Investigating the effect of Dez dam heightening and Ghadir projects on the reservoir thermal regime and water level variations

The Dez dam, with a volume of 3.3 billion cubic meters and 203 meters height, was built with the aim of generating hydropower, flood control and meeting agricultural demands. With the loss of about 700 million cubic meters of dam reservoir capacity due to sedimentation and approaching the level of sediments to the power plant intake, as well as increasing downstream water demand and reducing the reservoir inflow due to the upstream development projects, the optimal operation of the Dez Dam has faced problems. In order to overcome these problems, a plan to increase the height of the dam has been proposed. On the other hand, the Ghadir water supply project with design discharge of 24 cms for transferring water from the dam reservoir to some cities of Khuzestan province is underway. So far, the effects of these projects on the water level variations and thermal regime changes in the dam reservoir have not been studied.
In this paper, first by using the two-dimensional CE-QUAL-W2 hydrodynamics and water quality model, the sensitivity analysis, model calibration and confirmation for simulation of water level, thermal profile and total dissolved solids was performed. Then, the effect of Ghadir project in water level changes in the wet and dry conditions, before and after dam heightening while reducing and not reducing the reservoir inflow has been studied.
By performing model sensitivity analysis, it was found that the model showed the highest sensitivity respectively to the shading parameter and empirical coefficients a, c and b with the sensitivity index values of 3.25, 2.58, 2.34 and 1.23, respectively. After the sensitivity analysis, the model was calibrated for water level, temperature and total dissolved solids. The results showed that the calibration mean absolute error of water level, temperature and TDS was 9 cm, 0.79 °C and 15 mg/l, respectively. On the other hand, by examining the effect of power plant and Ghadir project on thermal stratification, it was observed that the thermal profiles do not experience a significant change due to the inability of the shear forces to overcome the buoyant forces. Also, by evaluating the effect of Ghadir project while reducing the reservoir inflow on the water level in different hydrological conditions, the positive effect of implementing the Dez Dam heightening plan to reduce the effects of water transfer and the possibility of continuous operation of the dam reservoir, Appeared. In this regard, in the hydrological conditions of 2014, which is an average of the hydrological conditions, the decrease in reservoir inflow and operation of the Ghadir project, leads to decrease in the water level of 10.68 meters and 7.8 meters, respectively. By comparing the water level in the conditions of increasing and not increasing the height of Dez Dam in dry and wet periods, improvement of about 50% and maximum improvement, respectively, in maintaining the minimum operational water level in the conditions of increasing the dam height can be seen. Therefore, in case of non-implementation of the Dez Dam heightening project, operation of Ghadir project to supply drinking water to parts of Khuzestan province will face a serious challenge.