The Scour Bridge Simulation around a Cylindrical Pier Using Flow-3D M. Ghasemi 1 , S. Soltani-Gerdefaramarzi 2* 1. Ph.D. Student of Water Engineering, Department of Water Engineering, Isfahan University of Technology, Iran 2. Assistant Professor, Factually of Agriculture and Natural Resource, Ardakan University, Iran A R T I C L E I N F O Article history: Received: 4 June 2017 Accepted: 25 August 2017 Keywords: Flow-3D, Scour, Cylindrical Pier, Rectangular Channel. A B S T R A C T Scour simulation is a way to prevent a bridge against the possible major damages. In this study, local scouring around a cylindrical pier in non-cohesive bed sediment was simulated with the aid of Flow-3D model capabilities. A rectangular channel with 0.4 m width and 1.0 m length and a vertical cylindrical pier with diameter of 0.03 m and height of 0.3 m were specified for simulation of scour depth. Simulations were performed at different flow rates of 5, 10, 19 and 30 L/sec., separately and parameters of flow velocity, fluid depth, Froude number, packed sediment height and changes in net sediment motion were investigated. According to the results obtained from the simulations and after 600 sec., maximum scour depths for different inflow rates were equal to 0.0, 1.3, 2.4 and 3.6 cm for 5, 10, 19 and 30 L/sec., respectively. It was also observed that the scour depth was comparatively higher in upstream while it was lower in downstream part of the pier. Comparisons between the simulated and observed scour depths, however, showed underestimates of 30% to 20% for upper and lower parts of the pier, respectively. 1. Introduction Scour or the removal of material from the bed and banks of streams and near the piers and abutments, always occurs due to flow acceleration, turbulence and erosive action of flowing water (Soltani-Gerdefaramarzi et al., 2013a and 2014). This process is affected by a large number of variables, mainly, the flow, fluid, pier and sediment characteristics. It can be divided into three main forms of general scour, contraction scour and local scour (Soltani- Gerdefaramarzi et al., 2013b). However, general scour (or evolution of the waterway) occurs naturally in river channels by aggradation and degradation of the river bed. A change in the river hydraulic parameters is known as the major cause for this type of scouring. Contraction scour occurs by reduction of channel’s cross-sectional area at the location of water structures (like bridge piers and abutments) that increases the flow velocity and bed shear stresses and finally, transport of the bed sediments (Briaud et al., 2012). Local scour is the third type of scouring that occurs in the vicinity of bridge piers or abutments. In such a scouring, downward flow is induced at the upstream end of the pier and leads to a localized erosion around the pier. It depends on the balance between streambed erosion and sediment deposition (Prendergast and Gavin, 2014). Based on this fact and the mode of sediment transport in the approaching flow, two types of local scour can be distinguished, namely clear water scour and live bed scour. In clear water scour, no sediments are delivered by the river approaching flow, while an interaction exists between sediment transport and the live bed scour process. Local type of scour has long been acknowledged as a major cause of the bridge failures around the world, causing major human and financial losses. For example, Alabi (2006) reported that the average cost for flood damage repair of the United States highways was $50 million per year. Additionally, it was estimated that at least $20 million have been invested on scour research in the United States in the last two decades (Briaud et. al, 2012). Alabi (2006) also cited that the associated repair costs of the bridge collapse in 1993 (due to scour) in the upstream Mississippi and downstream Missouri river basins were more than $8,000,000. Considering more severe and more frequent floods due to climate change, it is gaining an ever- increasing importance to mitigate the risk of bridge failure (Prendergast and Gavin, 2014). Monitoring of the local scour is a way to avoid major damages that may occur (Elsebaie, 2013) and to ensure the continued safe operation of the structure. But, most traditional monitoring techniques are based on installation of expensive underwater devices that may be damaged during a flooding event, when the highest risk of scouring exists. In such circumstances, computerized simulation of the scour occurrence can be introduced as a low cost/time consuming alternative for the prediction of the probable failures. The application of simulation software is in many ways similar to the set-up of an experiment, in which Computational Fluid Dynamics (CFD) methods are always used for the simulation of flow process by discretization and solving of Navier-Stokes and continuity equations for the ISSN: 2345-5608 Journal of Hydrosciences and Environment Available online at http://jhe.usb.ac.ir/ ________ * Corresponding author’s email: Ssoltani@ardakan.ac.ir