Development of one-dimensional numerical model of sedimentation with non-uniform granulation in settling basins using time-splitting method

Background and objectives: There are various methods to prevent the entry of suspended sediments into the irrigation canals, one of which is the construction of sedimentation ponds at the beginning of the irrigation network. Sediment efficiency, which is the ratio of the concentration of outlet sediments to inlet, should be determined before designing the sedimentation pond according to the geometric dimensions of the structure, hydraulic properties and characteristics of inlet sediments to be planned for management and operation. The purpose of this study is to present a one-dimensional model of sedimentation in the sedimentation pond by considering the sedimentation of inlet sediments, which in addition to estimating the trapping efficiency in the pond, also calculates changes in the pond floor, sedimentation of sediment outlet and sediment concentration in each section.
Materials and methods: n this study, in order to simulate sedimentation in the sedimentation pond, first a numerical model was developed and then the effect of the geometric factors of the pond on the sediment trapping efficiency was investigated. In the developed one-dimensional numerical model, the governing equations of flow and sediment were solved, respectively. To validate the results of the numerical model, El-Baroudi (1969) laboratory data were used, which include a rectangular sedimentation pond with a length of 1.2446 m and a width of 0.4572 m. For different discharges, El-Baroudi deposited 200 mg/l of sediment with a specified diameter into the sedimentation pond and after a certain period of time measured the concentration of the effluent from the pond. Also, field data of Nekoabad sediment catchment area of Isfahan (Shetab-Boushehri et al., 2010) were used for validation.
Results: The results of trap efficiency verifications showed that the mean value of the square root of the mean square error of the numerical model for El-Baroudi (1969) experiments was 6.15% and for the left and right sedimentation ponds were 8.13 and 6.77%, respectively. Also, the trapping efficiency was investigated for all sediment particles and for each group of particles in relation to the length of the pond. The results showed that with increasing the distance from the beginning of the pond, the amount of sedimentation efficiency increases so that at the end of the pond The sediments are deposited in the left and right basins, respectively. The floor level of the sedimentation pond also changes due to sedimentation of sediments. The rate of these changes in the floor of sedimentation ponds at the beginning of the structure is higher than the end due to the high concentration of inlet sediments. The amount of these changes in the bottom of the left and right sedimentation ponds using the numerical model simulation of the present study after three months was about 23.4 and 20 cm, respectively.
Conclusion: After developing and validating the numerical model, the performance of Nekoabad sedimentation ponds and El-Baroudi laboratory model were investigated using the numerical model. The results showed that the average sediment trapping efficiency for the left and right sedimentation ponds was 36.5% and 27.3%. Therefore, a large amount of sediment enters the irrigation network. In order to improve Nekoabad sedimentation ponds and increase sediment trapping efficiency, the length, width or depth of the pond can be increased. For this purpose, by increasing one of the above variables and keeping the other parameters constant, the numerical model was executed and the trapping efficiency was calculated. The results showed that increasing the length of the left sedimentation pond and increasing the width of the right pond of Nekoabad Dam has the greatest effect on the trapping efficiency.