United States Department of Transportation
i
Development of New Design Guidelines for Protection Against Erosion at Bridge Abutments-Phase II
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2020-01-01
[PDF-4.51 MB]
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Edition:Final Report May 2017-May 2018
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Abstract:Severe floods in the United States Midwest have caused major erosion at bridge abutments despite the fact that their protection design measures followed existing guidelines (e.g., HEC 23, 2001 and following updates through 2009). One likely possibility for the severe bridge erosion observed especially at small bridges is that the methodology to estimate the variables in the design formulas (e.g., for the minimum size of the riprap stone used for protection) recommended in these guidelines is oversimplified and/or does not account for complexities associated with these structured being placed in natural streams where, for instance, bank curvature effects may be important. The present research proposes a numerically-based approach for improving methodologies to design riprap protection measures at wing-wall and spill-through abutments. The mean flow fields and the bed shear stress distributions are obtained from fully three-dimensional, non-hydrostatic RANS simulations. These data are used to estimate the maximum bed shear stress over the riprap layer, the shear-failure entrainment threshold for the riprap stone and the other variables in the design formulas recommended in HEC 23 (Lagasse et al., 2001, Pagan-Ortiz, 1991). The numerically-based approach was validated for the case of wing-wall abutments placed in a straight channel based on data from laboratory experiments. The present Year 2 report describes how channel curvature and floodplain width affect the maximum bed shear stress over the region protected by riprap for wing-wall abutments placed at the two sides of a channel containing a floodplain at its two sides. A modified, two parameter, design formula is proposed that explicitly incorporates the effects of bank curvature and floodplain width.
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