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Numerical Modelling of Sediment Transport in Reservoirs using OpenFOAM Software – Case Study: Dez Reservoir

Aghajani, Amirsalar | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58075 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ghaemian, Mohsen; Shamsai, Abolfazl
  7. Abstract:
  8. The construction of dams at river mouths disrupts the balance between erosion and sedimentation rates, leading to sediment accumulation in dam reservoirs. Sediment accumulation behind dams reduces the reservoir's effective storage capacity, decreases water inflow to power plants, and endangers power generation facilities when sediment levels reach the dam's intake structures. This research investigates and numerically models sediment transport in the Dez Dam reservoir using OpenFOAM software. With a height of 203 meters and crest width of 212 meters, the Dez Dam is Iran's most important dam. For this modeling, the sediDriftFoam solver - an Eulerian approach solver that calculates transported sediment as concentration - was employed. Additionally, the study modeled the arrival time of density currents at the dam body using the driftFluxFoam solver. The research further examined and measured the dimensions of the scour cone formed by pressurized sediment flushing. Two models were created: one with small elements and another with larger elements of the dam reservoir. Due to high computational demands, the initial model could only simulate the upstream section (40 km from the dam). Results showed 12% sediment concentration in this area, aligning well with field measurements (17% concentration). The secondary model, with lower computational requirements, successfully simulated the entire reservoir from its commissioning in 1963 to 2002. Results revealed sediment concentrations of 20%, 50%, and 60% near the dam body in 1972, 1997, and 2002 respectively. Corresponding sediment depths were 36m, 90m, and 108m, resulting in sediment elevations of 206m, 260m, and 278m respectively. Given that the reservoir's irrigation outlet gates are at 222.7m elevation and pressurized flushing began in the mid-1990s, a scour cone formed near the dam body and around the outlets. Its dimensions were calculated for years when sediment levels exceeded 222.7m: in 1997, the cone measured 97m long and 52m deep, expanding to 111m long and 61m deep by 2002. The flushing process effectively dredged sediments near the dam, preventing sediment from reaching intake structures. The flushed sediment volumes in these years were 293,780 m³ and 452,996 m³ respectively - negligible compared to the annual 19 million tons of incoming sediment. Density current modeling showed that during early reservoir operation, such currents took 10 hours to reach the dam, reducing to 8 hours by 1997. By 2002, the reservoir volume had decreased from 3.3 billion m³ to 1.96 billion m³, and power generation capacity had declined due to reduced water inflow
  9. Keywords:
  10. Sediment Transport ; OpenFOAM Software ; Eulerian Method ; Pressured Flushing ; DriftFluxFoam Solver ; SediDriftFoam Solver ; Sediment Concentration ; Dez Dam Reservoir ; Scour Cone

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