جستجوی مقالات مرتبط با کلیدواژه « آبشستگی موضعی » در نشریات گروه « عمران »

One of the main factors in the collapse of the bridge piers in rivers is local scouring. By placing the piers in the direction of the streams, a complex three-dimensional flow is formed around the pier that has been the popular subject of research by many researchers. Methods of reducing the depth of local scouring are divided into two systems: 1. increasing the strength of bed materials around the piers by using more resistant materials, such as riprap, collar and gabion in the riverbed. 2. Reducing the strength of the main factors such as downward flow and horseshoe vortex by the deflector, blade and submerged Vane or changing the geometric shape. In the present study, the effect of the deflector shapes such as triangles and curved surface on the maximum scour depth around the pier under clear water conditions were investigated. General factors of bridge pier scour include down-flows, horseshoe vortex, and wake vortex. In general, the flow impact on the pier and its separation is the main factors that form scour holes around piers.

The experiments were done in the laboratory of Khuzestan water and power authority laboratory (KWPA), equipped with a flume with a length of 10 meters and a height of 500 mm and a width of 310 mm. The flume is equipped with an electromagnetic flow-meter with an accuracy of ±0.1 liters per second and a weir downstream of the flume to adjust the water level. In this study, natural river sand with uniform grain size (δg = 1.36), relative density Gs = 2.64 and the average particle diameter of 0.95 mm. In all experiments, water depth was considered 100 mm. In this research, three different models of PVC deflector surface (the deflector surface shapes such as triangles, curved and simple surface) with angle's face (θ= 15, 30 and 45-degrees). It should be noted that the angle of flow with the deflector head is calculated as α = 90-θ, which used to describe and analyze. The unprotected pier scouring investigated to represent a basis for controlling and comparing with the other scour and bed change conditions. A 12-hour control experiment was also conducted on the control pier to determine the experiment time (equilibrium time), and scour depth changes were recorded in the time unit during experiments.

The horseshoe vortex around the scour hole accelerates digging and transfers the sediments separated from the bed downstream with the main flow. The flow's separation from around the pier also creates perpendicular vortexes on the sedimentary bed known as wake vortexes. These vortexes are active behind the pier, separate the bed particles like a tornado, expose them to the flow, and help move sedimentary particles from the front and sides of piers downstream. The scour hole digging by the horseshoe vortex continues until the water volume inside the scour hole increases and exhausts the vortex energy. In this state, the scour depth changes negligibly over time and reaches equilibrium, and Figure 1 show the scour mechanism. In this study, to reduce the scouring depth pattern around the pier, three shapes of the deflector (the deflector surface shapes such as triangles, curved and simple) with three angles were used. The results showed that by reducing the head slope from 40 to 15 degrees, scouring depth decreases. For all deflectors with 15 degrees in the parameter (U/Uc=0.70), the percentage of the scouring depth reduction is close to 83 to 89 per cent. In the parameter U / Uc=0.96 near inception motion that is the most critical state and the value most comparable to the particle incipient motion, the deflector with triangle surface shows a decrease of 85%, curved surface 77%, and simple 75%. By reducing the angle of the deflector, part of the flow lines didn't deviate towards the bed, which reduced the potential of the high-pressure zone created at the pier. This reduction in compressive potential reduced the flow velocity of the back vortices and, ultimately, reduces their ability to transport sediment downstream. Based on the results, the deflector with a triangle surface shape in all flow conditions had a better and lower scour hole depth than the carved and simple shape.

This study used a deflector structure to reduce and control the scour depth around bridge piers. The flow effect was investigated by implementing these protections and their impact in various relative velocities (U/U_c=0.97,0.83,0.70). The scouring pattern and sediment point bar created around the pier with The deflectors protection compared with the control pier (without protection) have less scouring depth due to minor deviation of flow streamlines and reduced disturbances around the pier. Finally, the deflector with a triangle surface shape has a better response to reduce the scouring hole. The results state that the deflector at the 15-degree angle significantly affects the flow deflection near the bed, corrects the flow pattern around the pier, and reduces scouring depth.

The scour has been studied in hydraulic science for many years, due to its complexity and lack of proper relationship, it has been the attention of hydraulic scientists. Hydraulic structures situated as an obstacle in front of water flow that changed the flow pattern in their vicinity and causing the local scour around the structure. There are various methods to reduce local scour around the pier. In this research, sacrificial pile and collar were used to reduce the local scour around inclined pier group. The results show that using of the sacrificial pile in front of the inclined pier group, the effect of the inclined pier angle on the reduction of the scour is lower. So that difference between the percentage of scour reduction for the first and second inclined piers in the angle of 5 and 15 degrees is zero and in vertical pier is 1.4%. The use of three collars with a diameter of 4b in pier group with the distance of 4b between piers and presence of sacrificial pile in front of the first inclined pier in distance of 3b compared to 2b show a greater scour depth reduction in front of the piers. In the numerical model, the use of inclined pier group with the distance of 3b between piers and sacrificial pile with distance of 2b shows lower turbulence intensity compared to distances of b and 3b.

Local scour involves removal of material from around piers and abutments as the flow accelerates around the obstructed flow area. Various methods have been suggested for the control and reduction of local scour around the bridge piers and abutment. Using submerged vanes is one of these methods. Vanes change the regime of bed load movement that led to control place of deposition and erosion. The function of vanes to reduce scour around the bridge piers and sediment movement in the region of vanes due to down flow in front of them are affected by arrays of the vanes. In the present study, Different lay out of arrangements (Parallel rows, two parallel rows, and two zigzag rows) Flat Submerged vanes with the ratio of length to height at an angle of 20 degrees relative to the direction of flow on protection of local scour around abutment and piers group with rectangular debris were laboratory investigation. All experiments conducted in uniform flow with clear water were be done. The results showed that the zigzag arrangement of the two three rows had the greatest effect to protection of local scour around abutment. In this case, vane distance from abutment and the row spacing with the relative spacing is 0/5 and 1.(The distance of the vanes from the abutment was La and the distance of the rows was 2La) that reduced the scour depth by 81%.

The bridge pier scour is one of the major causes of bridge destruction worldwide. In spite of extensive research in this field, the study of this topic has always been of interest to researchers and it is necessary to study different methods to counter it. The collar is a simple and inexpensive structure for protection against bridge pier scour. In this study, the effect of collar size and height on bridge pier scour and determination of the best collar performance conditions in steady and unsteady flow conditions were investigated. The experiments used three different collar sizes and three different mounting heights relative to the substrate surface. After calculating the percentage of scour reduction in all test cases and comparing them, Investigation of time evolution diagram of scour hole and comparison of maximum depth of scour created in different states, It was found that increasing the size of the collar and decreasing the height of the collar installation relative to the substrate surface resulted in a further decrease in the depth of the bridge pier scour and this changes was more evident in the unsteady flow state. Also by comparing the two variables of size and height of collar installation, it was found that the height of collar with dimensionless ratio H/D=0.5 in steady and unsteady flow had the best performance and the greatest effect in reduction scour around bridge pier.

In this study, the performance and efficiency of cable in control of local scour around the pier of the rectangular bridge has been investigated by changing the flow collision angle. In this study, three types of piers, namely: 1.round corner rectangular without cables, 2.round corner rectangular with cable with 10 % of pier diameter with a 15-degree twist angle of cable (second type pier), 3.round corner rectangular with cable with 15 % of pier diameter with a 12-degree twist angle of cable (third type pier), were used with four different flow collision angles, including 0, 5, 10, and 15 degrees.The purpose of this study is wrapping the cable around the pier and changing the flow collision angle from the downstream flow water and then its effect for the reduction of the depth of the final scour measure. In this experimental study, the effect of flow angles on the pier was investigated, so that the first type pier under zero degrees angle was compared with the second and third types parallel to the flow direction. The maximum scour depth in experiments was achieved at the 15-degree angle with the flow direction, and the effect of the cable in reducing the scour depth in this angle for type-2 and type-3 piers was 10% and 22%, respectively. Also, for type-2 piers, in experiments under 5, 10, and 15-degree angles with the flow direction, the scour depth was increased by 3%, 21%, and 37%.

Local scour is one of the main factors of the destruction of bridge piers in rivers before the life expectancy of these structures. One of the newest indirect methods for reducing the erosion and scour around the bridge piers is the air injection method which is based on counteracting with downward flow to create such conditions to prevent or reduce the flow impact on the bed and the destruction of bridge piers.Tipireddy and Barkdoll (2019) studied the effect of air injection in controlling bridge pier scour. The results of this study showed that the maximum scour depth decreased with increasing air injection rate until the ratio of air outflow velocity from the pores to flow velocity was 57.1 and then increased.The performance of air injection to reduce bridge pier scour has been investigated in only one study, considering all parameters except constant air outflow rate. The main objective of this study was to investigate the effect of air injection on reducing bridge pier scour considering the number and the location of injection pipes.

Experiments were carried out at the Hydraulic Laboratory of the Faculty of Water Engineering, Shahid Chamran University of Ahvaz in a flume with 10 m long and 0.74 m wide and 0.6 m deep. The bottom of the flume was covered by sediments with a mean diameter of 0.7 mm with a standard deviation of 1.21 indicating uniform sand. All experiments were performed in four Froude numbers of 0.18, 0.2, 0.22, and 0.24 at a constant depth of 15 cm under clear-water conditions. To reach these conditions, the flow depth was adjusted so that the ratio of flow velocity to the critical velocity is less than 95%. In order to determine the appropriate time for the experiments, a 12-hour test at a Froude number of 0.2 was performed and it was observed that about 90% of the scour occurred in the first 3 hours. In order to create an air injection system, porous pipes were selected with an internal diameter of 4 mm. Each pipe had 24 pores with a diameter of 2.2 mm at fixed intervals mounted on the upstream half of the pier and two ends were connected to the air compressor for injection. Experiments were carried out at air injection rate of 100 lit/min and in 7 different number and position of aeration pipes on the pier. The pipes were mounted in three general modes: one pipe, two pipes and three pipes. The installation level of the pipe on the pier was considered as three heights of 1/3h, 1/2h, and 2/3h from the bed.

A number of four control experiments (without the air injection system) was first performed at four specified Froude numbers. A total of 28 experiments were performed in the presence of the air injection system at the air injection rate of 100 lit/min. The aeration pipe at on the upstream half of the pier provides some protection for the sediments against the horseshoe vortices.Effect of aeration pipe position on scour depth and scour development around bridge pierTo investigate the effect of the position of the aeration pipe on the pier, the pipes were installed in single-pipe mode at three different heights on the pier and in the simultaneous usage of two pipes, the pipes were installed in three modes at the same three height. The transverse expansion of the scour hole for the bridge pier were plotted in two general modes using a single aeration pipe and two aeration pipes in four Froude numbers. In single-pipe mode, the maximum decrease in the scour hole depth in front of the bridge pier (25.5%) was observed at the pipe installation depth of 5 cm beneath the bed and for Froude number of 0.18. For the use of two pipes with the same injection rate and similar hydraulic conditions, the maximum decrease in scour depth of 11.8% occurred when the pipes were installed at depths of 7.5 and 10 cm beneath the bed.Effect of aeration pipe on scour depth in scour development around bridge pierIn order to investigate the effect of the number of pipes on the scour depth, the transverse expansion of the scour hole around the bridge pier was plotted in three general modes: single pipe (three mode), two pipes (three modes) and three pipes (one mode). The results showed that the lowest efficiency was observed to the state of the three pipes and the efficiency is to the one state..Effect of Froude Number on Scour Depth and Scour Expansion around Bridge PierThe shape of the relative scour depth was plotted against the Froude number for the use of an aeration pipe. The results showed that there is a direct relationship between the scour hole depth and the Froude number and the scour hole depth increases with increasing Froude number.

In this study, the effect of air injection system on the scour pattern and sedimentation around the bridge pier under clear-water conditions in a straight flume was experimentally investigated. All experiments were performed with four Froude numbers of 0.18, 0.2, 0.22, and 0.24 and 7 different numbers and installation levels of the aeration pipe at air injection rate of 100 lit/min. The results showed that aeration structure in the flume reduced the scour depth around the pier. The reduction of the scour depth at the front of the pier in the single-pipe test was 25.5% with an installation level of 5 cm, Froude number of 0.18, and air injection rate of 100 lit/min. The results of comparisons made to investigate the effect of the number of aeration pipes on the pier showed that the three-pipe mode had the lowest efficiency. This system reduced the volume of scour hole around the pier. Therefore, it can be concluded that the application of this structure can be an appropriate alternative in comparison with the commonly used protective structures, therefore, further studies are suggested.

The bridge failure caused by the local scouring phenomenon around its piers or abutments is a common phenomenon. Therefore, some methods should be used to prevent the destruction of these structures. Slots and collars are among the tools that can be used for this purpose. Therefore, by conducting 49 tests in this research, we will examine and compare the performance of these two tools with new approaches around bridge abutments using an experimental model in different flow conditions. The results show that although the most effective slot model can reduce the dimensions and depth of scour hole by 61%, it cannot postpone the start of the scouring phenomenon and take the scour hole away from the abutment surroundings. However, for the most effective collar and highest Froude number, the scour hole reaches the base point after 120 minutes from the start of the test. In this collar model, the maximum depth of scour hole is shifted to a more distant point from the abutment, and the percentage of reduction in the depth of scour hole at the base point and erodible bed is 56% and 96%, respectively.

The failure of the bridge caused by local scour, the importance and necessity of studying the prediction of scour depth and ways of reducing it. One of the important method to reduce the effects of erosion factors is to use a thin plate around the bridge pier called collar. Collar dimensions and the position have great factors on reducing local scour around bridge pier. In this research, collar with four forehead length (lca ) equal to B, 2B, 4B and 9B(B is width of the pier) are examined. The collars were installed in four levels equal to: on the bed, 0.12B under the bed, 0.5B and B above the bed. The experiments have been conducted with three Froud Numbers equal to 0.13, 0.16 and 0.19 in clear water condition. The experiments have been done on a flume with the length of 6 m, width of 73 cm, and a depth of 60 cm with a slope near zero in Hydraulic Laboratory of Shahid Chamran University of Ahvaz. The results showed that increasing dimensions of collar will increase the efficiency of performance and the best dimention lca (length of oblong collar in the front of the pier) were B and 2B. The best position to install the collar were on the bed and 0.12B under the bed. In these situations the collar had 100% and 88% efficiency in respectively reducing the scour depth.

Study of different scouring areas relative to flow velocity in Downstream Base in pairs of bases and Impact of cable protection method on these areas Abstract Bridge failure is a common phenomenon all around the world. Bridges are one of the most important structures which are under attention from many years ago. Bridge is a structure to cross over obstacles such as rivers or valleys. Investigation of scouring in water structures especially in bridges is absolutely important in river engineering. Failure of several structures in all over the world are usually due to structural consideration and giant scales on piers. Created procedure of scouring by group of piers are more complicated than one single pier. Increasing the resistance of bed materials and decreasing the power of erosion factors are the ways to stand against local scouring. To decrease the power of erosion factors (horseshoe and wake vortex), equipment such as collars, submerged vanes and etc. are being used. This study was conducted with freshwater on cylindrical piers. The experiments were with a constant discharge during 6 hours in hydraulic laboratory of Ferdowsi University of Mashhad using a flume of 10 m length, 0.30 m width and 0.50 m depth. Sands with median diameter of 0.72, special gravity of 2.65 grams per cubic centimeters and geometric standard deviation of 1.12 are used in the experiments. Range of flow rate was from 8 to 18 l.s-1. An adjustable weir in the downstream regulates the water depth in the channel. The area for conducting the experiments in the channel has 1 m length and 10 cm bed height, which is 6 meters away from the beginning of the channel. Scouring procedure of downstream pier was investigated base on flow velocity. Results show that depth of local scouring of downstream pier can be categorized in 4 zone based on flow velocity: 1- No scouring occurrence zone, 2- Synchronized scouring zone, 3- Transitional zone, and 4- Deviance zone. Following previous investigations, effects of application of rolled cable over piers on reduction of scouring around two piers and zones of downstream pier (zones related to velocity) were examined. In this study, the efficiency of using rolled cable around the piers to decrease scouring is investigated. Results show that rolled cable, scouring will decrease. In fact, rolled cable around the piers will decrease the power of down flow, horseshoe and wake vortexes. Results show that scouring was reduced around piers due to application of rolled cable. So that cable at its best state reduces the maximum scour depth by 50 percent for downstream pier in the situation distance 3D and 54 percent for downstream pier in the situation distance 5D. Reduction of scouring depth of downstream pier was less than upstream piers. Scouring depth of downstream pier with rolled cable was 7 and 26 percent reduced in comparison with piers (No rolling cable was used) located at 3D and 5D from it. Because of scouring reduction as consequences of rolled cable, downstream zones were significantly changed that finally caused synchronized zone removal. Keywords: Local Scouring, Base group, Protective method, Downstream Base, Scouring zones

The result of 40 experiments in the field of scouring by three dimensional wall jets were conducted in this paper. Experiments were carried out at the different values of densimetric Froude number that ranges was 3.9 to 6.6, a range of tailwater condition was 4 to 15 times the dimensions of the jet and for the three same cross-section nozzle with different dimensionless axial distance from the channel wall in ranges of 0.5 to 17.3. At first in this paper the scour mechanism caused by the wall jets was described. The effect of jet distance to the channel side wall was also investigated. The results of the paper verified with the previous studies. Laboratory observations and results show that the nozzle distance from the channel side wall caused the jet deviation towards the near wall and asymmetric in flow regimes. The factors has significant influence on scour due to jets as the maximum scour depth increased by decreasing of the distance of the nozzle to the side wall. Also in most cases the scour longitudinal parameters decreases by increasing the distance of the nozzle to the side wall. It was found that tailwater depth has an influence on scour longitudinal parameters. According to the results it can be note that the effect of jet location on scour is significant and can be considered as an effective parameter in scour cause by wall jets.

Local scour at the culvert outlet is common event. In this study the effect of different angles of wing walls flare on scour in hydraulic conditions for type 1 and 4 flows at downstream of circular and box culvert investigated. Wing walls flare 15, 30, 45, 60 and 75- degrees to the outlet centerline were considered in this study. Result showed 15- degree flare wing walls reduced scour depth, scour length and mound height. So, 15- degree flare wing wall in box culvert and type1 flow reduced scour equal 35.3% to the control test. Also, 30- degree flare wing walls in box culvert and type1 flow has shown a good performance in reducing scour depth equal 46%. Wing walls flare angle 60 and 75- degree, did not have good performance in reducing scour depth at downstream of culverts. In type 1 flow, scour can be reduced by using of wing walls with any angles. In type4 flow, only 15- degree wing walls flare reduced maximum scour depth equal 30% in circular culvert and 10% in box culvert. Also, the dimensions of the scour hole on downstream of the circular culvert is more than box culvert. So, in the circular culvert and type 1, scour depth increased equal 35% compared to box culvert and it was observed equal 95% in type-4 flow. The dimensions of the scour hole in type 1 flow is higher than type 4 at all angles, while the mound height in type 4 is higher than type 1 flow.

Every year by flooding in various parts of the world a large number of bridges are destroyed just when its need to used widely. Most of this bridge destruction are due to scour around their piers. Erosion and the movement of the bed materials due to water flow is called scour. During this process, a hole is developed around piers and progress to reach to a depth of equilibrium gradually. For this research a laboratory flume with 6m length and width of 73cm located in the hydraulic laboratory of the Shahid Chamran University of Ahvaz was used. In order to reduce the scour around pile groups a flat plate with small thickness called collar are installed around the middle set of the pile groups. The collars used were netted with four percent
0% (simple collar) of 30%, 50% and 70% and they were installed in three levels, on the bed, 0.5D and 1D above the bed. The experiments were conducted under clear water condition and three Froude Number were equal to 0.12, 0.14 and 0.16 and the results were compared with the case of piers without collar. The results showed the simple collar located on the bed and netted collar with 70% of opening located on the level of 0.5D relative to the bed surface in both cases the reduction of scour in the front pier were equal to 86 percent compared to non-collar and these cases had the highest performance

The main reasons of bridge failure are local scour around the piers. In fact, they will be appeared as a scour hole in the river bed. Moreover, lack of control during the time, local scouring will threaten stability of structure. Therefore, determining of depth and dimension of the scour hole, also finding effective factors on scouring are important. Lots of researchers have studied the local scour around the bridge piers. Actually, they have proposed lots of appropriate techniques to control and to reduce scour around piers and bridge abutments. These proposed approaches are divided into two methods which are altering the flow and bed-armoring. In this research, the local scour around the bridge piers in a cohesive soils have studied. Particularly, the cohesive soils in form of mixture of Bentonite (montmorillonitic clay from 0 to 15%) and bed sediment (fine sand) used. In fact, this approach is a solution to challenge bed-armoring against existence shear stress. As a result, the proposed method will be used for controlling the scour. The experiments were carried out at the hydraulic laboratory of the Water Engineering Department, at Shiraz University, Iran. The laboratory flume was a rectangular cross section with 18 m length, 1.2 m wide and 0.4 m deep. According to channel geometry, the discharge and the depth of flow were determined to be 0.034 m^3⁄( s ) and 13 cm, respectively. In all experiments, the discharge and flow depth were constant. This study focused on the best compaction conditions, optimum clay content and the bed shear strength. The undrained shear strength of the soil was measured using an in situ miniature van shear apparatus. The best compaction conditions including optimum moisture and compaction energy. From standard Proctor test, the optimum initial water content Wopt and the optimum dry density ρdopt was determined. The compaction was expressed as the ratio of dry density ρd to maximum dry density ρdopt of the mixture. Therefore three relative compaction equal to70%, 80% and 90% used. Also, three water content optimum initial water content, optimum initial water content – 3% and optimum initial water content + 3% are used. The results indicate that for smaller clay content (5%) the shape of the scour hole was similar to that in sand sediment and is regular and symmetrical. For clay content equal to10%, scour hole is geometrically much irregular compared to that observed in sand bed and it is observed that a steeper slope of the scour hole in sediment mixture. The slope became steeper whit an increase in relative compaction. In fact, the higher relative compaction and the higher clay content increases shear strength and decreases the maximum scour depth. The samples compacted at optimum water content creates a structure with the most resistant to scour. The erodibility of samples compacted dry of optimum water content is less than samples compacted on the wet side of optimum. The results showed that under conditions which the amount of Bentonite equals at least 10% of dry weight in the mixture (Bentonite and sand sediment), relative compaction equal s to 90% and and water content equals to optimum moisture, simultaneously the local scour reduces by 100% in single pier. In addition, the influence of the compaction and type of clay mineral was investigated. The results show that Bentonite was more effective than Kaolin. If the 15% of dry weight Kaolin clay mixed with bed sand sediment to be used and relative compaction equal to 90% and water ontent equals to optimum moisture, simultaneously, the maximum scour hole was reduced only by 34%.

Every year many bridges during river floods are destroyed due to local piers and abutments scouring. Several methods have already been proposed and applied to reduce and control the depth of scour. For instance using riprap, collar, piles in front of piers, slot at piers, cables, submerged plates and modified pier shape. In the current study, the local scouring around a cylindrical pier has been investigated under normal and using different methods for scour reduction conditions by FLOW-3D numerical model and RNG model is used to estimate the flow turbulence parameters. Evaluating and validating the numerical model are carried out using the results of the physical model. In the following, amount of local scouring behind and in front of the pier in different 11 proposed scour reducing conditions is compared with solo pier. Applying riprap condition with 13% scouring of solo pier has the lowest scouring and using solo pile with half pier diameter in upstream with 26% scouring of solo pier has the highest scouring.

There are different techniques to control and reduce local scour around the bridge piers. In this study the effects of collar shape, size and depth of placement on the pier were investigated. The effects of sacrificial piles, slot and collar separately and combinations of double and triad of them on the scour depth were studied. The result showed that the square collar was more effective than circular and lozenge collar in decreasing the scour depth. The installation of collar below the bed level, especially in the depth of -0.1 D was more effective than -0.4D and on the bed level in decreasing the scour depth. The maximum reduction of the scour depth occurred for square collar which equaled to 70% and the next was sacrificial piles with 46.67% and then the slot with 31% had the highest effect, respectively. Combination of sacrificial piles and square collar on the pier with 75% reduction was most effective in decreasing the scour depth.

Scour around bridge piers is one of the most important fields of hydraulic research. Local Scour around bridge piers and abutment is one of the main reasons of bridge failures. In the field of scour at the bridges, researches are more focus on scour of piers in compare with abutment. While the available information on the bridge failures required cost, the major problems are related to abutment. Therefore, most costs have spent in this part. One of the methods used to reduce of scour around bridge piers and abutment is installing collar. The collars protect the river bed against vortex flow in the vicinity of the piers and abutments . In this research the effect of two types of collar, include complete and L-shaped collar, were investigated on abutment scour reduction and application diagram is proposed. Results show that increasing dimensions of collars will increasing the efficiency of performance. Comparing of both collars indicated, that if the length of collars is larger than half the length of the abutment, the L-shaped collar is more effective in compare with complete collar. The L-shaped collar doesn’t prevent sediment movement due to lift vortex in downstream of abutment and for this reason it is shown more effective performance than the complete collar. Also, using the L-shaped collars with less area is more cost effective.

Riprap layer is the most commonly employed countermeasure against scouring around bridge piers. Due to the large area of high intensity flow around a pier, riprap layer extent is large, especially in the case of skewed rectangular piers. Stable riprap size is designed so that it can resist the highest flow forces in the critical region around a pier. However, as the strength of flow forces around a pier is different, smaller riprap sizes can be used in the areas with lower flow forces to get optimum design. In the present study, extent of riprap layer with different sizes around bridge piers is investigated. Three different rectangular piers with or without an attached protective collar aligned with the flow and skewed at different angles are tested. The ranges of aspect ratio for the piers and skew angle were 3 to 7 and 0o to 20o, respectively. The optimal configurations of riprap extent for each pier condition with different sizes were determined. Experiments showed that in the case of aligned rectangular pier without a collar only 8% of the area around the pier is critical and the remaining 92% area can be protected with about 38% smaller riprap stones. As the skew angle of the pier increases, the critical area increases too. In case of protected pier with collar, the collar prevents the region around the pier and therefore, the extent of riprap layer decreases compared to unprotected pier with similar flow condition.

Woody debris accumulates in front of bridge piers, reduces the flow area, deviates the flow and increases the velocity around the bridge piers. Debris accumulation in front of bridge piers increases the downward flow, and shear stress around the bridge pier, therefore, the scour hole depth increases and bed degradation accelerates. Most of previous researchers focused on the scour depth around the bridge piers, and less researchers have investigated the effect of debris accumulation on the scour depth. To the best of our knowledge, the effect of debris accumulation on the scour depth has not been reported in previously published literature, as explored in this work. The purpose of this study is to experimentally investigate the effect of accumulation of woody debris in front of a square pier with a parabolic nose on the scour depth. The experiments were performed in clear water condition, with and without debris accumulation and with 20, 30 and 40 l/s discharges and for different debris dimensions. The experiments were performed in a sixty centimeter width channel at the hydraulic laboratory of Shahrekord University. Previous field studies showed that most debris accumulates in front of bridge piers in rectangular shapes, therefore three different rectangular shapes debris are designed and are placed in front of bridge piers during the experiments. Since, performed experiments at twelve hours showed the maximum equilibrium scour depth occurs at the first seven hours, therefore, all experiments are done in 420 minutes. The results showed that when woody debris is placed over, at, and below the water surface respectively, the ratio of scour depth to the scour depth of control sample is 2.2, 2.36 and 1.44, respectively. Moreover, when the percentage of blockage (ratio of the occupied flow area by debris to the flow cross section) is 30% and when debris is located below the water surface, the maximum scour depth will occur (2.36 with respect to the control sample). Also, the comparison between a square pier with parabolic nose and a sharp nose piers at the same hydraulic condition, show when there are no debris, the scour depth around the square pier with a parabolic nose is less than the scour around a sharp nose square pier (1.5 times). For the case in which debris are placed in front of bridge pier, the scour depth, width, and length around a square pier with a parabolic nose are significantly decreased with respect to a sharp nose square pier (2.3 times). Also, using dimensional analysis an equation is presented for predicting the maximum scour depth around a square pier with a parabolic nose in presence of woody debris accumulation. The results show that there is good agreement between predicted and observed scour depth.

The piers’ shape may be considered as one of the key factors in controlling the scour process around it، as this mainly changes the stream flow pattern around the structure. Furthermore، piers with variable section in depth may also be of interest in reducing the scour around them. The aim of this study was to evaluate the effectiveness of the piers with lateral boundaries fitted by velocity profile due to steady flow، in reducing local scour، when compared to the circular and conical shaped piers. The section of the piers was circular and the widths or diameters of these piers reduce from bed to water surface with a slope fitted by velocity profile. Due to increasing flow discharge close to the water surface، the flow which has maximum velocity in this region will then pass around the piers easier than the cylindrical pier and its velocity is reduced. Therefore، in using these piers، the dynamic pressure in upper regions decreases relative to that of the cylindrical pier and there will be a weaker down flow in front of pier which will consequently cause decreasing of the scour depth. The models tested in this study were three models with logarithmic profiles fitted by (d50=0. 78، 1. 1، 1. 3 mm) and three models with conical profiles by (degrees، in which φ is the angle of lateral slope). Then، the relative scour depth for these models compared with the cylindrical reference model and the maximum scour depth decreased with the rate of 55. 9%، 65. 4%، 73. 2%، 38. 5%، 47. 2% and 52. 7%، respectively. These tests have been performed in a wide flume at hydraulic research laboratory and under clear water conditions over12 hours using relatively uniform sediments particles. For the numerical models، the maximum scour depth around the piers was simulated using Flow-3D model. It was found that the numerical model results were in reasonably good agreement with the experimental data.