Effect of Bridge Pier Shape on Scour Depth at Uniform Single Bridge Pier

Bridge pier scouring may lead to the bridge failure and the shape of bridge pier itself is one of the main factor to control scouring around bridge pier. The amount of sediment which is removed from the boundary of bridge pier is called bridge pier scouring and the depth up to which sediment removed is called scour depth. Scouring results, the exposure of foundation, eventually leading to bridge failures. Assessment of the maximum scour that can be faced by pier has a vital role in the design of bridge foundation. Different techniques are used to minimize the scouring effect in order to control the bridge failure. In this study effect of bridge pier shape on scouring was checked on four different shape i.e. circular, oval, square and octagonal. Each pier shape was tested against three different values of flows i.e. 16, 22.50 and 29.50 liter/sec. Uniform bed material of standard deviation ( g = 1.21) and median size (D 50 = 0.28 mm) was used for all tests. Scour is measured with the help of point of gauge and bed is levelled before every experiment. Experimental results show that minimum scour is observed for octagonal bridge pier and maximum for square bridge pier while circular and oval faced intermediate scour depth relative to square and octagonal bridge pier. It is concluded that scour has reduced to 22% when octagonal pier was used compared to circular one.


Effect of Bridge Pier Shape on Scour Depth at Uniform Single Bridge Pier
Recent studies show that the impact of flow originates the formation of horseshoe vortex and separation of flow from bridge pier leads to the rising vortex. These two are the main factors causing bridge scouring [4]. Two main countermeasure for minimizing scour are bed armoring and flow altering. In the former case, the objective is to reduce the scour inducing mechanisms using hard materials or any sort of physical barriers such as rock riprap. In later case, the purpose is to either prevent the formation of the scour inducing mechanisms or to shift the scour away from the vicinity of the pier.
Raudkiviand Ettema [5] recognized that scouring process has three phases i.e. initial phase, principal phase and equilibrium phase.Scouring process is said to be in equilibrium phase when scour depth does not change with time.The concept of equilibrium scour is uncertain and different approaches has been adopted by different authors present to deal with it. Melville [6] reported that equilibrium scour can be achieved in a limited time but Franzetti [7] did not agree with this concept. Critiana and Lanea [8] concluded that scour also depends upon shape and alignment of pier with flow direction.
Lança [9]  [10] studied the effect of pier shape and alignment on local scour and reported that maximum scour is faced by rectangular pier. He also concluded that length of rectangular pier has no effect on scour depth. Melville [6] found that circular and oval pier behaves in almost same manner with 10%. Some researcher gave the concept of shape factor to relate scour depth of different pier shape. Al-Shukur [11] while both researcher used shape factor one for circular pier.
Piers are divided mainly in two categories uniform piers and non-uniform. Pier with a constant section throughout their depth are termed as uniform while non-uniform pier may have a slab footings and pile foundation by Melville [2]. This study was related to check the impact of the uniform bridge pier shapes on local scour. The fine sand with D 50 = 0.28 mm, having specific gravity 2.79 and geometric standard deviation ( g ) = 1.21 was used as bed material. Yanmaz [12] stated that if  g  1.30 than sand will be uniform. In sand used in this study was uniformly graded. Melville [6] reported that if pier width to grain size ratio is more than twenty-five (25) then sediment size does not affect the scour depth.

EXPERIMENTAL SETUP
Four piers of different shapes (square, circular, oval, octagonal) of equal width were used as pier model as shown in Fig. 3

since it was recommended by Chiew and
Melville [13], that the pier diameter should be less than 10% of flume width in order to avoid the influence the walls which leads to contraction scour. Hence the calculated width in each case was 7cm.

Effect of Bridge Pier Shape on Scour Depth at Uniform Single Bridge Pier
Flow was measured by a rectangular notch as per ASTM standard D5242-92 [14]. Where "H" was measured head above the crest, "P" crest height above the bottom of the channel "L"Crest length and "B" was the channel width as shown in Fig. 4.

Effect of Bridge Pier Shape on Scour Depth at Uniform Single Bridge Pier
In current study values of P, L and B were calculated to be 40, 40 and 100cm, respectively. The standard equation used for discharge measurement given by ASTM standard D5242-92 [14], is: Where H e was Effective head (H e = H + H) and "H =0.003ft (0.001 m) as mention in ASTM standard D5242-92. L e was Effective crest length ( L e = L + L) and "L can be calculated from Fig. 5. C e was Discharge coefficient and was calculated from Fig. 6 as described in ASTM standard D5242-92 [14].

RESULTS AND DISCUSSION
Experiments were carried out for three different heads over crest for equal time and discharges were calculated by using Equation (1) Fig. 9.
Scour around the oval pier was monitored and found to be (5.8, 6.3 and 6.5cm) for flows 16.16, 22.50and 29.50 lit/ sec respectively. Longitudinal profile of scour for oval pier is shown on Fig. 10.
Minimum scour was observed when octagonal pier was used. Scour was found to be 5.4, 5.7 and 5.9cm for flows 16.16, 22.50and 29.50 lit/sec respectively. Longitudinal profile of scour for octagonal pier is shown on Fig. 11.
Melville found that circular and oval piers behave in almost same manner with 10% difference and in current study maximum difference that was observed was 8.97%. Some Researchers used shape factor to relate the scour around different type of piers. Shape factor 1.2 was reported by Cristina Fael [8] for square nosed bridge pier shape while shape factor 1.11 was suggested by Al-Shukur [11] in current study it was found to be 1.10-1.15 relative to scour around circular pier.

CONCLUSIONS
This study was based on the effect of shape of pier on scour depth. Four different shapes were investigated and it was concluded that: (i) Minimum scour was observed in case of octagonal pier.
(ii) Scour has reduced to 22% when octagonal pier was used compared to circular one.
(iii) Maximum scour was observed when square pier was used.

NOTATION
Following notation are used in this research study.