There Are 31,000 US Bridges With Feet Underwater

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

Underwater Bridge Inspection | Cathodic Protection & Anodes I remember a February inspection on the Meramec River, water at 44 degrees, visibility eight inches. The diver came up holding a piece of the upstream nose of pier 3. It wasn't chipped; it was gone. The entire round had eroded down to the reinforcing steel. That is what underwater bridge inspection is for: finding the damage you cannot see from the deck. There are 31,000 bridges in the United States with their feet underwater—permanently submerged foundations carrying the weight of a highway on your state's rivers, lakes, and tidal creeks. A bridge's 'feet' are its pier bases, pile caps, and the piles that punch into the riverbed. You don't inspect them with binoculars. You put a diver in the water, usually blind, and you trust the training and the checklist. I've led these inspections for state DOTs and the feds for over twenty years. This is how it works, how it should work, and the mistakes I've made along the way.

What 'feet underwater' means

The National Bridge Inventory catalogs just over 600,000 bridges. Of those, roughly 31,000 have substructure elements below the water line for the entire year. That's the number that keeps me up at night, because each one requires a dive team, a plan, and a report. The 'feet' are the spread footings or pile-supported foundations. You can't see them from the deck, and you can't see them from a boat if the water is murky. You have to touch them.

The rule that sets the clock

Federal regulations are clear. Under 23 CFR 650.311(b), the baseline underwater inspection interval is 60 months. That's the default, not the maximum. Your state may extend to 72 months, but only if the bridge meets the criteria in 650.311(b)(1)(iii)(A) and (b)(2)—typically a low-risk structure with no known scour issues. The routine above-water inspection is 24 months, so you'll likely see the bridge five times before you dive it twice. Don't confuse the two intervals. And the person in charge must meet the qualifications in 23 CFR 650.309, which for underwater bridges means an FHWA-NHI qualified team leader. Your recreational divemaster card does not count.

What the team leader actually does on site

The team leader is not the person who just signs the report. Before the dive, I review previous reports, scour data, and any channel changes. On site, I walk the deck with a trowel and a sounding rod to sense how the structure feels. Then I set the dive plan: entry point, buddy assignment, task list, and communication signals. The diver works blind, using fingertips to feel for delamination, spalls, and cracks. I also run a tape measure down the face to check for settlement relative to the waterline. The leader's job is to make sure the diver comes back with data, not just a story.

Scour and the damage we miss

Scour is the number-one killer of water-crossing bridges. Flowing water removes bed material around a footing until it is no longer supported. We measure it with a probe or sonar, but the real skill is knowing what the pre-storm depth was. A common mistake is to compare scour depth to the original ground at the time of construction. The original ground is gone. We need the last inspection's cross-section. During a dive, I have the diver dig a hole at the upstream face to see if the underlying soil is loose or dense. A loose layer that goes down two feet is a warning sign.

When the water won't cooperate

Every inspector has a story about high water, zero visibility, and a deadline. The hard truth is that you cannot inspect what you cannot access. A diver in a dry suit in 44-degree water with eight inches of visibility is limited. When I had that Meramec job, I made the mistake of sending the diver down without a full pre-dive briefing on what the pier nose should feel like. We found the missing footing, but only because the diver was thorough. Now I write a detailed 'touch map' that tells the diver exactly how far along each face to sweep and what texture to expect. It is not glamorous. It works.

Frequently Asked Questions

Why can't you just use sonar or a camera? Sonar and ROVs are great for locating big problems, but they can't feel a hollow-sounding delamination or a crack that's hairline but growing. A diver's hand is still the best sensor for concrete. How do I know if my inspection team leader is qualified? Look for proof of FHWA-NHI underwater bridge inspection training, not just a dive certification. The requirement is in 23 CFR 650.309, and it is not satisfied by a PADI card. What happens if a bridge needs more frequent inspection? The 60-month interval is the baseline. If we find significant scour or movement, we can shorten the interval to 24 months or less, and we often do. The regulation sets a ceiling, not a goal.