Collaborative Research: Physics-Based Modeling of Bridge Foundation Scour: Numerical Simulations and Experiments

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This project is jointly funded by NSF and the Army Research Office (ARO) and seeks to develop and validate the first 3D, unsteady, numerical model capable of accurately reproducing bridge foundation scour. The basic premise of this work is that fluctuating hydrodynamic forces due to the foundation-induced unsteady coherent vortices drive sediment transport and scour and need to be modeled correctly. Available methods are incapable of capturing the inherently unsteady physics of the problem as they either rely on qualitative descriptions, empirical correlations or employ statistically stationary computational models. To overcome these shortcomings, a research partnership is established among St. Anthony Falls Laboratory (SAFL), Virginia Tech (VT), and the US Army Corps of Engineers (USACE) WES facility. The objective is to integrate the latest developments in 3D coherent-structure resolving numerical modeling of turbulent junction flows with state-of-the-art laboratory capabilities and instrumentation, which permit simultaneous measurements of instantaneous flow quantities and pressures with the corresponding spatial and temporal development of the scour hole. A novel Eulerian model of bedload transport will be developed, which employs Lagrangian ideas to account for the effect of near-bed fluctuating hydrodynamic forces into Exner's equation. Preliminary work has demonstrated the ability of this model to reproduce the highly dynamic evolution of the scour hole including the formation of complex bedforms. The following tasks will be accomplished in this project: a) experimental validation of the hydrodynamic model, b) experiments for monitoring the flow structures simultaneously with the scour hole evolution, instantaneous pressures on the pier and on sediment particles over a wide range of pier diameters, sediment sizes, and flow characteristics, with some of them representative of near-prototype conditions, and c) further development and validation of the new unsteady model of bedload transport. The laboratory experiments will be carried out at thirteen pier-diameter based Reynolds numbers (ranging from 4x104 to 6.7x105), while the numerical model will explore these as well as a wider range of Reynolds numbers. This project will advance, in a collaborative effort, the development of a computational model capable of scour prediction in practical flow conditions, as well as advance our knowledge and understanding of the phenomenon, including upscaling effects. The outcomes of the project will benefit society by providing a powerful computational tool that can be used to study and develop mitigation strategies for the bridge scour problem, which has resulted in more bridge failures than all other causes in recent history and has the potential to seriously impair the nation's transportation infrastructure. The numerical model will also enhance significantly our research infrastructure by providing a tool that can be used to tackle a wide range of stream restoration problems. In this regard the model can be applied to develop improved criteria for stream restoration studies that account for the flow structures around boulders and other obstructions affecting stream habitat quality. The potential impact of this work to stream restoration and outreach activities will be greatly facilitated through cross-disciplinary interactions with the NSF National Center for Earth Surface Dynamics housed at SAFL.