اثر جانمایی سد و قفل کشتیرانی مارد بر هیدرولیک جریان و رسوب‌گذاری رودخانه کارون

Background and objectives: Hydraulic control structures constructed in tidal rivers can modify the balance among flow energy, sediment transport capacity, and bed stability. These modifications are more complex than in ordinary fluvial systems because flow direction, water level, flow reversal, salinity intrusion, and suspended-sediment concentration vary over time and affect erosion and deposition zones. The Mard River, located in southwest Khuzestan Province, Iran, is connected to the Karun and Bahmanshir river systems and is important for water supply, salinity control, agriculture, industrial water use, and navigation in the Abadan-Khorramshahr region. A navigation lock is a hydraulic structure that allows vessels to pass between two different water levels through a controlled chamber. In such a system, lock operation and the presence of a hydraulic obstruction can change local flow velocity, water residence time, and sediment transport conditions. Therefore, the siting of the Mard Dam and navigation lock may reduce flow velocity, increase residence time, decrease sediment transport capacity, and intensify deposition in the navigation channel. The main novelty of this study is the integrated evaluation of dam and navigation lock siting effects on flow hydraulics, fine-sediment behavior, bed variation, and effective navigation depth in a tidal river using the CCHE2D numerical model and scenario-based analysis. The objective of this study was to numerically assess the effect of Mard Dam and navigation lock siting on the flow field, bed variation, and sedimentation pattern of the Mard River.
Materials and methods: Geometric, hydraulic, and sediment data were collected and processed for numerical modeling. The geometric data included river alignment, thalweg, longitudinal profiles, cross sections, and the computational channel domain. Hydraulic data included discharge, water level, upstream and downstream boundary conditions, and Manning's roughness coefficient. Sediment data included suspended-sediment information, sediment-rating data, bed-material gradation, and sediment characteristics at the Mard Dam and navigation lock station. Part of the information was extracted from reports and datasets provided by the Khuzestan Water and Power Authority and from the thesis data, while complementary spatial information was obtained through field surveys. The suspended-load rating curve of the Mard Dam and navigation lock station was extracted from the available data of the Khuzestan Water and Power Authority for the 2019-2020 period and was introduced into the model as the sediment boundary condition. No new sediment-rating curve was fitted or generated; instead, institutional data were used to define the incoming sediment load. After data preparation, the river geometry was reconstructed and meshed in the CCHE-Mesh environment. A structured, non-uniform grid adapted to the river geometry was generated, with a denser grid near the proposed structure because of expected velocity changes, local flow concentration, and possible sedimentation or erosion. The CCHE2D numerical model was used to simulate flow hydraulics and sediment processes. This depth-averaged two-dimensional model solves the governing flow equations in plan view and provides spatial outputs for velocity, depth, water-surface elevation, bed shear stress, and bed variation. HEC-RAS was also used in selected sediment scenarios to compare transport functions, including Ackers-White, Engelund-Hansen, Laursen-Copeland, Meyer-Peter and Muller, Toffaleti, Yang, and Wilcock. Three scenarios were defined. The first examined the mesh and basic hydraulic parameters. The second evaluated the effect of siting on sediment transport and bed variation. The third compared transport functions and assessed sedimentation behavior. Flow discharge was not simulated as an independent variable; rather, it was introduced as the upstream boundary condition, and the modeling focused on reproducing the flow field and analyzing its influence on sedimentation.
Results: The modeling results showed that the siting of the Mard Dam and navigation lock directly affects velocity distribution, water-surface elevation, bed elevation, and sediment-transport capacity. Spatial model outputs indicated that bed elevation and water-surface changes were not uniform along the modeled reach; therefore, different parts of the river show different hydraulic and sediment responses. The velocity-output map showed that flow velocity varied from very low values to approximately 3.77 m/s. Zones with higher velocities indicate greater sediment-transport capacity and a lower probability of deposition, whereas low-velocity zones may act as sedimentation areas. The spatial comparison of bed elevation, water-surface elevation, and velocity maps showed that the lowest-velocity zones do not necessarily coincide with the zones of maximum bed-elevation change. Thus, sedimentation in the Mard River should be assessed by integrating several hydraulic and sediment-related outputs rather than using a single variable. The bed-material gradation analysis indicated that the dominant bed material of the Mard River consists of very fine sand and silt. This correction is scientifically important because fine sediments behave differently from coarse materials and are strongly affected by reductions in flow velocity, decreases in bed shear stress, and increases in water residence time. Under such conditions, low-velocity zones created or intensified by the hydraulic structure can reduce the transport capacity of the flow and promote deposition of fine sediment within the channel. The longitudinal channel profiles also indicated that bed variation is spatially non-uniform, and local bed changes may reduce the effective navigation depth. A quantitative comparison between periodic hydrographic data and sediment-modeling results showed that the hydrographic channel depth was 30.5 m, whereas the sediment-modeling result was 28.5 m. The absolute difference was 2.0 m, corresponding to a relative reduction of approximately 6.56% in effective channel depth. The scenario comparison further showed that modifying or reducing the hydraulic obstruction effect of the dam could increase mean flow velocity by about 50% and decrease sedimentation potential by about 40%. These values were calculated as relative indices based on the reprocessing of scenario results and represent the comparison between the base condition and the modified condition in terms of the hydraulic effect of the structure. In addition, the comparison of sediment-transport functions indicated that the Engelund-Hansen function had the closest agreement with the measured bed profile at the Mard Dam and navigation lock section.
Conclusion: The findings demonstrate that sedimentation in the Mard River is not only the result of sediment supply or natural river behavior; it is strongly controlled by the siting of the dam and navigation lock and by the hydraulic influence of the structure on the flow field. Reduced flow velocity decreases sediment-transport capacity and increases the probability of fine bed-material deposition. Conversely, improving the hydraulic condition and reducing the obstruction effect can activate the flow field, increase sediment transport capacity, and reduce effective channel-depth loss. Considering that the dominant bed material consists of very fine sand and silt, sediment management in this reach should focus on controlling low-velocity zones, monitoring bed-elevation changes, evaluating flow residence time, and planning targeted dredging operations. The quantified results, including a 6.56% reduction in modeled effective channel depth, a 50% increase in relative flow-velocity index, and a 40% decrease in relative sedimentation-potential index, indicate that sedimentation should be assessed during the planning and siting phase. Therefore, relying solely on dredging is not sufficient. A more effective strategy should combine appropriate siting, hydraulic-condition control, periodic monitoring of bed elevation, analysis of bed-material gradation, water-level management, and targeted dredging plans. Overall, two-dimensional numerical modeling combined with siting scenarios and sediment-transport-function comparison provides an effective decision-support tool for reducing uncertainty in the design and management of river control structures, maintaining navigable depth, and improving the sustainable operation of tidal river systems in southwest Iran.