Simulation of salinity distribution due to the active salt karst in Gotvand Dam using system dynamics and determination of the parameters influencing salinity amount

Abstract

Abstract An important issue in the operation of the Gotvand reservoir is the presence of salt formation in this reservoir. The present study has sought to identify whether it is possible to modify the operation curve of the Gotvand Dam to reduce the destructive effects of the Anbal salt dome and increase the operation of this reservoir. To achieve this goal, the performance of the Gotvand Dam reservoir was modeled from the time it was used (from August 2011 to March 2017), as well as the behavior of active salt karstic activity inside the reservoir with the dynamic system tools. The results showed that during the simulation period, the average amount of corrosion of the salt dome in contact with the reservoir water was 20.1 cm per day. Besides, 43.72 million tons of salt has been accumulated in the reservoir. Analysis of the results showed that 39% of the reservoir salinity is due to the dissolution of the salt formation. The rest of the salinity enters the reservoir with the upstream runoff. Based on the results of sensitivity analysis, changes in water level in the reservoir is the most important factor determining the salinity distribution parameters in the reservoir and the best remaining solution to reduce the effects of the salt dome inside the Gotvand Reservoir is to implement the curve of the optimal use of the reservoir for water released through the Gotvand Dam and the upstream basin of this dam. Keywords: Reservoir Operation, Quality management, Water withdrawal from different layers of the reservoir, Vensim. Introduction Most of the karst areas in Iran are in the Zagros Mountains, which is one of the rainy areas and due to the favorable weather conditions, large karst areas have been formed in these mountains. The Gotvand Dam is one of the embankment dams with clay core, which was built in the Zagros zone, folded with evaporative rocks. The outcrops of Gachsaran Formation in Gotvand dam reservoir, which is located five to nine km upstream of the dam axis and on the left bank of the river, have a large volume of thick and soluble salt strata with many cavities and caves and its surface is in direct contact with groundwater, surface water and dam lake water. Assessment of environmental effects resulting from the construction of Gotvand Dam has shown that the construction and operation of the Gotvand Dam have caused salt drainage into the water, increased salinity of groundwater in the region, dissolution of soluble substances, production of toxic substances and increased pollution of organic matter. The present study sought an answer to the question of whether it is possible to optimize the operation of the Gotvand Dam by modifying the command curve to reduce the destructive effects of salt karst. Materials & Method In this research, simulation of water quality conditions and interaction of dynamic phenomena in the reservoir and how to dissolve the active salt karst in it was done according to the curve of the reservoir operation by the dynamic system method and using Vesim software, in the first 68 months of the reservoir operation period. On the other hand, considering the changes in the volume of reservoir water due to different inlet flow rates upstream and also changes in water velocity in the reservoir, the behavior of the model was compared with the reality using the collected field information of the Gotvand reservoir. As the quality of reservoir water is affected by the type, location and how to operate the reservoir water, a solution to increase the optimal operation of the Gotvand Dam was presented. Discussion of Results & Conclusions As the results show, 43.72 million tons of salt were accumulated in the reservoir during the simulation period. The karst dissolution coefficient was also observed to change from 35.46 to 599.86 kg/s. Besides, the average rate of the salt karst corrosion in the reservoir water was 22.1 cm/day, while the mean discharge rate of the salt (Qsalt) flowing into the dam was calculated to be 3923.94 m3/day. In addition, the best level of water release from the lower gates of the dam to obtain quality water from the reservoir, instead of releasing water from the lower gate level, was estimated to be 147 m.Based on our research findings, the parameters of salt dissolution coefficient and flow rate of water entering the dam reservoir were the most important factors affecting the behavior of salinity distribution in the reservoir. Through the quality management and flexible operation of the dam reservoir and by changing the command curve of the Gotvand dam reservoir (in the form of releasing water from the different levels of the reservoir), the operation rate of the dam reservoir increase by 57% on average, compared to the typical case of releasing water from the lower gate valves level). The results showed that larger variations in the reservoir’s water surface level in a short period of time further increased the salt concentration due to in the increase in the hydraulic gradient of the reservoir, water circulation in karstic pores and cavities, and hence the dissolution of the salt karst. Accordingly, a quite linear relationship exists between the water surface level in the reservoir and the hydraulic gradient. It should be noted that in a real reservoir, in addition to surficial dissolution, other factors such as water penetration into cavities, regional fractures, and slip of salt layers into the reservoir are involved in the introduction of salt into the reservoir water, which are not considered in simulation models. Therefore, the actual value of the salt dissolution coefficient is greater than the dissolution coefficient of the simulation model. However, the main source of salt entering the reservoir is the surficial dissolution due to the contact of the salt layers with the volume of the reservoir formation, which has been observed to occur slowly, continuously and dynamically. Finally, from the SD model results analysis of the Gotvand Dam, it can be understood that as the volume of the reservoir water increases, a greater amount of salt karst is dissolved. In brief, by controlling the amount of water flowing into the Gotvand Dam’s reservoir via controlling the amount of water flowing out of the Dez Dam, which is situated on the upstream side of the Gotvand Dam, and by controlling the amount of water flowing out of the Gotvand Dam’s reservoir, and to put it differently, by implementing the optimum rule curves for the Gotvand and Dez dams, the salinity of the water flowing out of the Gotvand Dam’s reservoir can be considerably reduced.

Keywords

• reservoir operation
• quality management
• water withdrawal from different layers of the reservoir
• vensim