Post-Flood Emergency Bridge Inspection
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama On the morning after the 2006 flood in central Pennsylvania, I stood on an approach slab that was sagging like a hammock. The wing wall had tipped out into the river, and the footing was hanging in the air. That bridge was a 90-foot steel beam span, built in 1956, and the flood had moved a large chunk of the bank with it. I've done a lot of inspections since then — roughly 400 underwater inspections, give or take — but that first one taught me the difference between checking a bridge and actually seeing the water. You can't do this job from the deck. You have to get your hands into the mud and your head into the data. The flood is still working even when the rain stops, and the decisions you make during the emergency inspection can keep a community alive or get someone killed. So I treat every post-flood emergency inspection as a structured, methodical activity, even when the pressure is on.
Start before you get to the bridge
I never leave for a flood site without pulling the stream gauge data first. The USGS has real-time stations on most major rivers, and I want peak discharge in cubic feet per second and the time of the crest. That tells me whether I'm dealing with a five-year event or a fifty-year event, and it changes what I bring. For a big event, I pack a probe, a mask and fins, and a dry suit if the water's cold. The numbers only go so far. I also call the county emergency manager and ask if anyone has drone footage or cell phone video from the day of the flood. Often it's shaky, but it can show a logjam that later broke loose and slammed into a pier. That's the kind of thing you'd never see from the deck. I remember one event where the video showed a car wash shed floating into a bridge. We closed it for a week, and the shed was the least of the damage.
What I'm actually looking for
From the deck, I'm comparing the channel to the last inspection report. If there's a bar of sediment that wasn't there before, or a bank that's lost a dozen feet of dirt, I note it. Then I get in the water. For wading-depth bridges, I use a long steel probe — a half-inch rebar rod with a flattened end — and I feel for the hard surface under the mud. I probe around the abutments, wing walls, and any exposed piles. It's slow, dirty work. For deeper bridges, I bring divers. The ones I trust use a full-face mask with communication, so I can talk them through what I need. I ask them to drive a fiberglass probe into the riverbed to measure the scour depth. We record the depth below the water surface, then subtract the known elevation of the footing top. That gives us the original scour versus the current one. It's not high-tech, but it works. Here's a digression: people forget to look up. Debris caught on the superstructure can be just as dangerous as scour. A log jammed between girders can push a span sideways, and it can also create a dam that releases without warning. I've seen a three-foot diameter walnut tree wrapped around a pier cap like a piece of licorice. That tree came from somewhere upstream, and I guarantee the owner didn't report it.
Tools that make the job possible
When the water's too murky to see your hands, I use side-scan sonar. A SonTek M9 or an EdgeTech 6205 gives you an image of the bottom even in zero visibility. I've also used an acoustic Doppler current profiler to see how the water moves around a pier. The vortex action underneath can tell you where the river is pulling material out. But sonar is not a substitute for physical probing. It's a hint. You always need to verify with a diver or a probe. And sometimes the best tool is a simple sounding line. You drop a weight, feel it hit the bottom, and swing it around to see if there's a sudden drop-off. That's how I found an eleven-foot scour hole next to a pier footing that was rated for six. That bridge had been inspected less than a year earlier, and it passed. The flood rearranged everything.
Making the call — open, restrict, or close
The public wants an answer fast. The county roads superintendent wants an answer faster. But I've learned to take the time to classify the damage against the AASHTO Bridge Evaluation Manual. I look for four things: loss of vertical support, lateral displacement, excessive rotation, and settlement. If any of those is severe, I close the bridge. If it's moderate, I restrict the load or the number of lanes. If it's minor, I open it with a speed limit and a follow-up inspection within two weeks. What I won't do is drive my truck over a bridge and pronounce it safe just because the deck looks fine. I've seen a bridge pass a loaded dump truck one day and then fail the next due to waterlogged soil giving way under the approach. So I write the restriction order based on the substructure condition, not the pavement. And I document it all: photos, videos, notes, and a rating sheet. I've also learned to check the approach roads themselves. A flood can undercut the pavement beyond the bridge, and that's the first thing a driver hits. A bridge can be sound and still be inaccessible. So the inspection covers the full approach prism, not just the span.
Frequently Asked Questions
How soon after a flood should a bridge be inspected? As soon as the water drops to a safe level for wading or diving, but we often do a visual deck inspection during the peak. The priority is to close any obviously compromised bridge. The formal emergency inspection can start within 12 hours if it's safe, but you need to be careful about unstable soil and floating debris. What's the difference between routine and emergency inspection? An emergency inspection is triggered by an event like a flood and focuses on whether the bridge is safe for public travel. It emphasizes the four critical failure modes. A routine inspection is a full check of all components, usually every two years. Can you reopen a bridge after a flood without sending divers? If the water is shallow and clear enough to wade, you can probe the footing from the surface. But if it's deep or turbid, I'd never sign off without a diver or sonar. You can't see the worst damage from the deck. Who pays for the emergency inspection? For state-owned bridges, DOT funding covers it. For county and municipal bridges, FHWA often provides emergency relief funds, but you have to document everything to get reimbursed. That's why we take a ton of photos.