Sediment Particle Size Distribution behind the Check Dams of the Lashkaran Watershed, Salmas, West Azerbaijan

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 4438' to 4440' east longitude and 3817' to 3818 ' 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