SETRIC: A MATHEMATICAL MODEL
FOR SEDIMENT TRANSPORT IN IRRIGATION CANALS

SETRIC (Sediment Transport in Irrigation Canals) simulates the water flow, sediment transport and changes of bottom level in an open, upstream controlled irrigation network with a main canal and several secondary canals with or without tertiary outlets. Various flow conditions along the canal network and during the irrigation season can be simulated. The mathematical model predicts the water flow and sediment transport together with the variations in bottom level and is based on an uncoupled solution of the water flow and sediment transport equations. The model is able to simulate the sediment entrainment in an irrigation network under changing flow conditions and sediment characteristics during the whole irrigation season and over one or more years. Moreover, it can evaluate the effects of the interrelation between irrigation practice (operation and maintenance) and sediment deposition. The direct effect of irrigation practices on the sediment deposition or erosion of earlier deposited sediment include changes in discharge and in sediment load, the effect of flow control structures, controlled entrainment, operation and maintenance activities, diverted sediment load to the farmlands, etc. Sediment deposition in canal reaches will affect the hydraulic aspects, for example water level variation, water distribution at outlets and flow control structures. The model predicts the sediment transport behaviour for specific water requirements and delivery schedules, defines the amount of sediment that will be deposited or eroded, will determine the required capacity and recommend the most efficient operation of sediment removal facilities. The model can also be used as a tool to select improved operation plans that will result in minimal deposition in the canal systems that have been designed without considering any sediment transport criterion; The water flow in irrigation canals during an irrigation season and moreover during the lifetime of the canals is not constant. Seasonal changes in crop water requirement, water supply and variation in size and type of cropping pattern are frequent events during the lifetime of an irrigation canal. The operation of gates to adjust to the variation in supply is normally gradual and allows sufficient time to move from one steady state to another steady state. Moreover, the Froude number in irrigation canals is normally small to maintain a stable water surface and the celerity of the water movement is much larger than the celerity of the bed level change. Therefore, the changes in the sediment morphology are much slower than the changes in the water flow and the water flow and the sediment transport calculations can be uncoupled. The model schematises the sub-critical water flow as quasi-steady and solves the gradually varied flow profile by the predictor-corrector method. The model separates the roughness on the canal bed and side slopes, it computes the roughness on the bed using the van Rijn method that is based on flow conditions and bed form and grain related parameters (bed form length, height and sediment size). The equivalent roughness is determined by taking into account the sidewall effect. The sediment continuity equation is solved numerically by the Modified Lax scheme. The Galappatti’s depth-integrated convection-diffusion, suspended sediment transport model predicts the actual sediment concentration at any point under non-equilibrium conditions. The model uses an asymptotic solution for the two-dimensional convection equation in the vertical plane and assumes only the vertical diffusion terms and a depth-averaged concentration. The total sediment transport under equilibrium condition follows from one of the three sediment predictors, namely Ackers-White, Brownlie or Engelund-Hansen. The total sediment transport capacity in the non-wide irrigation canals is corrected for the B/y ratio and side slope. The numerical solution of the one-dimensional sediment equations uses the friction factor predictor, continuity equation for sediment and the sediment transport predictor and requires boundary conditions for the hydraulic and sediment transport calculations. The general framework of the computer program consists of hydraulic, sediment and irrigation aspects, which are determined by the various input data. The computer program presents the results of the hydraulic and sediment transport calculations in either tables or graphs; on screen, on paper or in files. The tables or graphs include the hydraulic results (initial and final water depths, discharge, bed width and bottom slopes) and the sediment transport results (initial and final equilibrium and actual concentrations, initial and modified bed levels and deposited volumes). Further information in the theoretical background and use of the model could be found in:

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