Sonar Scour Monitoring vs Diver Inspection
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Dana Whitcombe, technical reviewer.
Underwater Inspection | Commercial Diving Contractors Alabama The Missouri was running at 45,000 cfs when the boat's multibeam suddenly showed a 12-foot drop-off right at the pier. I'd done this for years; my gut said it was wrong. So we sent a diver down, and she found we were staring at a stacked pallet of riprap, not a scour hole. That was the day I cooled off on believing sonar pictures without ground truth. Sonar and diver inspection aren't actually competing tools. They're two ways of seeing a bridge's feet. Over roughly 400 underwater inspections, I've learned where each one works and where it gets me into trouble. The short version: sonar gives you the what's-there, divers give you the what's-under-there. For scour, the what's-under-there often matters more than the pretty picture.
What Scour Actually Does
Scour is the removal of sediment around a bridge substructure by flowing water. It's not a single hole; it's local, contraction, or general scour, and it can change the support of a pier overnight. We worry about local scour at the pier, because it creates a hole that can extend call it three to four times the pier width. If that hole gets deep enough, it exposes the footing and the piles, and the bridge loses its bearing capacity. To measure scour, you need to know where the riverbed is relative to the footing and whether there's a void underneath that you can't see from above. A vertical measurement within half a foot is usually good enough for comparison year to year, but the real issue is tracking a trend. A bridge that lost two feet of bed last year is not the same as one that lost two feet of bed over a decade.
Sonar: Sweep Wide, Then Come Back
For a quick inventory, sonar is unbeatable. I've used side-scan sonars from Klein and EdgeTech, and multibeam systems from Kongsberg and Teledyne, typically running at 400 to 600 kHz. In clear water, a multibeam can produce a digital terrain model that resolves features on the order of a tenth of a meter. That's plenty to see a scour hole. With RTK GPS positioning, the boat can cover a mile of bridge in an hour, and repeat passes give you a difference map from year to year. But sonar has limits that a lot of people don't talk about. In turbid water, the sound washes out. In the Mississippi and Missouri, I've seen the bottom disappear within 20 feet of the transducer. And sonar can't see under a footing overhang—the exact place where scour creates a void. It also gives you the shape of the bottom, not the hardness or the resistance. A layer of loose sand can look solid one day and be gone the next. A quick digression: I've had engineers call a rock ledge a concrete footing on a sonar image. When the water is dirty and the return is hot, everything looks like the bottom. So if you're doing serious scour work, you have to ground-truth sonar with tactile measurements. That means divers.
Divers: The Human Feel
A diver is the ground truth. On a typical inspection, the team sets a line over the pier and the diver swims it, using a sounding rod to measure bottom elevation at fixed intervals. We use a chain drag to sweep for exposed piles, a probe to push into the material under the footing, and our hands to feel for voids and soft spots. In zero-visibility water, divers work by touch; I've seen an experienced diver describe the shape of a hole better than any multibeam image. The downside is that divers are slow, expensive, and vulnerable. A two-person dive team with a chase boat can inspect a footprint of maybe 100 feet of pier in a shift, while a sonar boat is covering the entire bridge. Depth and current limit what you can do; commercial divers have working depth limits and air, and river currents above 2 knots get dangerous. There's also human error, and let's not pretend otherwise: a diver who's been cold all morning might miss a void. That's why we use both. Sonar for the broad stroke, divers for the details that matter.
Comparing Them Honestly
So what's the real difference? Coverage and resolution, or rather, coverage versus tactile certainty. Sonar gives you a continuous map of the riverbed, with vertical accuracy in the range of a tenth of a foot if conditions are good. Divers give you point measurements that are very accurate, but you have to know where to put them. Sonar tells you where to look. Divers tell you what's actually there. Cost and mobilization push the decision. A sonar survey is typically faster, uses a two-person crew, and can be done from a small boat. A dive team needs divers, tenders, a decompression chamber if it's deep work, and a safety plan. That's a heavier lift. But if the bridge is fracture critical or has a known scour issue, the cost of a missed void is far higher than the cost of a dive team. The other factor is the question you're asking. If you need to know after a flood whether a pier has been undermined, sonar can give you a quick relative elevation change. If you need to know whether the soil is still holding the footing, you need a diver to probe it. That's not an either/or; it's a sequence.
My Rule of Thumb
Here's my practical rule, and I'll caveat it with depends on the plant. For a low-risk bridge with clean water and no known scour history, I start with sonar every two years. That gives me a spatial map and catches any new hole. But if the bridge is on a critical roadway, has shift or exposed piles, or has had a record flood, I send divers every time. And if the flood is the one we've never seen before, I send both, often within the same week. You also need a baseline. The first time you inspect a bridge, do the full diver survey and create a detailed bottom profile. After that, sonar can track the trend. But the first call has to be tactile. I've seen bridges that looked stable from above and had a void under the footing the size of a couch. The bottom line is that sonar and divers are complementary. Treat anyone who sells you one as the only tool with suspicion. For scour, you need the wide view and the human hand.
Frequently Asked Questions
Which is more accurate for measuring scour depth? For a single point at the bottom, a diver with a rod is hard to beat—sub-foot accuracy is standard in calm water. Sonar can achieve similar accuracy over a large area, but it depends on water clarity and bottom type. In practice, we use sonar to find the anomalies and divers to measure the critical points. Can sonar replace divers for underwater bridge inspection? No. Sonar cannot see under a footing or feel how much resistance the soil has. FHWA's underwater inspection requirements don't mandate a specific method, but for rating a bridge, you often need tactile information. I use sonar as a screening tool, and divers for the final answer on critical features. What about fixed sonar for continuous flood monitoring? A bridge-mounted sonar transducer can give real-time riverbed elevation every 15 minutes, which is an excellent early warning during a flood. But turbid water and floating debris can block the signal, and the sonar only sees one spot. You still want a diver or boat survey after the flood to capture the full picture. How often should scour inspections happen? Typical state programs run underwater inspections on a five-year cycle for routine bridges and three years for critical ones, with immediate checks after floods. That's the general FHWA guidance, though details depend on a state's plan. Sonar lets you stretch the interval confidently if no change shows; divers handle the no-change part.