5 th National Congress on Civil Engineering, May 4-6, 2010, Ferdowsi University of Mashhad, Mashhad, Iran Secondary Currents in a Bend Channel With T-Shape Spur Dike Seyed Abbas Mousavi Naeini (1) , Mohammad Vaghefi (2) , Masoud Ghodsian (3) 1- Msc, Hydraulic Engineering, Civil Engineering Department, Tarbiat Modares University 2- Assistant Professor of Hydraulic Structure, Civil Engineering Department, Persian Gulf University 3- Professor of Hydraulics, Water Engineering Research Center, Tarbiat Modares University mousavi_abbas@yahoo.com vaghefi@pgu.ac.ir ghods@modares.ac.ir Abstract The three-dimensional flow filed was experimentally measured in a laboratory flume, using Acoustic Doppler Velocimeter (ADV). Experiments were conducted in a 90 degree channel bend with a spur dike in three positions (30◦, 45◦ and 75◦) and the three-dimensional time-averaged velocity components were detected at different cross sections. The interaction of the spur dike with the secondary current in the bend and the effects of spur dike position in bend on vorticity and its value was investigated. Keywords: flow filed, spur dike, secondary currents, vorticity. 1. INTRODUCTION Spur dikes are constructed transverse to flow and extend from the bank into the river and are often utilized for increasing water depth, bank protection and rehabitation of flood plan. The flow field at a spur dike is coupled with a complex 3D separation of approach flow upstream and a periodic vortex shedding downstream of the spur dike. The complexity of flow increases with the development of the scour hole. Outer banks of river bends are usually associated by scour. As a result lateral migration of channel may take place. The spur dike may be used in a channel bend to control the bank scour and its lateral migration. The scour depth estimation has attracted considerable research interest, and different prediction methods exist at present. However, most of the investigators have focused on scour at spur dike in a straight channel, such as: Ahmad (1951, 1953), Garde et al. (1961), Gill (1972), Richardson and Stevens (1975), Rajaratnam and Nwachukwu (1983a), Shields et al. (1995), Kuhnel et al. (1999), Kothyari and Ranga Raju (2001) and Barbhuiya and Dey (2004). When a spur dike is placed in the outer bank of a bend, the scour process becomes a complex phenomenon. The flow field at bridge piers has been well researched but study on flow field at spur dikes has been limited. Kwan (1989) and Kwan and Melville (1994) used hydrogen bubble technique to measure the 3D flow field in a scour hole at a wing-wall abutment. It was observed that a primary vortex, which is similar to the horseshoe vortex at piers, associated with a down flow being main cause of scouring at abutments. Also, Rajaratnam and Nwachukwu (1983b) and Ahmed and Rajaratnam (2000) explored the flow fields at spur dike and abutment placed on a planar bed. Barbhuiya and Dey (2003) studies flow field in scour hole around abutments in straight channels. Dey and Barbhuiya (2005) studied the flow field at semi circular abutment. The 3D flow filed at a vertical wall abutment was studied by Dey and Barbhuiya (2006). Review of literature shows that in spite of the importance of spur dike in curved channel, less attention have been paid to study the flow field around a T shape spur dike in a bend. It is obvious that there is lack of knowledge regarding the scour and flow pattern around the spur dike in a curved channel. However, no attempt has so far been made to study the scour and 3D flow field at a T-shape spur dike in a curved channel. In this study, results of scour and 3D flow field measurements by an Acoustic Doppler Velocimeter (ADV) in a scour hole at a spur dike located in a 90 degree channel bend under a clear water regime are reported.