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Title: Using computer models to design gully erosion control structures for humid northern Ethiopia

Author
item Langendoen, Eddy
item ZEGEYE, ASSEFA - Cornell University
item TEBEBU, TIGIST - Cornell University
item STEENHUIS, TAMMO - Cornell University
item AYELE, GETANEH - Bahir Dar University
item TILAHUN, SEIFU - Bahir Dar University
item AYANA, ESSAYAS - Bahir Dar University

Submitted to: International Conference on Hydroscience and Engineering (ICHE)
Publication Type: Proceedings
Publication Acceptance Date: 7/2/2014
Publication Date: 9/28/2014
Citation: Langendoen, E.J., Zegeye, A.D., Tebebu, T.Y., Steenhuis, T.S. 2014. Using computer models to design gully erosion control structures for humid northern Ethiopia. International Conference on Hydroscience and Engineering (ICHE). Sep 28-Oct 2, 2014, Hamburg, Germany, pp. 1137-1145.

Interpretive Summary: Gully erosion is a major cause of land degradation in northern Ethiopia. Classic gully erosion control measures such as check dams have been unsuccessful in halting gully formation and growth in the humid northern Ethiopian highlands. Scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers at Cornell University and Bahir Dar University, Ethiopia, are using the USDA, ARS computer models BSTEM and CONCEPTS for developing more effective gully rehabilitation measures for humid northern Ethiopia. These models were applied to investigate the possibility to stabilize a rapidly expanding gully in the Debre-Mewi watershed, about 30 km south of Lake Tana through regrading and planting of vegetation. Data were obtained directly from field topographic surveying, measuring flow and sediment transport, shallow ground water sampling, and laboratory investigation of soil mechanical strength. Given the cohesive strength of the soils (> 19 kPa), BSTEM calculated stable slopes that varied between 75 degrees for a bank height of 3 m to 47 degrees for a bank height of 6 m. CONCEPTS simulated flow velocities of 7 m/s and shear stresses exceeding 100 Pa at the upstream portion of the regraded gully head. A demonstration project has started that uses bamboo sheets to protect the soil surface against such forces. The introduction of computational modeling technology of increasing complexity, such as BSTEM and CONCEPTS, could have major benefits to develop cost-effective erosion control measures in developing countries such as Ethiopia.

Technical Abstract: Classic gully erosion control measures such as check dams have been unsuccessful in halting gully formation and growth in the humid northern Ethiopian highlands. Gullies are typically formed in vertisols and flow often bypasses the check dams as elevated groundwater tables make gully banks unstable. Check dams also have been unable to halt the formation and headward migration of head cuts, which often exceed heights of 3 to 4 m. Structural measures built from concrete, steel, and riprap, such as high drop structures with plunge pools, are too expensive for the local farmers to construct. Here we used the U.S. Department of Agriculture (USDA) computer model BSTEM to determine stable slopes of the gully head and banks in the Debre-Mewi watershed, northern Ethiopia, to arrest a rapidly migrating head cut exceeding 3 m in height with downstream gully depths exceeding 6 m. We then used the USDA model CONCEPTS to determine the required protective cover to prevent erosion of the regraded head cut. Given the cohesive strength of the soils (> 19 kPa), BSTEM calculated stable slopes that varied between 75 degrees for a bank height of 3 m to 47 degrees for a bank height of 6 m. CONCEPTS simulated flow velocities of 7 m/s and shear stresses exceeding 100 Pa at the upstream of the regraded gully head. A demonstration project has started that uses bamboo sheets to protect the soil surface against such forces. The introduction of computational modeling technology of increasing complexity, such as BSTEM and CONCEPTS, could have major benefits to develop cost-effective erosion control measures in developing countries such as Ethiopia.