Development of new design guidelines for protection against erosion at bridge abutments - Phase V

The most common type of protection measure for bridges in the United States is to place riprap stone in the form of an apron around the base of bridge abutments and piers. This project will propose new riprap sizing design formulas to protect wing-wall and spill-through abutments against erosion from flood damage. The project will be completed in four main tasks. The first task will evaluate the performance of existing riprap design formulas recommended by HEC 23 for abutments placed in a straight channel and make recommendations for improvement (e.g., accounting for the effect of the floodplain width on the critical Froude number). The second task will use a numerical approach based on fully three-dimensional RANS simulations to determine the critical Froude number for shear failure of the riprap stone for cases when the abutments are placed in a curved channel. The third task will be to propose new riprap design formulas that can be used for wing-wall and spill-through abutments placed in straight and curved channels. The fourth task will be to numerically investigate how the critical discharge for shear failure of the riprap stone varies with increasing flow depth for cases with bridge deck overtopping and to make recommendations for riprap sizing at bridge sites under pressurized flow conditions.

This project aims to evaluate the performance of the formulas for riprap sizing at wing-wall and spill-through abutments presently recommended in HEC 23 and to propose modifications that will ensure the new design formulas can be applied to a wide range of flow (e.g., open channel and pressurized flow) conditions and geometrical parameters (e.g., straight and curved channels, channels with no floodplain and channels with a wide floodplain) at the bridge site.

Maintaining roads operational requires reliable and safe transportation infrastructure design. This is especially the case for bridges that are river-crossing structures that need to remain operational during floods and other extreme weather events. Severe erosion can endanger the stability of the embankments and induce scour beneath the foundations of the bridge abutments and piers. In extreme cases, this may lead to bridge failure. Such erosion problems were reported at many bridge sites even at those where riprap protection measures followed existing design guidelines (HEC-23). The availability of more accurate and general riprap-sizing design equations that can be effectively used in natural environments to protect bridge abutments against erosion will result in a significant reduction of the risk of failure of bridges during natural disasters and will reduce the costs of maintaining the bridges operational. The present validated numerical approach to generate the data needed to develop new design formulas is much less expensive compared to the classical approach based on conducting scaled model studies in the laboratory.

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