When floodwater races past a bridge pier, it can dig away the riverbed around it — scour, one of the main ways bridges fail, and a threat to public safety and interstate commerce. Measuring it the traditional way means sounding from the bridge or surveying the riverbed during a flood, which requires getting instruments close to the piers when the water is at its most dangerous.
From 2019 to 2021, the U.S. Geological Survey (USGS) and the Montana Department of Transportation tested an alternative: video cameras.
How it works
Large-scale particle image velocimetry (LSPIV) estimates how fast and in what direction the water surface is moving by tracking things floating on it between video frames. The video is tied to surveyed reference points on the banks to give it a scale; software then measures how far particles travel between frames and how long it takes. With the stream's cross-section, surface velocities can be turned into streamflow estimates.
LSPIV has been used to estimate streamflow in real time; newer methods apply it to indirect streamflow estimates, water-level triggers that record floods, and support for other measurements.
The four study sites. Credit: U.S. Geological Survey.
The four bridges
| Site | River | Town |
|---|---|---|
| A | Beaverhead River | Twin Bridges |
| B | Jefferson River | Three Forks |
| C | East Gallatin River | Belgrade |
| D | Yellowstone River | Livingston |
Each is also monitored for scour by other methods. The goal was to see whether LSPIV could make bridge scour predictions more accurate.
The equipment
The cameras and computers are small and low-power, running on batteries recharged by solar panels. Recording gear — battery, modem, router and computer — fits in a small waterproof box on the bridge or bank, and videos are stored or sent by cellular modem.
Recording equipment in its waterproof box, and a camera mounted under the Yellowstone River bridge. Credit: U.S. Geological Survey.
Cameras can be placed in several spots: mounted outside the bridge to measure incoming flow and the angle at which it strikes the piers, or on the bank, framing the whole stream surface from bank to bank to estimate total streamflow.
Analyzing flow approaching a pier: surface velocities and the velocity profile in front of it. Credit: U.S. Geological Survey.
What it can do
- Continuous, hands-off data through the whole hydrologic cycle, hourly, with no one near the water.
- Safer flood measurements of velocity and direction near piers, where other methods can fail or be too dangerous.
- Better design: bank-mounted cameras might reveal how flow varies in space and time, helping design bridges and piers.
- Cheap coverage: the cameras are fairly inexpensive, which could help study flooding on remote, intermittent, ungaged streams where storms are rare but intense.
- More uses: phone videos of floods to check streamflow estimates; tracking ice dams and jams; and watching the deepest part of the channel, the thalweg, shift after high flows — a sign of whether structures may be at risk.
What limits it
- Velocities can't be measured if the camera views the water at less than 15 degrees from horizontal — it must be mounted high enough.
- Strong wind can distort surface velocities.
- Vegetation on the banks or in the river can block the view.
- Highly turbulent water gives inconsistent readings.
- It doesn't work at night or when ice covers the water surface.
Sources
Based on "Evaluating the Use of Video Cameras to Estimate Bridge Scour Potential at Four Bridges in Southwestern Montana," USGS Fact Sheet 2022–3040, U.S. Geological Survey, in cooperation with the Montana Department of Transportation; a work of the United States government in the public domain. Full text.
Licence: CC0 1.0 (public domain) · Adapted from pubs.er.usgs.gov
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