EVALUATION OF LOCAL SCOUR DEVELOPMENT AROUND PRISMATIC AND NON-PRISMATIC BRIDGE PIERS WITH DIFFERENT SHAPES

In this study, laboratory experiments were conducted to evaluate the local scour around piers. Effects of many parameters on the maximum depth of scour and scour pattern around piers have been studied. The study considered the effects of upstream flow conditions, shape of pier, side slope angles, and type of soil. Three cross sections for pier (circular, elliptical and oblong) were used for different velocities and discharges to find the scour depth. Two types of uniform cohesionless soil with median grain sizes of d 50 =0.25 (bed soil of Al-Kufa river), and d 50 =0.66 were used as bed material. Four angles of pier side slope were selected 0° (prismatic pier) and 5°, 10° and 15° (non-prismatic piers). It has been observed that for circular cross section pier, the maximum reduction for the scour depth at the upstream of the pier with a side slope of 15° for the soil of d 50 =0.25mm was about 63.64% and for the soil of d 50 =0.66mm was about 54.55%. The reduction value was increased up to 80% and 75% for the same angle of pier side slope (Ɵ=15°) with oblong cross section for the soil of d 50 =0.25mm and d 50 =0.66mm respectively.


INTRODUCTION
Scour is a natural phenomenon caused by erosive action of the flowing water on the bed and banks of alluvial channels. Construction of bridges in alluvial channels causes a contraction in the waterway at the bridge. Contraction in the waterway causes significant scour at that site. Mohamed et al. (2007) studied a physical model to simulate the local scour around piers. Data gathered from experiments conducted on the physical model is used to study the effects of flow depth, velocity of approach, angle of pier inclination, and pier shape on the local scour depth. Negm et al. (2009) introduced a tool that maximize the reduction of the maximum local scour around bridge pier and hence ensure greater safety of the structure against the harmful local scour around the bridge pier. A rectangular collar of fixed width was used. Pourahmadi and Hakimzadeh (2011) conducted an experimental study for various slopes of conical piers using median size of sediment 0.78 mm. Shalmani and Hakimzadeh (2014)  The major goal of the present study is to evaluate the depth of local scour around bridge pier under different shapes of pier (circular, oblong, and elliptical), and side slope angles (0°, 5°, 10°, and 15°) which are defined with respect to the vertical axis. Also, study the effect of using different types of soil on the local scour around pier.
For clear water flow conditions approach, the maximum scour depth (ds max.) at a pier may be a function of the following parameters as shown in equations 1: (d s )=f {B, S, σ g , ρ, ρ s , g, µ, v c , v, y, d 50 , D 1 , D 2 , L 1 , L 2 } Where, B=the width of the channel, S=slope of the channel, =geometric standard deviation, ρ=density of fluid, ρ =density of the sediment, g=gravitational acceleration, µ=dynamic viscosity of fluid, vc=critical velocity, v=mean approach flow velocity, y=flow depth, 50 =medium particle grain size, D1= Top pier diameter, D2=Bottom pier diameter, L1=Top length of pier, L2=Bottom length of pier.
Using non-dimensional analysis, equation 1 can be written as: d s /y = f (B/y, S, σ g , v/(gy) 0.5 , ρ/ρ s , ρvy/ , v/v c , d 50 /y, D 1 /y, D 2 /y, L 1 /y, L 2 /y) As channel width is constant (40 cm) for all present cases, it can be disregarded B/y. Effect of changing channel width is implicitly considered in v/vc. Simplifying the equations 1 and 2, and eliminating the parameters with constant and negligible values, and applying the assumption (effect of viscosity and relative density), equation 2 can be written as: ds/y = f (Fr, v/v c , d 50 /y, D 1 /y, D 2 /y, L 1 /y, L 2 /y)  The flow depth is controlled by an adjustable tail gate at the downstream of the trap basin. There is also a sharp crested rectangular weir to measure flow discharge. This weir is 0.4 m in width, and 0.25 m in height. All depth measurements were carried out using a point gauge with accuracy ±1 mm. It is mounted on a carriage. This carriage can move through the working area transversely and longitudinally. To ensure conformity of measured discharge from weir with applied (actual) discharge, a calibration has been conducted. The standard method of capacity is used to examine the applied flow rate from which the accumulated volume of water is measured by known-volume container and stop watch. Fourteen different discharges were taken to increase the accuracy of the obtained relationship (USBR, 2001). A calibration curve is plotted as shown in Fig. 2, and the obtained relationship is used to calculate the actual required value of discharge.

Fig. 2. Discharge calibration curve.
Two types of uniform texture, cohesionless were used as bed material. Each type was prepared by sieving analysis, the first type was fine sand which represent the soil of bed soil of Al-Kufa river with a medium particle size (d50) of 0.25 mm and = 2.15, the second type was prepared by sieving analysis with a median particle size of 0.66 mm and σ g = 1.37.
In the experiments, wood plates were used for all present models of piers with four different side slope angles (0°, 5°, 10° and 15°) and three cross-sections (oblong, circular and elliptical) as shown in Fig. 3. The angles are defined with respect to the vertical axis. The Schematic presentation for all prismatic and non-prismatic models are shown in Table 1. To prevent any roughness, these models were smoothly finished and painted. The location of these models (piers) was chosen to be within the second third to achieve a well-established flow. Every pier was fixed vertically in the center of the channel base through the sand bed.
In order to find a more realistic equilibrium scour time, eight preliminary tests have been made on prismatic circular and oblong piers for two types of soil (  Steady subcritical flow is used for performing all the experiments at a clear water conditions with plain bed. The flume fills in the working section with 10cm thickness of the sand bed relative to flume solid bed, and this might be a useful tool to scrape the sand bed to a desired level, i.e., 10cm at any point of working section. Point gauge was used to check all levels. The tail gates is raised and the working area is filled with water by hose from downstream portion of the flume in order to allow any air bubble to peculate out of the bed to avoid any settlement around the pier, and to prevent any abrupt high velocity which causes the disturbance in the sand bed after starting pumping.
After starting pumping, the tail gate is gradually lowered until the required water depth established in the flume. At the equilibrium time the scour depth is recorded by using a point gauge at the upstream nose of pier, at which the expected higher scour occurs. After this, the flow is stopped and the flume is drained slowly to avoid any change in the scour hole, the sand is allowed to dry and the required measurements of sand bed upstream, downstream longitudinally and transversely are recorded.
A total of 96 runs with total time of 384 hours have been carried out throughout the experimental work. Nine parameters studied influencing scour depth. Some of those parameters are related to the pier as angle (expressed of diameter and length), and shape. Others are related to flow conditions like depth, velocity and Froude number, also with regard to sediment.

Effect of Pier's Shape on the Scour Depth
The shape of pier has a direct influence on the scouring process. Also, it is observed that, the scour at upstream of the pier is larger than at the downstream, this is due to the strength of the horseshoe vortex which is proportional to pier shape.

Circular Pier
Oblong Pier Fig. 5 shows clearly, that there is a match in the value of the scour (ds) for different velocities for elliptical and oblong shapes, and this will be focus to study the other parameters on the circular and oblong piers only in the present study. Also, it can be seen that the oblong piers caused scour depth less than the circular piers in range of 5.3% to 25%. The contour map and 3D representation of scour hole are also shown for comparison, see Fig. 6 and 7.  pier, (b) oblong pier, (c) elliptical pier.

Effect of Side Slope Angle on the Scour Depth
To investigate the influence of side slope angles on the scouring process and hence on the depth of scour at the upstream of bridge pier, four side slope angles are used in the present  Fig. 8 and 9 illustrate the influence of side slope angles (using prismatic and non-prismatic piers) on the scour depth at upstream of the circular and oblong piers. It can be seen that, the scour depth tends to decrease with increasing side slope angles. Fig. 10 shows that the depth of scour for pier side slope angles (Ɵ=0°, Ɵ=5°, Ɵ=10°, Ɵ=15°) with the oblong section is less than these of circular section. Fig. 11 shows that the reduction of scour for pier side slope angles decrease with increase in velocity for non-prismatic circular and oblong piers. This indicates the use of non-prismatic piers practically in low-lying rivers with velocities.
It was found that the reduction in scour increase by 75% and 54.6% for oblong and circular sections respectively for median particle size (d50) of 0.66 mm and in the rate of 80% and 63.6% for oblong and circular sections respectively for finer sand with a medium particle size (d50) of 0.25 mm. It can be concluded that, the use of non-prismatic oblong cross-section piers could contribute effectively to the reduction percentage of scouring effect.  The contour map and 3D representation of scour hole are also shown in Fig. 11, 12-a and 12-b, and 13-a and 13-b.

Effect of Soil Type on the Scour Depth
Two different type of cohesionless (soil) as bed material are used in experimental work. Fig.   15 illustrates the influence of sediment size on the scour depth at upstream of the pier for the same flow depth and velocity. It can be seen from that figure the scour depth tends to decrease with the increase in sediment size. Also at lower stages of flow velocities the lag between two curves seems greater than that at higher velocities, this may be attributed to that at higher velocities the action of down flow velocity increases leading to increasing the size of "rim" that forms at the base of scour hole in front of the pier. Hence, the large particles can be picked up from the rim leading to an increase in the scour depth as shown in Fig. 16. Figs. 17-a, 17- 18-a and 18-b show the effect of soil type on the contour map and 3D representation of scour hole.