عنوان مقاله [English]
Changes in the morphology of the riverbed at the confluence are a function of parameters such as the amount and direction of velocity, discharge and fluxes and the angle of collision of two different rivers, which is easier to measure during floods due to sudden changes in water flow and sediment. . Research focuses more on laboratory studies, while on land it may also be influenced by the morphological shape of two rivers. However, the normal flow of the river, especially with water exploitation, makes it difficult to observe bed changes. Occurrence of floods in the western region of Iran in 1398 will allow the study of a confluence of 110-degree meander to Meander of Gamasiab and Qarasu rivers.
The Gamasiab River flows in northwestern Iran in a northeast-southwest direction, near the Harsin in a mountainous region, in a northwest-southeast direction. The Qarasu River joins the Meander Gamassiab from outside Meander as it flows through the arch.
The study on this link is based on the SRH-2D model. For this purpose, first water and sediment data were gotten from the water organization and then sediment samples were taken in the field and the river channel was survey with two-frequency GPS. By examining the sediments in the laboratory and entering all this data into the software, different models were implemented and finally the most accurate model was selected. The results of SRH-2D model were validated by ground controls and comparison of satellite images before and after the flood.
Results and discussion
The spatial variation of the Froude number in the Chanel occur greatly in the first few hours of the flood. The variations disappear at the peak of the flood and the high but subcritical Froude number are set to only a few points, and when the flood subsides, almost uniform and subcritical characteristics usually prevail throughout the width of the Chanel. This situation is also seen in the pattern of shear stress distribution, so that it finds very small spatial fluctuations at the end of the flood. However, the most important thing that happens during the rising and recession of flood in the bed and riparian is the first that deposition occur at the first hours of the confluence point and then in the middle of the Gamasiab,s bed after 12 hours. The deposition is ended at the peak and transfer to the riparian. The most important effects of this sedimentation are bars that have expanded during the flood. Point bars are the most important among of the bars. However, in this place, the development of Meander Gamasiab is limited by Qarahsu river and since the steep slope of the valley does not allow to sedimentation inside the river, deposition is done on all zones of inner arch. Instead, Qarahsu take on this task and place sediment on the inner surface of the arch.
Comparison of the results of this research with the results of the recent research which took place in the same flood valley (Gamasiab-Dinver confluence) with a distance of 37 km above this research shows relatively different results. However, It should probably expect less capacity to make change in morphology in Gamasiab-dinver than the same flood happens in downstream, in Gamasiab-Qarasu, due to less flood volume (700 m3/s at the peak of flood in Gamasiab and 200 m3/s in Dinver) and flood peak time (approximately 100 hours after the onset of flood), but the mode of evolution during the flood is such a way that the role of channel morphology is emerged very important. This needs to focus to following differences:
1- Difference in sedimentation site - in that study, during the same flood, the stagnation area and the meander Gamasiab concave area were among the most active areas in terms of sedimentation, while in this study, the stagnation area did not experience much morphological changes and both the Gamasiab and its sub-branch (Qarasu) convexity area underwent many changes in terms of deposition.
2 - Difference in sedimentation time - in that area sedimentation expands after the peak of floods, while in Gamasiab- Qarahsu is limited after the peak of sedimentation.
3- Difference in erosion development - Erosion in terms of how it spreads in the Gamasiab-Dinever starts from the lower reaches of the main canal and continues retrograde upwards. But in this study, erosion starts from parts of the sub-branches and then, when the flood reaches its peak, the conjunction site becomes the most important place for the production of scour hole and then it develops downstream.
4- Difference in erosion development time - scour hole development timing in Gamasiab - Dinver continues until the peak of flood and at this time, it almost stops, while in Gamasiab Gharasoo, the erosion reaches its maximum development at the hours after the peak of the flood.
At the confluence of mountain rivers, which are environmentally similar to the study area, that is, mainly one meandering branch receives another branch at a large angle, the structure of landforms is formed up until the peak of floods and erosive features develop in the middle of the canals almost some hours after the peak of the flood, while only a few kilometers above, when another confluence with the same river but with a different morphology and topography took place, the type and the form of evolution took place in a different way. The result is that the morphology of the canal produces various variables in terms of the angle of collision of the two branches, the method of collision (meandering or non-meandering), the position of the junction (point of collision of the two meanders), the valley constraint, the slope of the canal sides and previous landforms. They can affect the hydraulic factors of flow and sediment and play important roles in the production of new canal landforms.