A Dam Can Be Inspected Without Ever Lowering the Water
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Cathodic Protection & Anodes | Offshore & Subsea Diving October 2017. I was standing on the crest of a hundred-foot concrete gravity dam in the Pacific Northwest, rain on my neck and a client who wasn't convinced. He kept looking at the reservoir like it might need to disappear. "We can't lose the water," he said. "The downstream fish runs, the irrigation contracts, the cold-water pool for the hatchery - if I drop this pool, I'm not doing it again." He was right. Dewatering a dam for inspection becomes a project inside the project: temporary bulkheads, massive pumping, months of permits, and a price tag that makes a CFO flinch. We didn't need any of it. We had sonar, an ROV, and a dive team already suited up. By Friday we had a full picture of everything below the waterline, and not a single cubic foot of the reservoir was lost. That's the trick. You don't have to empty the lake to know what's under it. You just have to match the tool to the visibility, the depth, and the question the engineer wants answered.
Start with the Sonar, Not the Divers
The first tool in a no-dewater inspection is side-scan sonar. On that job, we towed a 600 kHz unit from a small aluminum boat, running overlapping lanes so we covered the entire upstream face in a grid. Side-scan sends out high-frequency sound pulses and reads the echo. It can't tell you whether a crack is 3 millimeters or 30, but it shows you where the surface changes: missing concrete, exposed rebar, debris piles, or a gap that looks like it's been there since the 1970s. That sonar pass gives you targets. It tells you where to send the ROV and where the divers need to spend their bottom time. If you skip this step, you are hunting the entire face by hand. You'll find stuff eventually, but you'll be bottom-tending for days. Work the sonar first. It's faster, it's cheaper, and it gives the client the first layer of proof.
Get the ROV in for the Visual First Pass
Most reservoirs aren't crystal clear. But where visibility is a few feet, an ROV with a high-def camera and a scanning sonar head does a lot of the grunt work. It can fly down a trash rack, inspect the joints in the concrete, and video the intake structure without a diver getting wet. The catch is current. Large drafts on a power dam can pull an ROV toward the trash rack and wedge it. I've seen an Obsidian with a smashed thruster and a $20,000 repair bill because somebody didn't respect the intake. That's why you use a tether with a quick disconnect and a pilot who knows to approach on the downstream side or along the wall, not straight into the flow. You also need a topside winch for tether management. The ROV is a tool, not a toy. It will give you excellent images, but you need to run it like a survey, with lane spacing and overlap, and record everything.
Divers Are for the Zero-Visibility, High-Touch Work
In the deeper or murkier sections, there is no camera. We're talking visibility measured in inches. That's when you go in by hand. My divers work in teams of three: one in the water, one on the dive stand, and one as standby. The in-water diver is surface-supplied with a full face mask that lets them talk continuously to me at the surface. They carry no scuba tanks, so they can't get overwhelmed by bottom time limits the way you might think; we're taking it slow. The protocol is a line and a pattern. We run a descent line to the bottom, then the diver works a grid, sweeping a distance of about three feet per pass. They follow the concrete surface with their hands, feeling for a loose aggregate, a change in the profile, a hollow sound from a tap hammer, or a suction feeling that suggests a crack. It's like braille reading a sixty-foot-high wall. It takes patience. But a diver's fingers can reach into a joint that a camera can't and tell you whether the seal is pulling away or just discolored.
Inspect Gates and Seals with a Purpose-Made Protocol
The critical points on any dam are the gates, the trunnions, and the seals. A gate that doesn't seat properly means water loss, sediment passage, or worse. To inspect without dewatering, divers get inside the gate bay and work along the seal surfaces. We use mechanical feeler gauges and underwater callipers to measure the gap between the gate leaf and the embedded frame. On a recent job, we found a deteriorated rubber seal that was literally delaminating. It looked fine on the old video from the surface, but in the tactile check, the diver could peel off a layer like skin from a sunburn. That's the kind of detail you can't get from a camera. We also check the cathodic protection system, if there is one. The anode bed connections need to be measured with a voltmeter set to the half-cell potential, and a diver can read the measurements right into the comms. We log every reading into a waterproof tablet on the float. The gate shafts, trunnion clearances, and lubrication points are all accessible underwater, and the dive supervisor makes sure the measurements are repeated at three different depths or angles to give the client a confident profile.
Document Everything So the Client Doesn't Ask to Lower the Water Later
The final step is the one that sells the whole method: the report. We deliver annotated video, still photographs, a side-scan mosaic, diver log sheets, and a summary of findings. On deeper, cleaner jobs, we use photogrammetry - overlapping still frames stitched into a 3D model that the engineer can rotate in a viewer. That model has come a long way. I've handed a utility a digital grid of the dam face with a measurement tool that can quantify a crack from the office. The report needs to be clear enough to satisfy any state or Federal dam inspector. We write it like a standard inspection report, with the condition ratings (accept, monitor, repair) and we include the method so they can see why the data is trustworthy. When you do that, the client doesn't ask to drop the pool. They ask for a multi-year schedule of repeat inspections.
Frequently Asked Questions
Won't a dewatered inspection see more than an underwater one? Potentially, but not as much as you might think. Underwater, you get sonar and tactile access to everything below the waterline. What you miss is the above-water concrete, which you can inspect from the crest or a scaffold. The critical zones - the upstream face, the intake structure, and the gates - are all accessible underwater. In many cases, the risk of a bad dewatering (an unstable dam, a leaking bulkhead) is greater than the risk of a thorough underwater survey. How long does an underwater inspection take compared to dewatering? A typical dam face might take two to four days with sonar and divers, depending on size and access. Dewatering alone can take weeks of planning and pumping. The on-site time is shorter, but the report preparation takes another week. Overall, you often save two months and hundreds of thousands of dollars. Is it safe to send divers around moving intake gates? We never work near moving gates. The plant knows our schedule, and the gates are locked out and tagged out while we are in the water. The dive supervisor coordinates with the operations department, and there's a dedicated lockout/tagout procedure. If a gate cannot be isolated, we refuse that part of the inspection. Inland dive safety is not a game.