Two-Dimensional Numerical Simulation of Bed Level Variation around Vertical Wall Bridge Abutments
Abstract
An accurate prediction of bed level variation and especially of the mechanism of local scour hole development around bridge abutments is of paramount importance, in river engineering, for a safe design of the construction. In the present research work, a two-dimensional, explicit, finite-volume numerical algorithm, which combines the hydrodynamic equations of viscous, unsteady, free-surface flow in rivers with the continuity equation for the conservation of sediment mass is used to simulate scour depth variation in the region of vertical wall abutments. The capabilities of the applied numerical model are demonstrated by comparing the computed results with available measurements of bed formation in the region of three orthogonal abutments, with different widths, normal to the flow direction. All the experimental results were conducted in a laboratory flume in Technological Educational Institute of Thessaly and scouring depths were obtained in the vicinity of each construction, for different inflow discharges and flow duration. Numerical simulation results of the maximum scour depth and of the developed scour whole area are satisfactorily compared with the experimental measurements. Comparisons show the accuracy and the validity of the applied two dimensional, movable bed numerical techniques.
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Introduction
The most common cause of bridge failures is from floods scouring bed material from around bridge foundations. Scour is the engineering term for the erosion caused by water of the soil surrounding a bridge foundation, piers and abutments. The basic mechanism causing local scour at piers or abutments is the formation of vortices at their base which removes bed material from around the base of the construction. Extensive research has been conducted to determine the depth and location of the scour hole that develops from the vortex that occurs at the abutment, and numerous abutment scour equations have been developed to predict this scour depth [5]. Numerous experimental investigations have been performed on the study of the flow, the bed level variation and mainly the scour mechanism in rivers and especially around bridge piers and abutments [9], [13], [7], [11], [2], [4] and others.
Besides experimental studies, several numerical investigations using Reynolds averaged Navier-Stokes equations of the flow have been developed to examine the flow structure in the hole of local scour and the development of local scour. Threedimensional model provides the most realistic simulation of flow field under turbulence conditions adjacent to bridge piers and abutments. The development of the three-dimensional scour hole around a cylinder by solving simultaneously water flow field with sediment calculation was numerically simulated [12]. A 3-D time-accurate RANS solver with a nonlinear k-ɛ closure with wall functions was used to predict the scour evolution around an isolated vertical abutment (spur dike) in a channel [10]. Local scour depth around bridge pier and abutment, using the commercial solver Fluent with a user defined function for the calculation of channel bed elevation changes, was also numerically simulated [8].
The objective of this research work is to investigate bed formation in alluvial channels as well as in regions around vertical wall bridge abutments. For this purpose a two-dimensional, fully coupled hydrodynamic-sediment transport model was applied using an explicit finite-volume numerical technique [3]. The results of the model are verified by comparing them with available measurements of bed level variation around vertical wall abutments in uniform sediments under clear water scour conditions. The range of water discharge and width of the abutment was sufficient in order new and existing codes properly depict their capabilities against the satisfactorily compared results.
Conclusion
A two-dimensional, explicit, finite-volume numerical model has been applied to simulate bed level variation and maximum scour around bridge abutments in alluvial channels. The numerical predictions were backed by available experimental measurements and a sensitivity analysis was performed in order to test which is the most convenient empirical bed-load formula for the current hydraulic and sediment transport conditions. The applied numerical technique, directly coupling hydrodynamic and bed morphology equations, proved to be computational time consuming. It is stable, reliable, and accurate and can be applied to problems with complicated geometries. The numerical technique itself turned out to be flexible concerning its response to handle rapid changes of sediment transport at the boundaries and especially at regions of bridge constructions. Comparisons between computed results with measurements of scour depths at the region of vertical-wall abutments, in uniform sediments under clear water scour conditions, are graphically presented and can be used by other researchers to assist in the development of new and the refinement of existing codes for computing river bed morphology variations.