Experimental and computational investigation of local scour around bridge piers Ali Khosronejad, Seokkoo Kang, Fotis Sotiropoulos ⇑ Saint Anthony Falls Laboratory, University of Minnesota, 2 Third Ave. SE, Minneapolis, MN 55414, United States article info Article history: Received 5 May 2011 Received in revised form 27 September 2011 Accepted 27 September 2011 Available online 25 November 2011 Keywords: Bridge pier Turbulent flow Local scour Immersed boundary method Numerical modeling Sediment transport abstract Experiments and numerical simulations are carried out to study clear-water scour around three bridge piers with cylindrical, square, and diamond cross-sectional shape, respectively. To handle movable-bed channels with embedded hydraulic structures, the fluid–structure interaction curvilinear immersed boundary (FSI-CURVIB) method is employed. The hydrodynamic model solves the unsteady Reynolds- averaged Navier–Stokes (URANS) equations closed with the k-x turbulence model using a second-order accurate fractional step method. Bed erosion is simulated by solving the sediment continuity equation in the bed-load layer using a second-order accurate unstructured, finite-volume formulation with a sand- slide, bed-slope-limiting algorithm. Grid sensitivity studies are carried out to investigate the effect of grid resolution on the predictive capability of the model. Comparisons of the simulations with the experimen- tal data show that for all three cases the agreement is reasonable. A major finding of this work, however, is that the predictive capability of the URANS morphodynamic model improves dramatically for the dia- mond shape pier for which sediment transport is driven primarily by the shear layers shed from the pier sharp edges. For piers with blunt leading edge, on the other hand, as the circular and square shapes, the URANS model cannot resolve the energetic horseshoe vortex system at the pier/bed junction and thus significantly underpredicts both the scour depth at the nose of the pier and the rate of scour growth. It is also shown that ad hoc empirical corrections that modify the calculated critical bed shear stress to enhance scour rate in the pier leading edge need to be applied with caution as their predictive capabilities are not universal but rather depend on the pier shape and the region of the flow. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Flows around hydraulic structures in natural mobile-bed chan- nels are characterized by arbitrary geometric complexity, due to the inherent morphologic richness of bed bathymetry and a wide variety of hydraulic structures (e.g. barbs, bridge piers, abutments, spur-dikes, etc.), and are dominated by energetic coherent vortical structures. Such vortices interact with sediments on the bed leading to local scour, which could endanger streambed stability and under- mine the structural reliability of embedded hydraulic structures. Lo- cal scour is one of the major failure modes of bridge piers. Briaud et al. [7], for instance, report that as of 1999 more than 1000 of about 600,000 bridges in the United States failed with 60% of these failures being due to scour. Furthermore, local scour could be an important factor in the design of offshore wind and hydrokinetic turbine farms. According to Sumer [52] the cost of a wind turbine foundation in environments where scour protection is a key design issue could be more than 30% of the total turbine design and installation cost. The role of unsteady coherent vortices as the primary mecha- nism for initiating scour around bridge foundations is well estab- lished and documented in several previous studies. Baker [5] was among the first to visualize the turbulent horseshoe vortex system (THSV) that develops in the upstream junction of a cylindrical pier with a flat, rigid bed for Reynolds numbers up to 90,000 (based on the pier diameter D and the approach flow velocity) and was able to identify primary and secondary vortical structures wrapping around the pier. Dargahi [9] employed a visualization technique based on hydrogen bubbles to also study the flow around a cylin- drical pier on a flat rigid bed. His experiments revealed the pres- ence of an intricate THSV system consisting of up to five highly energetic necklace-like vortical structures in the front of the cylin- drical pier, which appeared to form, interact with each other and merge in a periodic or quasi-periodic manner. In a follow-up study Dargahi was the first to study and describe qualitatively the flow patterns upstream of the pier mounted on a mobile bed along with the initiation and progression toward equilibrium of the scour pro- cess [10]. Several other experimental studies focusing on the dynamics of the THSV system for cylindrical piers on a flat rigid bed have also been reported by Devenport and Simpson [11], Agui and Andreopoulos [2], Doligalski et al. [13] and Seal and Smith [49]. Martinuzzi and Tropea [32] and Hussein and Martinuzzi 0309-1708/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.advwatres.2011.09.013 ⇑ Corresponding author. E-mail address: fotis@umn.edu (F. Sotiropoulos). Advances in Water Resources 37 (2012) 73–85 Contents lists available at SciVerse ScienceDirect Advances in Water Resources journal homepage: www.elsevier.com/locate/advwatres