Effect of canal operation on sedimentation and erosion in irrigation canals : A case study in Sunsari Morang irrigation system, Nepal

Irrigation is a key to sustainable agriculture as it supplies the water to the crops to fulfil their crop water requirement. There is increasing pressure in irrigation as water is being scarce for various reasons and the agricultural land is sinking in size due to industrialization and urbanization. Moreover, there is need for providing sufficient food for increasing population with less water from a smaller area. Conversely this can only be achieved if the irrigation fulfils the so-called four criteria: adequacy, equitable, reliable and efficient. The irrigation schemes do not always run with the design discharge and to fulfil the water requirement for different growing stage of a cropping period the canal is operated leading to change of the flow. This variation of flow will generally lead to the siltation in the canals which will ultimately decrease its capacity and block the turnouts leading to poor serviceability. The problems associated with sediments have been led to very costly investments in maintenance and rehabilitation work and yearly billions are invested for this purpose. Sunsari Morang Irrigation System (SMIS), Nepal is not being able to provide its service to planned command area of 68000 ha because of various reasons among which sediment is a major problem. After few years of operation billions have been invested in order to solve the issues of sediment in the canals. Therefore, a study for the analysis of behavior of sediments according to canal operation would be a good asset in the field of irrigation. For the analysis a mathematical model Delft3D will be used which is capable of analyzing the flow and sediment transport for both cohesive and non-cohesive sediments for non-steady flow in non equilibrium condition coupled with gate operation using feedback control tool. In process of studying the sediment transport in the SMIS, the following questions arise, what is the sediment deposition pattern along the canals in SIS? Is there any effect of change in inflow in morphological change in Irrigation canals? Does gate operation affects the erosion and deposition pattern in the irrigation canals? And can process based model like Delft3D be used to address the sedimentation and morphological processes in irrigation system? To answer these questions, the following objectives have been set: • To study sediment transport and morphological changes in the SM irrigation canal • Analyze the effects of changes in sediment inflow on morphological behavior in the canal. • To analyze the effect of gate operation in the erosion and deposition pattern of sediments in the canal. This research attempts to fill the knowledge gap relate to canal operation and sediment transport in irrigation canals. This research mainly focuses on the effect of flow with different sediment concentration along the canal and effect of gate operation in the sediment erosion and deposition pattern. The analysis was done with three approaches therefore, scenarios were developed accordingly. First a reference model was developed to analyze the sediment transport and morphological changes in S MIS. Then scenarios with constant sediment inflow and variable sediment inflow were prepared. Lastly, scenarios for the gate operation were developed and simulated. The results for the showed that sediments mainly deposited along the upstream reach of the canal and erosion was spotted downstream of the desilting basin. Erosion was mainly caused by sudden contraction of the canal section after the basin. High deposition was observed near the head of the branch canal and it is suspected that this is caused by the orientation of the branch canal. It was observed that the sediment transport along the canal cross - section is not symmetrical. As most of the researches had been done in 1D model, it can be said that pure representation of the sediment transport along the irrigation canal is not done. Therefore, it is crucial to use multi - dimensional model in order to visualize the changes in canal bathymetry in lon gitudinal as well as traverse direction. For constant sediment inflow, four scenarios with concentration 0.15 kg/m3, 0.3 kg/m3, 0.9 kg/m3 and 1.5 kg/m3 were prepared. When the flow with constant sediment concentration was discharged from the upstream, it was observed that the desilting basin was able to trap most of the sediments and the flow as well as transport parameters were similar for all the scenarios. The result obtained from the scenario with flow passing with variable sediment concentration showed that the deposition that happened at one time can be eroded at the other. Also, as the concentration increases the hydrodynamics is affected more which in turn have effect on the sediment transport. Furthermore, it was found out that the deposition pattern somewhat mimics the actual deposition pattern in the field. In addition, as the most of the sediments were deposited in the upstream area, it is advised to focus the Operation and Maintenance activities on the upstream section of the canal. Lastly, it was found out that the gate operation does have effect in the sediment transport in the irrigation canal. The erosion and deposition were affected not only at the upstream and downstream of the gate but also have influence on the main and branch canal. More deposition was observed on the branch canal when the flow was lower. The deposition upstream of the gate is higher when fewer gates are open while more erosion at the downstream. It was found out that the selection of the gate to open for diverting the same amount of discharge affects the flow as well as the sediment transport in the canal. Therefore it is concluded that yes, gate operation definitely affects the sediment transport along the irrigation canal