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Scour around a bridge pier is one of the major possible causes of the hydraulic failures of a bridge. A typical sediment-water-structure interaction at the upstream junction of the bridge pier initiates a vortex system, the primary vortex of which is responsible for local scour around the pier. The growth of the primary vortex can be arrested by retaining it on a rigid surface as on a collar plate. A number of experiments covering various sizes of collar plates were carried out in an effort to minimize scour around a bridge pier. A collar plate of size three times the diameter of the circular pier, 0 or larger when placed at the average bed level assures I 00% scour reduction while a collar plate of size 20 placed 0.10 below the bed does not allow scour depth to go beyond it. Anticipating degradation of general bed level, a group of multiple collar plates of smaller size were employed at the laboratory scale to reduce the scour considerably. A group ofthree collar plates of size 1.50 having an inter-plate spacing C, of0/6 with the middle plate placed at the average bed level reduces scour by 83% in comparison to the scour depth of an unprotected pier. The composite device, apart from being axis symmetric, is highly suited to degrading river beds.

The study of scouring around bridge piers is very significant and plays a very important role in the safe and economical design of bridge piers. The term scouring is a process by which bed particles around the periphery of the pier get eroded and removed over a certain depth resulting in the formation of a scour hole around the bridge piers. This paper presents a detailed investigation of local scouring around six different shapes namely rectangular, circular, chamfered, joukowsky, oblong, and sharp-nosed of bridge pier models for selecting the best and most economical bridge pier shapes based on local scouring. For the experimental studies, local scour depth is measured around all six different shapes of bridge piers with varying discharges and velocities of 0.0169 m3/s to 0.0355 m3/s and 0.17 m/s to 0.30 m/s respectively. It is found that scour depth around the rectangular shape of bridge pier is larger due to development of strong intensity of horseshoe vortex caused by flat exposed frontal area and scour depth around the sharp nose shape of the pier is smaller because of flow is bifurcated due to smooth curvature caused less intensity of development of horseshoe vortex. In the comparison of obtained results, it is reported that scour depth around a pier is exactly proportional to velocity of approach flow and exposed frontal area of the bridge pier. The result of this study can be likely used to help in selecting the best shape of the bridge piers as sharp nose shape of bridge piers is better among the other six bridge pier shapes for design purposes. Key Words: Local scour, shape of bridge pier, fine bed material, scour depth

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Excessive scouring of sediment around bridge piers plays a major role causing failure of waterway structures. To prevent scouring around bridge piers, it is necessary to determine the maximum scour depth. In the past, researchers have conducted extensive laboratory work around flow structures with the help of scaled models. But most of the work has been carried out around an isolated pier and the work around a group of piers is relatively much lesser. To fill up the gaps in information, an experimental study was conducted in a recirculating water flume 15 m long, 0.4 m wide and 0.6 m high containing uniform sediment of mean size, d 50 = 0.30 mm. Experiments on cylindrical pier models of size 42 mm were run at a velocity equal to 0.89 times the critical velocity, u c. Three different arrangements i.e. Tandem, Side by Side and Staggered have been used to analyze the effect of spacing and orientation on scour. From the initial observations it was concluded that, in the case of Tandem arrangement when the spacing between the piers was 16 times the diameter of the pier and more, both the piers showed independent behavior. But if the spacing between the bridge piers is 0D, then both the piers behave like a single pier and the scour depth was found to be 41% greater than that of a single pier. However, when the clear spacing was greater than 1.5 times for side by side and 2.5 times for staggered arrangement, the piers behaved independently.

02-0.068, 0-0.0035, 0.013-0.139, 0.002-0.06, 0.001-0.09, 0.003-0.08, 0.075-0.488, 0,001-0.389

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