The Federal Highway Administration's Hydraulic Engineering Circular No. 18 (HEC-18), "Evaluating Scour at Bridges," has the best available guidelines for estimating bridge scour; however, the proposed Woodrow Wilson Bridge across the Potomac River near Washington, D.C., has a number of features that are not adequately covered by existing guidelines and that made it a special challenge for hydraulic engineers. The new bridge will have fewer but much wider piers than the existing bridge; the piers are designed to have exposed pile foundations to be capped near the water surface; soil borings indicated that there are some relatively erosion-resistant layers below the stream bed in the vicinity of the proposed new bridge; and the bascule piers that support the drawbridges will be protected from vessel impact by very large dolphins and a fender system. Researchers saw this bridge as a unique opportunity to provide technical support and to implement the latest technology for predicting scour depths to be used for the design of foundations (piers) for the new bridge. They sponsored the development of a three-dimensional (3D) sediment transport model that allows them to extrapolate physical model results from conditions that were not feasible to test in a flume. Two different methods were used for extrapolating model results to full scale: a geometric scaling procedure, and a procedure that uses the ratio of the structure width to sediment size as a scaling parameter. Evaluations using soil analyses were very effective for determining scour elevations for several of the piers. The Surface Modeling System--Finite Elements Surface Water Modeling System (SMS-FESWMS) model was a very useful tool for evaluating the more complex flow patterns and for making sensitivity analyses for what would happen if the construction trench fills or does not fill prior to a major flood.


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  • Accession Number: 00790639
  • Record Type: Publication
  • Files: TRIS, USDOT
  • Created Date: Apr 10 2000 12:00AM