Assessment of small scale irrigation structure failures; case of Weira, Hadiya zone,SNNRP, Ethiopia

Abstract

Many irrigation structures have been designed and constructed in past years. But different researcher shows that most of design irrigation structures do not perform their proposed uses, because of different reasons. It has been observed in various researches that some of the schemes have failed to serve the purpose for which they are designed. Therefore it is important to deal with cause of failure of this structure. The assessment of problems of small scale irrigation scheme failure with the main objective to assess the problems of failure of small scale irrigation scheme in case of SNNP Region, Hadiyya zone in weira scheme was carried out. Failures of weira irrigation project were assessed by using primary and secondary data. Primary data were collected through field data measurement, questionnaires and observation. Secondary data were collected from metrological data, other official documents and design documents. Some of the problems attributed to the institutional, social and operational problems are lack of adequate community participation in planning and designing, absence of WUAs, proper handing over problem and lack of proper training and absence of proper maintenance, evaluation and monitoring issues. Moreover, hydrologic failure was evaluated by Soil Conservation Service (SCS) method. The findings related to hydrologic failure revealed that the peak flood used for the design of the weir is smaller than the result obtained in this investigation, for irrigation scheme structure of 50 years recurrent intervals were 118m3/s the calculated value of flood for 50 years 155.75m3/sec. Therefore, the other major causes for the damage of the irrigation structures and overtopping of the canals and weir were as the result of the flood that came from catchment areas of the streams. Hydraulic failures were evaluated by lacey’s formula. Actual value of RL of the upstream and downstream bottom of scour hole (m), upstream and downstream anticipated scour, scour depth, manning’s roughness coefficient, discharge in canal and velocity, were compared with the designed value. The finding of these scheme failure shows that there was main canal siltation, damage on scouring sluice gate, downstream scouring and damage on downstream apron.