| چکیده انگلیسی مقاله |
One of the most important goals in watershed operations is to reduce soil erosion and Soil loss 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 the watersheds. One of the most important measures of watershed management to control erosion and water loss in watersheds are Checkdams that play a special and important role in reducing the sediment load of the river. The purpose of constructing sediment dams is to prevent the entry of sediment particles caused by the erosion of the upstream lands into the main river. Particle size distribution is one of the most important physical characteristics of sediment materials, which is used as an important factor in sediment management in watersheds. Comparing Checkdams with other water and soil conservation measures to reduce sediment, it seems that Checkdams are the most effective way to quickly reduce the entry of coarse sediments into the river. Considering the role of sediment traps in trapping sediment particles, studying the sediments behind these dams is very important in terms of particle size distribution characteristics. The purpose of this research is to investigate the size distribution of sediment particles behind the Checkdams in Lashkaran Salmas watershed.
Materials and methods:
Lashkaran watershed is located in the northwest of Salmas city in West Azarbaijan province. According to country division, it is located in the Urmia Lake River basin and its geographical coordinates in UTM unit are in the range of 468517 to 472227 east longitude and 4237415 to 4239229 north latitudes. The area of this watershred is 363 hectares. First, with field monitoring, the location and dimensions of the structures and the height of the accumulated sediment behind the structures were checked and measured. To determine the size distribution of sediment particles, sampling was done from the sediments behind each Checkdam at two depths of 0 to 25 and 25 to 50 cm. In total, 32 sediment samples were collected from eight dams. The samples were transferred to the laboratory and after air drying, they were passed through a 2 mm sieve. Particle size distribution of sediment samples was done by hydrometric method.
Results and Discussion:
The results show that in order 3 streams, the particle size distribution does not follow a specific and regular pattern, and the average of sand, silt, and clay is 65.7%, 28.2%, and 6.1%, In the order 4 streams, the average sand is 80.4% and silt and clay are 16.5% and 3.2%, respectively. This amount is 82.7%, 9.8% and 7.6% respectively in the first first order.respectively. In general, the average percentage of sand particles, followed by silt, is the largest volume of particles that make up sediments. In the studied watershed, Checkdams have been built on the streams with three order, 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 sediment size distribution of the third order steams showed that the amount of changes in sediment particles does not have a regular trend, but in general, the amount of clay and silt increases with increasing distance from the outlet of the watershed and the amount of sand particles decreases. The fourth order stream has a high flow intensity and it is thought that it carries larger particles. Along the way, approaching the outlet, the amount of coarse particles decreases due to the presence of Checkdams and the accumulation of sediments behind the dams. As the distance from the outlet increases, the amount of clay and silt tends to decrease, and sand tends to increase. The first order stream has the lowest flow intensity. The amount of clay is constant with increasing distance and silt has increased and sand has decreased.
Conclusions:
Considering that the average particles of sand, silt and clay are 69.5%, 24.7% and 5.8%, respectively, it indicates the performance of Checkdams in controlling coarse-grained sediments such as sand. The results show that the dams upstream of the watershed have restrained the larger particles. But this trend is not shown in other Checkdams. It seems that the type of dam and its sequence are more effective in sediment changes. Also, due to the short distance between the dams, the runoff carries fine to coarse particles, and regular diameter differentiation cannot be obtained. |