Underwater Seawall Inspection: What a Diver Finds That You Can't See
By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.
Seawall Repair | Underwater Inspection The first time I swam a seawall that had been "checked" from a boat was on a private dock in St. Augustine back in 2011. The owner showed me a clean concrete face, a neat row of tie rods, and a video from a drone that looked brand new. I slipped under the dock, ran a gloved hand along the waterline, and felt the round edge of a 6-inch gap behind the sheet pile. The backfill had washed out, and the wall was just a concrete curtain hanging on steel. We had a chat about anchors that afternoon. What you can see from the top is mostly the story you want to hear. The water hides the truth. And the only way to get the truth is to put a person under there who knows what to touch, what to scrape, and what to ignore. In this post, I'm going to talk about what a commercial diver actually does when you hire them for an underwater seawall inspection. Not the fluff from a marketing page. The real thing, including the tools, the tricks, and the things that will cost you money down the road.
What a Diver Is Actually Looking For
We don't just swim along and look. We work the wall in grids, usually a 5-foot section at a time. I start at the mudline and move up to the cap. The first thing I'm checking is scour. In a tidal pass with a 4-knot current, you can get a black hole under the toe of the wall that's two feet deep and four feet wide. That's the stuff that pulls a sheet pile out of the ground. Give or take six inches, but you get the picture. Then I'm feeling the face of the wall. On concrete, I'm looking for spalls, cracks wider than a 1/8-inch rule, and exposed rebar. On timber, I'm probing with a stainless steel awl to see if the wood is punky or solid. A #2 pencil will push right through a rotten pile. I carry one on my slate, and it works better than any electronic tester. Voids behind the wall are the big one. I use a 3/8-inch steel rod with a bent hook on one end. I poke it through any gap or gaping seam and feel for the backfill. If it goes in easy and comes back wet, that's a red flag. The backfill should be dense, not soup. I've seen homeowners pay for a new concrete cap while the whole wall was floating with air behind it. No point polishing the top of a hollow tooth.
The Tools I Carry Underwater
My basic kit is a drysuit, a double hose regulator — I'm old school, but many divers use a modern octopus — and a slate. But the real tools are simple. A few pieces of schedule-40 PVC pipe cut to different lengths let me tap the wall and listen for hollow spots. The sound changes when the concrete is delaminating. You can hear it through the water, but it's subtle. I also carry a 10-inch stainless steel probe with a T handle. If you push it into the mud and it stops at silt, that's fine. If it goes down soft all the way to the hilt, that's more than a foot of fluff and the toe is unsupported. Sometimes I bring a small underwater camera like a GoPro on a stick for documentation, but I don't trust it alone. The camera sees what the light shows. It doesn't feel the soft spot behind a timber or the rust flakes on a tie rod that are falling off like duck feathers. I need my hands. And yes, I've been bitten by a moray eel once, so I keep them out of holes unless I'm sure. That's the digression. More than you wanted to know, but every old dock has a hole story.
Why a Camera or Sonar Isn't Enough
There's a lot of love for ROVs these days, and they're good for some things. If you want to look at a stretch of wall in 30 feet of silted-out water and just see if the cap is still there, a small ROV with a light will do. But an ROV can't push a probe into a crack and tell you whether the backfill is sand or clay. It can't feel the difference between a solid pile and one that sounds like a watermelon when tapped. And it certainly can't reach into a void behind a protruding flange and scoop out a sample of what's in there. I've done inspections alongside camera guys. The camera will show a crack. The diver will find that the crack goes all the way through a 12-inch concrete cap and the steel is rusted to swiss cheese. Those are two different levels of detail. If you're deciding whether to patch or replace, you want the swiss cheese answer. Sonar is even worse for this. Side scan can show large undercut features, but the resolution is measured in inches, not fractions. And a lot of seawall failures are the size of a fist. You need a hand, not a transducer.
How to Use What the Diver Finds
Once the inspection's done, you get a report. A good diver will mark each location with a numbered tag or flag, and give you a list of every defect with a severity rating. Usually it's three categories: cosmetic, structural, and urgent. Cosmetic is stuff like scaling or small spalls. Structural means the wall is moving or losing support. Urgent means call a pile driver now. I've seen walls that need urgent repair but the owner wanted to wait for the next budget cycle. That's how a dock becomes a pile of rubble. If the report says you have voids behind the wall, the fix is usually pressure grouting. That's a separate operation, but the inspection will tell us the volume. Give or take a couple of yards. If it's a scour problem, you might need riprap or a mud-sill. And if the wall itself is leaning, you're looking at tiebacks or a whole replacement. The inspection doesn't fix it, but it tells you which way to go. My advice to any homeowner or property manager is to get a proper underwater inspection every three to five years, especially after a named storm. A hurricane will do more damage below the waterline than above it. I've seen "minor" storms move a wall so that it was one foot out of alignment, and nobody noticed until the cap started to crack. That's the hidden world. How often you need it depends on the plant, the water flow, and the age of the structure. Call it every three years.
Frequently Asked Questions
How often should I have my seawall inspected underwater? I recommend every three to five years for a normal residential seawall, and after any major storm or when you see cracks, leaning, or washout next to the cap. If you live on a high-current waterway, make it every two years. Can't a camera or drone do everything a diver can? No. A camera can show you the surface, but it can't feel the difference between solid backfill and a void, or test the integrity of timber. A diver adds tactile information that a lens never picks up. What do you check first when you're under a wall? Scour along the toe, then voids behind the face, then cracking, corrosion, and the condition of tie rods and anchors. The toe is where most walls die. Do I need to drain the water to inspect a seawall? No. That's the whole point. A diver works in the water. Draining a portion of a waterway is expensive and usually impossible. If there's a gate, you might dewater a section, but for 90% of seawalls, you hire a diver.