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45 CHAPTER 6 THE SRICOS-EFA METHOD FOR INITIAL SCOUR RATE AT COMPLEX PIERS 6.1 GENERAL front of the cylinder and the model. The application of the SSIIM Model can be found at http://www.sintef.no/nhl/vass/ The initial scour rate is an integral part of the SRICOS vassdrag.html. Method because it is one of the two fundamental parameters Wei et al. (1997) performed a numerical simulation of the used to describe the scour depth versus time curve. The other scour process in cohesive soils around cylindrical bridge piers. fundamental parameter is the maximum depth of scour, which A multiblock chimera Reynolds-averaged Navier Stokes was studied in Chapter 5. The initial rate of scour for a given (RANS) method was incorporated with a scour rate equation complex pier scour problem is obtained by first calculating to simulate the scour processes. The scour rate equation linked the maximum shear stress max existing around the pier before the scour rate to the streambed shear stress through a linear the scour hole develops (flat river bottom) and then reading the function. The simulation captured the important flow features initial scour rate on the erosion function obtained in the EFA such as the horseshoe vortex ahead of the pier and the flow test. Therefore, the problem of obtaining the initial rate of recirculation behind the pier. A reasonable agreement was scour is brought back to the problem of obtaining the maxi- found between the progress of the scour depth obtained in the mum shear stress around the pier before scour starts. This flume experiments and predicted by the numerical simulation. problem was solved by using numerical simulations. The sim- Wei et al. found that the value of the critical shear stress has a ulations performed and the associated results are described in significant influence on the scour process around a cylinder in this chapter. The goal was to develop correction factors for cohesive soils. The final scour depth and the time necessary to giving max for a cylindrical pier in deep water (Equation 6.1): reach it increase with decreasing critical shear stress. Based on a number of parametric runs, they also presented an empirical max = 0.094 v 2 - 1 1 formula for the maximum streambed shear stress for a cylin- (6.1) log R e 10 drical pier in deep water. Dou (1997) simulated the development of scour holes These factors include the effects of shallow water depth, around piers and abutments at bridge crossings. A stochastic pier shape, pier spacing, and angle of attack. turbulence closure model (Dou, 1980), which includes an iso- tropic turbulence, was incorporated into a three-dimensional 6.2 EXISTING KNOWLEDGE ON NUMERICAL flow model, CCHE3D, developed by the Center for Computa- SIMULATIONS FOR SCOUR tional Hydroscience and Engineering at the University of Mississippi. The factors that reflect the secondary flow motion Hoffman and Booij (1993) applied the Duct Model and the generated by the three-dimensional flow are adopted to mod- Sustra Model to simulate the development of local scour ify the sediment transport capacity formula originally devel- holes behind the structure. The flow model upon which Duct oped for estimating general scour. Dou's study also includes is based is a parabolic boundary-layer technique using the some investigations on sediment incipient movement in local finite element method. The Sustra Model is used to compute scour and includes some laboratory experiments. the concentration field following the approach used by Van Roulund (2000) presents a comprehensive description on Rijn and Meijer (1986). The computational model results the flow around a circular pier and the development of the were compared with experimental data. The results (i.e., flow scour hole by numerical and experimental study. The numeri- velocities, sediment concentration, and bed configurations as cal model solves the three-dimensional RANS equations with a function of time) showed an agreement between the exper- use of the k - (SST) turbulence closure model. The method imental data and the computational model. is based on a full three-dimensional bed load formulation, Olsen and Melaaen (1993) simulated scour around a cylin- including the effect of gravity. Based on the bed load calcula- der by using SSIIM, a three-dimensional free-surface flow and tion, the change in bed level with time is calculated from the transport model. The SSIIM Model solves Reynold stresses by equation of continuity for the sediments. For the experiments, the k - turbulence model. The authors observed and reported the scour development from a flat bed to the equilibrium of the that there is agreement between the pattern of the vortices in scour hole was videotaped and this visual record was used to