Investigating and Modeling of Gotvand-Olya Dam Challenge and Management Solutions

Investigation and Simulation of Upper-Gotvand Dam Challenge and Management Solutions


Introduction
Upper Gotvand dam is one of the Iran’s largest dams, has been constructed on Karun River, southwest of the country. This dam has been located in the distance of 380 kilometers from the outfall of Karun River and 12 km northeast of Gotvand city, in Khuzestan Province. The important and well-known challenge about Gotvand, is the existence of massive salt formations (known as Gachsaran formation) in the dam reservoir that have been raised from the beginning of its impoundment. Predicted that with the submergence of these formations, the reservoir water be salted and caused extremely undesirable environmental consequences at the downstream.
In order to attainment of proper operational options, the reservoir salinity stratification studies should be done. By implementation of reservoir water quality management, water can be extracted from different layers and also, the saline layers directed outside the reservoir as a water flow with lower concentrations and the risks of using saline water at downstream, can be prevented. On the other hand reservoir water quality management and assessment of its feasibility, need a comprehensive knowledge of the flow hydrodynamic coupled with salinity stratification and salt accumulation process in the reservoir. Hence, the main objectives of this study, are three-dimensional simulation of hydrodynamics and salinity/temperature stratification of Gotvand dam reservoir and presentation of the management solutions to attain this goal.
This study investigates the feasibility of reservoir quality management, regarding to the present challenge, by three-dimensional simulation of hydrodynamics and salinity of Gotvand dam reservoir and predicts the future situation, also provides management strategies. The main innovations of the presented study, include the three-dimensional simulation of hydrodynamics and salinity of Gotvand dam reservoir, calibration of dissolution rate of salt formations by using of the remarkable measured data at dam reservoir and presentation of management strategies.

Methodology
Hydrodynamic and salinity/thermal stratification in dam reservoir, is a three-dimensional phenomenon, hence for exact calculation of variables at the surface and depth, a three-dimensional model is required. Also, the position of salt formations in the dam reservoir (which have spread on one side of the reservoir) and the need for stratification calculations in depth, necessitate application of a three-dimensional model. In this study, for simulation the mentioned phenomenon, a commercial model which has been developed by the Danish Hydraulic Institute (DHI) was used. MIKE 3 introduced since 1998 and due to its great capabilities in simulation of flow hydrodynamics and water quality, have been used in several studies. Three-dimensional simulation of hydrodynamics and salinity/thermal stratification of the Gotvand dam reservoir needs various data and parameters. It worth to mention that, for provision, verification and preparation of required data, too much time and cost were spent, which is one of the strengths of this study.
In order to generate the horizontal mesh of the solution domain, two types of grids have been used: flexible rectangular and triangular grids. The main path of the river covered by high-density rectangular grids and the reservoir surface by triangular ones. In order to create the vertical mesh of solution domain 46 depth layers were selected, a combination of 40 Z-Level layers and six Sigma layers with equal thickness.
Salinity sources were placed at the exact location of salt formations in the reality, in order to consider dissolution of these formations in the model. The point sources covered the entire surface of salt formations with consideration of the wall slope with a distance of 50 meters and 5 meters height difference.
The most important part of this study is calibration of the dissolution rate of salt formations. The dissolution of salt formations in the dam reservoir is a complicated and nonlinear phenomenon, due to the effect of numerous factors, such as different water levels in the reservoir, reservoir hydrodynamics, salinity gradient and temperature, etc.

Conclusion
Several strategies have been presented to solve the present challenge, including removal of salt formations, preventing the contact of reservoir water with salt formations to reduce the dissolution rate, via some coatings such as geomembran or clay blanket, construction of conveyance pipelines at the reservoir bed, in order to transfer salt to the Persian Gulf and finally the reservoir water quality management.
In this study, the reservoir water quality management, through water extraction from various layers, has been investigated. Due to complex circumstance of the problem, it is difficult to predict the future condition of the dam. Here, a 3D numerical simulation was implemented for accurate prediction of the reservoir water quality conditions with respect to time, in the interaction with salt formations, and salinity /thermal stratification into the dam reservoir obtained. Using the model results, the reservoir can be monitored in terms of salt accumulation, so the quality management strategies can be offered.
Based on the results of dissolution rate calibration, it was found out that the amount of dissolution was not constant during the impoundment and varies according to water extraction method (decreasing and increasing at the rate of inflow and outflow and the outcrop surface of the salt formations). In this study the minimum and maximum value of dissolution rate obtained as 0.5 and 7 centimeters per hour, respectively.
The most effective factor in the variations of dissolution rate is variations in flow hydrodynamics in the dam reservoir, due to its three directional variations in different stages of impoundment, results in various dissolution rates. Also, the results of thermal stratification of the Upper Gotvand dam reservoir indicate that due to extreme variation in temperature in the dam reservoir site and because of highly variable salinity at different depths of the reservoir, there are different patterns of stratification cycle and overturning.
Another important point is that at the time of simulation, with the assumption of equilibrium dissolution rate as 0.5 centimeters per hour and consideration of imported salinity of upstream boundary and salty rivers leading to the reservoir, the salt accumulation amount in the reservoir will be 875 kg/s, which failure in discharge of that, results in a severe salinity gradient and serious challenges for the Upper-Gotvand dam. Discharge of accumulated salt by downstream outflow can prevent from salt accumulation in the reservoir, so the amount of flow discharges and corresponding salinity concentrations have been proposed. However, this act will follow the degradation of downstream water quality and relevant consequences; including the loss of agricultural land efficiency, degradation of water quality at downstream and environmental impacts. As a solution, the discharge of salinity could be conducted in the non-cultivation seasons and according to the associated standards, that need further investigations at downstream.