Bridge Pier Scour in Tidal Environments

Abstract

Scour depths in estuaries are often estimated using imprecise models that were developed for riverine systems and do not consider the various processes responsible for the removal of sediment in a tidal system. In a tidal or estuarine environment, the flow changes directions with the tide resulting in scour patterns that are quite different from those in upland rivers. At the other end of the spectrum of models used to determine pier scour are the multi-dimensional hydrodynamic models that include the tidal processes; however, these models are unattractive because they require extensive and costly data inputs and are characteristically computationally intensive and very complex. Also, not much accuracy is added to the process by the use of hydrodynamic models as the results they produce are used as inputs to less accurate empirical local scour equations. In order to efficiently enhance the safety of bridges spanning tidal waterways, a multi-component continuous model (WAVES) was developed to predict bridge pier scour in tidal estuaries and facilitate a risk-based design approach for bridges. The WAVES program is the result of the application of accepted theories regarding the mechanism of bridge pier scour in general, along with the combination of tidal and riverine hydraulics. The basis of the model is the novel approach used in the quantification of the effects of the downflow and horseshoe vortex that most experts in the field identify as the agents responsible for scour around bridge piers. Through the performance of this study, several conclusions were drawn. First, a number of scour models and equations currently in use may be inappropriate for bridge pier design and analysis in tidal environments. Second, a temporal model for tidal environments that provides a time history of scour significantly improves the usefulness of scour predictions over single-event models. The methodology employed by the WAVES program provides these improvements at reasonable costs. Third, the WAVES program facilitates a risk-based design approach by yielding estimates of the probability of scour depths over time and will help policy makers and designers to make important decisions regarding the design of safer new bridges and the safe use of existing bridges.