Dam Inspection Underwater: Spillways, Gates and Outlet Works

By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Dana Whitcombe, technical reviewer.

Underwater Inspection | Commercial Diving Contractors Alabama I remember a morning on a dam in eastern Tennessee, water temperature 44 degrees, visibility just under a foot. I was hanging onto a tainter gate's skin plate with a magnet, trying to run an ultrasonic thickness gauge on the steel while the whole structure hummed with the current. The client had a number in mind for how much steel was left. The hum was telling me we were there long enough. Underwater dam inspection isn't a sightseeing dive or a reef survey. It's an engineering job with a dive mask. You're looking for cracks, voids, corrosion, and anything that's going to let water go where it shouldn't. Spillways, gates, and outlet works are the business end of a dam. They don't shut down nicely, and they never let you forget the pressure above you. I've been doing this since 1996, out of a van in Oklahoma, later out of a trailer in the Pacific Northwest. I've done nearly 200 dam jobs. The basics don't change: clear work order, a crew that knows the plan, and a healthy respect for big water.

Before the Dive: Read the Dam Like an Engineer

Before the boat goes in, I pull the as-built drawings and the maintenance history. That sounds dull, but it keeps you from looking for the wrong thing. Call it a reconnaissance dive without getting wet. You need to know where the gate seats are, where the trash racks are, and which intake is going to be operating while you're working. Good plans show all that. Then we look at the flow. A concrete dam on a river might have a spillway that only runs during floods. But an outlet works pipe can be at the bottom of a reservoir and release water for downstream needs. We coordinate with the plant operators. They'll give us a window — sometimes a few hours, sometimes a whole day. I've had a gate close on a Tuesday and we had to be out by noon Thursday. Depends on the plant. Diver safety starts topside. We use surface-supplied air via a Kirby Morgan helmet or a full-face mask, and a hard-wire comms link to a tender. We set a downstream current meter a few feet ahead of the work. If the current exceeds a knot or so, we don't go in. It's that simple.

Spillways: Where Concrete and Water Meet

Spillways are the overtopping release. The concrete apron and sidewalls take a beating from floating debris and cavitation. We inspect the concrete by feel and by instrument. First, a sounding rod – basically a 5-foot steel pipe – to tap the surface. A solid hit sounds clean; a hollow hit tells you there's delamination behind it. Then we probe cracks with a small underwater video camera on a stick. Ultrasonic thickness testing is the number game. We use a Cygnus Instruments gauge with a single-crystal probe, taking readings on a grid over the affected area. The probe needs a clean surface, so we often scrape away a spot with a putty knife. Give or take 0.1 inch depending on the operator and the gauge. One thing most people don't think about: algae and zebra mussels. They cover a spillway surface and make a visual inspection useless. We work in wetsuits or dry suits depending on the temperature, but the scrape job is always a hand job. And you learn to hate zebra mussels. They're sharp and they get in everything.

Gates: The Big Doors

Gates are the big doors that hold water back. A tainter gate might be a quarter of a mile around at some flood control dams; a massive steel curve with a trunnion pin up top. Our job is to check the skin plate and the side seals. The plate corrodes from the inside out, so we look for pitting and leaks. We run the ultrasonic gauge over the plate in a grid pattern, and we watch for weld cracks along the frame. One digression: a gate is not just a chunk of metal. It's a moving structure. The hoist cable runs over a sheave at the top, and if that cable is bad, the gate might not close. On the underwater side, we check the cable for rust, broken strands, and fish-bites. Yes, fish-bites. A big carp can take a chunk out of a wire rope if it's in the right spot. Then we check the guide rails and the bearing pads. These need a smooth flat surface. We use a straightedge and a feeler gauge taped to an extension pole. You can feel the wear. Depends on the plant age, but a gate with a new seal will still allow a couple gallons per minute by a tag weld. That's normal.

Outlet Works: The Tight Spots

Outlet works are the real underwater oddballs. They're pipes and valves that run through the bottom of the dam. Some are small, 36 inches in diameter. Others are big enough to drive a pickup through. The flow can be massive if someone opens a valve, so we inspect them either with the pipe dewatered (empty) or with a diver on a tether, and a strict rule: no open valves on the line. Visibility inside a pipe is usually zero. That's where we use a Tritech Gemini sonar to get a 3D map of the interior. The sonar can see a partially clogged screen or a fallen pin in zero visibility. We still send a diver in, but only after the sonar shows no major hazard. In a 36-inch pipe, you're going feet-first with a bailout bottle and a pull-back rope. The helmet scrapes along the bottom. It's claustrophobia test number one. I've seen an outlet works gate with a missing hinge pin from a 40-year-old valve. The gate was angled, allowing a jet of water to erode a hole in the concrete that was the size of a basketball. That's why we get paid. You can't see that from the top of the dam.

Reporting: Making the Invisible Visible

The report is the product. I shoot video with a high-definition camera on the helmet, and I write notes on a slate with a grease pencil. But the real data is the ultrasonic readings and the exact locations. I mark the coordinates on the drawings with a ruler. The engineer home office needs to know that reading #23 was 4 feet from the left side of gate #2, 10 feet downstream. If I forget to mark it, the reading is worthless. Lo-tech goes a long way: a torpedo level and a tape measure underwater are still the tools that win the day. We take photos on a digital camera in a housing, but a video feed to the surface saves us time. The client watches live and tells us to spend more time on a spot. At the end of the week, I sit in a motel room and type the field report. No fancy software. I use a template with the drawings attached. And I always hedge when I have to: "It appears that the coating is disbonded" not "The coating failed." Comes down to uncertainty.

Frequently Asked Questions

How deep can you actually dive on a dam inspection? We're normally on standard air to 130 feet. Most spillways and gates are in the 20 to 60-foot range. Outlet works can push you deeper, and then we switch to mixed gas or use an ROV first. Depends on the plant. Can a ROV replace a diver for this work? Not yet. An ROV is great for a first look, but tapping concrete, taking ultrasonic readings, and feeling a seal need a human hand. We send a camera down first, but a diver still ends up touching the structure. How much does an underwater dam inspection cost? It's not a fixed per-square-foot number. You're paying for a dive team, a boat, and the engineering report. The model is a day rate per diver plus mobilization and equipment. For a typical spillway job, you might get a proposal for a lump sum once the depth, current, and access are known. What takes the longest on these inspections? Surprisingly, the bottom time is only a few hours. The long part is coordinating the flow window, waiting for a gate to close, and setting up the dive system. I've spent more time on the bank than in the water.