10 Underwater Structures Nobody Inspects Until They Fail

By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.

Underwater Inspection | Cathodic Protection & Anodes June 2007. A pump station on the San Joaquin. They'd asked me to install a new strainer cage on the suction line. Water was a muddy 58 degrees, maybe eight feet of visibility. The foreman said, 'While you're down there, check if the old rack is loose.' It wasn't loose. It was gone. Not the rack--the entire concrete miter groove where it seated. Silt had undercut the foot. Nobody had asked for a structural inspection. They just wanted a strainer. That's the pattern I've seen for almost three decades. We dive to fix the symptom--a clogged screen, a jammed gate, a scoured pile--and we surface with a list of three other things that are about to fail. Some structures in fresh water get eyeballed every year. These ten don't. They're not mysterious or exotic. They're just underwater, and out of sight. This isn't a horror story. It's a checklist. I'm going to tell you what these things are, why they actually fail, and what you can do to catch them before a phone call wakes you up at 2 a.m.

In the Flow

1. Intake trash racks. Every hydro plant has them. They get cleaned with a rake, but nobody looks at the steel bars themselves. Watch for loss of section from corrosion at the watermark, and check the welds where the bars meet the frame. Use a diver with a hammer and a thickness gauge. A 'ping' that sounds like a tin can means the bar is gone from the inside.
2. Dam low-level outlet gates. They sit under 40 feet of water and get operated once a year for a flush. If the paint is gone, the steel is pitting. Photograph the rubber seals close up--a strip that's missing will let a jet of water through that erodes the seat in one season. If you have a fixed gate, get a diver to run a finger along the seat. He'll feel the pitting before any instrument will.
3. Stilling basin baffle blocks. The water always looks churned there, so nobody assigns a diver to go feel the concrete. Cavitation vapor bubbles collapse with enough force to flake concrete off like a sunburn. You'll see white limestone-colored patches of fresh aggregate. That's your clue. Use sonar first, then send a diver to probe with a rebar locator.

On the Other Side of the Pier

4. Bridge pile bents. Inspectors look at the caps and girders from a boat. Underwater, the mudline hides where the pile actually started. Scour is the problem. A pile driven 20 feet into clay might have 6 inches left. Tap the pile below the water with a mallet. A hollow sound means the concrete is disbonded. Measure from the top to the mudline with a tape on a pole. Write it down. If that number changes between visits, you've got scour.
5. Fender dolphins. These take the hits--but we inspect the above-water timbers and forget the through-bolts at the bottom. A corroded bolt in saltwater will snap after a boat bumps the same corner twice. Divers should do a manual torque check on each bolt. There's no gauge. If a 7/8-inch bolt twists with a screwdriver, you've already lost it.
6. Sheet pile tie rods. This one belongs in a different category entirely. The tie rods go from the heavy sheet pile wall to a buried anchor pile. You cannot see them--they're below the water and behind the mud. No one ever inspects them until the wall starts to move. Use ground-penetrating radar on the deck to locate the rod, then excavate a small hole near the wale to expose the nut. If the nut is a rust ball, you're on borrowed time. There's no permanent fix. You pull the rod or anchor the wall with helical tiebacks.

What Goes Down Must Go Out

7. Outfall diffusers. Sewage plants and industry pump through them. By the time you see weird bubbles, the diffuser ports have been chipped by debris for years. Have a diver with a still camera take a picture of every port from below. Look for crescent-shaped chip lines around the opening. That's sand and shell erosion. But the real issue is the flexible connection joint at the shore end. If it's pulling apart, you lose your dilution before you lose the pipe.
8. Clearwell baffles. These are the concrete holding tanks that let chlorine do its work. They're drained and scrubbed every few years--but nobody inspects the underside of the baffle walls when the tank is empty and clean. That's the wrong time. Fill the tank halfway, put a diver in the corridor, and look for cracks at the waterline and at the connection to the floor. A crack from above is one thing. A crack that weeps water is a new path for short-circuited flow. Use a dye test with a camera to see where the water moves.
9. Floating dock anchors. A floating dock moves around all day, so you assume the anchor chain is okay. But half the time the chain wears against a rock or a concrete block, and a single link loses cross-section. Do this once a year: let the dock drift to slack, then mark the chain at the top and pull it up in sections. Or--if it's too deep--do a sonar survey and send a diver with a calliper to measure link thickness.

The Lock That Lied

10. Lock miter gates. I saved this for last because it's the one most people believe they're already inspecting. They operate the gates, listen to the closing, and check the above-water gear. But the sill bar that the gate seals against is underwater. When that bar gets a nick, the gate rattles for years while the concrete underneath erodes. You can't feel it from the operator's cabin. Get the diver to swim along the full length of the sill with a straightedge and a flashlight. The flashlight will show any gap. If you see one, the rubber seal is pressing into a void. That's a $500,000 repair that would have cost $20,000 if caught early. There you have it. Ten things I've seen fail. You don't need a full ROV team or a dry-dock. You need a diver who's willing to touch the stuff, a tape measure, a hammer, and a camera. And you need to schedule that inspection before the problem schedules it for you.

Frequently Asked Questions

How often should these structures be inspected? It depends on service life and environment. For critical intake structures, a visual inspection once a year is cheap insurance. For passive things like sheet pile tie rods, start with a baseline and then do a follow-up every five years. In saltwater, cut those intervals in half. Why can't I just use an ROV? An ROV sees what you point it at. It can't feel a loose bolt or tell you whether a concrete patch is surface staining or actual delamination. In turbid water, ROVs are nearly blind. A diver with a hand is still the best sensor for corrosion and cracking. What's the most common failure you see? Erosion behind the scenes--scour under piles, cavitation behind baffles, corrosion inside bolt connections. The structures don't often collapse without warning. They just start moving or leaking first, and nobody's measuring the movement.