نشریه تحقیقات مهندسی سازه های آبیاری و زهکشی
سال بیست و یکم شماره 79 (تابستان 1399)

Seepage flow through the base of hydraulic structures such as concrete dams causes different issues in these types of structures. It may cause under-scouring of foundations that are constructed in previous material like loose sands or non-cohesive soils. Moreover, seepage flow creates uplift pressure that may destroy the whole structure by overturning or sliding. Engineers have tried to reduce upward pressure created by seepage flow using different methods and techniques. Normally, applying drainage systems such as well-graded granular material, geo-materials, and artificial filters could control the moving of tiny soil particles from spaces between large particles. However, selecting optimal location of the mentioned elements to control seepage forces as well to fix bed material under the foundation of a hydraulic structure is the main challenge for engineers and designers. Nevertheless, by using appropriate filters and drainage system, moving of bed material can be controled; this method couldn’t control the harmful impacts of seepage flow singly. Since the last decades, researchers have proposed different methods to control uplift forces by using cut-off walls due to increasing seepage passway. This techniques drops the hydraulic gradient of seepage flow. Employing the relief well is another practical method to break high pour water pressure and reduce acting uplift pressure to the hydraulic structures.Conducted studies showed that the closer the drainage systems to the toe of the dam, the lower pressure (Yaghan, 1998). Ahmadi et al, (2010) confirmed that the best location for relief well and the downstream cut-off wall is 0.28L and 0.72L from the toe, respectively in homogenous materials to reduce uplift pressure and seepage flow (L is the length of the dam foundation). The literature review revealed that almost all the research and studies in the field of seepage problem and uplift issues beneath concrete structure have been done under homogenous material, meanwhile, in nature, the base materials are in non-homogeneous conditions especially in the layered form with different permeability and properties. Therefore, in this research, we tried to study the effects of non-homogeneities of foundation materials in the seepage and uplift problem beneath concrete dams.

To study the effects of drainage and cutoff wall application in concrete or diversion dams and to determine their optimum location simultaneously, the hydraulic behavior of cutoff wall with installed drainage system in different depth at the non-homogenous sandy foundation has been studied experimentally and numerically. At the prepared experimental setup seepage rate and piezometric pressure under 4 different hydraulic head have been recorded using by 18 mounted piezometers. To study the various condition of non-homogeneities of foundation material as the ratio of permeability of the upper layer to the lower seep/w, a module of Geostudio as a finite element base mathematical model has been used. Calibration and validation of mathematical model have been conducted based on observed experimental data by modifying the permeability of each layer to reach the same seepage flow rate in both model and experimental setup. The optimal location of cutoff walls also has been determined numerically. To study the under scouring phenomenon beneath the dam, critical seepage gradient has been checked under different heads of water at upstream and downstream of the dam.

The results showed that employing a drainage system within two cutoff wall led to increase in seepage flow and at the same time a significant reduction in uplift force and exit gradient in compare to the ordinary state (without drainage well). As well, increasing the depth of drainage installation makes the growth of seepage flow and dropping in the exit gradient but it does not have any effect on the uplift pressure. Also based on achieved results, in case of implementation of two cutoff walls with drainage, the optimum location for the upper cutoff wall would be exactly at the heel and the down cutoff should be installed at the distance of 0.73 of lengths of foundation from the heel. In current condition, all three parameters as seepage flow, uplift pressure, and exit gradient take in the minimum magnitude.

Assessment of the achieved results about the effects of non-homogenous material of the foundation of the dam in uplift pressure acting to the dam proved that layered strata didn’t have significant effects on the pressure distribution and its magnitude. However, analyzing of flow-networks showed that the equipotential lines lean horizontally in the case of the ratio of permeability of the upper layer to the lower is smaller than one. In fact, in such cases, seepage streams lines tend to pass through the lower layer.

The authors would like to express special gratitude to the vice-chancellor of research and technology of Urmia University for provhding the opportunity and facilities to conduct this research.

Any structure that is in the flow path and establishes a simple, specific and definite flow relationship and depth around it is called a flow controller. Weirs are structures that raise the water level behind them and create control sections and are simple means of measuring discharge. Bazin & Schwalt (1898) were the first to conduct experiments on rectangular broad crested weirs. Since then, many researchers have done a great deal of research on a variety of weirs. Among them we can mention: Woodburn (1932), Tracy(1957), Smith(1959), Abou-seida & Quraishi (1976), Hager&Schwalt (1994), Sargison & Percy (2009),….Due to the importance of flow measurement in open channels, the purpose of this study was to investigate the hydraulic performance of inclined weir, to determine the discharge conveyancecoefficient in this kind of structures in free and submerged flow condition, and to provide relationships to predict the discharge conveyancecoefficient.

Based on Equation 1, we can determine the weir discharge. In this equation Q is the discharge (m3 / s), Cd is the discharge coefficient (no dimension), B is the weir length (m), g is the acceleration (m / s2), and H is the height of the water over the weir (m).Also rewrite from the equation 1 to form 2 by defining it   as the discharge conveyancecoefficient (Mohamed, 2010):In this study, the equation 2 is based on the calculation of the discharge throughput on weirs. The following two equations for free and submerged flow are presented using dimensional analysis.The experiments were conducted in a plexiglass flume (Fig. 1) made of the British ArmField Company with a length of 15 m, a width of 30 cm and a height of 50 cm. A total of 60 experiments (30 free-flow and 30 submerged-flow experiments) were performed on the inclined weir.

A) Free flow condition
In this series of experiments, with increasing H/Lw ratio, the discharge conveynce coefficient of all three sloping weirs increased, but the coefficient of conveynce discharge of rectangular broad crested weir decreased (Fig. 1). This figure shows that among inclined weirs are most sensitive to H/Lw changes.Also comparison of the inclined weir with the rectangular broad crested weir indicates that the rectangular broad crested weir later is less sensitive to H/Lw. Equation 5 was derived to calculate the discharge conveyancecoefficient (Cf) in inclined weirs under free-flow conditions. B) Submerged flow conditionMultiple regression was used to investigate the interaction of the extracted dimensionless parameters on the discharge conveyancecoefficient and to provide a mathematical relation to predict these values. Equation 6 was derived to calculate the Cs coefficient in inclined weirs under submerged flow conditions. Figure 2 shows the computational and observational Cf, Cs in free and submerged flow conditions. The scattering of these points relative to the 45 ° line shows that the correlation coefficient of the experimental and computational values for free and submerged flow are equal to 0.9 and 0.91 respectively.

The results show that:- Increasing in H/Lw ratio leads increases in discharge conveyance coefficient of all three sloping weirs and decreases in rectangular broad crested weir. - For a constant value of the H/Lw ratio, the highest discharge coefficient is related to the upstream and downstream slope weir model (SCW-UD-1) and the lowest discharge coefficient is to the upstream slope weir model (SCW-U-1).-The upstream and downstream slope weir are most sensitive to H/Lw changes and rectangular broad crested weir is less sensitive to changes in H/Lw ratio. - As the H/p < /em> ratio increases, the discharge conveyance coefficient of all three sloping weir models increases.

One of the most extensively investigated phenomena in hydraulic engineering is the hydraulic jump that numerous investigators have studied it over the past century. There are different types of stilling basins, including Standard stilling basin USBR, Stilling basin SAF, Stilling basin with continuous sill and stilling basin with perforated sill noted. Each basin, depending on the intensity of the hydraulic jump, usually needs components to reduce the length of the hydraulic jump as much as possible while shaping it at a specific location. These components include the chute Block, Baffle Piers, and perforated sills that is the subject of this study. These types of dampers are the most common energy dampers in dams and irrigation and drainage networks and generally have high efficiency of over 60% in energy dissipation. The purpose of this study is evaluation of the effect of second perforated sill opening on flow pattern and hydraulic jump characteristics in horizontal stilling basin dissipation of energy and required tail water depth. Also determine the optimal distance of two perforated sills from the beginning of the stilling basin with a fixed height for perforated sill.

Experiments were carried out on one and two perforated sills in the Shahid Chamran University Lab in Ahwaz, in a Plexiglas flume and an iron tank. Experiments were carried out so that, given the minimum and maximum flow rates in the flume, the height of such discharges was marked on a deck mounted next to the storage tank at the beginning of the flume. After determining the optimal distance of a single perforated sill, the two perforated sills were tested in such a way that the second wall was located at distances 10, 20 and 30 centimeters from the first perforated sill. After investigating the results and determining the best perforated sill pair in increasing energy loss and decreasing basin length, the best perforated sill pair with constant distance between them were moved to the jump toe at three distances of 50, 60 and 70 centimeters from the first wall to give the best The distance for a pair of perforated sills are obtained. In all experiments to avoid the splashing conditions, a forced jump was created so that the jump would first be submerged and then a full (non-submerged) jump would be formed by opening the end gate of the flume. Experiments were carried out in the form of 32 tests for different discharges in range of 47.3 to 145.5 lit/s and Froude number in ranges of 3.6 to 11.2.

Maximum energy loss compared to free hydraulic jump increases 10%, i.e. perforated sill energy loss rate reached 83.2% at Froude number 11.2. The length of hydraulic jump in the stilling basin with the single perforated sill is reduced to 2.2 times the secondary depth of the hydraulic jump. In other words, the perforated sill reduces the jump length by 64.5% compared to the free jump. In this study experiments were conducted to evaluate the effects of two perforated sills with circular holes and fixed opening percentage to 50 for the first perforated sill and different opening percentage to (12.5%-25%-50%) for the second perforated sill on flow pattern and hydraulic jump characteristics in horizontal stilling basin. It should be noted that several tests were performed to confirm the perforated sill pair with relative distances 60 and 70 centimeters from the jump toe at the mentioned positions and showed that the perforated sill pair still with relative distances 16.7 and 23.3 had the greatest effect on decreasing the relative length of the jump. Based on the results of experiments, an analytical expression was developed for the prediction of the length of hydraulic jump in the case of two perforated sills. Results of experiments on two perforated sills showed that they can only reduce the length of hydraulic jump to an acceptable level that the distance between they provide the conditions for creating a stable jump and the length of jump do not decrease by reducing the distance between the sills.

Results of experiments show that two perforated sills with opening percentage (50% - 12.5%) increase relative energy loss up to 83.7%. Also the length of hydraulic jump in the stilling basin with two perforated sills with opening percentage 50 is reduced to 2.15 times the secondary depth of the free hydraulic jump. The results showed that two perforated sills with opening percentage of 50 for the first and second perforated sills were selected as the best twin of sills to create the perfect and sustainable jumps. In comparison with the other considered couples causes the less erosion potential in downstream of stilling basins due to transferring of the hydraulic rollers from the bed to the surface of flow.

In the recent decades, Dams construction have been developed due to increasing of population and growing need for water. However, sediment deposition inside the reservoirs has always caused to reduce the efficiency and the life time of the reservoirs. Several methods by which the life enhancement of storage reservoir can be made are: watershed management, dredging, flushing of sediments from reservoir, sediment routing/sluicing, sediment bypassing and density current venting; these methods are used independently or in combination (Breusers et al., 1982). Sediment flushing is a technique whereby previously accumulated and deposited sediments in a reservoir were hydraulically eroded and removed by accelerated flows generated by opening the bottom outlets of the dam. Flushing sediments through a reservoir has been used for a long time and has been practiced successfully and found to be inexpensive in many cases (Atkinson, 1996; Fruchard & Camenen, 2012). However, the engineers are generally interested in implementing the applicable measures for increasing the sediment removal from these reservoirs which encounters the excess deposition problems. In this study, by installing a prismatic structure named "sediment chamber" in front of the bottom outlet, the effects of this structure on increasing the pressurized flushing efficiency were investigated.

The experimental setup consisted of an elevated rectangular tank for the reservoir (1.5 * 1 * 1.2m) and for the sump. The reservoir was 1.5m long, 1 m wide, and 1.2m deep. The diameter of circular orifice of reservoir outlet was D= 5 cm. The bottom outlet center was 30 cm above the reservoir floor. The reservoir drained into the sump through the orifice and the flow was re-circulated from there. The space between the bottom outlet invert and the reservoir bed was filled with sediment. Non-cohesive sediment (sand with median size 0.51 mm) was used. A prismatic structure named the "sediment chamber" in front of the bottom outlet was used to increase the pressurized flushing efficiency. In front and side walls of this structure,  vertical slots with different opening width (b), number and arrangement (position) were considered. Slits were selected in three modes of single, binary and triple with 1.25-7.5 cm openings (b/D=0.25, 0.5, 1, 1.5 and D= the outlet diameter). The tests were conducted under two constant head of 20 and 30 cm above the bottom outlet. The tests were run until the bed topography reached equilibrium state, involving negligible sediment motion within the scour hole. The test duration lasted 1 hour. At the end of each test, the transported sediments were collected and weighed after drying in the oven.

Experimental results showed that in models with two front slot and a total opening of 5 cm (b/D=1), the flushing efficiency showed a 100% increase with increasing of slot distance. Also in models with two symmetrical side slots and a total opening of 5 cm (b/D=1), with going away the slots from the side wall of the sediment tank, the flushing efficiency increased by 50%. In the models with three symmetric slots and a total opening of 5 cm (b/D=1), increasing the number of slots could not increase the flushing efficiency. In the models with two combined slots (a slot in the front wall with a slot in the side wall) with a total openings of 5 cm(b/D=1), the flushing efficiency showed a significant increase, so that the flushing efficiency of this model showed a 50% increase with respect to the models with two symmetrical side slots.

In this study, the effects of a prismatic structure, sediment chamber, in front of the bottom outlet on increasing the pressurized flushing efficiency was investigated. The results showed that the model with two combined slots (combination of a slot in the front wall and a slot in the side wall) with a total openings of 5 cm (b/D=1) had the best performance of sediment flushing and increased the flushing efficiency more than 21 times, compared to the control model.

Unlike industrial facilities, irrigation and drainage infrastructure are both scattered and expensive compared to the revenues of irrigation management organizations. Also, these assets (canals and structures network) have specific hydraulic properties in terms of their function and role. Since irrigation organizations must also consider the commercial aspects of irrigation and drainage services, it is necessary to consider the details of the costs incurred in providing these services. Each service and facility must be managed based on its durability and life cycle to provide the services at the lowest cost. Maintenance extends the life of the equipment and in other words delays depreciation and failing. Maintenance plays an important role in the reliability, quality of production, risk reduction and increased efficiency. In the present situation, the lack of anticipation of structures failing, and its timely maintenance in most irrigation networks, reduces the life of network components, which is a major investment for them. Therefore, in this study, using the existing relationships and criteria and using statistics and probabilities, in addition to assessing the maintenance status of offtake structures in Behbahan irrigation network, the failure pattern of structures is predicted using maintenance scenarios. Then, the research results are presented for timely planning and repairs to reduce the risk of failure and reduce the costs imposed on the studied structures.

In this study, the status of the off take structures along the canal path in the northern Behbahan irrigation network on the right bank of the Shohada diversion dam has been studied. The off take structures in the Behbahan irrigation network are used to adjust the amount of water flowing into the canals to coordinate the water level and the flow rate with the amount of water demand. The network has one main canal (about 20 kilometers long) that supplies water to the second channel, A1 to A12. Many maintenance issues such as failure times are not accurate. Many of the factors in these systems are random. But it is possible to define them as random variables and assign them probabilistic distributions. To improve the maintenance condition, a maintenance program should be used that not only reduces operating and maintenance costs but also reduces the risk of failure. Therefore, three scenarios including low reliability, high reliability, and current maintenance conditions are defined.

Since it is impossible to achieve the conditions of a structure to its initial conditions, it is therefore assumed the target of reliability is 90%. To compare the scenarios in terms of maintenance costs, the cost of each scenario is calculated. The cost of repair in the S1 scenario(a current condition in the present network) is higher than the other two scenarios, with approximately 1.7 times the S2 scenario and 2 times the S3 scenario. Therefore, the north of Behbahan network is not in favor of maintenance. On the other hand, the S3 scenario is better than the S2 scenario because it will reduce costs by 20%. The results show that in order to reduce maintenance costs, structures in an irrigation network must be monitored on a continuous basis and be inspected regularly (not at the time of failure). Maintenance of the Behbahan irrigation network is done only at the time of failure and therefore costs are high to maintain the network. Improving maintenance and planning for monitoring structures in the irrigation network will reduce failure rates, increase reliability, and reduce operating and maintenance costs.

Asset monitoring alerts management to the status of assets and enables it to plan and act on any asset and its role in providing services. Also, asset status information and statistics are used as an indicator for prioritizing costs and investments and scheduling them. In this study, using repair and maintenance statistics of offtake structures in the North of Behbahan irrigation network, their physical status, and probability of failure were evaluated and based on the maintenance scenarios, solutions were provided to maintain and increase performance, reduce costs and increase their reliability. The results show that predicting the failure time of the structures and their timely maintenance not only reduces the failure rate but also reduces maintenance costs. Although periodic maintenance (not only at breakdown time), although it increases the frequency of repairs, due to the low cost of repairs, it not only reduces the cost of the entire network by 20% to 100% but also increases the reliability of the structure's performance. And it reduces its failure rate by 20 to 80 percent.

The simplest form of non-linear weirs, the labyrinth weirs enjoy a high foot width as a disadvantage. A recently welcome type of the weirs, the piano key weir (PKW) possesses smaller foot width thanks to having ramps, overhangs, and parallel sidewalls (Crookstoon et al., 2018). Several investigations were carried out on the performance of a PKW. Surveying on PKW with Wi/Wo=1.2, Anderson and Tullis (2013) got the fact that applying total head (Ht) in the equation of Lamperier (2009) estimates the discharge higher by 10.1% on average and by 10.9% at most. In addition to the study of physical models, the 3D numerical simulation was done on PKW by Lefebvre et al. (2014) and Pralong et al. (2011b) yielding the results conforming with the experimental data. Most of the hitherto conducted researches were focused on RPKWs with few attention paid on the hydraulic of TPKWs. In this research, the impact of increase in the key angle and the weir's height in TPKWs on the total upstream head and on Cd were investigated. Also, the results obtained with reference to this type of weir were compared to the rectangular type as well as the recent works in the literature.

Laboratory Model The experiments were conducted in the flume of hydraulic laboratory of Water and Power Organization of Khuzestan. The flume was of 7m length, 0.6m width, and 0.5m height. Five models were employed for the experiments in the free-flow condition. These include RPKW and TPKW with sidewalls making angles 4º, 8º, 12º, and 14º with the flow direction. Experiment Process Following the installment of one of the models in the flume, the flow was regulated with a certain discharge by means of a pump and a triangular weir. After the flow was stabilized, the water level at the upstream together with the water head on the weir (H) was taken. Then, the total head (Ht) and discharge coefficient were computed through the following: Where, Ht (m) designates the total upstream head at the upstream, H (m) stands for the piezometric head relative to the weir elevation, and Vu (m/s) is the flow velocity at the upstream.

The relation between the ratio Ht/p < /em> and Cd is illustrated in Figure (1) for all the models used in this research. The maximum amount of the coefficient Cd for RPKWs and TPKWs with key angles 4º, 8º, 12º, and 14º were attained, respectively as 0.54, 0.58, 0.56, 0.611, and 0.59. This guarantees that the maximum discharge coefficient belongs to TPKW with sidewall angle 12º. The figure also substantiates that Cd takes less values in RPKWs respective to TPKWs, a fact agreeing with the findings of Safarzadeh and Noroozi (2014). Figure (2) draws the curve of Ht/p < /em> against Cd for three height ratios P/Wu. As observed, for all the ratios, first the Cd value grows and reaches its maximum, and then decreases. The decrease in Cd values occurs also along with an increase in the ratio P/Wu. The differences among the values of Cd in various ratios of P/Wu broaden as the ratio Ht/p < /em> increases. So, there is a reverse relationship between Cd and P/Wu.

In this research, the discharge coefficient and the total heads of trapezoidal and rectangular piano key weirs were studied and compared. Three cycles, total crest length of 1.65 m, the ratio Wi/Wo=1, and three height ratios 0.75, 0.875, and 1 were adopted for the key piano weirs. The sidewall angles of the key for the trapezoidal piano key weirs were selected as 4º, 8º, 12º, and 14º. The most important results obtained were as follows:- In a certain discharge, the total upstream head of TPKW is less than that of the rectangular counterpart.- The discharge coefficient value in TPKW is greater than in RPKW. - The highest discharge coefficient 0.611 corresponded to the 12 º piano key weir. - The Lampiere's equation (2009) provides an acceptable solution for type A and in particular situations. - An increase in P/Wu leads to a decrease in discharge coefficient value.- The maximum discharge capacity of the 12 º TPKW is 2.35 times that of ogee weir, and it is 2.8 times that of the linear sharp-crested weir.

Many of irrigation and drainage systems in the world have poor performance or efficiency. Evaluation of irrigation and drainage systems may help managers to find managerial and technical problems. Some researchers have developed evaluation indicators for quantifying the irrigation systems characteristics. Molden and Gates (1990) extended four water delivery performance indicators based on the adequacy, efficiency, equity and dependability of water delivery. These water delivery performance indicators have been widely used over the world until now. A problem with these indicators was the final judgment about the overall performance of the system when some of these indicators show good and the others show the poor performance of the system. Shahrokhnia and Olyan-Ghiasi (2019) found a solution for this problem extending an overall water delivery performance including the four previous indicators. Water delivery performance in Doroodzan Irrigation Network has been controversy between farmers and managers. In this study the overall water delivery performance indicators were measured based on the previously extended indicators in Doroodzan Irrigation Network and discussed.

For evaluation of water delivery performance in Doroodzan Irrigation Network, the water delivery information in four years during 1993 to 2015 were used. The overall water delivery performance indicators which calculating based on adequacy, efficiency, equity and dependability of water delivery, were measured and evaluated. A new water delivery performance indicator named “summation water delivery performance indicator” was also extended in the present study and used for the evaluation of water delivery in Doroodzan Irrigation Network. The two overall indicators, overall water delivery performance indicator and summation water delivery performance indicator were used and compared for evaluation of some of the other irrigation and drainage schemes over the world.

The average of the four years measured indicators (during 1993 to 2015) for Doroodzan irrigation and drainage network showed that the adequacy of water delivery was about 0.79 which was categorized as poor, also was near the border value 0.80. The efficiency of water delivery was equal to 0.85 which is exactly the border value between fair and good category. The equity and dependability of water delivery were equal to 0.43 and 0.30, respectively, both stood on the poor class of performance. Therefore, the overall water delivery performance indicator and summation water delivery performance were equal to 0.73 and 0.91 which stay on the poor class of performance. It can be concluded that the overall performance of water delivery in Doroodzan irrigation network had been poor during past 22 years period of study. The overall water delivery performance indicator in related four main canals MC, RBPC, LBPC and RBSC were about 0.74, 0.75, 0.67 and 0.73, respectively. The summation water delivery performances of thosefour main canals were 0.95, 1.01, 0.69 and 0.90, respectively. The amount of overall and summation water delivery performance indicators in the main canals of the system showed that the main canals MC, RBPC, LBPC and RBSC also stay on the poor class of performance for the studied time period.

The four major water delivery performance indicators adequacy, efficiency, equity and dependability have been successfully used for the evaluation of irrigation and drainage networks, worldwide. However, they are some separate indicators and shall be used together for the better evaluation of the systems and related judgments. The recently developed indicator (overall water delivery performance indicator) and the new indicator developed in the present study (summation water delivery performance indicator) which are based on combination of adequacy, efficiency, equity and dependability, were used for overall evaluation of Doroodzan irrigation network and some others over the world. Doroodzan irrigation network and the four related main canals stood on poor category of performance over past 22 years period of study. The variation range of summation water delivery performance indicator is zero to 2 which is wider than the variation range of overall water delivery performance indicator (zero to 1). Therefore, the summation water delivery performance indicator can be better than the overall water delivery performance indicator to compare the irrigation systems that have little performance differences. The summation water delivery performance indicator is simpler than the overall water delivery performance indicator in shape and calculation

One of the most extensively investigated phenomena in hydraulic engineering is the hydraulic jump that numerous investigators have studied it over the past century. There are different types of stilling basins, including Standard stilling basin USBR, Stilling basin SAF, Stilling basin with continuous sill and stilling basin with perforated sill noted. Each basin, depending on the intensity of the hydraulic jump, usually needs components to reduce the length of the hydraulic jump as much as possible while shaping it at a specific location. These components include the chute Block, Baffle Piers, and perforated sills that is the subject of this study. These types of dampers are the most common energy dampers in dams and irrigation and drainage networks and generally have high efficiency of over 60% in energy dissipation. The purpose of this study is evaluation of the effect of second perforated sill opening on flow pattern and hydraulic jump characteristics in horizontal stilling basin dissipation of energy and required tail water depth. Also determine the optimal distance of two perforated sills from the beginning of the stilling basin with a fixed height for perforated sill.

Experiments were carried out on one and two perforated sills in the Shahid Chamran University Lab in Ahwaz, in a Plexiglas flume and an iron tank. Experiments were carried out so that, given the minimum and maximum flow rates in the flume, the height of such discharges was marked on a deck mounted next to the storage tank at the beginning of the flume. After determining the optimal distance of a single perforated sill, the two perforated sills were tested in such a way that the second wall was located at distances 10, 20 and 30 centimeters from the first perforated sill. After investigating the results and determining the best perforated sill pair in increasing energy loss and decreasing basin length, the best perforated sill pair with constant distance between them were moved to the jump toe at three distances of 50, 60 and 70 centimeters from the first wall to give the best The distance for a pair of perforated sills are obtained. In all experiments to avoid the splashing conditions, a forced jump was created so that the jump would first be submerged and then a full (non-submerged) jump would be formed by opening the end gate of the flume. Experiments were carried out in the form of 32 tests for different discharges in range of 47.3 to 145.5 lit/s and Froude number in ranges of 3.6 to 11.2.

Maximum energy loss compared to free hydraulic jump increases 10%, i.e. perforated sill energy loss rate reached 83.2% at Froude number 11.2. The length of hydraulic jump in the stilling basin with the single perforated sill is reduced to 2.2 times the secondary depth of the hydraulic jump. In other words, the perforated sill reduces the jump length by 64.5% compared to the free jump. In this study experiments were conducted to evaluate the effects of two perforated sills with circular holes and fixed opening percentage to 50 for the first perforated sill and different opening percentage to (12.5%-25%-50%) for the second perforated sill on flow pattern and hydraulic jump characteristics in horizontal stilling basin. It should be noted that several tests were performed to confirm the perforated sill pair with relative distances 60 and 70 centimeters from the jump toe at the mentioned positions and showed that the perforated sill pair still with relative distances 16.7 and 23.3 had the greatest effect on decreasing the relative length of the jump. Based on the results of experiments, an analytical expression was developed for the prediction of the length of hydraulic jump in the case of two perforated sills. Results of experiments on two perforated sills showed that they can only reduce the length of hydraulic jump to an acceptable level that the distance between they provide the conditions for creating a stable jump and the length of jump do not decrease by reducing the distance between the sills.

Results of experiments show that two perforated sills with opening percentage (50% - 12.5%) increase relative energy loss up to 83.7%. Also the length of hydraulic jump in the stilling basin with two perforated sills with opening percentage 50 is reduced to 2.15 times the secondary depth of the free hydraulic jump. The results showed that two perforated sills with opening percentage of 50 for the first and second perforated sills were selected as the best twin of sills to create the perfect and sustainable jumps. In comparison with the other considered couples causes the less erosion potential in downstream of stilling basins due to transferring of the hydraulic rollers from the bed to the surface of flow.