Numerical simulation and Experimental investigation of halite karst dissolution in dam reservoirs

The presence of halite karst systems in dam reservoirs can be one of the crucial sources of pollutants for the water resources. Karstic regions are those with dissolving caves. If dam is constructed in the regions with reactive rocks it is likely to be endangered by karst phenomenon, dissolution and corrosion of current waters. construction of dams and reservoirs in these formations are associated with high risk. Comprehensive and accurate understanding of the salt dissolution processes facilitates the description of salt karst formations dissolution process in dam reservoirs.This research was performed based on three basis: field studies, experimental investigation and numerical modeling. For determination of dissolution coefficient, the physical model of reservoir was built in form of trapezoidal channel and samples of salt rock in the reservoir were used in the physical model of karst formation. This model was implemented in different conditions and the results of dissolution amount and salt concentration were measured for different time scales. At the next stage, the physical model was simulated in numerical model for determining of dissolution coefficient. The numerical model ran with different dissolution coefficients and results were compared with experimental results. With this comparison and adaptation of results, the dissolution rate of halite karst determined and then used in the numerical simulation of main reservoir. With simulation of main reservoir using dissolution coefficient obtained from laboratory study, the effect of salt karst formation on reservoir’s water quality was analyzed. The influences of effective parameters such as dissolution rate, air temperature, discharge and relative humidity on the water quality of dam reservoir were studied along with carrying out a wide range of sensitivity numerical analysis. To this purpose, a versatile finite volume tool ‘MIKE’ was used. Using the experimental study and numerical simulation and implementation of results, the dissolution coefficient of salt karst formation was calculated about 0.2 cm/h. Numerical model results showed that early months of operation were not critical to the project and salt concentration in reservoir was in the range of 0.8 to 0.9 of allowable limit. But considering the changes occuring in reservoir’s conditions, the process of salinity distribution increases and salt concentration increases with time. By comparing the salt concentration in reservoir and in numerical model, it was found that the average error percentage of concentration values was  about 13.65 percent and the numerical model showed the values lower than the reservoir in most times.The sensitivity analysis of parameters affecting the salinity distribution, showed that these parameters have a significant impact on salinity distribution and salt concentration in reservoir. It was found that with 10 percent changes in studied parameters, dissolution rate with 21%, air temperature with18%, discharge with 21.1% and relative humidity with 6.1% caused changes in salt concentration of reservoir. According to the results it is evident that dissolution rate and discharge were the most important factors that influence the behavior of salinity distribution in dam reservoir.