What Happens During an Underwater Bridge Inspection

By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Dana Whitcombe, technical reviewer.

Underwater Inspection | Commercial Diving Contractors Alabama The water dropped to 42°F before we got the call. The dive supervisor said we had maybe 20 minutes of bottom time left, and we hadn't cleared the pier face yet. That's when I learned to listen hard for the tap-tap-tap of the diver's hammer against concrete. Underwater bridge inspections aren't for the faint-hearted. They're a required part of keeping bridges safe, and they're a lot less glamorous than they sound. It's cold, murky, and dangerous work. But it's also the only way to see the parts of a bridge nobody else ever looks at. If you don't go down there, you're missing half the foundation — literally.

Why Go Underwater at All?

Every bridge in the United States that falls under the National Bridge Inspection Standards (NBIS) has to get a full inspection every 24 months at most. That includes the substructure — the piers, abutments, and footings. And a good chunk of that sits below the waterline. So we go down there, not because we're brave, but because federal law and common sense demand it. A routine bridge inspection from topside can see cracks, spalls, and rust on the superstructure. But it can't tell you if the water is silently sucking out the sand and gravel around a pier's footing. That's called scour, and it's what knocked out the Schoharie Creek Bridge in 1987. So if a bridge has any kind of water under it, we have to inspect those submerged members. Period.

The Crew and the Gear

The whole operation runs on a dive team. I'm not the diver — I'm the engineer who rides along, briefs the divers, and reads what they find. A typical crew has a dive supervisor, a working diver, a tender who handles the air hose and lines, and me on the bank or on a boat. We use full-face masks, dry suits in cold water, and hard-wired communication headsets so the diver can talk to us while working. I've used Kirby Morgan helmets and suits from companies like DUI and Whites. Visibility is usually zero. I've seen water so full of silt that a diver can't see the flashlight in front of his face. So he works blind a lot, feeling along the concrete with his gloved hands, tapping with a hammer. Noise is all in your ears — the hiss of the regulator, the thump of the hammer, and the occasional crackle of the dive supervisor's voice. It's a weird way to inspect anything, but it works.

What We're Actually Looking For

We're looking for anything that threatens the strength of the substructure. The big one is scour — the erosion of bed material around the foundation. We measure the depth of the water around each pier and compare it to the original design (channel bed elevations). A drop of three feet can make a bridge 'scour critical' in the paperwork. And we look for undermining — where the scouring has undercut the footing so it's no longer resting on solid ground. Cracks are next. Hairline cracks on a pier can tell a story if we track their width and direction. Spalls — missing chunks of concrete — expose rebar, and that rebar corrodes in saltwater or even freshwater. Sometimes we find debris, like old tires or logs, wedged against a pier. That changes the hydraulics and can cause extra scour or local vibration. And we always inspect the piles, the timber or precast concrete members that carry the load deep into the ground. I've seen timber piles with their outer layer eaten away, leaving a wasp-waist that's barely holding. One digression: locals sometimes think we're out there catching something. I've had a kid throw a fishing line over my probe kit. That's fine — it's their river too. But if you see divers with hammers on a bridge, we're not hunting catfish.

The Tools and Techniques

The first tool is always a side-scan sonar or an acoustic image from topside, especially if the water is moving fast or deep. That gives us a rough picture of the bottom. Then the diver goes in. For visual work, we use a high-intensity dive light and, in clearer water, an underwater camera. But in zero visibility, we rely on tactile methods. The diver runs a chain drag along the concrete — a clean, ringing sound means sound material; a dull thud means delamination. He also uses a large hammer to tap and listen, a method we've used for decades. We also use ultrasonic thickness testing on steel piles or steel jackets. That requires a sensor to be pressed against the steel, and the diver has to clean the marine growth off first — not a glamorous job. And I've used a diving ROV like a VideoRay or SeaBotix. ROVs are great for reconnaissance and reconnaissance before sending a diver in, but they can't do much in bad current or tangled debris. So a lot of the time, it's still a person down there feeling around. It's slower, but it's more reliable.

Turning Notes into a Report

After the dive, we regroup. I sit down with the dive supervisor and review the diver's verbal notes, the video, and the measurements. We mark up the as-built drawings we brought from the office. Then I go back to my desk and write the underwater inspection report. This report becomes part of the bridge's permanent file — it gets reviewed by the next inspector, and it feeds into the state's asset management system. In the report, I document every finding, photograph the critical defects, and assign a condition rating based on the FHWA coding guide. I also recommend follow-up actions — maybe a more detailed investigation, a scour countermeasure, or just a shorter inspection interval. For example, if we find active scour, I might schedule a repeat inspection in 12 months instead of 24. That's how we keep the public safe. It's not glamorous, but it's real, and it's something I've done four hundred times now.

Frequently Asked Questions

How often are underwater bridge inspections required? Under the National Bridge Inspection Standards, most bridges must be inspected every 24 months. If a bridge is found to be scour critical or has known issues, the frequency can be shortened to 12 months or even after major flood events. Do divers inspect every bridge underwater? Not every bridge. It depends on the substructure and water conditions. Bridges over shallow, slow waterways might only need a walk through, but any bridge with water deep enough to hide foundation problems gets a dive or ROV inspection. Can an underwater ROV replace a diver? ROVs are helpful for initial surveys and are safer in some situations, but they can't feel or listen the way a diver can. In zero visibility or heavy debris, a diver with a hammer and chain is still the most reliable way to detect issues. What happens if we find scour at a bridge? We don't panic — we investigate. We compare the measured depths to the design values, check if the footing is undermined, and assess the risk. If it's serious, we might close the bridge immediately or recommend a temporary countermeasure like riprap. Then we set a shorter inspection interval.