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Sari Agricultural Sciences and Natural resources University
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Introduction and purpose: Rivers are complex systems in which all kinds of chemical, biological and physical processes have taken place and are changing under the influence of various factors and variables in terms of dimensions, shape, direction and pattern. The changes that occur in the conditions of the rivers have many effects on the river ecosystem. Carrying out any activity in rivers requires knowing the rules governing the river and predicting the river's reaction to it in order to avoid the related harmful consequences. It is usually difficult to understand the processes of rivers by measuring hydraulic parameters on a real scale. On the other hand, sediment transport modeling is also a very complex and difficult matter, because the information that is used to predict bed changes is basically uncertain and the theories used are also experimental and highly sensitive to a wide range of physical variables. They show themselves. The high costs of laboratory equipment and the limitation of the use of measuring devices are among other reasons that limit the use of physical methods and lead experts to mathematical and numerical modeling to simulate the flow inside water channels. Continuous change is one of the governing principles of every river, the change in flow conditions also causes change and displacement in other geometric characteristics of the river. Due to the fact that rivers are often moving in their alluvial beds, due to the shear stress in the bed, different types of bed forms have been formed in the river bed. The formed shapes cause a part of the surface water flow in the river to enter the porous environment below it and return to the surface water flow after oxygenating and feeding the benthic organisms. This type of currents that arise from the mixing of surface current and subsurface current in the porous environment under and around the river is called hyperic current. The surface, subsurface, and underground water system and exchanges between them are in three levels: point, interval and watershed. Fallen tree trunks are common structures in rivers. One of the factors of creating hyperic exchange is the presence of pressure gradient at the border of surface flow and porous medium. The pressure gradient is caused by various factors such as obstacles in the flow path or bed forms. Depending on the magnitude of these factors, it will affect the amount of exchange and the depth of the hyperic expansion. The first step in understanding the phenomenon of hyperic and its application is to examine the changes in the characteristics of this area, including the amount of current exchange, depth and retention time. Therefore, the objectives of this research are: to investigate the effect of natural obstacles created by tree trunks on hyperic characteristics and the effect of the arrangement of natural obstacles created by tree trunks on hyperic characteristics.
Material and Methods: The current field research was carried out in the summer and winter seasons of 1400 in Garambadesht river of Gorgan to investigate the effect of fallen tree trunks on the river path as a natural flow barrier in different conditions of tree trunk thickness (thickness 30-60-90 cm). Garmabadesht River, as one of the most important sources of drinking water for the city of Gorgan, originates from the slopes of Yazdaki Mountain at a point 27 km southeast of Gorgan and continues to flow north. Then it passes through the high and complex heights and enters the eastern plains of Gorgan. In order to carry out the present research, piezometers were installed in the upstream and downstream of the tree trunks, and then evaluated using a numerical model in the Comsol software environment, compared to the simulation of the hyperic flow to estimate the amount of exchange flow.
Findings: This study obtained convincing findings regarding the correlation between piezometer observational data and numerical simulation results. Investigations showed that there is a 91% correlation between piezometer observation data and simulation results. Based on this, the computational exchange flows from the numerical model were investigated. The findings showed that the amount of exchanged flow in blocked conditions is higher than in non-blocked conditions. This issue shows that tree trunks can have a significant impact on the dynamics of hyperic flow, its important consequence is the direct impact on river ecosystems, especially in relation to the preservation of coastal vegetation and aquatic habitats. Also, the investigation of the retention time of the flow lines in three obstacle states shows that the increase of the obstacle in the flow path has resulted in the increase of the retention time, because with the increase in the height of the obstacle, the flow lines have become deeper and their length has increased, hence the time Durability has also increased.
Conclusion: The results indicated that the maximum amount of equilibrium discharge occurs in the case where the thickness of the tree trunk is 30 cm in winter. Also, the amount of exchange flow with obstruction is higher than the amount of exchange flow without obstruction. The equilibrium flow rate in winter is higher than the exchange flow rate in summer. Also, the investigation of the penetration of flow lines shows that with the increase in the thickness of the barrier, the penetration rate of flow lines has increased. Considering the vastness of the research field, it is appropriate to conduct more research to discover more understanding of its mechanism.
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Type of Study: Research | Subject: حفاظت آب و خاک
Received: 2023/09/24 | Revised: 2023/12/22 | Accepted: 2023/12/23

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