Investigation of hydromorphological changes of Karaj River due to the implementation of water resources development and river engineering projects

Introduction:
One of the most important topics in river engineering is the study of the morphological condition of the river, which deals with the expression of the geometric shape, bed form, longitudinal profile of the canal, cross sections, deformation and displacement of the river over time. In fact, rivers and streams are a completely dynamic system, and their position, shape, and other morphological features are constantly changing over time (Rangzan et al., 2008).
So far, several studies have been conducted to evaluate the effects of river engineering and water resources engineering projects on the hydrodynamic status of the river (e.g., Yamani et al., 2007; Arshad et al., 2008; Vaezipour et al., 2020; Asghari Saraskanroud, 2013; etc.). The aim of this study is to evaluate these changes according to the implementation of various projects in recent years in the Karaj River that have imposed significant morphological changes on the river system and will change its behavior and response in the future.

Methodology:
In this study, possible morphological changes in the Karaj riverbed due to Karaj river canalization projects, construction of Alborz recreational lake and artificial recharge have been investigated. The models used in this research include one and two dimensional HEC-RAS flow models (4.1 & 5), HEC-RAS sediment model and RVR Meander model for morphological simulation. In these modellings, three hydraulic scenarios were performed to investigate the flow status in the initial state (without designs), within the channelization design and within the artificial recharge project, and modeling of bed changes (sediment) and morphology were performed separately.

Results and Discussion:
Scenario 1- According to the results obtained from the implementation of the two-dimensional model, the areas located on the right bank of the river, especially the area located in Chamran Park, as well as residential areas located at the beginning of the study reach due to being located in the river area are significantly flooded. One of the main reasons for this is the low altitude of the areas located on the right bank of the river.
Scenario 2 - In this scenario, the hydraulic flow is evaluated during the implementation of the Karaj River channelization and Alborz Lake projects. The results of two-dimensional modeling in the area of the main outlet indicate that the velocity of the current passing through this area is extremely high. According to calculations, the values of velocity along the longitudinal section of the channel vary between 2.5 to 4 meters per second, which is a significant number. Such velocity values can lead to erosion of the riverbed and destruction of the riverbed at the outlet of the waterway.
Scenario 3- In this scenario, the effect of constructing lakes of artificial recharge plan of Shahriar plain on the flow pattern and sedimentation process of Karaj river has been evaluated. According to the results, the proposed structures increase the water level, reduce the velocity and provide suitable conditions for water infiltration in the riverbed. Turkey nest (dam in the earth) reduce the velocity of the flow and penetrate more and more into the ground, but it is necessary to mention that due to the significant sedimentation of the river and the presence of sand factories upstream, a lot of fine sediments (caused by washing materials in the mines) will enter the lake of each of these structures.

Conclusions:
The most important results can be summarized as follows:
• Backflow upstream of Alborz Lake due to insufficient dimensions of the lake intake and the constriction caused by the implementation of the channelization project, will cause significant energy losses at the beginning of the project and this will cause water return and flood spread upstream.
• The occurrence of turbulence and rotational flows due to improper angle of connection between the channel and the freeway bridges in the future will have a significant impact on the hydraulic flow and sediment of the river, especially in the area of the bridges.
• Turkey nest structures reduce the velocity of the flow and penetrate the ground as much as possible, but it is necessary to mention that due to the significant sedimentation of the river and the existence of sand mines in the upstream areas, a lot of fine sediments enter the lakes and it will reduce their permeability and their performance.
• Construction of wide structures with only one outflow weir will cause the formation of passive areas in the reservoirs. The existence of such areas, in addition to providing suitable conditions for sedimentation, will also create a wetland state.
• Due to the high velocity of the outflow from the lake, the riverbed has suffered significant erosion and this erosion is due to the concentration of flow in the bed and the left bank of the river and will greatly increase the possibility of bank failures.
• Sediment modeling showed that in a relatively short period of time, relatively large sediments occurred upstream of the Turkey nests. The results show well that the reservoirs are almost full of sediment to overflow crest height, and in practice the discussion of aquifer recharging will face a major challenge.
• Based on the morphological model outputs, the main axis of the river, especially downstream of the Karaj-Tehran freeway bridge, has undergone many changes over time, and this issue poses a major threat to the left bank of the river.
• Based on the future morphological simulation of the Karak River, the presence of the Karaj-Tehran metro bridge in the outer arch of the river will cause the river to flow to the right bank in the long run and this will cause the destruction of the right bank wall and this area will need to be protected.