The Day a Hydropower Tunnel Became a Giant Cannon

Added: 2026-05-14

[00:00:08]A five-mile tunnel and one simple  human error. Most water hammers happen when a turbine shuts down,  but this disaster was different.

[00:00:18]By the time the crew realized the intake  gate was closed, the tunnel was already half empty. They opened the gate to fix  it—and accidentally turned five miles of concrete into a weapon. This is the disaster  story of ladhon hydropower plant from 1950s.

[00:00:33]This simplified layout shows the complete water  conveyance system of the hydropower project.

[00:00:38]On the left side is the intake structure at  the reservoir, where water enters the system through the intake gates. The reservoir  operating levels are also marked here, showing the maximum and minimum water levels. From the intake, water travels through a long power tunnel approximately 28,650  feet in length. The tunnel has a very slight downward slope of minus 0.002, allowing  water to flow steadily toward the powerhouse.

[00:01:06]Near the end of the tunnel, a surge tank  is installed. This vertical chamber acts like a hydraulic shock absorber, protecting the  system from dangerous pressure fluctuations and water hammer effects during sudden  load changes or turbine shutdowns.

[00:01:18]From the surge tank, water enters the steep  penstock, a 611-foot pressure shaft that carries high-pressure water downward to the powerhouse. Finally, at the powerhouse, the water strikes the turbine runners, converting  hydraulic energy into mechanical energy, and then into electricity through the generators. Let’s look at the layout of the intake structure.

[00:01:37]At the top is the Gate Control House, where  operators control the large intake gate located deep below. From this building, a tall vertical  Air Vent and Access Shaft extends downward to the water tunnel, allowing both air movement and  worker access for inspection and maintenance. At the bottom of the shaft is a massive rectangular  gate measuring approximately 12 feet 6 inches by 9 feet 10 inches, which regulates the flow of water  into the tunnel. The blue-colored sections on both sides represent water inside the intake tunnel.  The drawing also marks the maximum, normal, and minimum water levels, along with elevation  references (EL numbers) that indicate heights at different points of the structure. In addition,  a slot for temporary closure is provided for maintenance activities, and the shaft itself has a  diameter of about 13 feet 8 inches in one section.

[00:02:27]Now you all know run of river Hydropower  plants operate on a simple principle.

[00:02:32]Water flows from the reservoir through  long tunnels toward the turbines."

[00:02:36]As the water accelerates… it spins the turbine…  which spins the generator… producing electricity.

[00:02:41]But the water inside the tunnel  is moving at enormous momentum.

[00:02:45]This water cannot suddenly  stop or disturb that flow without creating dangerous pressure forces. "This phenomenon is called water hammer."

[00:02:54]You can see here in this video what is water  hammer and how surge shaft prevents water hammer.

[00:03:00]But the Ladhon accident was different. "This time… the problem did not come from stopping water flow by turbine. It came from air.

[00:03:08]On the morning of the incident… operators  were lowering the reservoir level slightly for spillway testing. A diversion tunnel gate  downstream of the dam was opened to release water.

[00:03:17]"Later during lunch hour… engineers  decided to reduce that discharge."

[00:03:22]"Instructions were sent by telephone to  partially close the diversion tunnel gate.

[00:03:28]"But because the project involved  Greek, Italian, and American personnel… communication problems existed and because of  this communication problem the mistake happened Instead of adjusting the diversion  tunnel gate… the assistant gate operator accidentally lowered the MAIN INTAKE  GATE feeding the powerhouse tunnel.

[00:03:45]"The intake gate was almost closed." "Only about a one-foot opening remained."

[00:03:50]At that moment… one turbine  unit was still operating.

[00:03:54]"But now the tunnel could no longer supply  enough water and it started draining.

[00:03:59]At first, nobody fully understood what  was happening. Inside the powerhouse, operators noticed that the turbine  output was falling and the penstock pressure was slowly dropping. At the  same time, the water level inside the five-mile-long tunnel kept decreasing minute  by minute as the tunnel partially emptied.

[00:04:17]As the water continued moving downhill toward  the powerhouse, a massive amount of air entered the upper sections of the tunnel, creating an  extremely dangerous situation. The system no longer contained a solid column of water — it now  contained both water and trapped compressed air, and that completely changed the  behavior of the entire system.

[00:04:35]Eventually, the plant superintendent discovered  what had happened — the intake gate had been mistakenly closed. Operators immediately  decided to reopen it, but by that time the five-mile-long tunnel was nearly half empty.  This became the critical moment of the accident.

[00:04:53]As the intake gate reopened, water from the  reservoir rushed violently back into the tunnel while the large volume of trapped  air inside became suddenly compressed.

[00:05:02]The fast-moving water acted like a  giant hydraulic piston, forcing pressure waves through the tunnel system because the  compressed air could not escape quickly enough.

[00:05:10]Soon, the entire intake structure began shaking. Workers near the intake house heard deep rumbling sounds followed by violent  blasts of air mixed with water.

[00:05:26]Air pressure exploded upward through an access  shaft connected to the intake structure. Operators even removed a steel cover plate to relieve the  pressure, but the situation only became worse.

[00:05:36]The blasts became so powerful that workers  started running for their lives. Moments later, windows shattered, massive doors were blown away,  parts of the reinforced concrete floor collapsed, and finally the roof of the intake house  was lifted completely off the building.

[00:05:51]Eyewitnesses later reported giant water jets  shooting nearly 200 feet into the air. The entire event lasted about ten minutes — almost  exactly the time required to refill the tunnel.

[00:06:06]You might wonder why the plant's surge shaft  didn't prevent the disaster. Surge shafts are mainly designed to control pressure changes  when a water column is full. However, at Ladhon, the tunnel had partially emptied, allowing large  pockets of air to get trapped inside. When water rushed back in, it compressed that air violently,  creating unstable "air-water hammer" effects that the surge shaft couldn't absorb. Essentially, the  system turned into a massive hydraulic cannon.

[00:06:34]The aftermath was devastating. Three people  lost their lives, another was seriously injured, and the plant was shut down for two weeks for  expensive repairs. Interestingly, investigators found that while the tunnel survived the  pressure, the intake structure was destroyed by the trapped-air explosions. It was a catastrophic  result triggered by a simple operational mistake.

[00:06:59]This disaster teaches a vital engineering lesson:  hydropower plants are dynamic systems storing enormous energy. A single incorrect valve movement  or a communication error can unleash forces powerful enough to destroy infrastructure in  minutes. Today, modern plants use strict operating procedures, alarms, and emergency protections to  prevent this. The most terrifying part is that nothing failed structurally at first—the dam and  turbines were fine. What failed was the intake gate system control, proving that in  hydropower engineering, control is everything.