Investigation of flood effect on bank erosion of side wall between Tehran Kan river and ferbet hole.

River have always played a significant role in human’s life in a way that development of agriculture , desire for industrialization and urbanization could not have been possible without rivers . Since 1950 s the construction of structures, water control and convergence structures, bridges and structures crossing the rivers increased, and as a consequence the flow regime , sediment and the trend of rivers topographic and morphological changes altered greatly . Bank erosion play an important role in the alluvial river and its bed load concentration , while it is also considered on of the human threats in river engineering . Another phenomenon that causes changes in sediment transport rates is flooding, which naturally increases with erosion and increasing flow velocity, erosion power of the river, and in addition, sedimentation rates in lower slope areas will increase. For this purpose, bank erosion of river and hydraulic changes during flood was investigated by using CCHE2D ( National Center for Computational Hydroscience and Engineering, two dimensional ) model.

In this study the morphological change of river’s bed and bank was studied.The study area is about 4 km. The change mentioned are due to flood with 25 years return period which were studied in unsteady flow. Two-dimensional depth averaged CCHE-Mesh software was used for meshing and construction of the study area topography and CCHE2D model was used to simulate hydrodynamic , sediment transport and bank erosion . And CCHE has the ability to simulate one, two and three-dimensional flow in aquatic zones , especially natural rivers in steady and unsteady state . In addition , simulation of flow Hydrodynamics in alluvial streams and rivers , simulation of sediment transport in non-uniform sediment distribution mode , investigation of changes and retreats of riverbanks are among the model capabilities . Furthermore the possibility of the right bank erosion and change in flows direction due to the walls breaking was also studied. In this research, by using the model, the sensitivity of the flow characteristics namely water depth and flow velocity and sediment transport and result of bank erosion , in the River, are investigated in respect to the various Manning's roughness coefficient and three turbulent models i.e. two zero equation eddy viscosity models,depth averaged parabolic model , depth averaged mixing length model and k - e standard model that is based on two equation eddy viscosity models.

The novelty of this study is to investigate the bank erosion and the possibility of failure of the river bank wall in the study area in the flood with a 25 years return period in unsteady state. The results of erosion and sedimentation at different coefficients of manning and three different eddy viscosity models indicate that the overall erosion and sedimentation process is almost same for all of these different conditions, but there is a significant difference in their values and the amount of them . Finally, by comparing the erosion process that occurred in study area, the mixing length eddy viscosity model yielded the closest response, followed by simulations. In unsteady hydrograph of water level there is no significant change in the ratio of change in resistance coefficient . with change of about 50 percent , this coefficient of water level changes only about 10 percent and since the depth of river in the river for 25 returns is about It is one meter,It means that changes of about 10 cm is negligible. The results indicated the erosion of right bank of river after rail way near hole is happen. This erosion may result in the destruction of bank and lead the rivers flow to hole (in 25 years return period flood, for unsteady flow, erosion prevailed over sedimentation). Furthermore the high speed of flow in chute domain result in demolition of it’s structure. In bridge, extreme erosion may cause bridge to collapse. Also considerable erosion happen after Chute, in stilling basin and river bed . The velocity of about 12 m / s in the stilling basin is beyond the tolerance of the concrete structure and is likely to be eroded and destroyed.

Finally, in flood with 25 years return period after erosion and fracture of the coastal wall of River, part of the water flow diverts to the hole, initially causing water to enter in the hole , also cause the flow lines to reduce and this means to increase velocity and more erosion in bed and banks of river. A breakdown point that decreases the water level reduces the flow depth, increases the speed and ultimately increases the erosion and flooding power of the flood stream. In addition to the river bed and banks,the structure such as the chute is also damaged due to the high velocity of the flow when the flood passes over it.