The Emerald Mile: The Epic Story of the Fastest Ride in History Through the Heart of the Grand Canyon (42 page)

When work on the dam began in 1956, a set of diversion tunnels had been drilled at river level through the sandstone cliffs on either side of the river. By shunting the Colorado into these horizontal tubes, the construction site was kept dry while the dam was being built. Then, six years later, as work on the concrete edifice was nearing completion, the construction crews had to bore a
second
set of tunnels through the cliffs to serve as the spillways. Drilling through solid rock was extremely expensive, so Glen’s design team had opted to save some money by allowing the steeply inclined spillway tunnels to intersect with the lower ends of the nearly horizontal diversion tunnels. (This configuration can easily be visualized by placing one’s arm on a desk: the bone that runs from the shoulder to the elbow represents the new spillway, while the forearm between the elbow and wrist represents the lower portion of the old diversion tunnel.)
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This was an efficient arrangement, but it forced the water flowing through each spillway to negotiate a significant bend at the elbow, where the two tunnels met. Moreover, as the water thundered through the tunnels, it picked up enormous speed, which meant that the most violent hydraulic forces were concentrated at this elbow joint. The faint illumination that Burgi discerned as the cart descended was coming
up
the bottom of the old diversion tunnel, and reflecting off a pool of water that had collected directly at the elbow.

By now the cart had been trundling down the incline for almost ten minutes, and they were roughly 80 percent of the way to the bend. Here, something on the surface of the invert caught Burgi’s eye. He told the radioman to order the cable operator to stop the winch, then flicked the switch on his portable light and peered through the mist. On the surface of the tunnel, where the concrete should have been solid and smooth and unblemished, he could see something that had no business being there: a jagged, ugly-looking hole.

Phil Burgi wasn’t the sort of man who condoned the use of profanity, ever—even when the situation seemed to beg for execration. So when the words
“Jiminy Christmas” escaped his lips in a low murmur, what he was really saying was
Holy mother of God.

B
urgi thought he had a pretty good idea what might have caused that declivity to form, and to confirm his suspicions, he trained the beam of his light just
above the hole. Sure enough, there it was: a bump on the tunnel surface. It was tiny—only about a quarter of an inch high and small enough that he could have covered it with a poker chip. But as Burgi knew, that bump was connected to a story that, much like the narrative of the elbow joint, went back more than twenty years.

During the final phase of the dam’s construction, the interiors of the tunnels were lined with concrete, and sometime after the concrete had set—it was impossible to know precisely when—a small protuberance had formed on the rounded invert of the tunnel’s steepest section just inside an area known as Station 25.00, where Burgi was now suspended. Here, two separate sections of the tunnel’s lining had butted together to form a joint, and the bump that had taken shape along that joint was anchored directly on the centerline of the tunnel. Composed of calcite, a crystallized mineral formed of calcium and carbon ions, the bump was generated by excess moisture rising to the surface through cracks in the concrete, the same process that produces powdery deposits on basement walls and the sides of swimming pools (and which also creates stalactites on cave ceilings). This nodule wasn’t the sort of thing that an inspector would have been able to spot unless he was searching for it. It was about the size of a barnacle on the hull of a ship—and, like a barnacle, the calcite was extremely hard. Knocking it off would have required a hammer and a cold chisel.

This was the cause of all the trouble, although the mechanism was complex and stemmed from a peculiar property of moving water—one of those quirky and chaotic elements that made hydrodynamics so fascinating to Burgi.

When water traveling at high velocity strikes an irregularity on the surface over which it flows, tiny vacuum bubbles radiate downstream. When the bubbles implode, they send out miniature shock waves that reverberate through the water column. Each concussion is minuscule, but when the effect is multiplied by thousands of bubbles collapsing simultaneously over a sustained time, it can generate surprising impact. The effect is similar to what might be created by tapping away with hundreds of tiny ball-peen hammers.
This force, called cavitation, was first identified by the British Royal Navy when the propellers of its warships and submarines began to sustain heavy damage during World War II. As the British discovered, when cavitation runs unchecked for days or weeks, it can inflict severe damage on hard metals such as iron and steel.

Concrete doesn’t stand a chance.

Although the engineers who designed the Glen Canyon Dam were generally familiar with the challenges posed by cavitation, they had been unable to devise a solution to the problem. But by the summer of 1983, Burgi’s team at the Hydraulics Lab in Denver understood cavitation more deeply than anybody else in the world: what triggered it, the damage it could wreak, and the steps
necessary to contain it. The lab’s roster of fancy gadgets included an enormous vacuum cavitation chamber, painted baby blue, that looked like the pilothouse of a ship perched on steel legs. Thanks to the research Henry Falvey and a handful of his colleagues had conducted inside that chamber, Reclamation had actually come up with an answer to the problem—an expensive but elegant fix in which a cushion of air was injected into a spillway’s water flow to absorb the shock waves and neutralize the force of the collapsing bubbles. Because of budgetary constraints, however, this fix had been back-burnered in the bureau’s dams across the arid Southwest on the grounds that their spillways were so seldom used. In a grotesquely ironic twist of timing, the discussions for implementing
this repair at Glen Canyon had been scheduled to start on June 1, one day before the spillway gates had been opened. Now, as Burgi discerned, it was too late.

By the time Burgi located the calcite nodule, cavitation had been eating away at the east tunnel’s concrete lining for five days. The hole just beneath the nodule was small—only about five inches deep and barely big enough to hold an orange. But Burgi could see that it contained a small pocket of wet gravel, which was as disturbing as the discovery of the hole itself. At higher velocities, the water in the spillway would cause those bits and pieces of broken concrete to swirl around the bottom of the hole like loose marbles, scouring the edges and rapidly causing the hole to widen. As it expanded, so too would the violence of the scouring. Engineers call this a positive-feedback loop—a system that builds in intensity by feeding on its own effects. But what was even more disturbing to Burgi was that this damage had probably triggered a chain reaction along the lower sections of the tunnel.

By creating a disruption in the water train that was quite a bit larger than that from the original calcite nodule, the orange-size cavity had almost certainly generated a second and considerably larger explosion of cavitation-inducing bubbles. Here lay the start of a different sort of positive-feedback loop—one that, if the theory was correct, would create a sequence of ever larger holes deeper inside the tunnel. Sure enough, when Burgi tested this theory by aiming the beam of his light into the darkness below, he could spot the shadowy outline of the next link in the chain. The concrete immediately below the first hole was undamaged, but eighteen inches farther down was a second cavity, at least twice as wide as the first and nearly a foot deep.

The second hole was followed by another intact section that ran for several feet and yet a third hole, then an even longer gap and a fourth hole, whose circumference was bigger than that of all of the others combined. Inside each impact zone, the damage grew radically worse—the scouring had penetrated deeper into the concrete, and each deposit of gravel was larger and coarser than the next. As the cart continued its descent,
Burgi discovered a total of six holes,
each wider and deeper than the one directly above it, the lowest of which was so extensive—penetrating nearly four feet—that twisted pieces of rebar were poking through the rubble.

Finally, the cart could go no farther. They had not yet reached the elbow joint at the bottom, but the size of the sixth hole exceeded the width of the cart’s chassis and thereby blocked their progress.

As he hung there in the darkness, huddled under what remained of the shredded umbrella, Burgi pondered the scene before him and pieced together what had taken place. The cavitation had essentially leapfrogged its way down the tunnel with ever-expanding fury, creating a widening triangle of destruction. This was the source of the hollow thumps and the mysterious frying-bacon sounds that the Control Room team had picked up late on Sunday night.

The pattern of damage that had resulted was a classic double-echelon that hydraulic engineers refer to as a
Christmas tree—with the giant hole at the base and the calcite deposit at the star. This came as no surprise to Burgi. But he was astonished by the extent of the destruction. The size of the holes, and even more shocking, the speed with which they had formed were far worse than he had anticipated. Moreover, if his hunch was correct, the area of worst damage wasn’t even visible.

Somewhere within the nebulous depths of the pool at the base of the incline, down where the tunnel formed its elbow, a truly colossal breach was being excavated. Just how deep that hole might penetrate was impossible to say. But this, Burgi had no doubt, was where the spillway jet had drilled all the way through the concrete lining of the tunnel and begun mining out the soft sandstone of the canyon wall that supported the eastern abutment of the dam. Here lay the source of the orange-tinted discharge, as well as the chunks of debris that Gamble and White had witnessed flying off the end of the flip bucket and arcing into the Colorado.

The idea that such a minor aberration—a mineralized globule half the size of a walnut—could have initiated the dismantling of something as immense as the spillway of the Glen Canyon Dam was hard to believe. But the evidence was clear.

Dismayed by what he had seen, Burgi pulled out his notebook and, doing his best to shield it from the cascade, made a quick sketch of the Christmas tree with his best estimate of the measurements and dimensions. Then he told the radio operator to order the folks on the surface to start hoisting them back to the top. It was time to get out of the tunnel and give Gamble the bad news.

I.
If you cup your palm and turn it upward, you’ve also neatly captured the flip bucket.

The distant lightning glowed mutely

like welding seen through foundry smoke.

As if repairs were under way

at some flawed place in the iron dark of the world.

—C
ORMAC
M
C
C
ARTHY

A
S
the cart ascended toward the top of the tunnel, Burgi concluded that using the spillways was no longer an option. The moment they reopened the gates and permitted the flow to resume, the cavitation damage would resume. Judging by what he’d just seen in the east tunnel, a similar process was surely unfolding in its counterpart on the west side of the canyon, which was still passing water. In Denver and Salt Lake City, however, Burgi’s superiors had drawn precisely the opposite conclusion: it was no longer an option
not
to use the spillways. Even as Burgi clambered from the cart and climbed the ladder back to the dam’s parapet, Gerry Williams’s team at the River Forecast Center was once again increasing its estimates for the amount of runoff pouring into Lake Powell.

This left Gamble in a terrible predicament. With barely two inches of freeboard left on the spillway gates, the man in charge of Glen had roughly twenty-four hours to ponder his options before the river made the decision for him. If the gates remained shut, sometime on the night of Tuesday, June 7, the first
surge would come sluicing over the top of the gates and form a fifty-two-foot waterfall at the upper end of the tunnel intake. The gates weren’t designed to handle such a cascade, which would tear out the cable hoists and damage the trunnion arms. Then, with no way of raising the gates, Gamble and his team would be robbed of their ability to control the flow into the spillway tunnels, where the water would resume its stripping away of the concrete lining and drilling into the sandstone.

Clearly, the situation called for out-of-the-box thinking. What they really needed was for some kind of river fairy to fly over the surface of Lake Powell, wave her wand, and expand the size of the reservoir by a couple of million acre-feet so they could store some extra water while figuring out what the hell to do next.

That was a tall order. But as it turned out, Gamble and his team had something in mind.