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By Samuel H. Austin, U.S. Geological Survey
The problem
Virginia needs safe, cost-effective highway bridges. Over time, streamflow scours sediment from around bridge piers, and designs must allow for the scour a pier will face over the bridge's lifespan — which drives both safety and cost.
Until recently, Federal Highway Administration (FHWA) guidance estimated scour only for loose, granular, highly erodible streambeds. Where the bed resists scour, that overestimates it.
A new approach
Newer FHWA publications let designers account for the scour resistance of piers set in or on cohesive soil or erodible rock. But those evaluations need to know how long the bed is exposed to scouring flows over the bridge's life — information that the Virginia Department of Transportation (VDOT)'s standard hydrologic methods, and those readily available to the industry, don't provide.
This pilot study takes a first step. It estimates, over a bridge's design life:
- how long specific streamflows last; and
- the potential cumulative stream power.
From that, scour rates can be projected for individual piers — potentially saving a great deal on construction while keeping bridges safe.
Who does what
| USGS | VDOT |
|---|---|
| compiles, calculates and sums the hydrologic properties needed to estimate scour rates at test sites | supplies the non-hydrologic pieces: bridge locations and design specifications, properties of the cohesive soils at piers, geotechnical inputs, and preferred methods for estimating soil scour |
How it works
Scour depends on instantaneous streamflow, which changes minute to minute. Using statistical methods including Monte Carlo sampling, the study uses instantaneous flows measured since 1990 to estimate instantaneous flows across the full range of daily flows measured at each site.
Instantaneous flows matched to daily flows (DV). USGS.
How instantaneous flows are sampled. At each time step, a deterministic model takes the daily flow; a Monte Carlo step picks an instantaneous flow at random from the probabilities fitted by Maximum Likelihood Logistic Regression (MLLR). USGS.
Site-specific modeling: daily flows, with Monte Carlo–selected instantaneous flows at the peak. USGS.
These hydrologic statistics feed hydrologic modeling that will help estimate the cumulative scour at specific piers over their projected design life.
Deliverables
- analysis of daily and instantaneous stream discharge data;
- hydrologic design products;
- hydraulic geometry characteristics;
- evaluation of discharge probabilities;
- simulation tools for probabilistic and deterministic analysis of discharge–duration scenarios.
Contact: USGS Virginia and West Virginia Water Science Center, 1730 East Parham Road, Richmond, VA 23228 — usgs.gov/centers/va-wv-water.
Sources
Based on U.S. Geological Survey–Virginia Department of Transportation: Bridge Scour Pilot Study, by Samuel H. Austin, USGS Fact Sheet 2018–3006, U.S. Geological Survey; a work of the United States government in the public domain. The three figures are taken from the fact sheet's PDF, which the import had left out; its photographs, credited to a private individual, are not reproduced.
Лицензия: CC0 1.0 (общественное достояние) · По материалам pubs.usgs.gov
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