Stoplogs, Bulkheads and Dewatering for Dam Maintenance
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Working the upstream face of a Bureau of Reclamation intake a few years back, I watched a 12-ton stoplog swing off the gantry crane and settle into its machined slots. The water didn't stop; it just changed its sound. That's the moment you know your dewatering job is real. Stoplogs and bulkheads are the movable walls that let you take a dam's waterhandling system apart for maintenance without draining the whole lake. A stoplog is a stacked beam – steel, concrete, or wood – lowered one section at a time into slots in the pier. A bulkhead is usually one heavy unit, sometimes wheeled, built to seal an entire opening. Both do the same thing: isolate a penstock, intake, or gate area so you can pump out the water and work dry. This piece is about how to do that safely and sanely – from load calcs to pump setups to the moment you walk into a dry gate bay for the first time. Call it a working guide from someone who's set a few hundred of these in everything from mud to ice.
Stoplogs vs. Bulkheads: Know What You're Setting
Stoplogs are the workhorses of dam dewatering. You lower them into slots one at a time, starting at the bottom, and each one locks in with the one below. They're simple. A 4-foot by 8-foot steel stoplog might weigh 3 tons; a concrete one can be 8 tons. You stack them until you've reached the crest or the sill, and then you start pumping. Bulkheads are the big levers – a single welded steel gate, often with wheels or rails, built for a specific opening. They seal one opening only. At a gated spillway, you might pull a bulkhead into place when a stoplog stack is too weak or too slow. But bulkheads need a lot of headroom and a crane that can handle the weight. For most maintenance, stoplogs are the cheaper, faster answer. That's the core of dam stoplog dewatering: you're building a temporary wall from parts, not dropping one giant lid. It gives you flexibility. If one log leaks, you can pull it, re-dress the seal, or set an inflatable plug in front. You can't do that with a one-piece bulkhead without un-watering the whole thing.
Planning the Dewatering: Head, Load, And Seal Condition
Before you rig the first log, do the math. A 5-foot-high stoplog that's 10 feet wide, set with its center 50 feet below surface, is carrying over 80 tons of water load. That's not the crane's problem – the crane just sets it – but it's the slot's problem and the seal's. If the slot lips are spalled or the metal has zebra mussels in it, that load will find a way through. I've seen crews skip the slot inspection and pay for it. A dive team, or a camera pole if the water's clear, can tell you if the embedded steel is true. You measure with feeler gauges or ultrasonics if it's bad. Seal condition matters just as much. Old rubber seals lose their squeeze; wooden stoplogs swell when wet but rot when cycled dry. You need to know what you're setting against. Then you plan the drawdown. You don't just pump the bay dry and hope. You need to know the leakage rate, the drainage path, and what lies below the sill – silt, debris, a broken gate that fell in. You also need to know who controls the lake level upstream. If someone opens a gate above you, your stoplog set becomes a sieve.
Setting the Logs and Managing the Water
Setting stoplogs is a dumb job that gets hard when something goes wrong. You hang each log from a spreader or a single point, lower it into the slots, and feel it 'set' through the rigging. Some plants use a monorail and electric hoist; others use a hydraulic gantry or a truck crane. You watch the downstream water as you pump – expecting it to draw down, then seeing it stay level because you missed a log. Leakage past a stoplog stack is normal. A new set with good seals might weep 20 gallons a minute. A 40-year-old stoplog with deformed plating and blown seals can pass 800 gallons a minute. That's where dewatering pumps come in. You size the pump to the worst expected seepage, plus growth – call it 1.5 times. A 6-inch Godwin Dri-Prime will handle maybe 1,500 gpm at the heads you'll see. That's a good starting point for a small intake. If you have a mountain of water coming through, you need a big Flygt submersible or two. One digression: I once found a complete shopping cart in a penstock after we dewatered behind a stoplog. It had a tattered '10 items or less' sticker still on the basket. That cart had been sitting there for at least a decade. You find the oddest things when you lower the lake.
Pumping Systems and the Final Dewatering
The first stage of dewatering is easy: the water level drops until you hit the top of the stoplog. That means the pump is now fighting the leakage at the seals, not just filling the empty intake. You adjust the pump to hold a stable level just below the top log. If you pump too fast, you'll suck air and lose prime. If you pump too slow, the water sits and you can't reach the low-level drains. I prefer to run a pair of smaller pumps — say two 4-inch units — rather than one big pump, because you can throttle one down as the seepage decreases. The last foot is the trickiest. That's when silt and debris plug the pump strainer, and you're pushing a slurry. You watch the suction hose like a hawk. You switch to a smaller, high-head pump to chase the last few inches. Once the bay is dry, the real work starts: inspect the metal, the seals, the gate guides. That's where you often find cracks, cavitation, or a bent gate arm. But don't get so focused on the repair that you forget the stoplog is the only thing holding back a lake. Someone has to watch it – the seal pressure changes as the lake level changes. Set a watch at the pump and log every hour.
Safety and Common Mistakes
The worst mistake is thinking a stoplog is a watertight door. It's not. It's a baffle that slows water, and the pump is the thing that truly dries the bay. If the pump dies, the bay fills. An alarm and a backup pump aren't optional; they're the difference between a dry repair and a drowned one. Second mistake is ignoring uplift. When you draw the water down, the concrete floor of the bay may try to float if it's not drained or tied down. The hydrostatic pressure under the floor has to be relieved. You drill relief holes or keep the under-drains open. Check with the plant engineer before you start pumping. Third, don't assume all stoplogs are alike. A 50-ton-capacity hoist is not the same as a 100-ton crane. Check the rigging angles and the rated capacity for the exact lift. And remember, a stoplog catches wind and water; a sudden gust can swing it into the pier. You spotters on the deck and radio discipline. If you're diving while the stoplog is moving, you're not working – you're past good judgment.
Frequently Asked Questions
What's the difference between a stoplog and a bulkhead? A stoplog is one of several stacked sections; a bulkhead is a single one-piece gate. Bulkheads can be larger and give a tighter seal, but they require more overhead and a bigger crane. Stoplogs are cheaper and easier to store. How do you dewater a dam intake with stoplogs? You set the stoplogs in the upstream slots, close any downstream gate, start the dewatering pump, and pump the bay dry. The stoplogs reduce flow to the point where the pump can handle it. How much leakage should you expect past a stoplog? Depends on seal condition and head. A properly seated set may leak 10-50 gpm; old, worn logs can pass several hundred gpm. You size pumps to handle 1.5 to 2 times your estimate. Can stoplogs be set underwater? Yes, they can be lowered through the water and guided into slots by a diver. It's done routinely, but it requires careful buoyancy control and the diver must be clear when the log is moving.