Development of New Design Guidelines for Protection Against Erosion at Bridge Abutments and Embankments – Phase II

This research proposes a numerically-based approach to develop improved methodologies to design riprap protection measures at wing-wall and spill-through abutments. Such abutments are very common at small bridges where abutments are placed on the floodplain and no piers are present. The mean flow fields and the bed shear stress distributions predicted using high-resolution, fully 3-D RANS simulations 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). During the first year, the numerically-based approach was validated for the case of wing-wall abutments placed in a straight channel, for which detailed laboratory experiments are available. A limited number of simulations were also conducted for spill-through abutments. In the second year, a more comprehensive parametric study with varying floodplain width, ratio of abutment length to floodplain width and riprap stone size will be conducted for spill-through abutments placed in a straight channel. The goal is to fully evaluate the performance of Lagasse et al. (2001) formula and to propose modifications that will ensure the (modified) design formula is conservative enough when applied for a wide range of flow and geometrical parameters. In the second part of the study, the effect of bank curvature on erosion potential at wing-wall abutments will be investigated. The aim is to quantitatively understand how channel curvature amplifies the maximum bed shear stress over the riprap layer and then to propose a procedure based on the design formula of Lagasse et al. (2001) that will allow estimating the required size of the riprap stone needed to protect an abutment situated at the outer bank of a curved channel. Recommendations will be made for inclusion of these modifications in future versions of HEC-23.

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