جستجوی مقالات مرتبط با کلیدواژه ": آبشستگی" در نشریات گروه "فنی و مهندسی"

There are various methods to prevent river bank erosion, one of them is groin. These structures are used to control the natural movement of the bed and reduce the movement of sediments by reducing the power of the water flow. Groins are divided into permeable and impermeable types, the permeable form of which is used in rivers where the amount of suspended load is high. By reducing the speed of the flow in the groin field, the sedimentary materials are quickly deposited. This process and then creating a thick sedimentary layer, keeping the erosion flow away from the groin area and it provides the stable conditions necessary to protect the banks. In this study, the performance of parallel permeable groin and single permeable groin has been compared. For this purpose, a flume with a length of 5 meters, width of 30 cm and height of 30 cm has been used. Experiments have been conducted with 60 and 40% permeability rates in clear water conditions, and the observations from the experiments have been made in relation to the maximum scour depth and the changes in the topography of the flume bed have been compared.

In a laboratory study, a new arrangement for permeable groin has been presented in which the form and number of groin bars are different compared to the conventional direct form, and the effect of this new arrangement on the maximum depth of the scour hole was investigated at the groin nose. The permeability values of the groin group are considered equal to 40, 60 and 80% and the tests were performed in clear water conditions. The flow rate and bed conditions are considered the same in all tests and the behavior of groin to deal with local scouring of the nose under the change in permeability and the number of bars in the groin group in both radial absorbing and direct forms has been investigated and compared. The results show that among the tests conducted, the maximum depth of the erosion hole decreased with the increase in permeability, and also the performance of the provided radial absorbent arrangement is better and more effective than the direct arrangement in controlling the maximum depth of the erosion hole.

Groins are one of the ways to protect river banks against erosion. These structures prevent their erosion by diverting the flow from the river bank. In this study, a new arrangement of permeable groin is presented, which are called radial groin. In order to investigate the behavior and performance of this type of groin, a flume with a length of 5 meters, width of 30 cm and height of 30 cm was used. The experiments were conducted with a permeability rate of 60% in clear water conditions and in the back to the flow state, and the observations from the experiments were made in relation to the maximum scouring depth and the changes in the topography of the flume bed with straight and parallel arrangement of scours with the same permeability has been compared. The results show that the radial arrangement has performed better than the parallel and direct arrangement and has been more successful in creating scour pits with less depth.

The scouring phenomenon is one of the issues that are always important in the design of various types of hydraulic structures; among these structures is the groin. These structures prevent their erosion by diverting the flow from the river bank. The flow mechanism in rivers at the locations of groin is such that the bed of a river undergoes changes and erosion over time. In this study, the performance of the permeable radial groin with different numbers was investigated. Also this groin was comparison with the double and single direct permeable arrangement. For this purpose, a flume with 5, 0.3, 0.3 meter long, wide and high was used, respectively. The experiments were conducted with a permeability rate of 40% in clear water conditions. The observations from the experiments regarding the maximum scour depth created in the flume bed show that the number of groin in a radial arrangement has an effect on the maximum scour hole depth.

Stilling basins are used in the outlet of channels, chutes and culverts to dissipate the excess kinetic energy of incoming flow. One of the basins in which the energy of incoming flow is dissipated by impact, is USBR VI stilling basin. The USBR VI stilling basin was first introduced by Bradely and Peterka in 1995 and then was modified by Biechley in 1978. This structure consists of a middle wall and an endsill. Scouring around the structures that are located in the vicinity of erodible beds, such as stilling basins, has always been one of the most important problems related to these structures. Unlike other types of stilling basins, the studies carried out around this type of basin are limited, and there are still many hydraulics features of this type that have not been considered in previous researches. In this article, the effect of the shape of endsill on scour depth downstream of stilling basin is evaluated. Based on Beichley graph (Standard Design), the physical model of stilling basin was designed, constructed and installed in the hydraulic laboratory of Tarbiat Modares. Experiments were conducted in a 0.8 m wide, 0.9 m height, and 0.8 m length rectangular channel. The pump used in the experiments had a nominal flow rate of 400 cubic meters per hour (about 120 liters per hour), a head of 11.7 meters, a power of 22 horsepower, and an engine speed of 1450 rpm. In the design of experiments, the parameters including approach Froude number (i.e. 1, 1.5 and 2 times of standard Froude number on Beichley graph), the diameter of inlet pipe (i.e. De = 5, 8, and 12 centimeters), and endsill shapes (triangular, stepped and circular quadrant), in the form of 27 tests were assessed to study the dimensions of the scour depth. The observations revealed that in all three endsill shapes, the increase in Froud number has led to the decrease in scour index. the circular quadrant endsill had the lowest scour depth in the front of endsill and the least scour index, in the range of the Froude number of 2 to 6. In the range of the Froude numbers of 9 to 14, the triangular endsill causes the lowest scour index. In the relative diameter of inlet pipe equals to 10.16, for Froud numbers equal to 9.27 and 13.91, the triangular shaped endsill has the least scour index. in every endsills, decreasing the pipe’s diameter results in the maximum depth of the scour. Another important finding is that sediment bar is only formed in experiments conducted with inlet pipe’s diameter equal to 5cm for Froude number equal to Froude number on Beichley graph. The biggest amounts of the height of sediment bar and maximum scour depth are found for the stepped endsill and the smallest amounts of the height of sediment bar and maximum scour depth are found for the circular quadrant endsill. Subsequently, the non-dimensional equations according to the Froud number of incoming flow and the relative diameter of inlet pipe, were presented to estimate the maximum depth of the scour hole.

Protection of the downstream of culvert outlets against scour process, as a water conveyance structure, is a highly significant issue in design of culverts. In this study, the effect of bed litter length on downstream scour depth is investigated. For this purpose, experiments were carried out on the laminate with different lengths. According to the results, the particle landing at 22%, 33% and 66%, respectively, and the landing at particle landing of 2.5%, respectively. 15%, 20%, and 40% decreased particle depth by 20%, 25%, and 31%, respectively, indicating that bedding has a positive effect on decreasing downstream scour depth. The scour for the flooring considering the length of the layer showed that the scour depth decreases with the increase of the layer length due to the constant landing number.

In this laboratory study, the longitudinal changes of the flooring to the depth of the downstream scour of Calvert (square calvert shape), the changes in the length of the flooring to the depth of scour downstream of the calvert are discussed. For this purpose, experiments were performed on the floor with different lengths. According to the results, 22%, 33% and 66% were observed in particle 2 landing, 15%, 20% and 40% in 2.5 particle landing, respectively, and 20% and 25% in particle 3 landing, respectively. And reduced scour depth by 31%. This showed that changes in floor length and characteristics have a positive effect on the scour depth downstream of Calvert. Therefore, all these results have been studied in the laboratory environment and are real numbers and have been taken on a small scale.

The passage of water through obstacles such as bridge piers in the river, dock piers in the sea, and piers of any other hydraulic structures located in an open channel causes the formation of a boundary layer upstream of the obstacles and the separation of flow lines in the downstream obstacles, leading to the formation of vortex flows. The overlap of the vortices created by each of the obstacles leads to the formation of surface waves whose propagation direction is perpendicular to the flow direction. Under special circumstances when the frequency caused by the vortex of the obstacles is equal to the natural frequency of the structure's oscillation, a resonance mode is created and transverse waves with maximum amplitude are formed along the flume width. The formation of transverse waves with maximum amplitude can affect the safety and stability of hydraulic structures including bridge piers. To this end, recognizing transverse waves can reveal the reasons for the occurrence of some phenomena. Most studies on transverse waves have been conducted in a flume with limited width and a few waves. Thus, by conducting experiments in a wide flume and creating more waves, this study aimed to investigate the effect of waves in different modes on the local scour around the cylindrical piers of bridge.

The experiments were carried out in the Physical and Hydraulic Modeling Laboratory of the Faculty of Water and Environmental Engineering, Shahid Chamran University, Ahvaz, using a rectangular flume with a length of 16 m, a width of 1.25 m, a height of 0.6 m, and a zero slope with glass walls. The flume bed was covered with sediments with an average size of d50 = 0.7 mm. The experiments were conducted with cylindrical pier groups in three stages. The experiments in the first stage were carried out without sediments and the cylindrical piers were placed on a fixed bed without sediments. The first stage experiments aimed to measure the wave parameters (wave amplitude, wave frequency, and flow depth) in the resonance. In the second stage, the sediment experiments were conducted with the formation of transverse waves. After adjusting the water level and forming the desired wave, 4 hours of equilibrium time was considered. Then, the pump was turned off and the end weir was slowly opened. After the complete discharge of the flow, the topography of the bed was measured using a laser meter with an accuracy of 1 mm as 1×1 cm2. The sediment experiments were in the third stage with the removal of transverse waves (control experiments) which were performed to compare with the results of the second stage experiments. A glass sheet was used for this purpose. The glass sheet prevented matching the natural frequency of the channel and the frequency caused by the tier vortex.

To investigate the effect of transverse waves caused by cylindrical piers on the maximum scour depth, each scour in the experiment with transverse waves was compared with the corresponding experiment conducted without transverse waves. The results showed that in wave modes 1, 2, and 3, the maximum scour depth was greater in the case with waves than in the case without waves. The maximum scour depth in wave mode 1 experiments at Froude numbers 0.059, 0.057, and 0.055 was 71, 55, and 54 percent higher than the scour depth in the experiments without waves, and for wave mode 2, the maximum scour depth in the experiments with waves at Froude numbers of 0.117, 0.110 and 0.106 was 90, 76 and 66 percent higher than the maximum scour depth in the experiments conducted without waves. The maximum scour depth in the wave mode 3 experiments with waves at Froude numbers of 0.156 and 0.151 was 70 and 68 percent was higher than the scour depth in the experiments without waves. This study also examined the effect of wave mode on the maximum scour depth. The results indicated that with an increase in the wave number, the maximum scour depth increased in each discharge. On average, the maximum scour depth in wave mode 3 increased by 130 percent compared to wave mode 1 and by 43 percent compared to wave mode 2, and the maximum scour depth in wave mode 2 increased by 60 percent compared to wave mode 1. Also, the maximum scour depth in each wave mode increased with an increase in the wave amplitude, indicating the existence of a direct relationship between the wave amplitude and the maximum scour depth. In addition to the maximum scour depth, transverse waves also affected the scour volume, which was greater in the experiments conducted with waves than in the experiments without waves. On average, the scour volume was 4.3 times in wave mode 1, 3.5 times in wave mode 2, and 9 times in wave mode 3 compared to the wave-free mode.

The present study examined the effect of transverse waves mode 1, 2, and 3 on the local scour around the cylindrical piers of the bridge. The formation of transverse waves affected the scouring of cylindrical piers, and the maximum scour depth in the experiments conducted with waves was greater than in the experiments without waves. On average, the maximum scour depth increased by 60 percent, 78 percent, and 69 percent in the wave modes 1, 2, and 3 compared to the wave-free mode respectively. Moreover, with an increase in the wave number, the maximum scour depth increased in each discharge, with the maximum and minimum scour depths being found in wave modes 3 and 1, respectively. Overall, the changes in the scour volume followed the same trend as the changes in the maximum scouring depth. On average, the scour volume was 4.3 times in wave mode 1, 3.5 times in wave mode 2, and 9 times in wave mode 3 compared to the wave-free mode.

One of the most important reasons for the instability and failure of river bridges is the scouring around its piers and abutments during floods. In the recent years, the issue of local scouring has been widely studied by various researchers and different methods have been proposed to control scouring, such as the use of riprap, gabion, collars, concrete blocks, sacrificial piles, groove and cables. In this article, firstly, various methods of protection against scouring have been introduced and previous studies have been reviewed. In the following, from a practical and executive point of view, the use of riprap, concrete blocks, gabion and the construction of apron and cut off wall, which are among the existing and common methods to reduce bridge scouring in the presence of shallow foundations (foundations without piles) are reviewed and compared. All the mentioned methods are classified in the category of bed-armoring. The design and executive considerations are presented in each case and the advantages and disadvantages of each method are stated. Finally, according to the different conditions of the river and the bridge structure, the most appropriate method has been proposed in terms of execution, economy and maintenance.

Tides continuously change the water level in seas, and due to the continuity and dominance of currents caused by tides in bays, straits and estuaries, it is necessary to investigate the phenomena related to sediment transport and scouring in the surroundings of the structures located in the above areas will be illuminated more than before. In this research, by numerical simulation of the flow field and sediment transport, the scouring phenomenon under the pipelines caused by bidirectional currents has been investigated, and according to the values ​​of movement threshold speed, depth and number of tidal cycles to reach equilibrium, the characteristics of the scouring profile has been drawn. The obtained results indicate an increase in the depth and surface of the scour pit caused by the tidal current due to the alternating changes in the flow direction compared to the scour caused by the unidirectional current, and the vortex shedding process affects the final shape of the scour profile.

Spillways are one of the important hydraulic structures of a dam, which perform their duties during floods in order to maintain the health of the dam. In this research, the free ogee spillway of Sahand dam is analyzed with Flow-3D modeling software and the effective hydraulic parameters on the performance of this structure including the occurrence of cavitation and scouring of the stilling basin have studied and verified with lab model. The results show that cavitation occurs in three areas along the service spillway of Sahand Dam. The ogee crest part of the spillway, the part that slope changes in the middle of the chute and the flip bucket are the parts that have the problem of cavitation. For the ogee crest part, the tip curve should be modified and the cavitation-prone part should be made longer to create a vacuum and eliminate the possibility of cavitation. If this doesn't work, aeration should be done. In the middle of the chute, aeration should be used by creating a gap in the floor. Also, the end edge of the flip bucket should not be smooth and reshaped as a curved surface. The scouring depth for the flip bucket of the Sahand dam spillway is about 7.6 meters, and to deal with it, it is necessary to organize the downstream of the spillway using a excavated pool, protection and a stone riprap.

Weir is a structure that is made in the body or support of the dam to safe drain the excess volume of water in the tank. Weirs are mainly considered for free flow mode, but in some cases there is a possibility of immersion in them. Submersion in weirs occurs in two general and local ways. General submersion will occur if the downstream water level is higher than the weir crown level. This is more likely to occur for weirs in canals and rivers and if the weir acts as a diversion dam. Local submersion is observed in the downstream part of weir due to local flow conditions. Weirs are divided into linear and non-linear weirs based on the shape in the plan. Piano key weirs are the newest type of non-linear weirs which recently due to its advantages. Non-linear Piano Key Weirs enjoy not only a higher water passage but also a relatively simple and economic structure compared to linear weirs. All experiments of this research were performed in a channel with a long 10 m, wide 0.75 m and high 0.8 m in Tarbiat Modares University hydraulic laboratory. A view of the laboratory flume is shown in Figure (3). The required water was supplied through an underground reservoir. In this research, trapezoidal piano key weir types A, made of thermoplastic with a thickness 1.2 m, slope of the input and output keys 28 degrees and a height 0.2 m was used. The weir has 6 keys (3 input keys and 3 output keys) with the same slope in the input key and the output key. Uniform bed materials with an average diameter 2.2 mm were used. Flow discharge was measured with an ultrasonic flowmeter and flow depth and bed level were measured with a laser level gauge. According to the selected discharges, the flow depth upstream of the weir was considered to be more than 3 cm so that the effect of surface tension is not significant. In this study, experiments were performed with five discharges 30, 40, 50, 60 and 70 liters per second. Under submerged flow for each discharge, two percent submersion and under free flow for each discharge, the tailwater depth was considered to be 0.13 m. Flow characteristics are affected in case of weir submersion. During the test, after the flow hit from the input switch to the tailwater surface, due to the amount of tailwater depth, surface rotations (at low tailwater depth) and surface turbulence (at high tailwater depth) are observed. Part of the flow moves downwards and after hitting the bed surface, a weak rotational zone is created in the range of the input switches. The flow enters the downstream in the form of a submerged jet after the output switch and, due to the momentum to the upper fluid, causes a severe rotational zone in the range of the output switches. In the free flow, more turbulence is observed in the range of the output switches due to the intersection of the flow passing through the weir lateral wall and the falling current from the upstream and downstream crowns. Less tailwater depth in free flow limits the deep growth and development of the sedimentary hill in downstream of the scouring hole, and sedimentation occurs with longer length and lower elevation. However under the submerged flow, due to the greater tailwater depth and lower fall height, the flow strength is more depleted and the flow strikes the bed with less energy, and result in less scouring. In this case, the flow does not have the power to transfer all the sediments to the downstream and most of the sediments accumulate on top of each other and a sedimentary hill is created as a point. As a result, a sedimentary hill with a higher height and less length is visible under submerged current than free flow. Therefore, in the free flow than in the submerged flow, the length and depth of the scour hole occur more, and sedimentation occurs with less height and longer length. Under submerged flow, most of the sediment bed remains unchanged, while under free flow most of the sediment bed is affected by scouring and sedimentation. Of course, changes vary depending on the hydraulic conditions. In the early times, in both free and submerged currents, scouring occurs with greater intensity, but over time, its severity decreases and scouring reaches a stable state. In this case, sediments are rarely transported downstream. The surface of the sedimentary hill downstream of the scour hole in the open stream is almost smooth and in the submerged stream is sharp. The results showed that most of the scour hole changes occur in 20% of the initial time of the experiment and the changes in the free flow are faster than the submerged flow. The depth and length of the scour hole in free flow is greater than in submerged flow. Relative scour depth with increasing 77% in tailwater depth for Froude numbers of particle 0.029, 0.037, 0.045, 0.052 and 0.058, respectively 42, 45, 21, 28 and 27% and with increasing the tailwater depth 123% is reduced by 95%, 92%, 90%, 75% and 68%, respectively. With increasing 18% in the submersion ratio in the Froude number of particle 0.029 and increasing 96% in the submersion ratio in the Froude number of particle 0.058, the maximum relative scour depth decreases by 92 and 56%, respectively. The location of the maximum scouring is also a function of Froude number of particle and the tailwater depth. Maximum scour depth at the tailwater depth 0.13 m at a distance 0.18 to 0.30 m from the weir toe, at the tailwater depth 0.23 m at a distance 0.18 to 0.36 m from the weir toe and at a tailwater depth 0.29 m is at a distance 0.03 to 0.30 m from the weir toe.

One of the main goals in diverting water in rivers is to provide required water ‎with a minimum of sediments, which is usually met through installation of sediment control structures with different ‎arrangements to significantly decrease the sediment that enters the intake.In addition to sediment control,serious ‎attention should also be paid to impact of these structures and sediment control scenarios on the riverbank and ‎other relevant facilities located in vicinity of intake entrance. Using a laboratory scale canal,equipped with ‎recirculating sediment and included a 90◦lateral intake and a bed moving because of dunes movement, the effects of ‎these structures including sill,dike, and submerged vanes on the banks in the vicinity of intake entrance was ‎experimentally investigated at four different sediment control scenarios and different discharge ratio. In order to ‎accurately evaluate the scour near banks, after dynamic equilibrium was achieved at the bed due to continuous ‎changes of its elevation in these conditions,scour time series analysis was performed at upstream and downstream ‎points near the intake, along the banks on either sides of main canal.results show that different behaviors are ‎observed near intake due to various factors influencing the flow. While, in range of discharge ratios studied dike increases depth and length of scour zone at intake downstream by an average of about72%and‎‎34%, respectively, vanes reduces length of zone by37% and at discharge ratio of0.12, an ‎increase of about 66% in depth of scour zone is observed, while at discharge ratio of0.18, a decrease of‎‎24% in depth of zone occurs.‎

Bridges are among the most important and most prolific river structures ever used. The three commonly used methods to deal with the scouring phenomenon around the bridges are to place the piers in a lower level than the depth of the erosive pit, to reduce the power of the vortex produced around the base, to use the shrub protection cover, or to use a collar, gaps, or submerged plates around the base bridges. Submerged plates are structures that are installed at the bottom of the river with an angle to the main stream and to prevent river erosion, rivets and base structures of the water and waterway reformation and morphology of the bed, which produces a secondary vortex and Changing the flow pattern in the river bed and, consequently, changing sediment transport and erosion. In this research, the effect of square cross-sectional diameter and the effect of submerged panels on scouring are investigated using SSIIM software.

Evaluating the vulnerability of bridges to flooding is essential for risk-informed planning of their maintenance. This paper aims to develop a systematic model for investigating the behavior of bridges at the time of flooding, in which structural, geotechnical, and hydraulic parameters are considered. A three-dimensional finite element model of reinforced concrete bridge piers was developed, in which the material nonlinearity and the interactions between the pier and the surrounding soil and the flood water were considered. The parameters used in the model were validated based on experimental data from a single pile and a reinforced concrete column under axial and lateral loading. The validated modeling approach was then used to simulate an existing bridge pier, for which the structural, geotechnical, and hydraulic parameters were varied to evaluate the effective parameters. The results showed that the presented modeling approach is capable of providing reliable predictions of the performance of bridge piers at the time of flooding, which makes it suitable for practical vulnerability assessment of bridges. Moreover, it was observed that the behavior of the bridge piers against floods was more sensitive to geotechnical and hydraulic parameters than structural parameters, to the level that by changing the soil type from medium sand to loose sand, the lateral displacement of the structure is 1.87 times. Also, by increasing the longitudinal slope from 0.004 to 0.005 and decreasing the roughness coefficient from 0.025 to 0.021, the lateral displacement of the structure will change to 2.57 and 6.55 times its initial value, respectively.

Laboratory experiments were carried out to investigate the hydraulics, turbulence structure, and sediment removal capacity of bridge pier with different adjusted roughness. Four experimental models of the roughness were employed to measure the effect of the geometry of the roughness on the scour hole formation and depth. The effects of roughness geometry on the turbulence structure and local erosion through the bridges were studied for one sand sediment size. The three-dimensional velocity profiles and turbulence characteristics of flow downstream and upstream of the bridge pier were measured by Acoustic Doppler velocimetry (ADV) technique due to different adjusted roughnesses. After filtration process by WinADV software, the contour plots of the turbulence kinetic energy were depicted due to triangular roughness. The maximum longitudinal eroded section, which is commonly located by bridge pier corners, was measured through the experiments. The results of the scour hole measurements indicate that due to increasing the length of the roughness, the scour hole depth values decreased; however, increasing the vertical distance has the same effect on the scour hole formation. Furthermore, different geometries of the employed roughness illustrate that the triangular roughness has less scour hole depth than other experimental models with the same hydraulic condition. The detailed information of the magnitude, distribution, and probability of turbulence structures was extracted from time-series data using power spectra. The turbulence data were compared with the topography of upstream erosion. Analysis of the power spectrum density function indicated that the discontinuous adjusted roughness remarkably reduced the downward energy level of vortexes at the upstream of bridge pier, which is supposed that the gap opening through the roughness can increase downward jets impact. It appears that this condition reduces the turbulence kinetic energy near the bed elevation at the upstream face of squire bridge pier.

The bed formations of submarine pipelines models were measured to investigate the effect of design parameters such as the distance between two submarine pipelines and also the effect of adjusted rectangular spoiler due to different angles of adjustment. To investigate the hydraulic parameters, three experimental models of parallel pipelines with three distances were made through the hydraulic laboratory. By considering the threshold velocity, the bed profiles were measured experimentally by employing the point gauge apparatus due to changing the angle of considered spoilers. Results indicate that increasing the distance between two submarine pipelines is the reason why that the downstream scour hole was formed independently from the upstream scour hole, which was developed under the upstream pipelines. Also, due to different angles of spoilers, the scour hole of spoiler with angle 90o formed the same as the witness model (with no spoiler), showing that angle 90o has no marginal effect on scour hole formation process. However, the angle of 180o adjusted on two submarine pipelines made the maximum scour hole under the upstream and downstream pipelines. It is assumed that the spoiler 180o affects the downstream flow of the pipelines and by creating the region with low pressure behind the pipelines, the scour holes were plunged to the bed and the depth of the scour holes increased significantly. Also, the investigation of spoiler position on the two pipe lines illustrates that by adjusting the spoiler on the downstream pipelines, the downstream pipe was buried under the sediment. These conditions were observed by angles of 135o, 180o, and 225o. It is assumed that increasing the distance between two submarine pipelines diminishes the rate of self-burial conditions. It appears that by increasing the distance between two pipelines, the effect of the upstream pipeline was omitted because the downstream and upstream pipelines were scoured as two single pipes; thus, the upstream pipelines did not affect the downstream pipelines.

A side weir is a hydraulic control structure used in irrigation and drainage systems and combined sewer systems. The Piano Key Weir (PKW) is a new type of long crest weirs that have a relatively simple structure and high economic efficiency structures. Due to the advantages of this weirs, it is necessary to study and investigate the Scour around of these structures as a side-weir. The present study focuses on investigate the scouring around the piano key Side weirs of the Type A at a 30° Section of a 180° Alluvial curved channel for clear water conditions. The results showed that at the end of the Side weir, longitudinal bar in the middle of the main channel and a scour hole close to the outer bank are formed because of the changes in shear stress field. The depth of clear-water scour increases by time and approaches the equilibrium state asymptotically depending on approach flow velocity. The equilibrium depth of scour depends on the dimensionless parameters of flow intensity, flow shallowness, weir crest height, side weir length and the maximum value of scour depth occurs at a depth when the approach flow intensity is equal to 1.0. Also, the scour equilibrium depth in the dimensionless ratio increased L/rc = 0.175 compared to L/rc = 0.125 in different flow velocity of 12 to 35%, 10 to 39% and 18 to 26%, respectively.

Scouring is the erosion around structures that occurs due to complex eddy currents and creates a pit around the supporting foundations of structures (Melville, 1997). Depth of scour plays a vital role in 1) the degree of the potential destruction of flow around the structure and 2) designing the dimensions of the foundation of structures in the path of water flow. Therefore, determining the mathematical relationship between scour depth and affecting parameters (Barbhuiya and Dey, 2004) and understanding the scour mechanism around bridge piers, and accurately estimating scour depth for more and better protection of bridge piers against this phenomenon (Breusers et al ., 1997) is of great importance. Numerous comprehensive studies have been performed since 1953 to identify the effect of various factors on scouring rate and determine its estimation methods (Laursen and Toch, 1953; Shen et al., 1966; Garde and Ranga-Raju, 1985; Melville, 1995).To date, researchers have studied the effect of geometric and hydraulic parameters (Sturm and Sadiq, 1996; Kouchakzadeh and Townsend, 2000; Jordanova and James, 2003) and effective vegetation Young et al., 1993; Amir et al., 2018) on scouring, but the simultaneous effect of changing the length of abutments and vegetation on the amount of middle pier scour has been less considered. Therefore, in this study, the amount of scour around the middle pier of the bridge in compound sections in case of simultaneous change of abutment length and vegetation has been investigated.

Normalized parameters significantly deepen the understanding of various physical phenomena and allow the results of limited experiments to be applied to different situations. In this study, the maximum scour depth  was selected as the primary variable and expressed as equation ‏(1) in terms of effective parameters. Then, considering the parameters  as iterative variables, using Buckingham p-theorem, equation ‏(1) was written as a dimensionless relation ‏(2). Because the amount of channel slope, unit weight of sediments, uniformity coefficient and average diameter of materials, velocity and depth of flow, and length of bridge pier are constant in all tests; normalized parameters  and Froude number were removed. By simplifying and combining other parameters, dimensionless relations for degree of scour depth and volume were obtained as equations ‏(3) and ‏(4).A trapezoidal laboratory channel made at the Water Research Institute, ‎Fig. 1, was used for the experiments. The central pier of the bridge around which the scour was examined has a square cross-section with sides of 4 cm. Abutments on both sides of the channel were provided in 8, 16, and 24 cm sizes. Vegetation was studied using non-submerged cylinders in diameter and length of 1 and 20 cm respectively applied in rows in areas of ​​1, 2, and 4% of the floodplain area. ‎Fig. 2 shows the image of the laboratory channel. Experiments were performed in two time intervals of 3 and 8 hours to obtain equilibrium time and maximum scouring.

A total of 9 experiments were performed in which the depth and volume of scouring were determined. To name each experiment, the vegetation and abutment length were marked respectively with V and A. For example, V%1A80 refers to an experiment with 1% vegetation and an abutment of 80 mm length.‎Fig. 3 shows the changes in the longitudinal profile of the bed after scouring in different conditions of vegetation and the length of the abutment. Increase in the abutment length increased the height of material accumulation in all experiments. Increasing the percentage of vegetation leads to an increase in the volume of material accumulation in all samples due to increased bed roughness and turbulence of the flow and enlargement of the scour cavity. Also, it is observed that with increasing the percentage of vegetation and the length of the abutment, the length of scouring (after the hole in the direction of flow) increases.‎Fig. 4 shows that the depth and volume of scouring increases with increasing vegetation percentage. The highest scour volume occurred in 4% vegetation (the densest vegetation condition studied), which results in 9.2 cm and 7031 cm3 for experiments V%4A160 and V%4A240, respectively. The lowest scour depth and volume values were related to 1% vegetation and V%1A80 test, which are 4.1 cm and 766 cm3, respectively. Increasing the vegetation from 1 to 2% and from 2 to 4%, the scour depth at the pier of the bridge (with length and width 4 cm and 8 cm abutment length) increases by 14 and 57%, respectively, i.e., the scour volume, respectively increases 3.2. And 1.9 times.As the relative length of the abutment increases, the rate of deflection and the intensity of the flow in the main channel increase, after which the amount of scouring around the middle pier increases. As the percentage of vegetation in the floodplain becomes denser, the bed roughness increases, and the flow rate in the main channel increases, causing more scouring around the middle pier of the bridge (‎Figs. 5 ‎and 6).

This study investigates the effect of flood vegetation and abutment length on the scouring depth and volume in a compound channel. This study shows that with increasing the percentage of vegetation, the scour depth at the pier of the bridge increases, and with increasing the length of the bridge abutment, the depth and volume of the scour increases. By the way, vegetation and abutment length percentages are the most influential parameters in creating scour depth and volume in which the former has more effect.

The scouring mechanism at the abutment is very similar to the pier of a bridge; such that the decrease in current intensity near the upstream of the abutment, a vertical pressure gradient is created, this pressure gradient moves downwards to become the initial vortex, the size of which expands with the development of the scour cavity (Melville, B.W. 1997).The bridge pier near the abutment causes deepening of the scour near the support (Hong, S. 2005). However, the scouring depth is determined prominently by the scouring of the abutment and that this value may be greater than the scouring depth of a pedestal alone (Nyarko, K., & Ettema, R. 2011; Karami et al., 2017).The length of the abutment has an essential effect on its scouring depth, such that by doubling the length of the abutment, the scouring increases by about 18 times and decreases with decreasing abutment distance (Arab and Zomordian 1393; Anjum Rooz et al., 2017).In the present study, we applied FLOW-3D to build a numerical model to investigate the effect of changing the piers' distance from the abutment on the scour depth and scour rate around the two piers.

Flow-3D is one of the most potent applications in computational fluid dynamics. This research makes applications to solve the mean Navier-Stokes equations of time (Reynolds equations) using the finite volume method under a rectangular grid on the base of the Eulerian theory.The flow continuity equation is written as Equation 1. FLOW-3D calculates the load transport of each sediment type, including bedload transport, separately. FLOW-3D calculates the load transport of each sediment type, including bedload transport, separately. The dimensionless form of the bed load transfer rate in terms of n is written by Equation 2, in which dn is obtained from Equation 3.We used the laboratory results obtained from the research conducted by Hosseini et al. (2016) to validate the numerical model and adopted the study undertaken by Khosronejad et al. (2012) to model the pier scour. A rectangular channel with a length of 6 meters, a width of 1.21 meters, and a height of 0.45 meters; was used to model the bridge pier and abutment. ‎Fig. 2 shows the rectangular channel a pier and abutment placed in, and Table one describes the model configuration.

For the three-dimensional simulation of flow, the geometry of the field was discretized into a fine mesh. ‎Fig. 2 shows the boundary conditions assigned to the numerical model. Then, three turbulence models, K-ɛ, RNG, and LES have been investigated to compare the maximum scour depth in the rectangular channel for a flow rate of 0.052 m3/s, and their results are shown in Table 3.The results indicate that the LES turbulence model can estimate the maximum scour depth for the bridge pier and the abutment. Although the LES turbulence model takes more time to run, it is more accurate and presents a scouring pattern similar to the following laboratory sample justifying the computational costs. The maximum scour depth at the bridge abutment estimated by the LES model is about 74% of the measured value, while its maximum scour depth measured at the bridge's pier is 87%.‎Figs 3 and ‎4 illustrate the bed changes around the abutment and the bridge pier for the numerical and laboratory model. Comparing the bed changes and the location of maximum scour depth for the numerical model of pier and abutment and results of laboratory models shows that the numerical model has a good capability in simulating bed changes and local scour around each of the abutment structures and bridge pier.Comparison of scour time development resulting from numerical models around the abutment and bridge pier with laboratory models, ‎Fig. 6, shows that 70% of the maximum scour depth in the abutment area occurs in 20% of the initial scour time, which is relatively consistent for both numerical and laboratory models.Examination of substrate changes, ‎Figs. 7-‎11, shows that a deep scouring depth occurred for the abutment and at the upper-right edge of the forehead. And by reducing the pier distance from the abutment in the models, the maximum scours depth around the pier, and abutment structures have undergone significant changes.

Comparing the scour pattern and the location at which the maximum scour-depth for pier and abutment resulted by the numerical model with experimental, shows that the numerical model procured using FLOW-3D can well simulate bed changes and local scour around the bridge pier and abutment. Furthermore, increasing the relative velocity of the flow, strengthening the eddy currents around the pier and abutment, consequently arising the scour depth; as the relative velocity increases by 10%, the scour depth increases by about 50%. Moreover, when the distance from the base to the abutment decreases, the scouring depth around the rectangular abutment increases.