Hydraulic analysis of Thiba main canal reach in Mwea irrigation scheme, Kenya

Abstract

As population continues to increase especially in African and Asian countries, demand for more food is equally increasing. There have been challenges in meeting the food security needs in most countries especially those depending on rain-fed agriculture. Although a number of countries are addressing the shortfall by investing heavily on irrigated agriculture, the availability of irrigation water still remains one of the major challenges. Irrigation has been ranked as one of the activities that utilize huge amounts of water with global values estimated between 70-80% of the available fresh water. Kenya is one such country facing the challenge of having enough water available for irrigation. To supplement rain-fed agriculture in Kenya, a number of food crops are grown under irrigation. One of these food crops whose demand as a staple food has continuously increased is rice which in Kenya is grown in irrigation schemes. Mwea Irrigation Scheme (MIS) continue to face various challenges that include; high irrigation water demand, vandalism, inadequate system calibration and minimal measuring devices. Due to water shortage in addition to poor operation and maintenance of hydraulic structures within irrigation schemes, rice production is far below the required quantity. To address these challenges, this research focused on a number of strategies which include; improved design of the main canals, proper management and operation of hydraulic structures and proper scheduling of water release to the farmers. In order to realize these strategies, Hydrologic Engineering Center - River Analysis System model (HEC-RAS) was applied. The model was calibrated and validated using two sets of observed discharges and water levels. In addition the model was used to simulate the hydraulic behaviour of Thiba Main Canal (TMC) reach in MIS. The model was used to simulate different flows in the main canal for varied design discharges through the sluice gates and drop structures. Statistical and graphical techniques were used to assess the model against its performance. The model was finally used to estimate the potential capacity of the main canal reach. The results show that HEC-RAS model is capable of evaluating the canal hydraulics under steady state conditions. The results from this study further show that increasing the hydraulic resistance of Link Canal II (LCII) from 0.022 to 0.027 resulted in a decrease in estimated maximum capacity by 10.97%. In the case of Thiba Main Canal, increasing the roughness coefficient from 0.015 to 0.016 resulted in a decrease in estimated maximum capacity by 11.61%. The Link canal II and TMC were capable of only allowing flows of 9.9 m3/s and 5.7 m3/s respectively. These research findings would therefore be a basis for the scheme management and operators to improve on the operation, management and maintenance of the irrigation system for effective and efficient water delivery to the farmers.