4. Discussion
The basic function of the regulating structures is to convey the required amount of water to where it is needed, for example to the offtakes to the branch canals, and maintain a certain water level there. The reference scenario (SGO1) with all gates open diverts 29% of water to the branch canal. As expected, after closing two or three gates in the regulator, more water enters the branch canal at the first time-steps. Scenarios with two open gates divert 7% to 8% more water to the branch canal as compared to the reference case. Scenarios with one open gate, divert about 19% to 21% more water to the branch canal as compared to the reference case (
Table 5).
Scenario SGO5 diverts less water than scenarios SGO2, SGO3 and similarly SGO4 because in this scenario gates a and d are open, which are at the sides of the main canal where the flow velocity is less than in the middle because of wall friction. Opening gate b or c located in the middle of the main canal (scenarios SGO7 and SGO8, respectively), results in less water diverted to the branch canal than in scenarios SGO6 and SGO9.
Comparing the different gate operation scenarios with one or two gates open, it is clear that not only the number but also the location of the selected gate(s) has an impact on sediment deposition. Opening the gate which is at the side of the offtake leads to less sediment deposition at the upstream of the regulating gates (
Table 6). Opening the farther gate leads to higher sediment deposition at the intake. This deposition will not only alter the hydrodynamic characteristics but also morphologic characteristics of the canal, which can result in system inefficiencies as regulating gates are of crucial importance. The sediment depositions around structures are harder to remove and manage.
The results in
Table 6 show that when one gate is opened, the amount of deposited sediments in the upstream part of the main canal is higher than in scenarios with two open gates and the reference case, because in the latter scenarios more water, and hence more sediments, flow towards downstream of the main canal. Opening gates at the diversion side convert more water and sediments to downstream of main canal than when opening gates at the opposite side. Gates on the opposite side leads to having less sediment transport capacity and consequently higher deposition which is transported to the branch. The sediment stored at the branch canal is around 2% in the reference scenario while a negligible amount is stored in the other scenarios.
Among the scenarios with one open gate, the location of the gate has a small impact on the amount of sediment diverted to the branch canal. In scenarios SGO6 and SGO9 the sediment is slightly less compared to other scenarios. Although the difference is small, it is worth mentioning that gate operation does have an impact on the sedimentation and erosion of the branch canal.
Additionally, in scenarios of two open gates, the amount of water transferred to the branch canal in the case of SGO4 is more than the reference case, SGO2 and SGO3 and similar to SGO5 as shown in
Table 5. On the other hand, the amount of sediment transferred to the branch canal in the case of SGO4 is less than other scenarios of two open gates, as shown in
Table 6.
In scenarios of one gate open, the amount of water transferred to the branch canal in the case of SGO9 is more than other scenarios as shown in
Table 5. On the other hand, the amount of sediment transferred to the branch canal in the case of SGO9 is less than SGO7, SGO8 and similar to SGO6 as shown in
Table 6.
In practice, usually, it is preferred for gate operations to have less effect on the canal shape, in this case, the operation of the gate SGO4 seems a better choice among the scenarios with two open gates. The operation of the gate SGO9 seems a better choice among the scenarios with one open gate. This has ensured delivering a higher amount of water to the branch canal with fewer sediments.
A slight change in hydrodynamic characteristics changes the sediment transport mechanism and thus the canal morphology, which in turn changes the water flow features in the canal [
3]. The results show that because of gate operation the change in the flow characteristics alters the amount of sediment passing through each cross-section different for each scenario. This means that gate operation has a significant effect on the hydrodynamic as well as the morphologic parameters in an irrigation system [
3]. Gate operation can be used as not only the as diverting the water but also as a sediment management technique. The incoming flow through time can erode the accumulated sediment and flush it away. This is because of the change in the canal properties and the hydraulics of the canal. For this reason, it can be said that there is constant sediment erosion and deposition until the equilibrium state is reached, which can occur only in the case of non-cohesive sediments.
Operating the gate at the side of the offtake can minimize the sediment deposition at the entrance of the offtake; however, if the same gates are regularly opened, the canal geometry will be permanently altered due to the deposition and erosion resulted from this gate operation, which is not appropriate for equitable water distribution and for the proper system functioning. Additionally, the deposited sediments during one gate operation can be removed during the other [
16].
The results from comparison of the scenarios show that the number of gates opened as well as their location has an impact on sedimentation and erosion patterns in the both the main and branch canal. Not only the amount of sediment deposition but also the location in the cross section of the canal depends on the gate selection. Therefore, a canal operation plan along with the gate operation schedules can help to increase the canal efficiency and to reduce sediment removal costs. Alternating the gates to be opened during the irrigation period can help to reduce the sediments around the flow control structures, while at the same time delivering sufficient water to the branch canals. Sediment depositions can be eroded with the help of the canal operation itself, by alternating the gate to be opened, without investing extra money and labor for the cleaning process.
This paper considers non-cohesive sediments only. In reality, many irrigation systems take water from rivers which contain a mix of cohesive and non-cohesive sediments. This shall be considered in future research.
5. Conclusions
The study confirms that gate operations have a significant impact on the overall hydrodynamic and morphologic behavior of the irrigation system [
43]. The simulations using a multi-dimensional numerical showed that opening the gate on the side of offtake resulted in less deposition of sediment at the entrance of the offtake than when the gate on the other side of the offtake was opened. This means that the selection of gates to be opened for delivering a certain amount of water has a major impact on the spatial sediment erosion and deposition patterns, and it shows the benefit of using such multi-dimensional simulation models. An analysis using merely a 1D model, as has been the de facto standard until this date, would have missed such asymmetric channel developments.
Furthermore, the simulations showed that the gate selection did not only impact the main canal, but it also extended to the branch canal. Although a primarily one-dimensional morphological response was observed both upstream and downstream of the closed gates, those erosion and deposition patterns include a secondary asymmetric distribution along the canal and within the cross-section. Again, this supports the need for using multi-dimensional models for such situations in order to obtain a better estimate of the maintenance costs under various scenarios. While 1D models provide insight in the mean depositional pattern in the longitudinal direction of irrigation canals, asymmetric deposition patterns can only be resolved by including the second spatial dimension in the simulation.
All simulations reported in this paper were carried out in both depth-averaged (2D) and 3D hydrostatic mode using five layers. While the 3D mode provides a slightly better description of the physics involved, the running times are quite long for large irrigation canal networks. Compared to 1D models, the use of Delft3D run in 2D mode has a clear benefit by providing an estimate of the asymmetric development of the hydraulics and morphology, while avoiding the long simulation times of 3D models [
43]. However, for the simulations considered here the extra simulation time of the 3D models has not shown a significant benefit for this type of application.