نوع مقاله : مقاله پژوهشی
نویسندگان
1 گروه سازههای آبی، دانشکده مهندسی آب و محیط زیست، دانشگاه شهید چمران اهواز، اهواز، ایران.
2 گروه سازه های آبی، دانشکده مهندسی آب و محیط زیست، دانشگاه شهید چمران اهواز، اهواز، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
During flood events, a river naturally transports significant amounts of sediment from its upstream watershed. Due to the reduction in flow velocity in calm and deep areas such as dam reservoirs, coarser sediments carried by the flow tend to settle in a zone known as the plunge area of the turbidity current. The Dez Dam, located in the northern part of Khuzestan Province, plays a crucial role in providing drinking water, irrigation, and flood control. Investigating the turbidity plunge zone can yield valuable insights to support the management of sediment-laden flow discharge through bottom outlets and sediment bypass tunnels. In this study, the movement and behavior of muddy turbidity currents within the Dez Dam reservoir were simulated using the FLOW-3D computational fluid dynamics (CFD) code. The RNG k-ε turbulence model was employed to simulate field-scale conditions. Numerical modeling results indicate that for flood discharges exceeding 300 m3/s and reservoir water levels above 320 meters above sea level, the turbidity current plunging depth is formed at a distance ranging from 20 to 27 kilometers upstream of the dam structure. A comparison between the estimated plunging depths of the turbidity currents and previously developed empirical formulas reveals a direct relationship with flood discharge and an inverse relationship with the sediment concentration entering the reservoir. The model validation results demonstrated a strong agreement between the simulated plunging depths and those predicted by empirical relationships, with a determination coefficient (R²) of 0.938, confirming the model's high accuracy.
کلیدواژهها [English]
EXTENDED ABSTRACT
Flood events that deliver large volumes of sediment into dam reservoirs often generate high-density turbidity currents. These flows, upon entering the reservoir, interact with the less dense ambient water and move as underflows. Depending on flow conditions, they can travel considerable distances and reach the dam body, leading to significant sediment accumulation and a reduction in storage capacity. In Iran, the annual sedimentation rate is estimated between 0.5% and 0.75% of the initial reservoir volume, equivalent to approximately 175–250 million cubic meters (Meskar & Fazloula, 2013). These currents also negatively affect hydropower intake structures and outlet systems (Chamoun et al., 2016).Turbidity currents submerge beneath clear water when buoyancy forces surpass inertial forces, creating a plunging point and corresponding plunging depth (Schuch et al., 2021). Several studies have focused on modeling these flows. Kostaschuk et al. (2018) and Howlett et al. (2019) used DNS and Flow-3D to simulate velocity profiles and instabilities. Zayeri & Ghomeshi (2019) successfully modeled turbidity currents in Dez Dam, showing strong agreement with field data. Goodarzi et al. (2020) explored the effects of topography and inlet elevation, while Sun et al. (2023) examined how tributary inflows influence flow dispersion and plunging location.
Due to challenges in measuring turbidity currents, especially in real reservoir conditions, numerical modeling is essential. This study aims to assess the capability of Flow-3D in three-dimensional simulation of turbidity currents and estimation of plunging depth in Dez Dam. The results are compared with empirical formulations to validate model accuracy.
This study focuses on the Dez Dam reservoir, located at the confluence of the Bakhtiari and Sezar Rivers in the Tele Zang region, where the Dez River originates. The research utilizes FLOW-3D, a powerful computational fluid dynamics (CFD) software, to numerically simulate turbidity currents using the governing flow equations. The software is well-suited for modeling open channel flows.
Reservoir bed geometry and its variations were defined using hydrographic maps prepared by the Khuzestan Water and Power Authority in 2007. These maps, containing X, Y, and Z coordinates, were provided in CAD format. ArcGIS software was employed to develop a digital elevation model (DEM) and prepare the flow domain.The computational domain was structured using nine blocks, with a vertical grid spacing of 2 meters (∆Z = 2 m) and a horizontal mesh resolution of 15 meters (∆x = ∆y = 15 m). This setup enabled the application of the finite volume method for solving the flow equations.
A major flood occurred on January 14, 2007, introducing significant volumes of water and sediment into the Dez reservoir. After reaching the dam body, sediments were discharged through the bottom outlets. Consequently, data from January 13, 2007, were used as boundary conditions and for model validation of the turbidity current simulation.
In this study, following the validation of field data downstream of the plunging zone, the steady-state plunging depth was calculated, as illustrated in Figure 12. Subsequently, key flow parameters such as flow depth, velocity, and the densimetric Froude number were determined. A comparison of all results from the numerical model regarding the formation of turbidity current plunging with satellite imagery (Figure 1) indicates that for flood events exceeding 300 m³/s and a reservoir water level above 320 meters above sea level, the plunging zone typically forms between stations G and F.
It was also observed that for high sediment concentrations exceeding 400 mg/L, empirical criteria derived from laboratory data were not sufficiently accurate in predicting plunging depth in the Dez Dam reservoir. Therefore, the Fan (1960) criterion presented in this study is recommended as a more suitable and reliable approach for estimating the plunging depth of high-concentration turbidity currents in this specific context.
The main conclusion of this study demonstrates that the three-dimensional numerical model effectively simulates the dynamics of turbidity current propagation within the Dez Dam reservoir. The simulated plunging depth of the turbidity currents was validated using experimental data from previous turbidity flow studies, which encompass a wide range of sediment concentrations from low to high. This validation yielded a high degree of correlation, confirming the model’s reliability.
The results indicate that the plunging depth of turbidity currents is directly proportional to the 2/3 power of the unit discharge and inversely proportional to the 1/3 power of the sediment volumetric concentration (Cs). These validated outcomes further support the use of the Fan (1960) criterion as an effective predictive tool for estimating the plunging depth of sediment-laden flows in the Dez Dam reservoir.
For this research article, the individual contributions are as follows: Conceptualization, [Author A] and [Author B] and [Author C] ; methodology, [Author B]; software, [Author C]; validation, [Author A], [Author B], and [Author C]; formal analysis, [Author A]; investigation, [Author B]; resources, [Author A]; data curation, [Author B]; writing—original draft preparation, [Author C]; writing—review and editing, [Author B]; visualization, [Author A]; supervision, [Author B]; project administration, [Author C]; funding acquisition, [Author A]. All authors have read and agreed to the published version of the manuscript.
The data that support the findings of this study are available. For further inquiries regarding the data, please contact author’s email.
The authors are grateful for the financial support of the Research Council of Shahid Chamran University of Ahvaz (GN: SCU.WH1403.43525).
The authors avoided data fabrication, falsification, plagiarism, and misconduct.
The author declares no conflict of interest.
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