The accumulation of sediments in reservoirs of dams has always been one of the problems of reservoir dams. Accordingly, one of the approaches to address this problem can be referred to as the method of pressure flushing. In this method, during the course of flushing, generally the bottom drainage outlets of the dam are opened for a certain period of time and the reservoir water level is kept almost constant. When water discharges through the bottom outlets, and after performing flushing, a hollow or a hole in the form of a funnel or a cone will appear at the front of the outlet gate, that the size of this scour funnel depends on various parameters such as: discharge, depth of water inside the reservoir and the types of sediment deposited inside the reservoir. In this research, a physical model with specific dimensions was used to investigate the effect of cohesive and non-cohesive sediments on the form of flushing funnel. The result of this research shows that in non-cohesive sediment the erosion pattern was retrogressive, and while experiments are done with minimum water depth (40 cm) and maximum discharge (8 lit/s), the flushing funnel will be balanced after 9-11 minutes. of the bottom outlet can be presented in the form of exponential and function of water height and sediment were obtained.

Introduction: The deposition of sediments in reservoirs seems to be one of the fundamental problems in the operation of dams. An operation known as flushing, with two free and pressurized types, is used to discharge these sediments. In the free flushing method, all the water in the reservoir is drained from the bottom outlet and large amounts of the deposited sediments are discharged; however, this method does not perform well in the case of large dams. In the pressurized flushing method, the discharge of sediments is done under constant water height at the upstream of the bottom outlet. The efficiency of the free flushing method is superior to the pressurized flushing method; but it is not much common to be applied due to causing environmental problems resulting from the sudden outflow of large volumes of water and sediments in the downstream and is specifically used only for small reservoirs. Hence, due to its low efficiency, some strategies are needed to increase the efficiency of the pressurized flushing approach. A few researches have been conducted so far on the topic of increasing efficiency. By installing a group of cylindrical piles in the upstream of the orifice, Madadi et al. (2016) increased the efficiency of pressurized flushing by 250% compared to the control test (without piles). Also by installing a semi-cylindrical structure in the upstream of the orifice, Madadi et al. (2017) managed to enhance the efficiency of pressurized flushing by 450% compared to the control test. A new method has been provided in this study to examine the effect of using a square single-pile at the upstream of the orifice on the dimensions and the volume of the flushing cone in the pressurized flushing. Materials and Methods: The experiments were performed in a rectangular flume in the hydraulic research laboratory of the Faculty of Water Sciences and Engineering, Shahid Chamran University of Ahvaz, Iran. Three flow rates (Q) of 4. 17, 6. 39 and 8. 34 l/s were considered for the experiments. In all experiments, the level of sediments (Hs) was set constant at the level of the orifice lower edge. The water level in the flume to the center of the orifice (Hw) was considered to be 52 cm in all experiments. The diameter of the outlet orifice (Do) was also set to be 7 cm. The gradation of the sediments used was also considered fixed in all experiments (d50 = 0. 5mm). We set the experiment time as150 minutes in all cases. We utilized four different sizes of the side (1. 4, 2. 1, 2. 8, and 3. 5 cm or the corresponding ratio Dp/D0 equal to 0. 2, 0. 3, 0. 4, and 0. 5, respectively) aimed at examining the effect of square pile size (Dp) on the dimensions and efficiency of the score cone. We determined the pile placement distance from the orifice (Lp) in such a way to avoid any impact on the water level at the upstream of orifice and the outlet flow rate when discharging and also to be located at the closest distance from the orifice. This distance was calculated to be 4. 9 cm by performing successive experiments. The pile was installed with the highest impact on flushing at different distances from the orifice upstream to examine the effect of pile placement distance (Lp). These distances have defined as a ratio of the orifice diameter equal to 𝐿 𝑝 𝐷 𝑜 ⁄ = 0. 7, 1. 2, 1. 7, 2. 2. Results and Discussion: The control experiments were made in a state of non-installation of the pile at the orifice upstream. Revealed by the results, by increasing the flow rate from 4. 17 l/s to 8. 34 l/s, the volume of the flushing cone has increased by approximately 287%. Also, the length, width, and depth of the flushing cone have increased by 57%, 42% and 53%, respectively. The movement of sediments in the pressurized flushing and their outflow are made due to shear stress along the bed and two clockwise and counterclockwise vortices in the orifice upstream. As the flow rate increases, more sediments removed as a result of more shear stress caused in the bed and the strength of the vortices enhances as well. This leads to the removal and exit of further sediments from the orifice, which will increase the volume and dimensions of the sediment flushing cone. The results of the pile installation experiments demonstrated that the application of the square pile has significantly increased the volume of the flushing cone so that in the case of a flow rate of 4. 17 l/s and a pile installation with a side size of 3. 5 cm at a distance of 4. 9 cm from the orifice upstream, the volume of the flushing cone increased by approximately 362% compared to the control state. Also, in the same case, the depth, length and width of the flushing cone respectively increased by 120%, 57%, and 42% in comparison to the control state. When a pile is placed in the path of the water stream with a sedimentary bed, a series of downward currents are formed known as Horseshoe Vortices due to the collision of the flow lines to the pile upstream side, which causes the hydrodynamic scour phenomenon around the pile. Installing the pile at the orifice upstream causes the erosive sediments caused by the presence of the pile to exit from the orifice in addition to the removal of sediments from the sediment flushing phenomenon, increasing the efficiency of flushing. To explain this phenomenon, we can say that more downward vortices are formed in the pile upstream by increasing the pile dimensions. Also, the separation of the flow lines in this case increases and a low-pressure area is created with a larger area at the pile downstream. Thus, more sediments are detached from the orifice upstream and exit from it. It was found that the highest effect of the square pile dimension on extension of flushing cone is related to the size of 𝐷 𝑃 𝐷 𝑜 = 0. 5 ⁄ . Therefore, in order to investigate the effect of the pile distance from the orifice on flushing cone volume and dimensions, the measured flushing volume and dimensions related to the relative distance of 𝐿 𝑃 𝐷 𝑜 = 0. 7, 1. 2, 1. 7, 2. 2 ⁄ with the abovementioned pile size were only analyzed here. As the pile placement distance from the orifice upstream increases, the volume of the sediment flushing cone decreases; for the furthest distance the effect of pile placement almost vanishes. Conclusion: The study of the effect of installing a single square pile at the upstream of the orifice on the pressurized flushing efficiency indicated that the presence of a single pile can lead to an increase in the sediment flushing efficiency. The greatest impact of the pile placement (𝐿 𝑃 𝐷 𝑜 = 0. 7 ) ⁄ belongs to the largest pile (𝐷 𝑃 𝐷 𝑜 = 0. 5 ⁄ ). In this case, the volume of the flushing cone increased by about 362% compared to the control state. In other words, with the same amount of discharge of water from the orifice in the pile-less state, we can increase the volume of sediments discharge to a considerable extent by installing the pile.

One of the most effective techniques for removing the deposited sediments from reservoirs is pressure flushing which has only much local effects. It is often applied as a clearing process to remove sediment saround the entrance of intakes .In order to make rational design of bottom outlets and other sediment flushing structures, the understanding of the characteristics of a scour funnel in under pressure flushing is significant. The physical model was constructed in the Hydraulic Laboratory at the Water Sciences Engineerin Faculty of Shahid Chamran University of Ahvaz and by performing various experiments on the effect of bottom outlet shape on the volume and dimensions of flushing cone was experimentally investigated. For this purposes, the experiments on 4 bottom outlets with a circular, semicircular, rectangular and square cross-sectional area equal to 18 cm2, five water levels of 30, 45, 55, 65, 78 cm, resulting in five different discharges on the bottom outlet, was performed. The results of this survey revealed that the shape of the lower opening is an important parameter in hydraulic flushing, changing of which results in change of scouring diameter. Results also showed that for a specific water level in the reservoir and for a specific time the scouring dimension for square valves and semicircle valves are more than one of rectangular valves. The scouring dimensions of rectangular valves are more than one of circular valves. The difference is more perceptible in lower water heights (i. e. lower discharge). The length of scouring and the volume of flushing cone increases with incrsing head and is the least for circular valves.

Reduction of reservoir storage capacity due to high rate of sedimentation affects all the purposes of dam operation, such as hydropower energy production and seasonal flood control. Using some sediment management techniques are not economically and technically affordable and one of the main solutions, especially in arid and semi-arid regions, is pressurized flushing that faced with low efficiency as a main challenge. In this research a new structure named DBE was used for enhancing sediment removal efficiency. Therefore DBE structure with four different lengths and four diameters in three discharges mode was investigated. For carrying out the experiments, non-cohesive silica sediment with a median diameter of 𝐷 50=0. 73 𝑚 𝑚 was used and temporal development of sediment flushing cone was investigated. Finally the best dimensions of the structure that leads to creation of the maximum sediment flushing cone dimensions and the minimum scouring equilibrium time were presented. Also, time dependent dimensionless equations for calculating the sediment flushing cone dimensions were presented.

The low inflow rate of water into the dam reservoirs causes sedimentation, leading to a decrease in the service life of dam. As a result, the performance of the dam in controlling floods and generating energy through water release downstream will be affected. Additionally, sediment deposition near the bottom outlets and turbines causes their burial, leading to difficulties in their operation and utilization. Installing inclined plates in the upper of bottom outlets is suggested as a new method to increase the amount of flushed sediments in pressurized flushing. Awareness of changes in the upstream flow pattern of the orifice is of great importance. In the present study, the effect of installing inclined plates on flow pattern changes in the upstream of the orifice was investigated using the Flow3D model. In this study, a numerical model calibration was performed using the results of experiments conducted in the hydraulic laboratory of the Faculty of Water and Environmental Engineering at Shahid Chamran University of Ahvaz, and the RNG turbulence model was chosen for conducting the simulation. The considered variables include plate width, plate installation angle, and plate installation distance to the orifice. In total, 5 scenarios (including the reference test (i.e., without installing plates)) have been defined for the numerical model.The results showed that the installation of inclined plates against the orifice led to the creation of vortexes and the development of a low-pressure zone, resulting in an increase in the volume of flushed sediment. Also, reducing the width of plates, increasing the installation angle, and increasing the installation distance of the plates will lead to a decrease in the intensity of eddies and a decrease in the range of low-pressure zone, which reduces the effect of installing inclined plates to increase flushed sediment volume.Based on previous studies, the amount of sediment output in pressurized flushing is limited and confined to the vicinity of the dam body. Therefore, this method is not used to revive the dead storage capacity of the dam reservoirs. Ancillary facilities such as bottom outlets and hydro-power plant outlets are located near the dam structure, and sediment entry into these facilities can have destructive effects. Therefore, providing methods to increase sediment discharge from near the dam body can be highly beneficial. Additionally, by creating a low-pressure zone in the upstream of the outlet, a suction effect is created, which can be useful in increasing the volume of sediment discharged during turbidity current discharging from the bottom outlet.

Reservoir sedimentation is always known as a serious problem for sustainable use of reservoirs and the main factor of reservoirs storage loss. One of the most common engineering techniques for preserving the reservoirs storage capacity is the periodical desilting of reservoirs by hydraulic flushing. Review of the literature shows that the pressurized flushing operation is currently accomplished with low efficiency. In this paper, the effect of PBC structure on the sediment removal efficiency during pressurized flushing operation was experimentally investigated. In this way, PBC structure with four relative length and four relative diameters were used in the reservoir. Non-cohesive sand particles with D50=0.36 mm were used as the deposited sediments in the reservoir. The results showed that by using the PBC structure with LPBC/Doutlet=5.26 and DPBC/Doutlet=1.32, the flushing efficiency became 4.57 times more than that of reference test. By increasing the relative length of PBC structure, the maximum relative length and width of flushing cone increased, respectively, compared to the reference test while the variation of the maximum relative depth was negligible. In addition, all the geometric parameters of flushing cone had their maximum values at DPBC/Doutlet=1.32.

Pressurized flushing is one of the techniques for evacuating sediments from reservoirs. In this study, the impact of submerged vanes on performance of pressurized flushing were investigated. For this purpose, submerged vanes with convergent, divergent and combined arrangements in three distances from the bottom outlet (𝐿 𝑣 ), three middle distances (𝐿 ℎ 𝑟 ) and three heights above the sediment bed (𝐻 𝑠 𝑣 ) were used and the results were compared with the non structural test (reference test). The results showed that the submerged vanes by creating rotational flow and turbulence, enhanced the performance of flushing and also by evacuating much sediment below the bottom outlet, the amount of evacuated sediments increased in all experiments. As, in the convergent and divergent arrangements, the volume of evacuated sediments increased respectively 6. 5 and 48 times compared to the non structural test. Also, in the combined arrangement with two-row divergent of submerged vanes, in 𝐿 ℎ 𝑟 Do=0. 5, 𝐿 𝑣 Do=0. 3 and 𝐻 𝑠 𝑣 𝐷 𝑜 =1, the volume of flushing cone increased 51 times compared to reference test. Finally, by using a polynomial correlation with vane spacing, a non-dimensional equation for estimating the scour cone volume was proposed.

About 1% of the total storage capacity in the world’s reservoirs is lost annually due to sedimentation. Sediments can also block intakes in reservoirs and damage tunnels or turbines. One of the most effective techniques to remove these sediments is flushing, whereby water level is lowered sufficiently to re-erode deposits and flush them through the intakes. Outflow sediment discharge may well be related to the parameters such as the sediment characteristics in the reservoir, during flushing and geometry of flushing channel. In this study, laboratory experiments were performed on a 1-D reservoir model in a flume in the hydraulics laboratory of Shiraz University to investigate the flushing operation processes by using polymer particles. The polymer particles were lightweight and non-cohesive with an average grain size of about 2.40 mm and density of 1065.3 (kg/m3). The model was installed in a flume; 30 m long, 1 m wide and 0.75 m height. The length of the test section was 11.5 m, and sediments were placed at a length of 4.8 m long upstream from the dam position. Experimental runs have been performed for two flow conditions; 0.0004678 m3/s and 0.000628 m3/s. The very low inflow discharge helped for better monitoring and measuring of the effective parameters. A sluice gate was placed at the central bottom of the dam (as the bottom outlet) and was opened at a constant rate to make the complete drawdown. Results showed that the rate of sediment flushing is strongly associated with outflow rate, water surface gradient with the dam section and the width of the flushing channel. The results from this study were in agreement with that in the literature. It is considered that the low density of the particles causes them to behave as very fine and non-cohesive sediment particles, like loess sediments.

Sediment flushing of reservoirs is an operational technique, whereby previously accumulated sediments in the reservoirs are hydraulically removed by accelerated flow when the bottom de-silting outlets of the dam are opened. In this research, the process of sediment flushing is simulated by a three dimensional numerical model in which sediment and flow interaction are reflected in the reservoirs. Reynolds Averaged Navier-Stokes (RANS) equations are solved numerically by Finite Volume on a three dimensional grid and a standard k-e turbulence model is used. The resulting flow analysis is used as an input data for the sediment model. The convection diffusion equation for the sediment concentration is solved. The concentration equation derived by Van-Rijn is adopted as a boundary condition, resulting in a calculation of bed material load. The depth integrated mass balance equation is applied to find the bed changes. The results from the numerical model are compared favorably with the data from physical model studies available in the literature.

For desilting the deposited sediment, flushing method could be used in two different methods: Free Flushing and Pressure Flushing. In the latter, the previously deposited sediment would be flushed from the reservoir by opening of the outlets, and typically the scour cone "Flushing Cone" developed near the outlet in the reservoir. The scour cone geometry was influenced by many factors including water depth on the bottom outlets, outflow discharge through the bottom outlet, outlet geometry, characteristic of the sediments which deposited in the reservoir, etc. In this study, the effect of the outflow discharge through the bottom outlet and the water depth on the bottom outlets were investigated by non-cohesive sediment with d50=0.27mm. For this purpose, laboratory experiments were performed, so a physical model was constructed in the Hydraulic Laboratory at the faculty of Water Engineering and Science, University of Shahid Chamran, Ahvaz, Iran. The result of this research showed the flushed sediment increased with the decrease of reservoir's water depth, as the reduction of water depth equal to 31.5% caused the scour cone volume and its length to increase to 41.5% and 14.5%. Also the decrease of 76.6%, outflow discharge caused the scour cone volume and its length decreased to 20.3% and 49.9% respectively. Also based on the satirical analysis, non-dimensional relations for determining the flushing cone volume and its length are represented.