پژوهشنامه مدیریت حوزه آبخیز (Oct 2024)
Sediment Particle Size Distribution behind the Check Dams of the Lashkaran Watershed, Salmas, West Azerbaijan
Abstract
Extended Abstract Background: One of the major goals of watershed management operations is to reduce soil erosion and waste, and in the next step, to prevent the exit of eroded particles from the watershed. For this purpose, sediment control structures are usually built throughout watersheds. Check dams also play a special and important role in reducing the sediment load of rivers. Sediment dams are constructed to prevent the entry of sediment particles caused by the erosion of upstream lands into the main river. Particle size distribution is one of the key physical characteristics of sediment materials that is used as an important factor in sediment management in watersheds. Many quantitative and qualitative characteristics of sediments, such as porosity, permeability, transferability, chemical reactivity, and erodibility, are influenced by the size of particles and their mode of distribution. To achieve better results in watersheds and especially sediment trapping behind check dams, knowing the type of sediment and its characteristics helps in the better management of watersheds and water and soil resources. Comparing check dams with other water and soil conservation measures to reduce sedimentation, it seems that check dams are the most effective way to quickly reduce the entry of coarse sediments into rivers. Considering the role of sediment traps in trapping sediment particles, it is very important to study the sediments behind these dams in terms of particle size distribution characteristics. This research aims to investigate the size distribution of sediment particles behind check dams and to investigate the efficiency of sediment trapping dams in trapping sediments in the Lashkaran Salmas watershed. Methods: The Lashkaran watershed is located in the northwest of Salmas city in West Azerbaijan province. According to the country division, it is located in the Urmia Lake River basin, and its geographical coordinates are in the range of 44°38' to 44°40' east longitude and 38°17' to 38°18 'north latitude, with an area of of 363 hectares. First, the location and dimensions of the structures and the height of the accumulated sediment behind the structures were examined and measured through field monitoring. To determine the size distribution of sediment particles, the sediments behind each check dam were sampled at two depths of 0-25 and 25-50 cm. In total, 32 sediment samples were collected from eight dams. The samples were transferred to the laboratory, air dried, and then passed through a 2 mm sieve. The particle size distribution of sediment samples was determined by the hydrometric method. To quantitatively investigate sediment particle size distribution, an optimal model was selected from several particle size distribution models, and particle size distribution indices, such as D50, were calculated with the optimal model. Results: In the studied watershed, check dams have been built on the streams with three orders, including order one, three, and four. Seven dams have been built on the third-order streams, one dam on the fourth-order stream, and one dam on the first-order stream. The results show that the particle size distribution does not follow a specific and regular pattern in order three streams, and the average percentages of sand, silt, and clay are 65.7%, 28.2%, and 6.1%, respectively. In order four streams, the average percentages of sand,silt, and clay are 80.4%, 16.5%, and 3.2%, respectively. The respective percentages are 82.7%, 9.8%, and 7.6% in the first order. In general, the average percentage of sand particles, followed by silt, is the largest volume of particles making up sediments. The sediment size distribution of the third-order steams shows that the amount of changes in sediment particles does not have a regular trend, but in general, the percentages of clay and silt increase with increasing distance from the outlet of the watershed, with a decrease in the amount of sand particles. The fourth-order stream has high flow intensity and assumedly carries larger particles. Along with approaching the outlet, the amount of coarse particles decreases due to the presence of check dams and the accumulation of sediments behind the dams. As the distance from the outlet increases, the amount of clay and silt tends to decrease while sand particles tend to increase. The first-order stream has the lowest flow intensity. The amount of clay is constant with increasing distance, with increased silt and decreased sand percentages. Three efficiency coefficients were used to assess the efficiency of the models. The results of the sediment granulation distribution showed that check dams were most efficient in trapping sediments such as sand with a share of 69.5%. The results of examining the efficiency coefficients of particle size distribution models show that the Fredlund model with the highest coefficient of determination (0.98), the lowest RMSE index (0.03), and Akaike's statistic (59.54) is the most efficient model in measuring the distribution of sediment particle size compared to the other seven models. Conclusion: The average particles of sand, silt, and clay (69.5%, 24.7%, and 5.8%, respectively) indicate the performance of check dams in controlling coarse-grained sediments such as sand. The results show that the dams upstream the watershed have restrained larger particles, but this trend is not seen in other check dams. It seems that the type and sequence of the dam are more effective in sediment changes. Moreover, the runoff carries fine to coarse particles, and regular diameter differentiation cannot be obtained due to the short distance between the dams.
