The 31,000 Bridges With Submerged Substructures

By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.

Underwater Bridge Inspection | Cathodic Protection & Anodes First bridge I ever dove was a T-beam on a state route over a muddy creek in eastern Ohio. Visibility was zero, and my supervisor made me find the footing by feel before she'd let me write anything down. That bridge is one of roughly 31,000 in the National Bridge Inventory with a submerged substructure — and every one of them is subject to a federal underwater inspection schedule that most people misread. People hear 'underwater inspection' and assume it's just a dive with a camera. It's a compliance function. The rules are in 23 CFR 650.311, and they're specific about intervals, about who's allowed to lead the team, and about what doesn't count. I've signed off on about 400 of these, and I still see state and municipal engineers confuse the underwater interval with the routine one.

The 60-month baseline is exactly that — a baseline

Under 23 CFR 650.311(b), the baseline underwater inspection interval is 60 months. Not 24, not 48. 60. But it's a baseline, not a ceiling. The rule allows you to move to 72 months — not informally, but under stated conditions in (b)(1)(iii)(A) and (b)(2). Those conditions aren't a rubber stamp. They involve the bridge's condition, the water environment, and your inspection history. The key is to document why you're at 72 months and to be ready to defend it. Routine above-water inspection is different. That baseline is 24 months. And yes, it can be extended to 48 or even 72 months under the conditions in 650.311(b) — but your underwater interval doesn't move with it. I've seen a lot of plans that assume the underwater work happens on the same cycle as the routine one. It doesn't. There's no point trying to force every bridge into the same box. The rule's built for engineering judgment, but it's conditioned on evidence. If you're going to stretch an interval, you'd better have that evidence in the bridge file.

Who is qualified to look under the water

Here's where I sound like a broken record. The underwater inspection team leader must meet the qualifications in 23 CFR 650.309, not somewhere else. For underwater work, that means an FHWA-NHI qualified team leader. And no, your commercial dive supervisor's card from ADCI doesn't automatically make them that, even though ADCI does great work. The NHI course specifically covers bridge inspection underwater — scour, deterioration, structural behavior — not just how to dive safely. I've got an awful lot of respect for the dive community. But a PADI open-water card is about as relevant as a yoga certificate. You need someone who can tell a scour hole from a donut hole and make a load-rating decision in the field. We're not being precious; it's a liability question. If you have to back up your findings in court or before an audit, the first thing people ask is whether the team leader was qualified under 650.309.

The trap: NSTM is not underwater

One of the biggest mistakes I see in inspection reports is mixing up the underwater interval with the inspection category that used to be called 'fracture critical.' In the federal rule, that's now Non-Standard Tolerable Movement, or NSTM, and it lives in 650.311(c). That paragraph is about visual inspection of members whose failure would collapse the bridge. It's not about being underwater. I remember a county engineer who insisted his bridge needed an underwater inspection every year because it was fracture critical. Nope. Those are separate requirements. You can have a bridge that's NSTM and has no underwater elements at all. Or you can have an underwater bridge that's not NSTM. The intervals don't automatically align. Read (b) for underwater, read (c) for NSTM, and keep the file organized. This confusion costs money. I've seen agencies spend real budget on underwater dives for bridges that don't need them, while missing a true underwater bridge that's on their list. Get the classification right first.

What actually happens on an underwater inspection

Let me paint a picture so you know what compliance looks like in the water. We've got a two-person dive team in full-face Kirby Morgan masks, a surface-supplied air console, a video camera that's actually running, and someone topside logging every pass. We clear the mudline off a footing at depth, measure scour with a rod, and look for exposed piles, spalling, cracks, and any sign of settlement. If visibility's good we'll do a systematic grid; if it's zero, we work by touch and take samples. Call it 90 minutes bottom time on a typical substructure unit. The point is, the dive itself isn't the inspection. The dive is how you gather data. The inspection is the analysis and the written report. A lot of the work happens inside the dive trailer, between dives, comparing what we see with prior reports. And that's why the team leader qualification matters. It's not about being a good diver; it's about being a good engineer who can dive and interpret. One digression, if you'll tolerate it. I've had plenty of days where the water temperature was 48 degrees and the current reminded me why I like bridges over culverts. But the worst thing we deal with is silt. You can have a perfectly well-behaved river and still get zero visibility as soon as you stir up the bottom. That's when you rely on your training and your hands. Anyway.

How to stay compliant without getting burned

Start with a list. Your bridge inventory should flag every bridge with an underwater element and the date of its last underwater inspection. Then match that against the interval — 60 months baseline, but if you've got a bridge that meets the conditions, you can plan a 72-month cycle. Just make sure the FHWA-approved file contains the justification. It's not something you can declare at the dock. Also, maintain your staffing. The biggest bottleneck in this program is qualified team leaders. I've seen states struggle with two or three people covering hundreds of bridges. That means training is a compliance issue, not a nice-to-have. Budget for NHI classes, and keep your dive team current on OSHA 1910 Subpart T for commercial diving, even though the inspection rules live in 650.311. Finally, don't wait until the interval lapses to think about the next one. The agencies that stay clean are the ones that schedule the underwater inspection in the same planning cycle as the routine inspection, and they read the rule as a whole. There's no secret enforcement trick. It's just methodical work.

Frequently Asked Questions

Does every bridge in the U.S. Require an underwater inspection? No. Only bridges with submerged substructure elements. The sum is roughly 31,000 in the National Bridge Inventory. If your bridge has water under it but no permanent element below the waterline, the underwater inspection requirement may not apply. But if it has piles, footings, or piers that are normally submerged, it's on the list. Can the 60-month underwater inspection interval be extended to 72 months legally? Yes. 23 CFR 650.311(b)(1)(iii)(A) and (b)(2) lay out specific conditions. You have to document why the bridge qualifies and keep that justification in the bridge file. It's not a blanket allowance, and it's not an informal FHWA wink. Follow the rule. Does a commercial diving certification alone make someone a qualified underwater bridge inspection team leader? No. The team leader must meet 23 CFR 650.309, which for underwater work includes being an FHWA-NHI qualified team leader. Commercial dive certifications like ADCI are about dive safety, not bridge inspection. Both matter, but they're different qualifications. What is the difference between the 24-month routine inspection and the 60-month underwater inspection? Those are different intervals under 23 CFR 650.311. The routine above-water baseline is 24 months, with 48- and 72-month extensions possible under stated conditions. The underwater baseline is 60 months, and the only standard extension is to 72 months. People mix them up all the time.