Abstract
The Fengman Reservoir in China employs a distinctive approach termed two-dam coexistence to mitigate the risks associated with aging and hazardous dams. This method involves the partial demolition of the old dam and construction of a new dam downstream, which allows the remaining part of the old dam to act as a retaining wall for selective water withdrawal. Through a comprehensive study, long-term water temperature data were analyzed, along with the characteristics of the two-dam coexistence system, to comprehend its effects on thermal stratification in the main basin near the dams and the withdrawal water temperature (WWT) regime. The results revealed that the metalimnion shifted upwards, strengthening stratification, and the temperature gradient expanded from 0–0.9 °C/m to 0–10.5 °C/m compared to that under the one-dam condition. Vertical mixing was reduced, causing the temperature difference between surface and bottom waters to range from 6.8 to 26.8 °C. This enhanced the stratification stability by 1.8 times compared with the one-dam condition, with some annual variability. Thus, the buoyancy frequency decreased accordingly. Furthermore, two-dam coexistence influenced the WWT, resulting in higher WWT during the warming period (averaging 4.3 °C from May to June) and lower WWT during the freeze-up period (averaging 0.1 °C from January to February) than that of the one-dam condition. The most significant temperature difference between the main basin near the old dam and the one near the new dam occurred at the bottom of the reservoir (ranging from 3.5 to 12.0 °C), confirming that the old dam functioned as a retaining wall, allowing the thermal regime to change. Owing to the two-dam coexistence, the withdrawal layer shifted by 20.0 m from near the intake to close to the demolition elevation. These changes in stable stratification and variations in WWT play crucial roles in reservoir management. This study offers practical insights for treating similar aging and hazardous reservoirs and serves as a reference for optimizing water quality management strategies.
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