Investigation of local scour around unconventional bridge piers

Local scour around bridge pier is the leading cause of bridge failure, effecting significantly on the total construction and maintenance costs. Therefore, scouring around uniform pier with cylindrical shape is commonly used in the field. The shape of bridge piers has important effect on the local scour. A literature review showed that there is still a lack of experimental studies on the effects of non-uniform pier, especially under live-bed scour. The present study experimentally investigated the effect of bridge pier shape on scouring process. Three different shapes of bridge piers (10-4, 10-6 and 10-8 cm) were tested using with three discharges (38, 48 and 58 l/s) for duration of 3 hours under live-bed condition. The present experimental studies consist of two different orientations of bridge piers according to flow direction. These orientations are named as "upstream-facing round nosed pier (US-FRNP)" and "downstream-facing round nosed pier (DS-FRNP)" according to flow direction. The experimental results of oriented US-FRNP and DS-FRNP models are compared with round-nosed pier (10-10 cm) and circular pier (10 cm diameter) models. The results of comparison between the performances of bridge piers under different flow conditions reveal that downstream facing round-nosed bridge pier (DS-FRNP) is an effective countermeasure to reduce the depth of scour. An empirical relationship was also developed on the basis of obtained experimental results. In this study, the three-dimensional flow field inside scour hole is also experimentally investigated. The velocity field measurements were obtained using an Acoustic Doppler Velocimeter (ADV). Vector plots of the flow field in the upstream and downstream plane of bridge piers reveal that the existence of downward flow and subsequently vortex formed at the toe of the pier named horseshoe vortex and the wake vortex in the downstream. Scouring effect of downflow and horseshoe vortex was decreased by changing the orientation of bridge pier. The bed-shear stresses are also calculated from Reynolds stresses and Turbulent Kinetic Energy method.