Modelling of sedimentation dynamics in an underground canal in South West Kano Irrigation Scheme - Kenya

Abstract

The design of an irrigation canal is intended to ensure that water is conveyed with minimal erosion and sedimentation. The underground canal in Southwest Kano Irrigation scheme was designed to meet these conditions but over time it has been silted up to the extent that its conveyance capacity has significantly dropped. The sediment control structure at the inlet can only prevent bedload, consisting of large sediments, from entering the canal but cannot prevent entry of fine suspended particles. Deposition of a particle a canal starts when the vertical components of the bed-level turbulence are less than the fall velocity of a particle. Sediment transport capacity depends on the magnitude of these vertical components as they oppose the settling process in which the weight of the particle causes it to descend. This study was based on modelling sediment transport within the underground canal in Southwest Kano Irrigation Scheme using Hydrologic Engineering Centre – River Analysis System (HEC-RAS) model. This model includes Ackers-White sediment transport equation, which was used to estimate sediment transport capacity, discharge and deposition. The conceptual and physical parameters required in the HEC-RAS model were determined through calibration and direct measurement in the field respectively. The conceptual parameters for the Ackers-White equation were optimised using algorithms in the HEC-RAS model. The model was calibrated based on the current operational conditions of the canal followed by simulation using the calibrated model to determine the sediment discharge and deposition rates at different levels of flow in the canal. The simulation results showed that the calibrated HEC-RAS Model could predict the sediment sizes which were deposited at specific sections of the canal at different flow rates. Deposition occurred since the prevailing flowrates were below the critical velocity. Grain classes 8, 9, 10 and 11 should be screened from entering the canal, to ensure that sediment entering the canal is transported to the canal outlet without deposition. The scheme needs to improve on sediment control management to minimise deposition.