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Addis Ababa University (2013)

Hydraulic and Hydrologic Factors on Conveyance Efficiency : Case of Fantale and Tibila Irrigation Schemes

Adam, Tefera

Titre : Hydraulic and Hydrologic Factors on Conveyance Efficiency : Case of Fantale and Tibila Irrigation Schemes

Auteur : Adam, Tefera

Université de soutenance : Addis Ababa University

Grade : Master of Science (Hydraulic Engineering) 2013

In planning and designing an irrigation system, a major problem is to decide what efficiency to apply in the calculations. Since basic knowledge on this subject is lacking, it is common practice that this efficiency is either assumed or derived from existing irrigation systems. Obviously, the efficiency thus obtained is unlikely to suit the conditions of the project area in its future state. Most irrigation projects in Ethiopia are found to work below their expectation. One of the main reason behind is the difference in the conveyance & application efficiency envisioned (designed) and the actual values obtained in the projects design life. For irrigation projects having extensive length of main canal and secondary canals, the effect of conveyance efficiency is far more important than the application efficiency in deciding the project future performance. To evaluate the effect of conveyance efficiency, variation from the designed value, two irrigation schemes, Fantale and Tibila constructed in 2003 and 2008 respectively were used as case study. The designed value was taken from the respective design reports and the existing conveyance efficiency is obtained by measuring discharge, depth of flow, wetted perimeter, wetted area and bed slope of the canal at different locations. The conveyance efficiency given at the design stage was 95% for both schemes which was directly taken from FAO recommendations. However, the values obtained during the study were 34.4% and 89% respectively for Fantale and Tibila schemes. Such a decrease in conveyance efficiency is found to be due to hydraulic and hydrologic factors. The factor is hydraulic in a sense that the conveyance characters (roughness coefficient, hydraulic radius and bed slope) used in the designed stage has no longer exists in the current situation of the schemes. The factor is hydrologic in a sense that the variation in hydraulic radius from the designed values has resulted in increase in the water depth which intern increases the top width of the water surface which further resulted in increasing in seepage and evaporation losses. Thus a designed conveyance efficiency taken directly from standard books has effect on the overall performance of an irrigation project. Conveyance efficiency selected should be based on construction material, method and quality, managerial activities, technological and other factors affecting conveyance efficiency. Key words : Conveyance efficiency, Evaporation loss, Fantale & Tibila irrigation schemes

Mots clés : Conveyance efficiency Evaporation loss Fantale & Tibila irrigation schemes


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