Journal of Hydraulics
Year:2025 | Volume:20 | Issue:1|Papers Count:8

Introduction: The phenomenon of saltwater advance in coastal aquifers is a global problem. Until now, most of the laboratory studies carried out regarding the advance of sea saltwater have considered the beach boundary as vertical, while the penetration and advance of saltwater from non-vertical beaches to coastal aquifers is a common phenomenon. Laboratory models can simulate the phenomenon of saltwater advancing on sloping beaches. This research expands the past research by building a 3D laboratory model and creating sloping beach conditions. The main purpose of this research is to investigate the phenomenon of saltwater advance on a sloping beach in a laboratory and numerical 3D manner in permanent and non-permanent conditions. An image processing technique was used to analyze the images obtained from the progress of salt water in the laboratory model. Validation of laboratory results was done with SEAWAT model. Methodology: In this research, a laboratory model of a three-dimensional flow tank was built. The flow tank of this model included a central chamber containing the porous medium of the aquifer and two side chambers on the right and left side of the central chamber to adjust the saltwater and freshwater head. According to the purpose of this research, the grid plate on the saltwater side was connected to the bottom of the tank by a hinge to create a sloping beach. To create a porous medium, glass beads with a diameter of 1000 to 1200 microns and a specific weight of 2400 kg/m3 were used (Figure 1). The experiments were conducted in three stages. The permanent condition of the first stage was used as the initial condition for the second stage of the experiment. In the second stage (advance stages), the level of freshwater in the left lateral chamber decreased and in the third stage (recede stages), the level of freshwater was returned to its original state. In this research, an image processing technique was used to identify the behavior of saltwater advance in the porous medium and calculate the characteristics of the saltwater wedge. From all stages of the advance and recede Experiments of the saltwater wedge, images of the flow tank were automatically taken at intervals of 60 seconds. The level curve with saltwater diluted by 50% was considered as the boundary of the mixing zone of salt-freshwater and determined the characteristics of the saltwater wedge. After obtaining the numerical value equivalent to saltwater with a concentration of 50% (Equation 2) and the modified pixel values of each model image (Equation 1), the contour of saltwater with a concentration of 50% was drawn in MATLAB software and the position of the saltwater wedge was determined. SEAWAT finite difference model has been used to simulate the experiments performed in the laboratory model. The conceptual model and boundary conditions used to simulate the numerical model are shown in Figure 3. Results and Discussion: In this research, three indicators of length, height, and area of the saltwater wedge were considered as parameters representing the progress of the saltwater wedge. Pictures of laboratory observations and numerical model results of saltwater wedge advance and recede are shown in Figures 4, 5, and 6. A good match between the laboratory observations and the simulation results of the numerical model is observed. The reduction of the freshwater level at the beginning of the advanced stage of saltwater decreased the interaction forces exerted by the freshwater on the saltwater wedge. As a result, the wedge advanced into the laboratory model until reaching new equilibrium conditions (Figure 5). On the other hand, in the third stage, with the increase in freshwater level, the interaction forces applied by the freshwater on the wedge increased and the wedge was pushed back towards the sea (Figure 6).To further investigate the behavior of the saltwater wedge in the advance and recede stages, the relative displacement parameter of each index was used, which is a dimension less parameter and allows comparison independent of the unit and the total size of the change (Equation 3). Figure 8, shows that the toe length of wedge index reaches permanent conditions earlier in the recede phase of the wedge compared to the advancing time. This phenomenon was reported in experimental and numerical studies by Chang and Clement (2012), Lu and Werner (2013), Robinson et al., (2015), and Rezapour et al., (2018). However, the wedge height index reaches permanent conditions in both the advance and recede stages in an almost equal period. The value of each index over time and the relative changes of each index over time are displayed in a single figure (Figure 9). In the advancing stage, due to the decrease in the height of freshwater, the pressure field of freshwater decreases. Therefore, the salty waters are brought together. Due to the difference in density between saltwater and freshwater, as the depth of the interface increases, its advance rate increases in permanent conditions. As the rate of advance increases, the relative rate of advance decreases. Therefore, the height of the wedge, as the highest point of the interface, starts to move with the highest relative rate and reaches permanent conditions sooner. Accordingly, the toe wedge has the lowest relative displacement rate with the greatest advance.Conclusion: The present study proposes and presents a three-step hybrid method (laboratory model, image processing, and numerical simulation) to analyze flow patterns with variable density in porous media. The height of the wedge reaches permanent conditions in both advance and recede stages in almost equal periods. But the toe length of the wedge reaches permanent conditions much earlier in the recede stages than in the advanced stages. During the intrusion of saltwater, the height of the wedge reaches a permanent condition much earlier than the toe and area of the wedge, but during the recede stages, all three almost stop together. In addition to permanent conditions, the toe length index is also a suitable representative for expressing the amount of saltwater entering the coastal aquifer in transient conditions. Keywords: Seawater, Physical Model, Image Processing Technique, SEAWAT

IntroductionWeirs are usually made of impermeable materials such as concrete. These impermeable structures prevent the longitudinal movement of aquatic organisms and physical and chemical substances in the water, thereby preventing sediment accumulation behind the weir and negative impacts on the water ecosystem (Mohamed, 2010). According to the Francis equation, increasing the discharge coefficient or the effective length of the weir can enhance its hydraulic performance (Crookston, 2012). Various studies have been conducted on impermeable weirs. Using an arc-shaped and zigzag plan can increase the effective length of the weir, thereby improving its performance. The main idea of this research was to investigate the performance of a porous weir with an arc-shaped plan. Firstly, due to its porous nature, the weir increases the discharge coefficient (due to the presence of flow through and over it). Secondly, the arc-shaped plan increases the effective length of the weir, resulting in significant hydraulic performance. The flow through and over the weir and their mutual effects combine flow within an open channel and a porous medium, significantly increasing the subject's complexity. This research aims to experimentally investigate the discharge reduction factor (DRF) in submerged flow conditions in a porous weir with an arc-shaped plan under different hydraulic and geometric conditions. Methodology In the present study, two impermeable weir models of plexiglass were compared with corresponding porous weir models, and four porous weir models were constructed. The stone materials used in the porous weirs included five sizes ranging from approximately 7.13 to 31.75 mm in diameter, which were uniform and sharp-edged. In this research, various weir models were tested at different flow rates, and a total of 1704 experiments were conducted in the submerged flow condition and analyzed. To achieve this, the gate at the end of the flume was used to gradually increase the depth of submergence by reducing the gate opening, and the results were obtained for different submergence depths.As the weir became submerged, the water depth upstream was influenced by the downstream depth. The discharge through the weir in submerged flow conditions was smaller than the discharge in free flow conditions, and it is usually extracted using a discharge reduction factor from the free flow discharge as follows:Q_s=DRF × Q→DRF=Q_s/Q (1)The first step in the dimensional analysis is identifying the potential parameters affecting the hydraulic flow in a submerged arc porous weir. A general relationship consisting of dimensionless parameters has been derived by dimensional analysis and based on previous studies. This study neglected the number of weirs with a flow depth greater than 3 cm, considering the range of hydraulic conditions and flow depth over the weir (Horton, 1907; Novak & Cabelka, 1981). Although the flow conditions in this study were turbulent, the Reynolds number (Re) was used as an effective parameter in the relationship for the discharge reduction factor (DRF) based on the correlation between DRF and Reynolds number and by the studies by Mohamed (2010). Therefore, the final relationship for DRF can be simplified as follows.DRF=f(ξ,L_c/L_Arc,R_e,d_50/P,n) (2)Results and Discussion With an increase in flow rate, the value of the DRF decreases with a steep slope. At lower flow rates, the slope of the DRF is steep and gradually decreases with increasing flow rates. In the submerged condition, the discharge coefficient reduces the free-state coefficient by up to 68 percent. In other words, the discharge coefficient in the submerged condition is 68 percent less than the corresponding free-flow discharge coefficient. By increasing the materials' size, the DRF's value decreases, and the model becomes submerged earlier. Comparing the results of each model with the corresponding solid model, it is evident that impermeable weirs have less sensitivity to lower depths and take longer to become submerged compared to porous weirs. The results indicate that the linear porous weir model becomes submerged later and is less sensitive to lower depths than arc-shaped models. Despite the similar values, comparing arc-shaped models shows that with an increase in arc length, the sensitivity of the weir to submergence decreases.With an increase in flow rate, the DRF increases. With an increase in flow rate, the head over the weir crest increases, meaning the energy over the weir crest has increased. With increased energy over the crest, more energy is required for the weir to become submerged. Therefore, the weir becomes submerged at a greater depth and less sensitive to the submergence depth. The equation derived using the SPSS model has statistical parameter values of R2 = 0.78, RMSE = 0.074, and MAE = 0.068. Furthermore, the equation derived using the GEP model has statistical parameter values of R2 = 0.95, RMSE = 0.035, and MAE = 0.027.Conclusion In comparison with impermeable models, porous models consistently have a lower DRF. In impermeable models, the DRF decreases as the size of the materials increases. As the intensity of flow increases, the reduction in the DRF decreases. The results show that the discharge coefficient in the submerged condition is at most 68% less than the corresponding free condition. The investigation of the effect of arc shape on the DRF showed that the linear porous weir model becomes submerged later compared to models with arc and has less sensitivity to lower depths. The accuracy of the extracted relationship based on the GEP model is more suitable. It predicts about 95% of the data with an error of less than 10%.

Extended Abstract Introduction Various measurement tools are used to monitor water environments such as oceans, seas, rivers, and dam reservoirs. Acoustic tomography is one of the branches of remote sensing, which is a powerful tool for monitoring the characteristics of water resources, such as water temperature and velocity in different layers of water depth (in various water environments such as the ocean, sea, river, and dam reservoir). This technology is used with the aim of obtaining information from a desired area, without any interference in the characteristics of that area. In acoustic tomography systems with at least two devices, the waves are sent through the transmission part on both sides of the water environment. Due to the presence of two important boundaries of the bottom of the bed and the water surface, these waves propagate in the entire water depth. By calculating the travel times of each sound ray sent to the opposite station, the average sound speed is calculated for each path. These calculations are used to understand the temperature and flow rate changes as a function of water depth in a water environment such as a dam reservoir. The basis of the work of layering the water environment in depth to monitor changes in temperature and flow speed is the way sound rays propagate in that water environment. Amplitude-independent ray simulation is performed using conductivity-temperature-depth (CTD) data on the transmission line.Methodology Experiments and measurements were carried out in the reservoir of Latian Dam, which is located on the Jajroud River. In this study, two experiments were conducted with sound frequencies of 10 kHz and 30 kHz. In each experiment, two acoustic tomography stations were deployed in the dam reservoir. The approximate distance between the two stations was 1617 meters. CTD and bathymetry data were collected. The time of data collection was the 22 and 23 of October 2020. No valve of Latian Dam was open during data collection. The sound ray propagation pattern in the dam reservoir can be well approximated by the ray tracing method that only considers sound refraction (Snell's refraction law). Snell's law describes the refraction of sound waves in an environment where the speed of sound in separate horizontal layers varies with depth (reflection of sound rays in an environment with variable speed). Transmission losses and mirror reflections on the water surface and reservoir bed can also be included in this method. For this purpose, we first interpolate and draw the depth measurement data with a distance of 0.1 meters. Then we interpolate the CTD data including depth, temperature and salinity to a distance of 0.1 m. After obtaining the depth, temperature and salinity in every 0.1 meter depth, using the McKenzie relationship, the average depth reference sound speed is obtained. Ray simulation identifies transmitted rays (special rays) that are valuable for peak detection and for solving the tuned inverse problem.Results and Discussion It was done by processing the data obtained from acoustic tomography in the Latian dam reservoir and measuring the ray traveling time from one to two and two to one station. The correlation plot (signal-to-noise-travel time) was plotted for the data sent from the one-to-two and two-to-one stations with a frequency of 10 kHz. Considering the distance of 1617 meters between the two stations and the approximate sound speed of 1470 meters per second in water (considered as an approximation according to CTD data collection), the first peak travel time should occur in the range of 1100 milliseconds. Therefore, the plots were plotted between 1080 milliseconds and 100 milliseconds after that. It was observed that the first peak occurred for the data in the range of 1100 ms.To accurately measure the travel time (arrival time) of the first peak that will be used in the selection of the special ray, the first peak (the largest peak) was identified for each data and the travel time of the first peaks was calculated and plotted against the data acquisition time. Due to the fact that the travel time of the sound ray and the speed of sound in water are related to the temperature gradient the measurement was done with the frequency of 10 kHz and 30 kHz on two different days.400 rays launched from the first station were intercepted. 36 rays reached the second station. The results of the simulation of sound wave propagation patterns were investigated and validated by using CTD and by measuring the travel time of the waves. It can be seen that the difference between the travel time simulated by the ray theory method and the travel time obtained from the acoustic tomography and the calculations of the first peak is in the thousandth of a second, which is very insignificant and shows the accuracy of the simulation.Conclusion The propagation pattern of sound waves in the Latian Dam reservoir, which is considered shallow and fresh water, was intercepted, and as a result, three special rays were identified and validated. The results of this ray tracing were evaluated using the acoustic tomography system. The results showed that this model can simulate different sound paths with different propagation angles as well as the travel time of sound waves. Changing the frequency has no effect on the radiation pattern in the dam reservoir, but it affects the intensity of the signal.As a result, the radiation interception in the dam reservoir depends on the topography of the reservoir, water level, and bed as boundary conditions and thermal stratification and temperature, salinity, and depth. Finally, according to the propagation of rays in the entire depth of the tank, it was observed that the location of the transom should be placed near the water level of the tank, considering the shape of the cross-section of the reservoir.

Introduction: Tailing dams are responsible for collecting tailings from mining operations. Tailing dams are built to conserve water for use in mining and protecting the environment. These structures are often constructed using the tailing. This issue, along with factors such as the relatively long construction period and inadequate design and supervision, has resulted in a high number of tailings dam failures. Tailing dam accidents have occurred frequently in recent years. The current rate of failure of large tailings dams is about 2 to 5 cases a year. In the recent failure tailing dam on 2022, Nov. 7, the tailings escaped through an approx. 150 m wide breach of the eastern wall of the impoundment, which caused the release of 12.8 million m3 of water and tailings (World Information Service on Energy 2022. Tailings Dam Safety. https://www.wise-uranium.org/mdaf.html). Some tailings dams have a water pond near the dam crest and the water and tailings liquefy or not contribute to the tailing dam breach. Other tailings dams have no water pond or the pond is far from dam's crest. Water has no or less contribution to the tailings dam failure and the runout of tailings liquefy or not is significant. (Small et al. (2017)). Methodology: This research explores a local failure dam near the tailings dam abutment with a water pond in the large-scale experimental setup that consists of a tailings pond by 505 cm(length), 310 cm (width) and 64 cm (height). The uniformity and curvature coefficient of tailings are 2.21 and 1.44 and the tailings are not liquefaction. The longitudinal slope of bed is 2%. A gate with a width of 20 cm as a local dam breach is applied near the left abutment of the dam. An ultrasonic equipment measured the time series of water surface elevation. The experimental tests were carried out at different water surface elevations and repeated three times to ensure the validity of the results. The bed topography is measured by the Kinect before and after the dam breach. These bed elevations were performed to calculate the volume of the eroded tailings by using Civil3D 2019 software. Moreover, some videos are provided to recognize the flow pattern and bed erosion.Results and Discussion In all experiments, after the local failure of the tailing dam, a pit is created inside the tailings dam pond near the breach. At the beginning of the experiment, the scour pit created has no effect on the water surface elevation. After that, the pit effect is recognized on it. Then, there is water at the end of the pond just near the dam body. At the end, there is a small water height inside the pit and a hydraulic jump occurs in the scour hole. The grooves are created on the bed and the eroded sediments do not have any ability to move towards the dam breach downstream.The bed erosion pattern and sediment transfer rate were surveyed in water level variations of the tailings dam reservoir during a local failure near the abutment. The largest scour height has occurred near the local failure. Due to the complex nature of the sediment, the topography bed images of the same experiments are different in detail.Conclusion Water and sediment are quickly released from the local tailing dam breach and a scour hole forms in the bed near the local failure. In all experiments, the ratio of the volume of eroded sediment near the local failure to the water volume of the dam pond is about one percent. The variation of the water surface elevation with time is about 0.45. It was observed that the length of the scour hole in the dam body direction is more than the direction perpendicular to the dam body. The ratio of the length to the width of the scour hole is 3 to 3.3, approximately.

IntroductionNowadays, wastewater collection networks play a crucial role in the collection, transportation, and disposal of various types of wastewater, primarily because of their vulnerability to flooding and preventing future health, environmental, and economic problems. In recent decades, researchers have been exploring ways of dealing with the uncertainties associated with floods in these networks. An assessment of resilience and its components is an effective means of reducing vulnerability and increasing sustainability. This article presents an innovative approach to quantifying resilience based on penalty curves. This approach, based on the hydraulic performance curve, allows the user to explore the main components of resilience and better understanding of the performance, strengths and weaknesses of the sewer network during floods, showing the level of resilience closer to reality in the operation phase. Despite the overall damage to the network's performance caused by floods, the resilience assessment indicates that performance can be enhanced during the initial and final hours of floods. Furthermore, considering the importance of surface water networks in increasing resilience, despite the lack of such a network in the case study, Hendijan City, was hypothetically implemented for absorbing runoff. This network resulted in an average increase in resilience of approximately 9.86% and a decrease in resilience loss of approximately 17.3% on average, making it an appropriate urban infrastructure for flash floods.MethodologyTo assess the resilience of the wastewater collection network under the potential impact of floods, a resilience framework is employed. This model consists of three sub-models: hydraulic, hydrological, and resilience. The hydraulic sub-model pertains to the physical characteristics of the sewage network and the amount of generated wastewater. The hydrological sub-model is related to the characteristics of watershed sub-basins, precipitation patterns, and determines the amount of runoff generated by floods. Finally, the resilience sub-model is created using specified relationships, utilizing the output of the hydraulic-hydrological model of the wastewater collection network and hydraulic performance indices based on flow velocity penalty curves and conduit filling percentage. In this framework, these sub-models are integrated to establish a comprehensive evaluation of wastewater collection network resilience to floods.Results and DiscussionThe wastewater collection network has a critical line consisting of 26 sewage conduits, representing the highest combined flow of runoff resulting from both floods and generated wastewater. During a 24-hour simulation, these conduits exhibit the highest percentage of filling (h/d) and velocity (v). With an increase in the rainfall return period in each main scenario, the resilience decreases, and the resilience loss increases. In the 25-year and 50-year rainfall return periods, the reduction in resilience and the increase in resilience loss are less compared to the 5-year and 10-year rainfall return periods. This is because the input floods into the network mainly affect the time required to discharge these runoff flows. In the hydraulic performance curve, the robustness index, rapidity in response, and rapidity in recovery decrease with an increase in the rainfall return. However, the application of a storm water network as a resourcefulness index has a significant effect on improving other indices.ConclusionIn this article, the resilience of the wastewater collection network after the flood simulation was calculated using the Critical Performance Curve. The area under this performance curve indicates the level of resilience. Using the penalty curve method, the network’s performance was determined based on actual conditions, unlike other performance calculation equations that assume the ideal performance level (HPI=1) for the beginning of the simulation period. Therefore, this method resulted in a lower error in resilience calculation and appears to be more realistic. Furthermore, considering the resilience components that each cover a whole of the performance curve, a better understanding and clearer insight into the strengths and weaknesses of the network can be obtained. By identifying the hydraulic performance of the network at each moment, appropriate strategies can be implemented based on available resources and budget to enhance the network resilience. In this article, using a resourcefulness indice, in addition to the direct impact on robustness and rapidity indices, resilience increased by 9.86%, and resilience loss decreased by 17.3%, respectively. It was also observed that by using the penalty curve method, which calculates resilience based on the analysis of wastewater collection network flow, the occurrence of floods will always lead to a reduction in performance and a decrease in resilience throughout the simulation period.

Introduction:In the context of energy dissipation in stepped spillways, various geometric and hydraulic parameters play a role. These include the flow rate, step height, number of steps, inception point of free aeration, and width of the spillway. Researchers have explored different approaches to enhance energy dissipation in stepped spillways. These approaches involve modifying the spillway's geometry and structure, as well as introducing obstacles and roughness on the bottom and edges of the steps. Regarding changes in geometry, researchers have investigated altering the spillway angle, the angle of the bottom of the steps, and the transverse slope of the steps. However, previous research has not specifically investigated the location and shape of obstacles on the steps to assess their impact on wear and flow characteristics. Therefore, further research in this area is necessary. Methodology:In this research, a flume with dimensions of 10 meters in length, 1.2 meters in width, and 1 meter in height was used. The first 3 meters of the flume had a height of 1.2 meters. The maximum flow rate of the flume was 150 liters per second, and the flow rate parameter (dc/h) ranged from 0.37 to 1.06 to cover all three flow regimes in the stepped spillway. Two depth gauges with a measurement accuracy of ±1 mm were used to measure the depth of the spillway downstream and the depth of water upstream at the entrance of the spillway. The spillway had a total height of 87 cm, 8 steps, with a step height of 10.9 cm and a step length of 20.9 cm. The obstacles used in this research included a continuous obstacle with a square cross-section (CO), a right-angled triangle with a chord in the upstream (TU) and downstream (TD), an isosceles triangle (IT), and a combination of a square and a triangular barrier in the upstream direction (MTU) and downstream (MTD). The relative heights (h_o/h) ranged from 0.19 to 0.56, and the relative edge distance (L_o/L) ranged from 0.19 to 0.48. The obstacles were placed on all steps, and in some experiments, obstacles were placed only on step 5.Results and discussionAccording to the results of the flow boundaries, it can be seen that the placement of continuous obstacles with different shapes and heights in different places, due to the variability of the mentioned parameters, causes the displacement of the boundaries of the beginning of the flow. The change of flow boundaries under the influence of obstacles (continuous and discontinuous) at the edge of a stepped spillway was also reported by Kökpinar (2004) and Asghari Pari & Kordnaeij (2020 & 2021). According to the results of the present research, it can be stated that in the examined range, the placement of a continuous barrier with different cross-section shapes with different height and location at the bottom of the step also causes the change of the beginning of the flow boundaries compared to the control state, and in general, the tendency of the flow to expand And the durability is more in the transitional range.The results of energy dissipation for arrangements that have energy dissipation equal to or greater than the control state are given in Figure 1. In all the arrangements of the current research, in the nappe flow regime, the placement of obstacles with different shapes, heights and locations on the bottom of the steps causes an increase in energy dissipation compared to the control condition. In the transitional and skimming flow regimes of all three states, there was no change, increase and decrease in energy dissipation compared to the control state. In the skimming and transitional flow regime, the increase in energy dissipation was not observed only in the combined MTU and MTD arrangements, and in other arrangements, it was different according to the height and location of the obstacle.Conclusion- The placement of a continuous obstacle with different cross-sectional shapes with different heights and locations on the bootom of the step spillway changes the beginning of the flow boundaries compared to the flat step (F.S) and generally increases the tendency of the flow to expand and remain longer in the transition range.- The use of square obstacles (CO) has moved the inception point of free aeration (IP) to the downstream side compared to the flat step (F.S), but the triangular obstacles (TU, TD, IT) have moved the IP unchanged or to the upstream side compared to the F.S.. In the combined triangle and square obstacles (MTU, MTD), the IP has not changed compared to the F.S,. Also, all the arrangements that have been able to move the IP upstream compared to the F.S. have created more energy dissipation in the skimmimg flow regime than the F.S.- For the nappe flow regime, the energy dissipation results show the effectiveness of placing the obstacle with different shapes and heights in different places on the steps, but in the transition and skimming flow regimes, according to the arrangement used, energy dissipation has three modes of no change, increase and Or had a decrease in energy dissipation.- The results of BIV analysis show that according to the shape of the obstacle, the height of the obstacle and the location of the obstacle, the dimensions of the formed areas are different compared to the F.S. At a slope of 1:2, if a continuous barrier is placed on the edge of the step, only a rotating zone. is formed on the step, but with the distance of the barrier from the edge of the step in the range of lo/l 0.38 and 0.48, in addition to the rotating zone, the mixing zone is also formed. Is.

Introduction The construction of dams, in spite of great profits, have consequences too; including disruption of the river's sedimentary balance. This construction has increased the cross-section area available for flow, which causes a decrease in the flow velocity and sediment carrying capacity, which leads to sedimentation in the reservoir. In the reservoir of the dam, Density current control methods can include management (non-structural) and structural methods or a combination of them, which are selected for each dam specifically and according to its existing conditions. The first and most effective measure to control the density current is to control the erosion of the watershed and trap the upstream sediments in order to prevent the production of sediments caused by erosion. An effective method that has been studied by other researchers is the method of controlling the density current using an obstacle. The study of the effect of these obstacles has been carried out in extensive numerical and laboratory researches, Since the methods used in the past studies, such as creating an obstacle in the form of a rod with a high height, a plate blocking the channel in Head of the flow, can increase the possibility of blocking the flow, and then the water quality of the reservoir will be affected, it is very important to use obstacles that reduce the possibility of blocking the flow, and also have easy implemention and be more stable. The purpose of this research is to investigate the effect of obstacles in the form of side plates against the density current and its control.Methodology The experiments were carried out in a flume with a length of 12 meters, a height of 80 cm and a width of 40 cm. The density current used in this research was considered to be a saline flow with a concentration of 20 g/liter. The obstacles were made of glass with a thickness of 4 mm. The depth of the ambient water in the experiments was constant and equal to 50 cm. Also, in all experiments, the inlet flow rate of density current and the slope of the floor were considered constant and horizontal. The height of obstacles (h) used in this research were considered to be 3, 6 and 9 cm. Obstacles were installed as side plates on both sides of the flume with an installation angle (Ѳ) of 30, 60 and 90 degrees to the wall in such a way that their ends were on the middle axis of the flume and the distance between the ends of the obstacles (d) on the flume axis was fixed(10 cm)(fig 2). In the conducted experiments, the velocity of the flow Head and its depth concentration were measured at two depths of 4 and 6 cm, and the body concentration was measured at 7 points in the depth. After taking the samples, their salinity was read using a salinity meter and converted to concentration in grams per liter with the salinity-concentration calibration chart. Concentrated, unobstructed concentrated flow test was used as a control.Results and Discussion Based on the observations, when the density current hits the row of side obstacles, part of the flow continues its path through the obstacles and another part climbs, after descending for a while, it joins the rest of the flow and continues the path. It continues with a lower velocity and concentration, this decrease in concentration occurs with the entry of the surrounding fluid into the density current due to spreading caused by the collision of the density current with obstacles. It can also be seen that the efficiency of the complete blockage mode in controlling the density current is almost equal to the plates with an angle of 60 degrees, with the increase in height, the percentage of density current control has increased, so that at a height of 9 cm, the amount of density current control at an angle of 60 degrees is about 80 is a percentage. Creating obstacles in Head of the density current, in addition to the effect on the forehead, also affects the concentration profile of its body.By increasing the height of the obstacle in all working angles, the reduction of the concentration of the thick flow body is more. In such a way that at a height of 9 cm compared to the state where the flow passes without the presence of an obstacle, a large difference between the concentrations of the body in all four positions of the obstacle is observed.Conclusion In this research, the effect of installing side side plates on the control of the density current Head was investigated. Installation of side plates is effective in controlling it due to increasing the length of Headal contact with the obstacle. It should be noted that in real conditions, the density current is in the form of sediments and creating obstacles in Head of the flow will lead to a decrease in velocity and as a result sedimentation. Therefore, by completely blocking the flow path, the space behind the obstacle will be filled with sediment in a short period of time, and the efficiency of the installed obstacle will be lost in restraining the Head of the density current. But on the side side plates, an empty space is created between the plates, due to the presence of this space, part of the flow passes through there. The results showed that the greatest effect of obstacles on the control of the thick flow Head is in the case of installation angle of 60 degrees, and it is almost equal to the amount of control in the case of complete obstruction. Therefore, installing side plates with an angle can be used as an alternative method. Investigating the effect of installing obstacles on the shape of the concentration profile of the density current body showed that the greatest effect is related to the installation angle of 90 degrees, and with the increase in the height of the obstacle, the concentration of the thick flow body decreased more.

Introduction:During floods in many Rivers, the flow exits its main section and inundates the vicinity floodplains. The hydraulic flow in compound sections differs significantly from single channels. In such conditions, due to the change in flow section shape and the roughness difference between the main channel and floodplains, the flow structure in compound channels becomes highly complex (Yang et al., 2007; 2013). Vegetation in natural rivers is typically classified into rigid (trees) and flexible (grasses) plant. The stems of flexible plants can change shape with the movement of water flow, whereas the trunks of rigid trees remain unchanged against the flow without deformation (Terrier, 2010). The vegetation of floodplain is one of the main components that can influence the velocity distribution in compound channels (samadi Rahim et al., 2021; Hamidifar et al., 2016). this study, has investigated the flow field and the formation of secondary currents resulting from rigid and flexible vegetation in divergence compound channels with a mobile bed.Methodology:In order to investigate the effect of the vegetation of floodplain on the flow structure and the formation of secondary currents, solid cylindrical plastic rods with a diameter (D) of 10 mm were used to model the vegetation of trees and artificial grass with a height of 3 cm as flexible vegetation on They were placed in floodplains with a width of 0.36 cm. The distance between the rows of rods (lx) was considered constant and equal to 75 mm. While in order to change the vegetation density, the transverse distance between two rods (ly) was set at three distances of 50, 75 and 100 mm. In the main channel with a width of 0.24 meters and a height of 0.15 meters, siliceous sediments with a mean diameter (D50) of 1 mm created a mobile bed. In the floodplain, three divergence angles of 3.8, 5.7 and 11.3 degrees were set in such a way that in three relative depths of flow equal to 0.25, 0.35 and 0.45 and three densities of vegetation with the space ratio to 5, 7.5 and 10 were performed in a total of 36 experiments. The components of the local velocity of the flow in the middle section and at the end of the divergence of floodplain were measured by the 3D velocimeter of the Vectrino Profiler.Results and Discussion:In the non-prismatic compound channels with vegetation, the vertical isovelocity in the common area between the main channel and the floodplain indicates a strong velocity gradient in this area and confirms the formation of a free shear layer due to the Kelvin-Helmholtz phenomenon. Due to the presence of grass vegetation in the floodplain bed, the flow in floodplain is divided into two separate regions: the flow within the grass vegetation and the flow top of the grass vegetation. In divergent compound sections without vegetation, the transverse velocity of the flow in the main channel and the floodplain is significant and its positive. In the presence of vegetation, because the crest of the dune has reached the middle range of the divergence reach, the vertical component of the velocity in the main channel has become negative. As the distance from the floodplain bed increases, the drag force caused by the trees dominates and the contribution of the bed roughness in controlling the flow decreases. Therefore, the velocity profile in floodplain does not follow the logarithmic distribution and almost assumes an S shape. Vegetation has caused the formation of very weak secondary currents in the floodplain, especially in the reach of the end of the divergence. Also, the increase in vegetation density has caused the strength of eddies to decrease in the the main channel. The transverse gradient shows that the greatest effect of the secondary currents was in the common area between the main channel and floodplain, so that the increase in vegetation density confirms the formation of a stronger shear layer in this area. Vegetation on the floodplain weakens the secondary currents in this area. These results agree well with the studies of researchers such as Yang et al. (2007), Hamidifar et al. (2016) and Samadi Rahim et al. (2023). Also, the movement of eddies direction is constantly changing and the flow is very complicated. Therefore, local erosion is observed in the bed of the main channel and close to the common side wall, and on the other side, the height of the dune is increased. While in the main channel, there are two eddies in opposite directions. Conclusion:The Kelvin-Helmholtz instability causes the generation of a free shear layer in the intersection area. Also, the formation of points with a velocity close to zero on the floodplains shows the existence of excessive resistance of flexible and rigid vegetation compared to literature. The periodic positive and negative values of the transverse flow velocity in the floodplain range indicate the interference of Von Kàrmàn vortex streets in the space between the two vegetation elements. In the divergent compound channel with the vegetated floodplain, too weak secondary currents are generated, especially in the end of the divergence reach.