Underwater Bridge Inspection Requirements: What 23 CFR 650.311 Actually Says
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Bridge Inspection | Cathodic Protection & Anodes Last spring I stood on a sandbar in the Wabash River with a Kirby Morgan 37 tucked under one arm, waiting for the county sheriff to wave off a ski boat before I put a diver in. That bridge was on a 72-month underwater inspection cycle, and I still pulled the file and re-read the justification before signing the order. Truth is, the rule that lets us go to 72 months is narrower than a lot of people think. The rule is 23 CFR 650.311. Not 650.309, which is where inspector qualifications live, and not 650.311(c), which covers Non-Standard Tolerable Movement steel. The underwater interval lives in paragraph (b). And the baseline is 60 months. Period. There's a whole mythology out there that says '60 months' means you can't go past five years, and another myth that says '72 months' is a secret FHWA handshake. Neither is right.
The 60-Month Baseline and the 72-Month Exception
Under 650.311(b)(1)(iii)(A) and (b)(2), a bridge can get up to 72 months between underwater inspections, but only if it meets specific conditions the regulation spells out. Call it a documented risk-based call. In practice, that means a documented history of no scour, no structural or hydraulic deficiencies, and a written risk assessment from a qualified team leader. I don't even consider a 72-month interval unless the foundation has a clean record across at least two inspection cycles. That last requirement isn't in the rule, but it should be. The baseline, though, is 60 months. That's the default for most bridges. If a bridge has known scour or active movement, you go more often — I've signed 12-month underwater intervals for problem bridges. The 60-month number is the maximum, not the suggested cadence for everything that touches water. The same paragraph also sets routine above-water inspection at 24 months, and allows 48- and 72-month intervals under stated conditions. Don't let anyone tell you that 24 months is a hard cap. It isn't. But the extensions depend on the bridge's condition, not on how cold the water gets in January.
What 650.311 Doesn't Cover
People mix up underwater inspection with the old 'fracture critical' idea. FHWA retired that term years ago. Now it's Non-Standard Tolerable Movement, or NSTM, and it lives in 650.311(c) — not in the underwater paragraphs. NSTM is about steel members whose failure could bring down the bridge. It has nothing to do with depth. You can have an NSTM bridge with a dry creek under it and a non-NSTM bridge with forty feet of water around the piers. Inspector qualifications are in 23 CFR 650.309, not 650.311. The team leader has to be an FHWA-NHI qualified team leader. For underwater work, that means someone who has done the NHI course and has logged real underwater inspections. A PADI or NAUI card doesn't get you anywhere near this job. Neither does a fishing rod. And 650.311 doesn't say a word about equipment. Whether you dive with a Kirby Morgan 37 on surface-supplied air, use an ROV, or wade in with a probe, the rule only cares that the underwater portion gets examined and documented by someone who knows what to look for.
What the Owner Has to Document
The burden falls on the owner, which usually means the state or a local public agency. We have to maintain a written inspection program, set intervals for each bridge, and show FHWA that those intervals are justified. If you want a 72-month underwater interval, the justification has to sit in the bridge file and survive an audit. 'We've always done it this way' isn't a rationale. I sign the interval decisions. When I approve a 72-month underwater interval, I'm taking responsibility for the fact that scour or undermining could develop out of sight for six years while the bridge keeps carrying traffic. That's why I look at the history, not just the age. If there's any question, I keep it at 60 or shorter. State rules can be stricter. Some states won't let you go beyond 60 months, period. That's fine — the federal rule sets a ceiling, not a floor. If your state says 36 months, you do 36. The point of 650.311 is to make sure every underwater substructure gets a serious, documented look on a regular schedule.
What the Field Work Actually Looks Like
On a typical underwater inspection, we put together a dive team with a commercial diver, a standby diver, and a supervisor. We use surface-supplied air — Kirby Morgan helmets are common — because overhead environments and low visibility need more than a scuba tank. Under water, we're looking for scour holes, undermining, debris, cracking, spalling, and signs of foundation movement. We measure, photograph, and record everything. The hazard I watch most closely on any underwater inspection is differential pressure. Static head, not flow velocity. If a diver's body seals or partially seals an opening, the full head acts across that area at essentially zero flow. A 10-foot differential is about 4.3 psi across whatever area is sealed, and that adds up to thousands of pounds of force on a torso. So we isolate, lock out, tag out, blank off, and verify positive isolation before anyone goes near an intake. You never rely on 'staying clear' or hoping the flow stays low. The regulation is the skeleton; the fieldwork is the muscle. After four hundred or so inspections, I still go back to 650.311 every time someone proposes a 72-month cycle. The rule is specific, and my signature is on the line.
Frequently Asked Questions
Can the underwater inspection interval be 72 months? Yes, but only where the bridge meets the conditions in 23 CFR 650.311(b)(1)(iii)(A) and (b)(2). That typically means a documented history of no scour, no structural or hydraulic deficiencies, and a written risk assessment. It's not an informal loophole. Who can lead an underwater bridge inspection? An FHWA-NHI qualified team leader per 23 CFR 650.309. They need the NHI training and documented underwater inspection experience. Scuba cards from PADI or NAUI don't count. Is the routine inspection interval 24 months, period? No. The baseline for routine above-water inspection is 24 months, but 650.311 also allows 48- and 72-month intervals under the stated conditions. The same paragraph governs underwater intervals with a 60-month baseline and a conditional 72-month extension. What's the difference between 650.311 and 650.309? 650.311 sets the inspection intervals and methods; 650.309 sets the team leader qualifications. Underwater interval rules are in 650.311(b). Team leader requirements are in 650.309.