Skip to main content
ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #293474
Title: Assessing gully widening and its control in the Debri-Mawi Watershed, northern Ethiopia

Author
item Langendoen, Eddy
item TEBEBU, TYGIST - Cornell University
item STEENHUIS, TAMMO - Cornell University
item TILAHUN, SEIFU - Cornell University

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/5/2013
Publication Date: 5/17/2013
Citation: Langendoen, E.J., Tebebu, T.Y., Steenhuis, T.S., Tilahun, S.A. 2013. Assessing gully widening and its control in the Debri-Mawi Watershed, northern Ethiopia. In: Procedings of the 1st International Conference on Advancement of Science and Technology (ICAST-2013), May 17-18, 2013, Bahir Dar, Ethiopia. pp. 214-221.

Interpretive Summary: Gully erosion is a major cause of land degradation in northern Ethiopia. Monitoring of groundwater wells has shown that gully growth and widening may be controlled by groundwater elevation. Scientists at the USDA-ARS National Sedimentation Laboratory in collaboration with researchers at Cornell University and Bahir Dar University, Ethiopia, have used the Bank Stability and Toe Erosion Model (BSTEM), which was developed by ARS, to quantify groundwater effects on gully sidewall stability. They showed that during the 2008 rainy season, a rise in groundwater elevation of 1.5 m was sufficient to destabilize gully sidewalls in the Debri-Mawi watershed near Lake Tana, northern Ethiopia. Observed rises in groundwater elevation are as large as 3 m. Model application further showed that gully sidewall stabilization requires a combination of gully bottom control to prevent gully deepening, toe protection to prevent steepening of the gully sidewall, and the planting of vegetation on top of the gully sidewall to prevent the formation of cracks and lower soil water pressure. Small increases in gully depth or gully sidewall steepness, and cracks adjacent to a gully suffice to destabilize it. The BSTEM model and these findings will be used by researchers at Bahir Dar University and watershed managers to develop gully stabilization practices in northern Ethiopia.

Technical Abstract: The highlands of northern Ethiopia suffer from severe land degradation manifested by widespread gully and channel erosion and network development. Research on the geomorphic adjustment of similar landscapes in the midcontinental United States has resulted in the development of the computer models BSTEM and CONCEPTS, which have been used to assess the long-term evolution of incised channels and the stabilizing impact of conservation measures. These models express channel bank stability by a factor of safety (Fs). If Fs > 1 the bank slope is stable, otherwise it is unstable. The BSTEM model was used to: 1) evaluate the stability of the channel banks of a gully in the Debri-Mawi watershed near Lake Tana, Ethiopia during the 2008 rainy season; 2) test the hypothesis that groundwater dynamics are responsible for gully bank mass failure; and 3) evaluate measures to stabilize the gully banks. Model results show that the stability of the 2007 gully bank was controlled by the groundwater table. Factor of safety was typically greater than 1.3 for groundwater table elevations near the gully bottom, whereas factor of safety was smaller than 1 for saturated or near-saturated conditions even for high soil shear-strength values. The post-rainy season bank profile also has limited stability. Ongoing gully incision or steepening of the banks would destabilize the gully banks. Planting vegetation on the bank top has a limited effect; increases in factor of safety were only about 0.1-0.2. Stabilization needs to include a combination of grade control measures to stabilize the gully bottom, toe protection to prevent bank steepening, and planting vegetation on the bank top to reinforce the upper portion of the gully walls in order to limit the extent of tension cracks and lower pore-water pressure. Tension cracks could reduce factor of safety by a value of 0.2-0.3.