somayeh rahimi; mahmood kashefipoor; mahmoud shafai bajestan; ahmad fathi
Abstract
Introduction: Outer bank region is always exposed to destruction and scour due to secondary flow. Different methods are generally used to protect the outer bank. One of the most common and economical ways is spur dike. As an obstacle in the flow, spur dikes protect the outer bank through decreasing the ...
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Introduction: Outer bank region is always exposed to destruction and scour due to secondary flow. Different methods are generally used to protect the outer bank. One of the most common and economical ways is spur dike. As an obstacle in the flow, spur dikes protect the outer bank through decreasing the velocity and forming vortexes between them and as a result sedimentation along that area. However, the spur dike increases the shear stress and local scour around the spur, especially its nose.. This study investigates the scour hole dimensions around three types of spur dike including permeable, impermeable and bandal-like spur dike which is done as a combination of permeable and impermeable spur dike. Few studies were focused on field applications and laboratory experiments to investigate the practical applicability of the bandal-like structure in natural rivers. Rahman et al. (4) Studied the prediction of main channel degradation and local scour around hydraulic structures (impermeable and bandal-like spur dikes) under non-submerged condition. Teraguchi et al. (9) Investigated the influenced of submerged condition on flow characteristics and sediment transport process caused by bandal-like structures with different spacing and alignment under live-bed scour condition through laboratory experiments.
Materials and Methods: Experiments were carried out in the Physical Modeling Laboratory of Faculty of Water Science Engineering of Shahid Chamran University, Ahwaz. The main channel consisted of a 5m long upstream and a 3m long downstream straight reaches. A 90 degree bend was located between the two straight reach. The channel was of rectangular cross section, having a width of 0.7m and depth of 0.6m, with mild bends (ratio of radius to a width equal to 4) and it was filled with almost uniform sediment with a median particle size of D50=1.5 mm. The discharges were measured using an ultrasonic flow meter, which was installed on the pipe inlet of the flume. The water elevation was regulated using the sliding gate installed at the end of the flume. Plexiglas with a thickness of 0.01m was used for impermeable part of spurs and the permeable part prepared by using steel roll piles with 4mm diameter. The most erodible area along the bend was determined and after installing the spurs, the bed surface was leveled by a plate attached to the carriage mounted on the channel. Then the inlet valve was opened slowly and the gate at the end of the flume was first closed. The discharge increased to a predetermined value so that no scour occurs at the straight reaches of the flume. Each experimental case was carried out for 3 hours under clear-water scour condition. At the end of experiments, water was carefully drained out and measurement of bed topography was done using laser bed profiles.
Results and Discussion: The most erodible area along the bend was determined using the procedure described by the U.S. Army Corps of Engineers and in each experimental case specified the critical spur in terms of the maximum erosion around it that happened at the exit of the bend (sections of 80 to 90 degree of bend) and downstream straight reach in all conditions. The centrifugal force will occur has increased the water depth at the exit of the bend. This increase in flow depth is associated with longitudinal negative pressure gradient due to this maximum velocity occurs at the exit of the bend and by this high velocity flow the shear stress increases. The characteristics of the scour hole have been shown to be affected by Froude number and this parameter has a direct relation to maximum relative scour depth and dimensions of the scour hole. The results showed that by increasing the permeability percentage, the amount of maximum relative scour hole depth, length and width decreased. The amount of relative scour depth in permeable and bandal-like spur dike decreased (62% and 55%), and (87% and 76%) for permeability of 33% and 64%, respectively in comparison with impermeable spur dike.
Conclusions: The effect of hydraulic structures, with emphasis in the bandal-like structures, on the scour hole geometric dimensions were investigated in this study. Five types of structures (impermeable, permeable and bandal-like with a permeability of 33% and 64%) were tested experimentally. It was found that:
The increase of permeability, reduced scour rate significantly, such that, the maximum amount of depth, width and length of scour-hole was related to impermeable spur dike (with permeability of 0.0%) and the minimum amount belonged to the permeable spur dike with 64% permeability. The performance of bandal-like structures considering the erosion process around the structures show close results compared with permeable spur dikes.
Farhang Azarang; Abdolrasoul Telvari; Hossein Sedghi; Mahmoud Shafai Bajestan
Abstract
Introduction: The critical role of the rivers in supplying water for various needs of life has led to engineering identification of the hydraulic regime and flow condition of the rivers. Hydraulic structures such dams have inevitable effects on their downstream that should be well investigated. The ...
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Introduction: The critical role of the rivers in supplying water for various needs of life has led to engineering identification of the hydraulic regime and flow condition of the rivers. Hydraulic structures such dams have inevitable effects on their downstream that should be well investigated. The reservoir dams are the most important hydraulic structures which are the cause of great changes in river flow conditions.
Materials and Methods: In this research, an accurate assessment was performed to study the flow regime of Karkheh river at downstream of Karkheh Reservoir Dam as the largest dam in Middle East. Karkheh River is the third waterful river of Iran after Karun and Dez and the third longest river after the Karun and Sefidrud. The Karkheh Dam is a large reservoir dam built in Iran on the Karkheh River in 2000. The Karkheh Reservoir Dam is on the Karkheh River in the Northwestern Khouzestan Province, the closest city being Andimeshk to the east. The part of Karkheh River, which was studied in this research is located at downstream of Karkheh Reservoir Dam. This interval is approximately 94 km, which is located between PayePol and Abdolkhan hydrometric stations. In this research, 138 cross sections were used along Karkheh River. Distance of cross sections from each other was 680m in average. The efficient model of HEC-RAS has been utilized to simulate the Karkheh flow conditions before and after the reservoir dam construction using of hydrometric stations data included annually and monthly mean discharges, instantaneous maximum discharges, water surface profiles and etc. Three defined discharges had been chosen to simulate the Karkheh River flow; maximum defined discharge, mean defined discharge and minimum defined discharge. For each of these discharges values, HEC-RAS model was implemented as a steady flow of the Karkheh River at river reach of study. Water surface profiles of flow, hydraulic parameters and other results of flow regime in HEC-RAS model were obtained for the conditions before and after the construction of the Karkheh Reservoir Dam and then it was reviewed and analyzed.
Results and Discussion: By exploiting the Karkheh Reservoir Dam, the river flow was changed from the natural condition to the regulatory situation. The results indicate that the river flow was considerably declined because the regulatory effect of the reservoir dam which has contributed to the great alternations at hydraulic parameters of the river. For example, the mean annual discharge of the Karkheh River shows 44pecent reduction during the time period of simulating (after the dam construction in comparison with the natural river flow before construction of reservoir dam) in PayePol hydrometric station. Flow velocity of Karkheh River is influenced by discharge, slope of the river channel and geometry of cross section. By increasing the river flow, the flow velocity has increased and there is a significant difference between pre and post-dam condition at the mean velocity of river flow in different sections. The flow area is directly influenced by river discharge and there is a significant difference in the maximum defined discharge before and after dam construction. The width of water surface is a parameter of the geometric situation of the river cross section that also shows the maximum width of the cross sections, passing discharge through the desired cross section. Since Karkheh River has a relatively large water surface width, it has a high wetted perimeter. For this reason, the Karkheh river hydraulic radius is usually low. The significant reduction of all these quantities is for reduction of flow rate by construction of Karkheh Reservoir Dam. Studying the water surface profiles represents reduction of water level in the longitudinal profile of Karkheh River and water level of hydrometric stations by construction of the Karkheh Reservoir Dam. Also, due to the reduction of the discharge in the downstream of Karkheh Dam, all hydraulic parameters of the river such as flow velocity, flow area, width of surface water, hydraulic depth, shear stress and the hydraulic radius have been changed. In general, it can be concluded that the construction of a large dam such as Karkheh Reservoir Dam has a significant effects on the flow regime conditions at river downstream. Our survey would be helpful for environmental, geological and ecological experiments on effects of dam construction and for engineering next hydraulic structures on such rivers.
Vida Atashi; Mahmood Shafai-Bajestan; Ideh Golrokh
Abstract
Introduction: Flow patterns within the river bend is three dimensional. Occurrence of secondary flow due to centrifugal force and formation of helicoidally vortex in river bend usually causes the outer bank of river erodes whilst the sediment are deposited in inner bend which appears in the form of point ...
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Introduction: Flow patterns within the river bend is three dimensional. Occurrence of secondary flow due to centrifugal force and formation of helicoidally vortex in river bend usually causes the outer bank of river erodes whilst the sediment are deposited in inner bend which appears in the form of point bars. To reduce the river bank scour, many techniques have been developed which may be classified as covering technique and modified flow patterns methods. The W-weir is among such structures. In the present paper, by measuring three components of flow velocity with and without presence of W-weir, variation of flow patterns and shear stress distribution in a 90-degree sharp bend have been investigated. The main purpose of this study is to see the installation of different locations of W-weir in the bend on reduction of outer bank scour. In the present paper, by measuring three components of flow velocity with and without presence of W-weir, variation of flow patterns and shear stress distribution in a 90-degree sharp bend have been investigated. The analyses of data showed more uniform flow upstream of the weir and also revealed that the effect of transverse and centrifugal forces are modified in such a way that the secondary flow is diminished. The results showed that for 30, 60 and 90-degree bends maximum erosion depth in the vicinity of the outer bank with Froude number of 0.206 in comparison with 0.137 has increased up to 84, 90 and 118 % respectively. In both Froude numbers, installation of W-Weir in 30 degree has the most reduction in bed in comparison with 60 and 90 degree.
Materials and Methods: To reach the goal of this study a physical model of 90 degree sharp bend was constructed in the hydraulic lab of Shahid Chamran university of Ahvaz. The ratio of R(radius)/b(flume width) was less than 2 which shows a sharp bend. The W-weir was built with 1mm galvanized steel. Flume bed was covered with sediment of D50=1.5mm. The W-weir was installed at three different locations of 30, 60 and 90 degrees from the bend entrance. Two sets of tests were carried out with and without weir. For each location two different flow discharges (Fr= 0.137 and Fr=0.206) were studied. The flow depth for all tests were kept constant equal to 15cm. At the end of each test the flume was drained and bed topography was recorded using laser meter. Measured bed topographic data were used in SURFER and TECPLOT software to compare the results of the W-weir location
Results and Discussion:
a)W-weir in 60 degree location
b)W-weir in 30 degree location
a)W-weir in 90 degree location
Fig.1 Bed topography after W-weir installed(Q=10l/s)
The results showed that W-weir concentrated the flow toward the flume center thus the bed only will scour at the downstream of W-weir whilst the bed at upstream is neither of weir nor eroded. This is because the flow patterns within the bend has been modified in such a way that diminishes the strength of helicoidally vortex upstream of the weir thus the scour or deposition will not occur. The results of tests with Q=15 l/s also was similar with the exception that in these tests the Froude number is higher and the scour depth downstream of weir is much larger. The results also showed that the scour depth is much higher when the weir is installed at 60 degrees. The scour depth for weir at 90 degree location showed reduction of about 33% and 39% compare to the weir in 30 and 60 degree location respectively.
Conclusion: In this research, by assessing the cross velocities and the scour depth downstream of weir in 90 degree sharp bends and studying the impacts of w-shape weir on those parameters, following results were obtained: The W-weir can modify the flow patterns within the flume bend in which no scour and deposition is observed upstream of the weir. The scour downstream of weir with higher depth closed to the outer bank is observed in all tests. The scour depth is much higher when the weir is installed at 30 degree location whilst is minimum when the weir is installed at 90 degree location.
M. J. Nasr Esfahani; M. Shafai Bajestan
Abstract
Abrupt drop is a sudden lowering of the channel bed which is done for reduction of hydraulic jump length and to ensure that the jump will not be shifted outside the basin for various flow conditions. This type of basin is used in irrigation and drainage networks as an effective measure for dissipation ...
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Abrupt drop is a sudden lowering of the channel bed which is done for reduction of hydraulic jump length and to ensure that the jump will not be shifted outside the basin for various flow conditions. This type of basin is used in irrigation and drainage networks as an effective measure for dissipation of kinetic energy. Since the roughened bed of basin also can reduce the stilling basin length, the use of both abrupt drop on bed and roughness can reduce the jump length even more. Therefore it is the main goal of this study to conduct experimental tests on a large scale model to see how the roughness can affect on jump characteristics. To reach such goal tests conducted on a flume of 80cm wide, 15m long and 60cm high. The flow discharge ranged from 56lit/sec to 230l/sec providing Froude number in the range of 2.5 to 9.0. The results of this study show that the jump length reduces to about 36% of classical jump length. In comparison with jump in smooth bed of abrupt drop, the amount of reduction reaches as high as 53%. The observation al results show that the location in which the jump starts, is almost constant in all tests and the sequent depth ratios have almost the same value as in classical jump.
M. Bahrami Yarahmadi; M. Shafai Bejestan
Abstract
Abstract
In all river engineering projects, determination of flow resistance is essential. Most commonly, the Manning roughness coefficient, , is used to describe the flow resistance or relative roughness of main channel or flood plane. Over the past decades little or no study has been conducted to ...
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Abstract
In all river engineering projects, determination of flow resistance is essential. Most commonly, the Manning roughness coefficient, , is used to describe the flow resistance or relative roughness of main channel or flood plane. Over the past decades little or no study has been conducted to investigate the effects of particle shapes on Manning roughness coefficient. Therefore, it is the purpose of this study to see how and by what extend, the particle shape can affect the Manning’s coefficient. To do so three different sediment shapes (angle rounded particles and crushed particles and spherical artificial particles); each in 11 different sizes were collected and tested under different flow conditions. Tests conducted in a flume of 10 meter long, 25 cm wide, 50 cm height and constant bed slope of 0.0005. The results show that the effect of particle shape in larger particle size is less compare to the smaller particle size. As in sediments with size of 11 mm, values of Manning roughness coefficient ( ) for sediment particles shows great difference and in 21mm and 27 mm sediment particles, the Manning roughness coefficient ( ) in angle rounded and spherical artificial particles are equal nearly as the average difference between Manning's coefficient ( ) of these particles are found to be in the order of +1.65 percent and values of Manning roughness coefficient ( ) for crushed particles in 21mm and 27 mm sediment particles shows little difference with other sediment particles. The average difference between Manning's coefficient ( ) of crushed particles with natural rounded particles are found to be in the order of -6.2 percent.
Keywords: Flow resistance, Relative roughness, Manning roughness coefficient, Sediment particles.
S.M. Seyedian; M. Shafai Bajestan
Abstract
Abstract
Lateral intake is a hydraulic structure which is used for diversion of some portion of water from a river for the purpose of irrigation, storage and industrial. Most of lateral intakes from canal are installed at canal with inclined banks which has not received the attention of the researchers ...
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Abstract
Lateral intake is a hydraulic structure which is used for diversion of some portion of water from a river for the purpose of irrigation, storage and industrial. Most of lateral intakes from canal are installed at canal with inclined banks which has not received the attention of the researchers in the past. Since the flow discharge and sediment which enters the intake canal depends on the flow patterns at the head of the intake and the inclined banks can affect the flow patterns and so the sediments, this study has been conducted. In this study a series of experimental tests are conducted using canal of vertical bank and a series of tests with canal of inclined banks. In all tests the suspended sediment feed with a constant concentration upstream of the intake. Sediments which enter the intake canal were collected after each test and weighted. Using dimensional analysis a general non-dimensional relation was developed. By applying the experimental data it was found that the flow patterns at the upstream of the intake has been modified in such a way that more water from surface layers are diverted. Therefore less suspended sediment enters the intake. Also it was found that in all tests the amount of sediment enters the intake reaches its minimum value at Froude number equal 0.37. In low flow depth because of the effect of bed roughness, the suspended sediment enters the intake is higher compare to the higher flow depth for the same conditions.
Keywords: Lateral intake, Suspended load, Inclined banks, Sediment delivery ratio
S.M. Sajadi; M. Shafai Bejestan; M. Bina
Abstract
Abstract
Settling basins are essential hydraulic structures which have to be designed and constructed at all river water intakes to remove most of suspended sediments which enters the intake by flowing water. There have been many techniques for designing these structures. To simplifying the design procedure, ...
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Abstract
Settling basins are essential hydraulic structures which have to be designed and constructed at all river water intakes to remove most of suspended sediments which enters the intake by flowing water. There have been many techniques for designing these structures. To simplifying the design procedure, some of these techniques have been applied many assumption and some others are so complicated which are difficult to apply or needs many data for design purpose. There are also some applicable methods which have been developed based on experimental or analytical procedures, but their results are not well known. Thus it is the purpose of this paper to compare these available methods. To do so extensive experimental tests were conducted in a flume of m using three different sediment concentrations under three Froude numbers. Field tests also were conducted in an irrigation rectangular concrete canal of 200 meter long, 1 meter wide and depth. Field tests also have been conducted under there different discharges different sediment concentration. Then the experimental data was used to determine the trap efficiency of settling basins by different methods. The predicted values were compared to the measurement values. The result indicate that the method of Swamee and Tyagi over predict the trap efficiency. This method was found to be more sensitive to the input data. Result obtained by the methods of Jin and Kaveshnikov were found to be more close to measure data than Raju and Garde methods. In this study a coefficient of Jin method in irrigation settling basin was determined for best estimating of.
Key words: Trap efficiency, Settling Basin, Suspended Sediment
K. Esmaili; S.M. Kashefipoor; M. Shafai Bejestan
Abstract
Abstract
For investigating the effect of bed form in alluvial channels on the amount of bed load transport, a set of experiments was carried out in a tilting flume under unsteady flow conditions (hydrograph). The produced hydrographs were in triangular form with a maximum discharge of 30 to 45 lit/s/m. ...
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Abstract
For investigating the effect of bed form in alluvial channels on the amount of bed load transport, a set of experiments was carried out in a tilting flume under unsteady flow conditions (hydrograph). The produced hydrographs were in triangular form with a maximum discharge of 30 to 45 lit/s/m. Three size of sediment particles were used with equal to 1.5, 2.1 and 3.0mm, clear water without any feeding was supplied from upstream. To analyze the time varying of bed resistance, a numerical model with initial and boundary conditions was used and its outputs were depth, velocity and discharge. The results showed that the inflow made considerable erosion at the starting time of flowing near the channel inlet, and this is a cause of producing bed form, increasing roughness and bed resistance. However, by the time the erosion trend was declined with increasing discharge. Despite of increasing flow turbulence, Froude number and bed erosion decreased gradually when the discharge approaching to its maximum value increasing flow depth. In this process, because of bed elevation changing along the flume, Froude number at any point is different from the other points and bed particles move toward downstream as a progressive wave. The bed resistance coefficient, n, was increased at the start of inflow time and after a proper time it was again increased, and this matter has very important role in sediment transport for non cohesive beds.
Key words: Movable bed, Bed form, Coefficient roughness, Unsteady flow
K. Esmaili; M. Shafai Bejestan; M. Kashefipoor
Abstract
Abstract
To investigate the effective parameters and simulate the conditions under which sediments are transported in ephemeral rivers and compare it to perennial rivers a series of experiments were performed in a laboratory flume. The hydrodynamic equations were solved for initial and boundary conditions ...
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Abstract
To investigate the effective parameters and simulate the conditions under which sediments are transported in ephemeral rivers and compare it to perennial rivers a series of experiments were performed in a laboratory flume. The hydrodynamic equations were solved for initial and boundary conditions using numerical method. Triangle hydrographs with time duration of 40, 60 and 80 seconds and five different flow rates for each situation were considered. Three slopes of 0.006, 0.014 and 0.018 were selected. A fine and uniform sand with d50 of 1.5, 2.1 and 3 mm from river materials was used as the mobile bed. Clean water (with no sediment) was used. The results of 85 experiments showed that the bed slope was the most important parameter in sediment transportation and it could be used for predicting the sediment transport. The base time of hydrograph was also found to have an effective role in sediment transport, and the results showed that by increasing the base time the transported sediment was decreased. The bed form had a big role on the flow resistance and also affected the sediment transport. The sediment ratio of unsteady to steady flow found in this research was found to be similar with those of Tun Lee.
Key words: Ephemeral rivers, Perennial rivers, Bed load, Sediment
