Assessing Bridge Scour With South Carolina's Envelope Curves

Engineers predict scour at bridges with the equations of the Federal Highway Administration's Hydraulic Engineering Circular No. 18 (HEC-18), the current standard of practice. But the equations come from laboratory experiments, and applying them in the field carries uncertainty that laboratory researchers acknowledge and field investigations have confirmed.

HEC-18 therefore advises engineers to check computed scour depths and adjust any that look unreasonable. The best check is to compare them with scour that has actually happened in past floods — but such measurements rarely exist at or near the bridge in question.

Measuring scour at 200 bridges

To fill that gap, the U.S. Geological Survey and the South Carolina Department of Transportation (SCDOT) carried out three field investigations of historic scour at the state's riverine bridges. At more than 200 bridges they measured clear-water abutment, contraction and pier scour, and live-bed contraction and pier scour.

South Carolina's physiographic provinces — Blue Ridge, Piedmont, Upper and Lower Coastal Plain — and the bridge-scour study sites. USGS.

The measurements showed regional patterns and produced envelope curves: lines that bound the largest scour observed, which engineers can use alongside the equations to judge every component of scour. Four USGS reports documented how to use them and their limits.

Live-bed pier scour. The South Carolina envelope, y = 1.1b + 3.34 (scour depth y and pier width b, in feet, for piers up to 6 feet wide), sits below the one drawn from the National Bridge Scour Database, y = 1.5b + 4.1. Dashed lines are projections. USGS.

Clear-water abutment scour in the Piedmont against the length of embankment blocking a 100-year flood: the envelope y = −0.000009L² + 0.0276L, for embankments up to 950 feet. USGS.

One procedure

Because each report covered a different component of scour, a single integrated procedure was needed. A 2016 study (Benedict and others) set out to:

• combine the earlier findings in one guidance manual for applying the envelope curves, with a companion spreadsheet that automates the calculations;
• test the South Carolina curves against published scour data from outside the state;
• develop envelope-curve coefficients for the 500-year flood for each component of scour;
• merge the three databases into a format that could later go into a web map application such as StreamStats.

Who benefits

• SCDOT gets independent verification of the curves and a consistent, quick way to assess scour potential at any riverine bridge in the state.
• Other states with streams like South Carolina's may be able to use the procedure directly; where streams differ, it still offers guidance on how scour behaves.
• Science gains a better understanding of general and regional scour trends from field data in South Carolina and elsewhere.

Sources

• Stephen T. Benedict, Toby D. Feaster and Andral W. Caldwell, Assessing Potential Scour Using the South Carolina Bridge-Scour Envelope Curves, USGS Fact Sheet 2016–3065, September 2016, prepared with the South Carolina Department of Transportation. https://doi.org/10.3133/fs20163065
• Full study: Benedict, Feaster and Caldwell, USGS Scientific Investigations Report 2016–5121. http://dx.doi.org/10.3133/sir20165121