Colorado cavitation

In the summer of 1983, the Glen Canyon Dam began to shake. The structure fought to hold back a surging Colorado River. A winter of record snowfall in the mountains of Colorado and Utah melted and arrived at Lake Powell. Engineers nervously watched a steadily rising reservoir, and hoped the dam would hold. The biggest near-disaster in the history of man-made lakes in the U.S. was caused by the same phenomenon that makes your residential heating circulator sound noisy.  

That summer, for the first time since the Glen Canyon Dam was built, they had to open the spillways to release water. The spillways are basically two relief valves. One opens on each side of the dam when a radial gate moves upwards, allowing water to fall almost 600 feet down. If you are familiar with the kids’ game Hungry Hungry Hippos, picture the nose of the hippo slowly being pulled up by a rope to let water into the mouth. The gates can modulate how much water they allow down each spillway, within reason. Unfortunately, the water was rising so fast that the gates were in danger of being flooded over.  

The spillways are the best way to let extra water through. Water going over the top of the dam would wash away the turbine equipment at the bottom. Even a few inches of water flowing over the top could cause millions of dollars of damage and take the power plant offline.   

Dailykos.com reported that an internal U.S. Bureau of Reclamation memo said The Glen Canyon Dam could take a water level up to 3,708.40 feet in elevation. The water level peaked at 3,708.34 feet in 1983. The Colorado River was less than an inch from an "uncontrolled release," as the Bureau put it. While that may not have been a full collapse of the dam, the potential energy of the lake was so big that any new path of water was concerning.  

The Bureau's engineers were so worried about water going over the spill gates that they started adding plywood to the tops to extend them upward a couple extra feet. The sight of a plywood Band-aid was probably terrifying to anyone working in the structure of the dam at the time. Not soon after, the plywood was replaced with 8-foot steel extensions. 

The structural integrity of the dam wasn’t the main concern. The arch shape of this dam is a time-tested structure for holding weight. The more pressure you put on an arch, the more it distributes the weight into the walls on the sides of the canyon that it plugs. The scary part of this particular incident was the rapidly eroding spillway pipe. Cavitation midway down both the right and left spillway tubes was tearing apart the concrete pipes and digging into the sandstone sides of the Grand Canyon.  

The dictionary.com definition of cavitation is, “the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure, a frequent cause of structural damage to propellers, pumps, etc.”  

High Country News reported that the maximum spill rate that summer was 92,000 cubic feet per second split between the two tubes. The Bureau had previously stated that 30,000 cfs was the maximum allowed. This peak spill rate would have been just over 20 million gallons per minute down each of the 41-foot diameter tube, if they were spilling equally.  

The spillway tubes are of rebar reinforced concrete. The concrete pipe does not do as well with high flow rates as metal, especially when bits of concrete from the top of the tunnel break loose and sandblast the lower parts of the tunnel. The high velocity of water and debris in the spillway scooped through parts of the spillway walls as if it were ice cream.

What started the cavitation process? According to Kevin Fedarko’s book “The Emerald Mile,” a golf ball-sized bump of calcite at a concrete joint in the spillway. Like a burr in a piece of copper pipe, this created turbulence in the absurdly high flow of water.  

The cavitation pattern in the spillway was similar to skipping a rock across a pond —  except every time the rock hit the water, the circular splash grew bigger. The engineers onsite described the damage as a Christmas tree shape. The small calcite bump was the top of the tree. The cavitation implosions started right downstream and made a small divot in the concrete. The turbulence continued to bounce further down and created a bigger hole each time. Again and again, it dug deeper and wider, with the chunks of concrete from the first holes now sandblasting the tube further down. Eventually, this cavitation process dug a hole in the east spillway 134 feet long, 15 feet wide, and about 30 feet deep. All of this was created by a small bump at a joint.

If you had been at the top of the Glen Canyon Dam on June 28, 1983, you would have witnessed a terrifying sight. According to a High Country News article, “You would have seen the steady sweep of a spillway mouths suddenly waver, choke, cough, and then vomit forth half-digested gobbets of steel reinforced concrete.” 

The Bureau tried to play it cool about the whole incident. However, helicopters were dropping fliers into the Grand Canyon to warn rafters to camp high and beware of the increasing flow rate of the river. At some point, they stopped letting people launch new boats into the canyon. The Emerald Mile is the name of a boat that illegally launched into the river during this time. The crew knew it would give them a good chance to break the speed record down the nearly 300-mile span of the Grand Canyon. An average tour down the Canyon can take weeks. The Emerald Mile crew took less than two days.   

Cavitation in a boiler system probably won’t hit extreme conditions like the 1983 flood, unless you missed the sizing of your circulator by a mile. If one of your circulators sounds like it is continuously pumping rocks through, you may have a cavitation issue. Having turbulent water just upstream of your circulator could cause the problem. If you have low system pressure, you may also hear cavitation. A piece of copper pipe that you forgot to ream will also cause turbulence, and like the spillway, the damage through the wall of the actual pipe may be a bit downstream.   

 
An uncontrolled release of water in one of your heating systems hopefully wouldn’t cause a complete disaster ending in the Gulf of California, but any leak is a problem. The Glen Canyon Dam event in 1983 serves a cautionary tale to plumbers and engineers to double-check installation practices and system sizing. 

Max Rohr is a graduate of the University of Utah. He is the REHAU Construction Academy Manager in Leesburg, Virginia. He has worked in the hydronics and solar industry for 16 years in the installation, sales and marketing sectors. Rohr is a LEED Green Associate and the Radiant Professional Alliance (RPA) Education Committee chairman. He can be reached by email at max.rohr@mac.com and on Twitter at @maxjrohr.

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