Flow-3D Modelling of the Debris Effect on Maximum Scour Hole Depth at Bridge Piers

Bridge foundations are frequently inserted in riverbeds which are constituted by alluvial material. As a consequence of that, they are subjected to erosive processes that take place next to them. Such phenomenon is aggravated during flood events that can be caused either by the increase of water flow and/or by the increase of the debris carried by the flow that can pile up along bridge structural elements. The debris, mainly floating wooden materials, can partially obstruct river spans and accumulate near bridge piers and abutments which will have consequences in terms of reduction of flow capacity and an increment of the maximum scour depth near this structures. The dimensions, shape and location of the debris are characteristics that must be considered when predicting the scour hole depth. Until now, very limited efforts have been made to investigate these effects in a systematic way. The present study aims at analysing the effect of the debris on the maximum clear-water scour depth at cylindrical piers inserted in uniform sand, non-ripple forming sand, fully-developed turbulent flows in wide rectangular channel, using computational fluid dynamics software. For this purpose, debris with a triangular and rectangular shape, floating and on the sand bottom, were tested, using the k-ε RNG turbulence model and the numerical results were compared with those obtained experimentally in the Laboratory of Hydraulics and Structures of the University of Beira Interior by Dias (2018). With the results obtained throughout the numerical simulation it was possible to verify that the floating debris gives greater scour hole depth and the debris on sand bottom behave as scour countermeasure reducing the scour hole depth, as verified in the experimental work. Also, rectangular shaped debris seems to cause more erosion near the structures than the triangular shaped ones, which also seemed beneficial to the scour hole, by reducing it. However, the results obtained computationally are significantly lower than those obtained experimentally. These differences are analysed and the results are discussed.

Keywords

debris, cylindrical pier, local scour, RANS, numerical modelling.