

∗ A Piezometer which did not reach the bed rock due to deep soil layer

 

Runoff depths of September 2008 were measured manually at 10

minutes interval during a storm and on average every 6 hours to measure non-

storm flows. The velocity of runoff (m s-1) was determined by float method in

which a float was released on the water surface a distant of 4.2 m above the

weir outlet and the time required for the float to reach the weir was recorded.

The depth records and runoff velocity were subsequently converted to the

  11

corresponding discharge and runoff volume using a rating equation developed

for the gully and weir.

VAQ = Equation 1

Where Q is the discharge (m3 s-1), v is the velocity of runoff (m s-1) determined

by float method, and A is the cross sectional area of the weir or the flow (m2)

calculated by multiplying the base width of the weir, W, (m) by the runoff

depth, D, at the outlet (m).

Figure 2: The dimensions of a rectangular weir constructed at the outlet of the gully

Several 70 cm long metal rods marked at 10 cm intervals were driven

into the bed of the gully at the beginning of the study period rainy season at

the head cut gully segments and selected gully bases with 10 cm of the rods

exposed above the gully surface (Figure 3). The length of the exposed ends of

the metal rods were measured using the 10 cm intervals at the beginning and

end of the monitoring period.

  12

Precipitation in 2008 rainy season was measured daily using a

calibrated rain gauge at a standard runoff plot (Adet Research Center) about 1

km north of the sub ‘catchment in the monitoring dates. It was crosschecked

with five simple rain gauges that were installed 60 m north of the weir.

Infiltration tests were done with double ring infiltrometer at the north and south

gully branch upslope recharge areas separately during August and September

2008. The diameter of the inner ring and the outer ring measured 32 cm 53

cm.

Figure 3: The location of piezometers, the constructed weir, and metal rods installed in the head cut areas.

Monitoring Gully through Community Interviews

The evolution of the gully was studied in detail using the assessment of

gully erosion rates through interviews and measurements (AGERTIM) method

(Nyssen et al., 2006). The AGERTIM method has been used for the

assessment of gully erosion rates over the last decades in the absence of

historically written or photographic information (Nyssen et al., 2006). The

0 8 0 160 240 32 040Me ters

  13

selected gully was visited with five key informants in four age groups (farmers

of the age of 20, 30, 40, and 50 years) and the age of the various gully

segments was estimated through different questions. The key informants

located different segments of the gully head and the major changes that

occurred during the past three decades. Based on these different time

horizons an event-oriented calendar (Table 2) was prepared.

Table 2: Event oriented calendar used in the questions

The extent and location of the gully in its early stage was first

reconstructed with the oldest informants. Key events in the informants’ lives,

including the necessity to cross the road to fetch water from the rural-

developed well, appearance of a log over the gully and used as a bridge,

flooding of the road during the initial settlement of the nearby town, served as

time references for the progression of the gully.

The commencing points of the gully located at different ephemeral

springs were investigated in more detail. Information from the oldest key

informant (age around 50) were discussed and crosschecked with information

provided by younger informants. According to these interviews, major incisions

started in 1981 at the location of three ephemeral springs near the current

  14

junction of the north and south branch. The main branch started around 2004

and almost has joined the other gully system.

Gully Evolution and Erosion Rate

The current surface area of the gully system was analyzed from a 0.58

m resolution Quick Bird image from 2005 and compared with the area of the

current gully system. The extent of the gully was delineated with a Garmin

GPS (2 m overall accuracy) and compared with the gully area of the 2005

satellite image (Figure 4).

Figure 4: The location of springs, dyke and major active gullying segments in the Debre- Mewi gully system. Gully areas marked with red triangles expanded in width and depth during the 2008 rainy season.

The gully volume was determined from the cross sectional area of the

gully and the length of the considered gully segments. A hand held Garmin

Etrex global positions system (GPS) receiver (Garmin International, Inc.,

Olathe, Kansas) was also used to measure and determine the spatial

  15

coordinates for each gully segment. The gully segments were numbered

based on their location in the study area. The length of each segment between

two cross profiles, where the top width, bottom width and depth of the gully

segment were measured, using a 50m long surveyor’s tape. For each

segment, two or three top widths and three depths were measured at locations

where the gully cross-sections changed abruptly. Based on these

measurements an average gully volume of each segment could be estimated.

In addition, the shape of the gully was determined to understand the current

rate of gullying and to obtain whether parts of the gully stabilized naturally. The

cross-sectional shapes of the gully in the study site are almost trapezoidal or

V-shaped. They were usually larger in width than depth (large width to depth

ratio). The measurements were taken at the beginning of the rain (July,

considered as measurement of 2007) and end of the rainy season (October,

considered as measurement of 2008). Thus, the short-term erosion could be

estimated considering only the gully segment measurements taken during the

past rainy season. The eroded volume of each gully segment was calculated

using the cross sectional dimensions and the distance between cross

sections.

∑= ii ALV Equation 2

Where Li is the length of considered gully segment (m) and Ai is the

representative cross sectional area of the gully segment (m2).

Long-term gully erosion rates (t ha-1 yr-1) (RL) were calculated using the

estimated current volume (V) of the gully, the average bulk density (Bd) of

soils occurring in the contributing area, the time span of gully development in

years (T) and the watershed area in hectares (A).

  16

Equation 3

The soil bulk density was estimated in different locations and depths

throughout the contributing area of the gully using a cylindrical core sampler

with a volume of 98 cm3. Six core samples were collected at the twenty-four

piezometer locations and 12 from within the gully at active erosion sites.

Samples were weighed then dried at 105oC for 24 hours and weighed again.

An average of all the samples was taken as the bulk density value for the gully

watershed.

Short-term erosion rates (t ha-1 yr-1) (Rs) were determined to estimate

the rate over the duration of this study.

Equation 4

Where V is the current volume of the gully and Vo is the initial volume at the

start of the study period.

Erosion per unit gully surface (t m-2), (RP), were estimated as:

Equation 5

Where V is the current volume of the gully and AP: Plan area of the gully (m2).

PP A

VBdR =

TABdVV

RS)( 0−

=

TAVBdRL =

  17

CHAPTER THREE

3 RESULTS AND DISCUSSION

The Long term Evolution of Gully in Debre-Mewi

According to the respondents, in the earlier 1980’s, the valley bottom of

the study area was marshy, and grasses were grown all year long. The

hillslope area was covered with different tree species like Podocarpus

africana, Acacia spp. and other indigenous species. Two ephemeral and a

perennial spring were observed in the midslope gully segments. Currently

most tree species were cleared and eucalyptus plantations have expanded in

alarming rate since 1991. All respondents agreed on the incision location of

the current gully. Although one of the older and one of the younger

respondents disagreed in their statements, all other respondents confirm that

the locations of the two gully incisions were related to three springs in the

hillslope.

Since the beginning of the 1980s, one of the springs, which is located

approximately 272 m uphill from the current valley bottom gully (SP1 in Figure

4), showed running water throughout the year. One of the oldest respondents

mentioned that the water from this spring accumulated in a small pond and

served to hose down the maize seed and to prepare local beer for different

local ceremonies. According to the remaining respondents, it was also used as

a watering hole for cattle. Presently the pond is filled with “adebuha”,

sediments deposited from the surrounding agricultural lands and the slumping

of gully walls. According to the farmer, the time when the pond began to dry up

coincided with the incision of the gully in the valley bottom. It is hypothesized

that the drying up of the pond is likely caused through the lowered water table

in the gully.

  18

In 1981 during the Derg regime of the new settlement period, passage

between the nearby town and the villages “Shanko Bahir” and “Yemariam

Wuha“to Debre-Mewi town was made via a footpath above the second spring.

The second spring is approximately 373m away from the valley bottom gully

and approximately 78m away from the first spring (section SP2, Figure 4).

According to the local informants, after the fall of the Derg regime in 1990 and

1991, the marshy area around the second spring changed into a branched

gully with a northern and a southern gully branch as soon as settlers returned

to their previous homeland ,“Shanko Bahir” and “Yemariam Wuha“ from the

Debre-Mewi town . Furthermore, the edges of the gully downstream of the

second spring were used as a border for the farmland after the land

distribution in 1996.

The third spring (SP3), located near the head cut areas of the southern

gully branch is still actively eroding into the hillside as indicated by newly

developed, ephemeral side branches (Figure 4) .These gully arms may have

developed due to seepage flow or interflow. Decades ago the spring water

was seeping from the start of the rainy season (June) until the following April.

Recently, this period of seepage has decreased from June to February.

Although the increasing livestock density and depletion of the vegetation cover

is easily to blame for the decrease in spring flow as proposed by (Øygarden,

2003), as we will see later it is much more likely that a decrease in

groundwater table due to drainage by a gullies is the cause of the decreased

water availably for the springs. Thus, the change management practice is only

indirectly the cause: less vegetation increased subsurface initiating the gulley

resulting lowered groundwater levels and ultimately decline in spring flow.

  19

Short term Evolution of Gully in Debre-Mewi

The satellite image delineation of the gully system indicated the

planimetric area of the gully was 0.65 ha in 2005 while the GPS delineation of

the gully area showed the area had extended to 1.0 ha in 2007 and 1.43 ha in

2008 (Table 3 and Figure 5). The gully had increased by 43 and 60% within

three years and during the monitoring period of 2008, respectively. The total

length of the gully in the study site was 1319 m with a total volume of 24900

m3 in 2008 (Table 3). The northern gully branch is 427 m long has a volume of

1640 m3 in 2007 and increased by 41 % in 2008 to 2310m3. The southern

gully branch is 519 m long has a volume of 6670m3 in 2007 and increased by

41 % in 2008 to 8610m3. The intermediate gully, the most stabilized section of

the midslope gully has 186 m long and a volume of 3000 m3 and 3560 m3 in

2007 and 2008 respectively .It was increased by 19 %. The valley-bottom gully

segment was 159 m long and has a volume of 4816 m3 in 2007 in 2007 and

increased to 187 m long and 10,450 m3 in 2008, increased by 117%. The total

volume of the studied gully system increased approximately 46% during the

monitoring period. During the rainy season of 2008, a deep gully formed from

a former footpath that crossed the valley bottom branch of the gully. In this

gully branch, it was observed that the gully eroded at the beginning of the

rainy season due to sliding of the heavier and saturated topsoil on top of a

heavy silty clay layer, which functions as a semi-permeable layer and hinders

deeper percolation of infiltrated rainwater.

Most gully segments in the valley bottom and in the upper end of the

two gully branches in the midslope area were said to have been actively

eroding during the past 4 years as confirmed by one of the younger

informants.

  20

Table 3: Long and short-term erosion rates in different gully locations of the Debre-Mewi watershed

Figure 5: The evolution of the studied gully system in Debre-Mewi (2005-2008).

NOTE: RL, Area specific long-term gully erosion rate since incision RS, Area specific short-term gully erosion rate (2007- 2008)

 

  21

Saturated areas in the valley bottom, which were fenced by one of the

informants to grow grass for his cattle, as well as the communal grazing lands,

have shown accelerated gullying in recent years as shown in Figure 6. The

measurements with the piezometers, which are discussed in the later section

show that at these locations the water table is above the bottom of the gully.

Other changes that occurred in the watershed and could be possible causes

are a decrease of soil fertility and the expansion of croplands and greater

density of cattle

Figure 6: Actively forming gully at the most downstream end, the saturated valley bottom gully walls are actively eroding up to 5m deep and 26 m wide.

Gully Erosion Rate and Soil Loss

According to information obtained with the semi-structured interview the

approximate incision period of the gully segments ranged from one year for

the new gullies to 27 years for the old gullies. Thus, it was assumed that the

gully started to erode approximately 27 years ago. Local informants confirm

the alarming incision rate and the accelerated development of the gully since

2004.

  22

The estimated long-term average soil loss rate (RL) due to gully erosion

of 17 t ha-1 yr-1 was estimated for the midslope gully segments since the

incision period of 1981 to 2007 and increased to 21 t ha-1 yr-1 in 2008 (Table

3). In comparison, the valley bottom gully’s long-term erosion rates were 13 t

ha-1 yr-1 from 1981 to 2007 and 27 t ha-1 yr-1 from 1981 to 2008. These values

are very high for the region compared to the result of other studies (Nyssen et

al., 2006). The percent increase in the erosion rate of the valley bottom gully

(109 %) compared to the midslope gully (24 %) within one rainy season is

likely caused by slumping of the gully walls and the surrounding soil at the

valley bottom. However, the erosion rates indicate that gully incision was

slower at the beginning of the incision period (1980s) compared to the current

rainy season. The volume of gully at the beginning of the monitoring period

(2007) was less than by 8800 m3 to the end the monitoring period (2008). The

area specific short-term gully erosion rates (RS) between 2007 and 2008 were

approximately 128 t ha-1 yr-1 for the midslope gully and contributes to 1.7 cm

soil loss for its 16.5 ha watershed and 402 t ha-1 yr-1 for the valley bottom gully

contributes to 3 cm soil loss of its 17.4 ha watershed. The recent increase in

measured gulley erosion is in accordance with local information.

Figure 7 shows the depth of soil (cm) eroded during the monitoring

time. The higher erosion rate estimated for the valley-bottom gully relative to

the upper part of the gully (midslope gully) is most likely caused by higher rate

of landslides and land slumps, which increased the sediment production, and

the presence of stabilized gully segments in the midslope gully section. The

higher erosion rate estimated for the southern gully branch than the northern

gully branch in the midslope gully is related with the presence of the springs.

The average soil loss per unit surface area of the gully was estimated to be

  23

1.24 t m-2, based on measured gully volumes and the average soil bulk

density. The soil loss from the valley-bottom gully was estimated to be 2.9

t m-2 and 1.7 t m-2 for the midslope gully segments with an average of 2.2 t m-2

for the past 27 years. This indicates the severity of soil loss due to gully

erosion in the studied sub-catchment. The estimated erosion rates compare

well with gully erosion rates observed in other regions of the country, which

are in the range of 1 to 2 t m-2 (Daba et al., 2003).

Figure 7: The estimated soil loss from gully volume for the Valley bottom (VB) and midslope (southern (S) and northern (N) gully branches) gully in the two monitoring dates.

The average gully width and depth of the saturated valley bottom gully

segments was 22.1 m and 4.6 m, respectively, and 6.8 m and 2 m for the

midslope gully segments(including the north, south and intermediate gully

branches),. These measurements indicate that gullying occurred more

predominately in the valley bottom than in the hillslope areas.

  24

Table 4 and Figure 8 show the average dimensions of the various

cross-sections of the various parts of the gully. The intermediate gully changed

least while the valley bottom increased in size the most. The top width of the

valley bottom doubled in just one year. The gully at the valley bottom was

actively progressing upstream and the average depth increase greatly as

shown in Table 4 and Figure 8. The other sections of the gully only slightly

increased in depth. Average top width for the north and south branch was in

the order of 25%.

 Table 4: summary of the measured parameter average values and change in % for south, north intermediate and valley bottom gully branches.

  25

Figure 8: The relative positions, widths (top and bottom) and depths of gully segments compared for two monitoring periods (2007 and 2008). (a) Northern gully branch( starting from intersection point to the north), (b) Southern gully branch (starting from intersection point to the south) and (c) Valley bottom gully(from the bottom end to splitting point) at a distance of 159 -187m away the valley bottom gully was not present in 2007.

  26

Precipitation and Observed Discharge

The total precipitation from September 7, 2008 to September 26, 2008

was 97.5 mm. On September 13, it rained 23 mm with an average intensity of

17 mm hr-1 that caused the water to rise 70 cm at the weir flowing with a

velocity of 2.1 m sec-1. This rainfall event was similar to a late July storm that

caused gully wall slump. The hydropgraph of the September 13 storm is

depicted in Figure 9. The total runoff volume is 2,750 m3, which indicate within

the accuracy of the measurement that, nearly all the rain ran off. The

maximum suspended sediment concentration was 25.3 g l-1 occurring after the

peak flow.

Figure 9: A single storm event observed discharge for Debre Mewi gully system in Sep. 13 2008.

Gully Formation Mechanisms

In the next section, we will discuss how gullies are being formed and

widened. The mechanism for gully formation is different for the beginning of

the rainy season when the vertisols have large cracks that later in the rainy

  27

season when the cracks are closed and the ground water table has increased.

We will first shortly discuss the initial gully erosion in the beginning of the rainy

season and the occurrence of pipes and then discuss extensively the effect of

water table on gully formation.

Gully Formation due to cracks and pipes

After the dry season, the vertisol around the gully has wide cracks is a 5

to 8 m rectangular pattern (Figure 10). In addition, there are a number of soil

pipes from 10 to 30 m long with diameters of 5 to 50 cm in mostly lower lying

areas usually near the gully (Figure 10). Especially in the saturated areas of

the valley bottom, several holes, vertical and horizontal pipes, were observed

at the interface of the organic A-horizon and the underlying red clay horizon

(heavy clay soil). After the first rains of the rainy season, surface runoff from a

high intensity rainfall fills the cracks and pipes filled with water, wetting up the

soil through these cracks. Since the soil was dry initially and was loose and

non-cohesive, the pipes and cracks could easily erode giving the water a high

sediment concentration. Deposition of sand at the end of the pipes clearly is

indicative of the erosion process. In addition subsurface soil pipes and tunnels

(Zhu, 1997), which are seen as initiators of gully erosion (Jones, 1987; Nyssen

et al., 2006).

Slumping of the gully walls is facilitated when the top organic A-horizon

becomes saturated and heavy and looses its stability due to the higher

potential of the groundwater exerting pressure onto the overlaying saturated

soil profile. The soil consequently looses its stability causing the slumping of

the gully walls and the surrounding soil (Zhu, 2003). The existence of grayish

colored clay at different depths of the gully wall and at the pipe outlets

  28

indicates the level of the local long-term water table (Appendix 6). The relative

importance of this type of erosion becomes less as the rainy season

progresses because of closure of cracks.

Figure 10: The cracks, pipe and pipe out lets indicates the sub surface erosion and future incision of gullies.

Gully Formation due to High Water Table

During the progression of the rainy season, the water table builds up

and becomes the major cause for the gully widening and progressing uphill. In

the next section, we discuss the relation between water table height, gulley

depth for the actively widening gulley areas. For these active gully formation

areas the depth of the water table in piezometers is given in Figure 11.

Piezometers P23 and P24 are located in the valley bottom, P13 and P1 in the

  29

south gully and P16 in the north gully. All the piezometers associated with the

active gully area are listed in Table 5 and their location is given in Figure 3.

Table 5: The location of representative piezometers in the studied gully system

Location Representative piezometer/s Southern gully branch P18, P19, P13, P1 Northern gully branch P21, P16, P15 Valley bottom gully branch P23, P24, P26 ,P17

Figure 11: Water table in the active gully areas remained near the surface during the period of observation.

0

10

20

30

8/5/08 8/15/08 8/25/08 9/4/08 9/14/08 9/24/08 10/4/08 10/14/08 10/24/08

Prec

ipita

tion

(mm

)

-50

0

50

100

1508/5/08 8/15/08 8/25/08 9/4/08 9/14/08 9/24/08 10/4/08 10/14/08 10/24/08

Wat

er le

vel b

elow

gro

und

(cm

)

P1 P13 P23 P16 P24

  30

Valley bottom Gully

The depths of the gully (Figure 12a) and the corresponding widths

(Figure 12b) bottom is depicted before (2007) and after (2008) the rainy

season for the valley bottom area as a function of the distance of point where

this gully joins the main branch. The average water table depth for the most

nearby piezometers( from bottom to top P24, P23, P22 and P26 and P17) are

shown as well and indicate that the valley bottom is saturated close to the

surface while further upstream the water table is below the gully bottom.

During the 2008 rainy season the gully advanced backward past the

187 m mark (Figure 12a) and increased up to 20 m in top width (Figure 12b).

In this region, the water table was near the surface and approximately 4 m

above the gully bottom (Figure 12a). Upstream of the 187 mark, the water

table is below the gully bottom (Figure 12a) and the gully is stable as can be

seen from Figure 12b since the width is not increasing.

The difference between the water table and gully bottom in the first 200

m of the gully varies from were 3.5 m to 4 m (Figure 12a). This means that

under static conduction the pore water pressure near the gully advance point

is 5 m causing the valley wall to fail repeatedly. In gully adjacent to piezometer

at 272 m the water table and the depth were both approximately 120 cm and

the gully was stable. More upstream at P17 the water table was below the

gully bottom. The gully walls did not fail at this location as well.

  31

Figure 12: Gully dimensions before and after the 2008 rainy season for the valley bottom. a) Depths and average ground water table; b) change in top and bottom width and depth of the gully.

North Gully

The active gully erosion process in the North Gully is driven by the

ground water dynamics similarly to the valley bottom as shown in Figure 13.

0

200

400

600

50 159 187 244 272 322 373

Water de

pth to gu

lly bottom 

and gully dep

th  (cm

)

Gully  length (m)

Gully depth Jul. 2008 Gully depth Oct. 2008

Average water table

Valley bottom

??Dry

??DryP24

P24 P23

P22 P26

P17

P17

a)

b)

  32

The depth of the gully in 2008 is depicted in Figure 13a, and the change in

bottom and top width and depth during the 2008 rainy season in Figure 13b. In

addition, the height of maximum and average water table above the gully

bottom is shown in Figure 13 and is obtained by subtracting the water table

depth from the gully depth. Positive numbers mean that the water table is

above and negative numbers below the valley bottom.

Figure 13: Gully dimensions before and after the 2008 rainy season for the northern gully branch. a) Depths and average ground water table; b) change in top and bottom width and depth of the gully.

a)

b)

  33

Although the relationship is not as dramatic as in Figure 12, the general

trend is the same. Where the water table is approximately 2 m below the

valley bottom (at 130 m) the gully does not change. When at 201 and 231m

above the junction with the south gully, the water table is the farthest

(approximately 75 cm) above the gully bottom and the water seeps into the

gully the gully dimensions increase most. For water tables near the gully

bottom, some small changes in gully dimension were observed.

South Gully

The gully widening in the south gully is influenced by a sapprolite and

rock close to surface. The rock outcrop (called dyke) and sapprolite together

with the gully depth is shown in Figure 14a. Figure 14b shows that the most

active gully formation occurred at 263 m from the junction with the south

branch just uphill from the dyke. Ground water table flow is blocked by the

dyke and is therefore above the gully bottom. Only downhill from the dyke at

115 m from the junction is water table is below the ditch bottom.

Unlike the north and valley bottom gully where the water table above

the valley bottom caused collapse of the walls, there is no widening in the gully

in the section 300 to 400 m from the junction with south gully despite that the

water table is between 1 and 2 m above the gully bottom. The only severe

widening of the gully occurred just upslope of the dyke (187 m from the

junction) where the water table was approximately 3 m above the gully bottom

(Figure 14). Here the water cannot flow further downhill due to the dyke and

seeps into the gully and causing the bank to slip. Further upslope the soil is

underlain by the sapprolite and water is seeping through the sapprolite to the

  34

gully making the bank stable.

Figure 14: Gully dimensions before and after the 2008 rainy season for the southern gully branch. a) Depths and average ground water table; b) change in top and bottom width and depth of the gully.

Erosion of Stabilized Gullies

In stable areas of the gully where the water table is generally below the

channel bottom, some widening of the top of the gully is occurring. The typical

U shape of the active eroding gully with the 90 degrees wells becomes V

b)

a)

  35

shaped due to grazing animals in and around the gully and the inflow of direct

hillslope runoff into the gullies. The direct runoff originates in many cases from

the cultural ditches used by farmers in the cropland for erosion control and by

concentrated flow of breached stone terraces similarly to that reported by

(Esser et al., 2002). In some other cases rills developed in the middle slope

gully section formed with an intervals of 0.2 to 1 m and caused gully bank

erosion. Thus, although concentrated runoff from the surrounding

farmlands both widen the gully and forms new gully branches as proposed by

(Øygarden, 2003), high ground water is the main cause of rapidly expanding

gully systems in the flatter areas of the landscape.

  36

CHAPTER FOUR

4 CONCLUSIONS AND RECOMMENDATIONS

The present and past gullying process in the Debre Mewi watershed

was related to environmental changes that induce surface and subsurface

runoff from the hillside to the valley bottoms. Clearance of indigenous

vegetation in the 1990’s decreased annual evaporation causing increased

subsurface flow of water, thereby upsetting the delicate balance of

subsurface and surface flows in the watershed.

Gully erosion processes are driven by the local hydrology and

differences in soil properties. The actively eroding sections especially in the

valley bottom water table were in general closer to the ground surface on the

gully shoulder than in stabilized sections. Therefore, this research

suggests that the water table depths above the gully bottom are

responsible for the establishment of newly formed gullies in the vertisols

located flatter areas of the landscape. Once the gully is formed the water

table is being drawdown by the gully itself and the gully becomes more

stable and changes from a U to a V shape due to erosion of direct runoff

entering the gully. Additionally, erosion is caused by grazing animals or

food. Besides gully slumping due to high water tables, gully walls slide in the

beginning of the rainy season by water entering the wide cracks that are

formed during the dry season in the vertisols. Finally, many soil

pipes in the watershed play a role in the formation of gullies.

These findings suggest that erosion due to gully formation can be

decreased through conservation and protection measures that alter the

runoff response and the water balance so that the water table is lowered

below the gully bottom. This can be accomplished by reestablishing

  37

deep-rooted permanent vegetation on the hillsides and, grasses in the

saturated valley bottoms. Artificial drainage will lower the water table

but is only of limited applicability under Ethiopian conditions. Finally,

improved land use practices will decrease erosion after the gully is

formed but likely not prevent the initial gully formation after the

landscape is cleared.

  38

CHAPTER FIVE

5 REFERENCES

Billi, P., Dramis, F., 2002. Geomorphological investigation on gully erosion in

the Rift Valley and the northern highlands of Ethiopia.

Bocco, G., 1991. Gully erosiuon :processes and models. Progress in Physical

Geography 15,392-406.

Capra, A., Mazzara, L.M., Scicolone, B., 2005. Application of EGEM Model to

predict ephemeral gully erosion in Siciliy, Italy . Catena 59, 133-146.

Daba, S., Rieger, W., Strauss, P., 2003. Assessment of gully erosion in

eastern Ethiopia using photogrammetric techniques. Catena 50, 273-

291.

Esser, K., Vagen, T.,Yinebeb, T., Mitiku H., 2002. Soil conservation in Tigray,

Ethiopia Noragric Report No. 5, 21 pp.

Jones, J. A. A., 1987. The initiation of natural drainage networks. Progress in

Physical Geography 11, 207-245.

Márquez, A., De Pablob MA., Ovarzun, R., 2005. Evidence of gully formation

by regional ground water flow in the Gorgonum-Newton region (Mars),

ICARUS, 179 (2), 398-414.

Martinez- Casasnovas, J.A., 2003. A spatial information technology approach

for the mapping and quantification of gully erosion. Catena 50, 293-

308.

Mathys, N., Poesen, J., 2006. Gully erosion in mountain areas: processes,

measurement modeling and regionalization. Earth Surface Processes

and Landforms 31, 133-134.

Mc CornicK P.G., Kmara A.B. and Girma Tadesse, (Eds), 2003. Integrated

water and land management research and capacity building priorities

  39

for Ethiopia. Proceedings of MoWR/EARO/IWMI/ILRI international

workshop held at ILRI , Addis Abeba, Ethiopia, 2-4 December 2002.

IWMI ( International Water Management Institute), Colombo. Sri Lnaka,

and ILRI (International Livestock Research Institute), Nairobi, Kenya

267 pp.

Mitiku , H., Herweg, K., Stillhardt, B., 2006. Sustaiable land management –A

new approach to soil and water conservation in Ethiopia. Mekelle,

Ethiopia: Land Resource Management and Environmental Protection

Department, Mekelle university ; Bern, Switzerland: Center for

Development and Environment (CDE), University of Bern and Swiss

National Center of Competence in Research (NCCR) North-South,

269pp.

Nyssen, J., Poesen, J., Moeyersons, J., Luyten, C., Veyerety Picot, M.,

Deckers, J., Mitiku, H., Govers, G., 2002. Impact of road building on

gully erosion risk: a case study from the northern Ethiopia highlands.

Earth Surface Processesses and Landforms 27, 1267-1283.

Nyssen, J., Poesen, J., Veyret-Picot, M., Moeyersons, J., Haile , M., Deckers

J., Dewit, J., Naudts, J., Teka, K., Govers, G., 2006. Assessment of

gully erosion rates through interviews and measurements: a case study

from northern Ethiopia. Earth Surface Processes and Landforms 31,

167-185.

Øygarden, L.., 2003. Rill and gully development during an extreme winter

runoff event in Norway. Catena 50, 217-242.

Parkner, T., Page, M., Marutani, T., Trustrum, N., 2006. Development and

controlling factors of gullies and gully complexes, East Coast New

Zealand. Earth Surface Processes and Landforms 31(2), 187-199.

  40

Pathak , P., Wani, Sp., Sudi, R.., 2005. Gully control in SAT watersheds,

Global Teme on Agro ecosystem Report no. 15, Patancheru. 502324.

Andhra Pradesh, India: International crops research institute for the

Semi-arid Tropics 28 pp.

Radoane, M., Ichim, I., Radoane, N., 1995. Gully distribution and development

in Moldavia, Romania. Catena 24, 127-146.

Soil Science Society of America ,1996. Glossary of soils science terms. Soil

Science society of America, Madison, WI, USA, and 134 pp.

Tamene, L., Vlek, P.L.G., 2007. Assessing the potential of changing landuse

for reducing soil erosion and sediment yield of catchments: a case

study in the highlands of northern Ethiopia. Soil Use and Management

23, 82-91.

Valentin, C., Posen, J., Li, Y., 2005. Gully erosion: Impacts, factors and

control. Catena 63, 132-153.

Vandekerckhove, L., Muys, B., Poesen, J., De Weerdt, B., Coppe, N., 2001. A

method for dendrochronological assessment of medium term gully

erosion rates. Catena 45, 123-161.

Vandekerckhove, L., Poeson, J., Oostwould Wijdenes, D., Gyssels, G.,

Beuselinck, L., De Luna, E., 2000. Characteristics and controlling

factors of bank gullies in two semi arid Mediterranean environments.

Geomorphology 33, 37-58.

Vanwalleghem, T., Van Den Eeekhat, M., Poesen, J., Deckers, J.,

Nacchtergaele, J., Van, K., Slenters, C., 2003. Charactersrica and

controlling factors of old gullies under forest in a temprate humid

climate: a case study from the Meerdaal Forest ( Centeral belgium).

Geomorphology 56, 15-19.

  41

Zhang, Y., Wu, Y., Ciu, B., zheng, Q., Yin, J., 2007. Characteristics and

controlling factors of the development of ephemeral gullies in cultivated

catchments of black soil region, Northeast China. Soil and Tillage

Research 96, 28-41.

Zhu TX., 1997. Deep-seated, complex tunnel systems a hydrological study in

a semi-arid catchment, Loess Plateau, China. Geomorphology 20 , 255-

267.

Zhu TX., 2003. Tunnel development over a 12 year period in a Semi-Arid

catchment of the loess plateau, China. Earth Surface Processes and

Landforms, 28,507-525.

 

 

42

CHAPTER SIX

6 APPENDIX Appendix 1: The relative positions, and measured parameters of gully segments compared for two monitoring periods (2007 and 2008).

Top Width (m) Bottom Width Depth (m) Length (m) Volume (m3) Plan area (m2) # incision period 2007 2008 2007 2008 2007 2008 2008 Cumulative Shape 2007 2008 2007 2008 Valley bottom gully branch

1 2004 12 19.9 10.7 14.6 3.7 4.05 50 50 v-Shape 1110 2015 600 995 2 2004 10 26.4 7 15.1 4 4.9 109 159 v-Shape 3706 7050 1090 2878 3 2008 0 19.9 0 6.6 0 4.9 28.4 187.4 v-Shape 0 1385 0 565

sub total 4816 10450 1690 4438 Intermediate gully (part of valley bottom gully branch)

4 1982 20.15 20.3 20.1 20.4 0.5 0.6 56.9 244.3 U-shape 573 695 1147 1155 5 1982 20 20.5 15.9 16.05 1.20 1.3 27.9 272.2 U-shape 601 663 558 572 6 1981 11.7 11.9 5 5.3 2.15 2.5 50 322.2 U-shape 898 1075 585 595 7 1996 8.9 9 2.6 3 3.17 3.68 51 373.2 U-shape 930 1126 454 459

185.8 sub total 3001 3559 2743 2781 Northern gully branch

8 1996 7.6 7.8 2.8 3 2.95 3.25 17.8 391 V-shape 273 312 135 139 9 2002 7 7.35 2.8 3.15 1.8 1.95 59.2 450.2 U-shape 522 606 414 435

10 2002 7.35 7.38 4.35 4.35 0.9 0.95 52.5 502.7 U-shape 276 293 386 387 11 2006 2.5 4.55 1.1 1.6 1.25 1.7 71 573.7 v-Shape 160 371 178 323

12 2006 2.4 6.2 1.5 2.35 1.25 1.35 30 603.7 v-Shape 73 173 72 186 13 2005 1.7 2.6 1.25 1.4 0.6 0.7 67.4 671.1 U-Shape 60 94 115 175 14 2005 3 4.55 1.6 2 0.95 1.25 60 731.1 v-Shape 131 246 180 273 15 2005 3.6 4.4 2.1 2.5 0.75 0.9 68.7 799.8 v-Shape 147 213 247 302 sub total 1642 2309 1727 2221 Southern gully branch 16 2004 9.95 11.9 2 2.2 3.2 3.6 85 884.8 U-Shape 1625 2157 846 1012

 

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Top Width (m) Bottom Width Depth (m) Length (m) Volume (m3) Plan area (m2) # incision period 2007 2008 2007 2008 2007 2008 2008 Cumulative Shape 2007 2008 2007 2008 17 2003 12 12.1 6.65 6.7 1.8 1.9 30 914.8 U-shape 504 536 360 363 18 2004 13.8 14.2 6.6 6.65 2.5 2.8 97 1011.8 U-shape 2474 2831 1339 1377 19 2003 2.5 14.9 5.4 8.8 3.15 3.6 51 1062.8 v-Shape 635 1368 128 760 20 2003 7.4 7.5 3.15 3.2 1.7 1.75 31 1093.8 U-shape 278 290 229 233

21 1981 6.2 6.25 2.2 2.3 2.5 2.6 53 1146.8 U-Shape 557 589 329 331 22 2004 5.3 5.7 1.45 1.65 1.7 1.9 54 1200.8 v-Shape 310 377 286 308 23 2004 2 3.8 0.9 1.2 1.1 1.35 54.4 1255.2 v-Shape 87 184 109 207 24 2004 3.6 4.6 1.5 1.6 1.25 1.4 63.7 1318.9 v-Shape 203 276 229 293 sub total 6671 8609 3854 4883 total 16130 24926 16175 23763

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Appendix 2: The relation ship of daily discharge including base flow and rainfall recordings for twenty consecutive days (Sep. 7, 2008-Sep 27, 2008).

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Appendix 3: The studied saturated valley bottom gully water table was near or at the surface resulted the slumping of the gully walls and the surrounding soil (SG=Gully Section and P=Piezometer).

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Appendix 4: At the southern gully branch piezometer P19 down slope of the dyke, show a water table below the ditch bottom (SG 17 and P19) and the gully section was semi stabilized through out the monitoring period. While Piezometers P13 upslope of the dyke show a water table above the ditch bottom near or at the soil surface resulted the slumping of cropland.

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Appendix 5: Head cut migration and undercut deepening was contributed for the development of northern gully branch at a point where the water table was below the ditch bottom (GS 8-10 and P21) and the active gully section Piezometer P16 water table was above the ditch bottom.

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Appendix 6: The existence of grayish colored clay at different depths of the gully wall and at the pipe outlets indicates the level of the local long-term water table.

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Appendix 7: The springs are the splitting points of the studied gully system at different locations.