SEDIMENT ESTIMATION AND ASSESSMENT OF HYDRAULIC STRUCTURE OF HIRNA SMALL SCALE IRRIGATION SCHEME, WEST HARARGHE ZONE OROMIA REGIONAL STATE, ETHIOPIA

Abstract

Accelerated soil erosion and sedimentation is a serious concern worldwide problem and it is difficult to assess its economic and environmental impacts accurately. The problem of sedimentation is clearly felt in the irrigation canals, in dry years when there is scarcity of rain more silt-laden water is diverted to the canals which cause high reduction of canal capacity discharge. Hirna watershed at which Hirna small scale irrigation project found is one of the most erosion-prone watersheds in the eastern highland of Ethiopia which little attention is given. The objectives of this study were to estimate Hirna river watershed sediment loss and to evaluate the design concept of Hirna small scale irrigation canals in view of water and sediment transport. This study spatial predict soil loss rate of the watershed using Geographic Information System, Revised Universal Soil Loss Equation was used to estimate potential soil loss. Based on the analysis, the mean and total annual soil loss potential of the study watershed was 26.61 ton/ha/yr and 140,569.99 tons/yr respectively. About 36.60% (1,933.42 ha) of watershed was classified under high to very high soil loss class while the remaining 63.40% (3349.17 ha) of watershed categorized under slow to moderate soil loss class. The total soil loss at the outlet of watershed or at the weir site estimated as 25,311.80 tons. Beside spatial estimation of soil loss, field soil loss measurement by runoff controlling point was also done. The estimated mean annual soil loss from watershed with runoff pot method was 22.30 ton/ha/yr. which that the two value of soil loss estimated with runoff plot and spatial estimated with the help of the model was almost the same. Assessment of irrigation project and structural problem by measuring bed load, structural problem observation, and redesigning of the Hirna small scale irrigation were done to relate the capacity of the structure with the designed discharge and sediment deposited. As the assessment show the project have a problem of sedimentation due to improper design of flow velocity and canal slope. Finally the result show 116.80 tons of sediment was deposited in irrigation canal which have dramatically reduced the quantity of designed water depth into 41.67% and reduced designed discharge with 50.2%.