Applicability of Scour Equations in Tidal Areas

by J. R. Richardson, (M.ASCE),
E. V. Richardson, (F.ASCE),
B. L. Edge,

Abstract: Bridge scour and waterway instability in the coastal region where waterways are subjected to tidal flow can be subjected to mass density stratification, water salinity, sedimentation (littoral drift and/or riverine transport), unsteady reversible flows from astronomical tides and storm surge, as well as riverine flows. Nevertheless, bridge foundation scour depths can be determined and waterway instability countered using existing scour equations and geomorphology techniques. A major difference for non-tidal streams (riverine) is that the design discharge (50-, 100-, or 500-return period flows) has a constant value. Whereas, with tidal waterways, the design discharge for the same return periods may increase because it is dependent on the design storm surge elevation, volume of water in the tidal prism upstream of the bridge, and the area of the waterway under the bridge at mean tide. If there is erosion of the waterway from the constant daily flow from the astronomical tides which increase waterway area, the discharges can increase. An existing clear-water scour equation can be used to predict the magnitude of this scour, but not its time history. Recent experience indicates that this long-term degradation can be as large as 1 to 3 feet per year.

Subject Headings: Tides | Waterways | Scour | Bridge design | River flow | Water flow | Storm surges | Water discharge

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