Scour Has Dropped More Bridges Than Any Other Cause

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

Underwater Bridge Inspection | Cathodic Protection & Anodes April 5, 1987. The phone rang at six in the morning. I was a junior engineer in the New York State DOT bridge evaluation unit, not yet three years out of school. The trooper on the line said the Thruway bridge over Schoharie Creek was gone. I didn't believe him. I had crossed that bridge in the rain the Thursday before. It seemed solid as stone. Ten people died that morning. The cause wasn't a badly built deck or a failed weld. It was water moving gravel under a pier. That process is called scour, and it has dropped more bridges than any other cause in the United States. I've led roughly 400 underwater inspections in the 35 years since. I've worn out more dry suits than I can remember. But that first phone call is why I still wake up alert before a flood-toe inspection. Scour doesn't announce itself. It doesn't fail bridges in a spectacular collapse; it removes the ground underneath and lets gravity do the work. The bridge looks fine from the deck. People drive across it right up until the moment it isn't. This article is how I inspect for scour, how I read the holes, and how I decide when to close the road. If you take one thing away, take this: the river is always moving the foundation under your bridge. Your job is to look at what's left.

What scour actually looks like

Scour isn't erosion, at least not the slow bank-eating kind. Erosion strips soil over years. Scour digs a hole. When floodwater hits a pier, it speeds up and curls around the sides. The faster water picks up gravel, sand, sometimes boulders, and carries them downstream. A bowl-shaped trough forms at the footing. The bridge doesn't fail because the water is too strong; it fails because the footing is left standing on nothing. There are three flavors. Local scour is that hole right at the pier or abutment, caused by the flow wrapping around it. Contraction scour happens when the whole channel tightens, say under a culvert or a long approach, and the river lowers its bed uniformly across the width. Degradation is a slow, downstream lowering of the riverbed over decades, usually from upstream development or sediment trapping. When you're underwater, you have to recognize which one you're seeing. Each tells you a different story about whether the hole is still growing.

How to inspect a hole that's trying to kill you

Inspection starts with the file. You read the original plans, the last inspection report, and the flood history. Then you get in the water. In clear water, a diver can see the footing. In most working rivers, visibility is under a foot, so you use touch and force. We carry a sounding rod: a six-foot steel bar with a flat plate welded to the bottom. Push it into the gravel. If it stops after a few inches, the bed is firm. If it sinks with your weight, you've found a hole. For deeper or wider spans, we lower a sonar head from a boat. Sonar gives you a picture, but it can't tell you whether the bottom is cobble or fine sand. For that, we use a jet probe: a rigid pipe connected to a high-pressure water pump. The water lets the pipe work through loose material until it hits refusal. That refusal layer is the real floor. The distance from refusal up to the riverbed is the scour depth. The probe is cold, heavy work, and the only reliable way to find out what a bridge is standing on.

How to read what the river left you

Once you've mapped the hole, ask: is it still digging? A stable scour hole has an armored bottom. The water has washed away all the fine material, leaving coarse gravel and rocks that are too heavy to move. Armored holes are dangerous but predictable. An active hole is the opposite: clean sand, loose pebbles, and a bottom that changes shape between floods. We put a monitoring point on the pier and measure depth relative to it. After every significant flood, we go back and re-measure. If the depth increases by a foot or more, you have an active threat. The real check is whether the footing or piles still have enough soil above their bearing depth to keep lateral load. If the river has scoured down to the bottom of a spread footing, you're at critical. If piles are exposed, you need to know the required embedment. HEC-18 gives you the predicted scour depth. The inspection gives you the actual. When actual catches up with predicted, it's time to act.

What you do with the bad answer

There are a few moves, and none of them is hoping the flood is done. Monitoring is the cheapest. Install a simple probe or a fixed indicator; set a threshold. When the hole reaches that threshold, close the bridge until you can get inside. Monitoring only works if somebody actually reads it. I've seen engineers put in a beautiful gauge and then forget it exists. Countermeasures come next. The standard is riprap: angular stone dumped around the footing. It works by armoring the bed so the water can't pick up material. Size matters, and most people undersize it. You need rock the flood can't lift, typically evaluated with the formulas in HEC-23. In tight spots, we use grout bags: heavy canvas containers pumped full of concrete and stacked by divers to form a solid mat. There are also concrete aprons, gabions, and articulating blocks. The last resort is a structural fix: installing piles or pinning the footing into the rock. That is expensive and almost always takes a lane out of service. But it's the only choice when the footing is already exposed. Riprap does not replace lost soil. It only protects what's still there.

The judgment call

The hardest part of this job is not the diving. It's the phone call to the resident engineer who wants the bridge open for school buses. I've closed bridges. I've also re-opened them a day later after a quick countermeasure. The rule I learned at Schoharie is: if you think you need to check, check. And if you don't know what the riverbed looks like under a pier after a hundred-year flood, you don't know the bridge. We work to a standard: a full bridge inspection every 24 months, and an underwater scour re-check after any flood that crosses a specific river stage. That's the minimum. Many states do it more often. It sounds like paperwork until the day it saves a commuter from falling into a river. Scour is the leading cause of bridge collapse in America, but it's also the most preventable. You have to be willing to stand in the cold water and see what's true.

Frequently Asked Questions

If a bridge scores 'critical' for scour, is it about to fall? No. Critical means it needs urgent attention, not that it's falling today. The bridge can still carry traffic if you monitor it and manage the risk. But you need an engineering plan, not just a report. How often do you actually dive under a bridge? Federal rules require a thorough inspection every 24 months. For bridges with known or potential scour, that inspection includes an underwater component. After a big flood, we get out there immediately. The exact schedule depends on the bridge and your state's program. Can I just throw riprap around my footing and stop worrying? Riprap is a proven countermeasure, but only if it's sized and placed correctly. It doesn't replace lost soil, and it doesn't stop scour from moving deeper below it. You need someone who knows the design to look at it after every flood.