1. Effects of gully headcut treatment on sediment load and gully expansion in (sub) humid Ethiopian highlands
Assefa D.Zegeye1,2, Eddy J. Langendoen3 Seifu A. Tilahun4, Dessalegn C. Dagnaw4, Tammo S. Steenhuis1,3,
Introduction
Soil and water conservation in Ethiopia is as old as four decades which was started to combat severe soil degradation in the highlands of the country under the umbrella of the World Food Program ‘Food for Work’ (Herweg and Ludi, 1999). Decisions on which type of soil and water conservation (SWC) measures to use and where to construct them were not made by the farmers, and little attempt was rarely made to include their indigenous experience and knowledge (Herweg and Ludi, 1999). Most of the SWC measures introduced were addressing some of the erosion features such as rill and inter-rill erosion. Little attention, if any, has been given for halting gully erosion.. Gully erosion is more difficult and expensive to control than sheet and rill erosion. In the study area, two approaches are used for controlling gully erosion. First, through the application of SWC techniques in catchments (i.e. soil bunds), an attempt is made to reduce both peak runoff discharges and total runoff volumes (Dagnaw et al., 2015). In addition, the establishment of control measures in gullies, leading to sediment deposition and vegetation regrowth, creates new hydraulic conditions aimed at controlled runoff evacuation (Zegeye et al., 2014).
Site description
.Elevation ranges from 1800 to 2300masl
. Located at 11o21’58’’N and 37o24’25’’E
. Temperature 7.8 to 25.7oc., with a semi-humid climate
. Ten years average rainfall of 1238mm in unimodal distribution
. Situated 30km south of Lake Tana ,Ethiopia (Fig.1)
Figure1. Gully drainage networks and location of study gully
Methodology
To compute the effects of Gully headcut treatment (GHT) on sediment concentration and yield before and after GHT, two rectangular weirs, one at the inlet and the other at the outlet of the gully were installed prior to the rainy season of To understand the effect of GHT on sediment reduction, some headcut controlling techniques such as: regrading the side-walls and headcut to reduce headcut angle to 45o and planting plant species such as elephant grass and susbania on the regraded banks. In addition, to dissipate the flowing energy, one gabion check-dam beneath the headcut and others consecutive check-dams made of woods, at short distances below the headcut, were installed prior to the 2014 rainy season (Figure 2).
Figure 4. The visual sediment concentration comparison of the inlet and outlet
discharge of the study gully, August 6, 2014.
Conclusion
Gullies are a specific form of severe erosion typically caused by concentrated water flow as well as elevated ground water table on erosive soils.
Takeing action to prevent the formation of gullies and to stabilize existing gullies before they grow larger is the most important ways of controlling gullying. Once large, gully stabilization measures can be very difficult and expensive.
Maintenance of gully control structures is a very important point worth to be emphasized. Treated gullies should be checked regularly and the healing process monitored closely.
Damaged check-dams should be repaired immediately to avoid further damage and the eventual collapse.
 This study is a good indicator of what areas need to be targeted to reduce sediment loads.
The upper catchment SWC doesn't have significant role in sediment load reduction once a gully network has formed.
Generally, the gully erosion controlling measures can significantly reduce sediment yield to increase the lifespan of the downstream reservoirs.
Results
The GHT was successful in controlling the uphill migration of the headcut
The total sediment yield decreased by 50% after GHT.
The daily averaged sediment concentration decreased by 30 %
The sediment yield and concentration reduction at the outlet was mainly due to the headcut treatment
The left side-wall of the headcut that was not regraded effectively, migrated laterally about 1.5m.
In the (sub) humid Ethiopia highlands, where the soil saturates periodically, halting headcuts is more difficult than in semi-arid areas and more expensive.
Thus, designing measures to treat deep gullies in cost effective ways should be a priority in future research
lowering water table elevation and regrading the gully head and sidewall and effective bank toe protection using inexpensive materials
Figure 2. Gully headcut treatment started in May, 2014, in the Debre-Mawi watershed
References
Dagnew, D. C., Guzman, C. D., Zegeye, A. D.,  Tebebu T.Y., Menelik G., Solomon A., Zemale F.A., Ayana E.K., Tilahun, S. A., and Steenhuis, T. S., Impact of conservation practices on runoff and soil loss in the sub-humid Ethiopian Highlands: The Debre Mawi watershed, J.Hydrol.Hydromech.63, doi: johh
Herweg K , Ludi E., The performance of selected soil and water conservation measures— case studies from Ethiopia and Eritrea Catena 36, 99 –114
Lakew Desta and Belayneh Adugna, A Field Guide on Gully Prevention and Control, Nail Basin Initiative
Tebebu, T. Y., Abiy, A. Z., Zegeye, A. D., Dahlke, H. E., Easton, Z. M., Tilahun, S. A. Collick, A. S. Kidnau, S., Moges, S., Dadgari, F. and Steenhuis, T. S., Surface and subsurface flow effect on permanent gully formation and upland erosion near Lake Tana in the northern highlands of Ethiopia. Hydrol. Earth Syst. Sci., 14, 2207–2217.
Zegeye, A.D., Damtew, S., Tilahun, S.A., Langendoen, E.J., Dagnew, D. C., Guzman C.D., Tebebu T .Y., Steenhuis, T.S., Gully development processes in the Ethiopian highlands, Proceedings of the 2nd International Conference on the Advancements in Science and Technology (ICAST), Bahir Dar University, pp
Table 1. The soil loss, discharge and suspended sediment at the inlet and outlet of the studied gully
year
Soil loss from
bank erosion (t)
Sediment load (t)
Discharge (m3)
Sed.Conc. (g/lit)
Inlet
Outlet
2013
2466 99
1225
5571
14410
17.8
85.0
2014
1993 57
800
8187
13774
7.0
58.1
Affiliations
1Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA
2Amhara Region Agricultural Research Institute, Bahir Dar, Ethiopia,
3US Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, Oxford, MS 38655, USA,
4School of Civil and Water Resources Engineering, Institute of Technology, Bahir Dar University, Bahir Dar, Ethiopia.
Figure 3. Sediment concentration at the inlet and outlet of the study gully in 2013 and 2014
Acknowledgement
This research was supported financially by the International Foundation for Science (IFS), Cornell University and Ministry of Water, Irrigation and Energy, Ethiopia
Contact: Assefa D Zegeye, Tel: ,
December, 2015