Bridge Scour: The Number One Cause of Bridge Failure in the US
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Last October I dropped into the South Branch of the Potomac wearing a dry suit and a twenty-pound weight belt. The sonar on the chase boat said we had a scour hole four feet deep, but the truth was the bottom had shifted so much that our probe never touched solid riverbed. That's bridge scour. It doesn't announce itself. It just moves sediment out from under your piers. And it's the number one cause of bridge failure in the US, according to FHWA. Not overloading, not corrosion, not earthquakes. Water moving sand and gravel away from a foundation until the structure gives. I've been a state DOT bridge engineer for 26 years. I'm NHI-qualified to lead underwater inspections, and I've signed off on roughly 400 of them. This is not a textbook problem. It's a field problem. So let me walk you through exactly what scour looks like, how to find it, and what to do about it.
What Is Bridge Scour, Exactly?
Scour is erosion of the riverbed around a bridge foundation caused by flowing water. It's not one thing. We split it into three types. General scour lowers the entire riverbed over time. Contraction scour happens when the bridge itself narrows the channel, speeding the water up. Local scour, the nastiest one, happens right around a pier or abutment, where turbulent water digs a hole like a toilet flushing dirt out. The US Federal Highway Administration publishes the standard guidance in HEC-18, 'Evaluating Scour at Bridges.' We use that manual every time we sit down with a new structure. HEC-18 says that in a major flood, you can see local scour deepen by as much as 10 feet at a single pier in a day. Ten feet. In a day. That's enough to drop a 20-ton concrete pier cap into the water. Scour doesn't care if the bridge is a 50-year-old slab or a brand-new precast girder. If the foundation isn't deep enough, water will find it.
Why Scour Is the Number One Bridge Killer
FHWA has long said more bridges fail from scour than from any other cause. It's not a close race. The 1987 collapse on the New York State Thruway, Schoharie Creek Bridge, took out three people. The pier sat on a tab of glacial till that the creek scoured away over years. Since then, FHWA mandated scour evaluations for every bridge crossing a waterway in the US. Here's the thing most people miss: scour failures don't happen during normal flow. They happen in floods. You get a 100-year flood, the water rises, it screams through the opening, and in a matter of hours you've got a pier with no support. We don't always get a warning. The river just moves. We saw it again in 2015 when the I-10 bridge over the Pecos River collapsed in Texas. The river came up after a rain event, and the abutment backfill washed out. Not corrosion. Not rust. Water undercutting the approach slab.
How to Assess Scour Risk: A Practical How-To
Step one: dig out the structure files. What's the foundation type? Steel H piles? Concrete caisson? Spread footing on bedrock? That tells you how much room you have to lose. Step two: run a hydraulic study. We use tools like HEC-RAS, the US Army Corps of Engineers model. That gives you a predicted scour depth for a given flood. Step three: get in the water. Underwater inspections typically happen every five years, but for scour-prone bridges we go after every major flood. I've dived from Wilkes-Barre to Richmond. We put a crew in the river with a sonar unit — I've used Norbit's single-beam echo sounders and EdgeTech side-scan sonar. We also use dive teams from firms like Collins Engineers. You're not looking for concrete cracks; you're looking for voids under the footing, exposed pile tips, or a bed that dropped a foot since last year. One thing I've learned: the river doesn't care about your schedule. I remember diving under an old truss bridge in upstate New York and seeing a school of smallmouth bass swimming straight through what had been a solid sandbar a month before. The fish knew the bottom was gone before the equipment did. And here's a digression worth making. Most people think of a bridge as concrete and steel. But in a flood, the riverbed is part of the structure. If the bed lowers, you lose frictional support, and a pier can lean just enough to snap a girder. You're not just inspecting concrete; you're listening to the river.
Countermeasures That Actually Work
Once you find scour, you have a suite of countermeasures. The standard is riprap — graded stone laid around a pier. But it has to be sized correctly. Throw in pebbles and the river just moves them away. We use formulas in HEC-23 to pick a D50 stone size, often 18 to 24 inches in fast water. Underneath, we lay a geotextile filter fabric so water doesn't suck the fines out through the gaps. Concrete is another answer. A heavy concrete apron cast around the pier can armor the bed. I've seen that work on the Mississippi River bridges north of St. Louis. But it costs real money. The FHWA publishes cost ranges, but honestly it depends on the plant and access. Newer options include articulated concrete block mattresses and even fiber-optic sensors that watch bed elevation in real time. We put an early warning system on one bridge in Virginia that measures bed level every 15 minutes during a flood event. It emails the on-call engineer if the bed drops past a threshold. That's not cheap, but a single bridge replacement runs eight figures. Riprap costs less than a used sedan per cubic yard.
What You Can Do If You Own or Manage a Bridge
If you're responsible for a bridge — say you're a county engineer or a public works director — don't wait for the state to tell you there's a problem. Pull your inspection reports. Look for any mention of 'scour,' 'undermining,' or 'channel migration.' If the pier or abutment foundation is not buried deep enough, ask for a hydraulic assessment. The state DOT can help; that's our job. During high water, if you can safely do it, walk the banks. Look for turbid water around piers, exposed piles, or approach slabs that have settled. If you see a whirlpool near a pier, that's a bad sign. That's a vortex actively moving material. Call it what you want — scour, washout, undermining — it's the same thing. Water wins eventually. But with regular inspection, careful hydraulic analysis, and the right countermeasures, you can beat it for decades.
Frequently Asked Questions
What is bridge scour? Bridge scour is the erosion of riverbed material around a bridge foundation caused by flowing water, especially during floods. It can undermine piers and abutments, leading to partial or total collapse. How often do bridges need underwater inspections? Underwater inspections are typically required every five years, but bridges at high scour risk should be inspected after every major flood event, sometimes more frequently. What are the first signs of bridge scour? Look for visible erosion around abutments, exposed piles, muddy water swirling near piers, and settlement of the approach slab. A whirlpool near a pier is a classic sign. Can bridge scour be prevented? Scour can't be eliminated completely, but it can be managed with riprap armor, concrete aprons, and real-time bed monitoring, along with proactive hydraulic design.