FOR the past five years, drought-stricken Californians have implored the heavens for rain. Lately, their prayers have been answered more torrentially than many may have wished. This winter is shaping up to be California’s wettest on record. The snowpack on the Sierra Nevada (a crucial water supply for the long, dry summer) is almost double its normal depth, and near-empty reservoirs are filling fast to the brim. To many farmers and residents, the peripatetic visitor from the tropics responsible for their dousing—an “atmospheric river” (known colloquially as the Pineapple Express) that is hundreds of miles wide and carries more moisture than the Amazon—has outstayed its welcome.

None may wish an end to the deluge more than the good people living downstream of the Oroville Dam, 75 miles (120km) north of Sacramento, which came close to failing catastrophically on February 12th. Some 188,000 people in communities along the Feather River below the dam were given notice to evacuate, as engineers worked feverishly to prevent an uncontrolled release of water from the 770-ft (235m) high dam—the highest in America.

With yet more storms on their way, the main spillway of the dam was opened, so the fresh runoff from Lake Oroville’s 6,000 square mile (15,500 square kilometre) catchment area could be accommodated. Normally, this would be a routine undertaking. But no sooner had water from the reservoir begun to gush down the paved spillway than a huge crater—big enough to swallow a house—was torn in the concrete half way down the 3,000-ft chute. Water diverted by the jagged hole hundreds of feet long plowed gullies in the surrounding hillside and sent boulders and concrete debris crashing down into the hydroelectric generating station 700 ft below.

Alarmed, the dam’s operators resorted to the one backup measure they had hoped to avoid: let the water in the reservoir rise until it flowed over a weir leading to an unpaved emergency spillway off to the side. In all the years since the dam was opened in 1968, the unpaved emergency spillway had never been tested, let alone used in anger. The conventional wisdom was that the bedrock supporting the hillside was stable and impervious enough to withstand anticipated amounts of water rushing down its earthen surface without undue erosion.

Not everyone was convinced, though. In response to Oroville Dam’s relicensing application in 2005, three environmental groups filed a motion with the Federal Energy Regulatory Commission (FERC), warning there could be a “loss of crest control” at the top of the emergency spillway if the unpaved hillside were to suffer serious erosion during an emergency. Were that to happen, a large hole would be torn in the lip of the dam, unleashing a torrent of water that would engulf the power station below, inundate towns downstream, and breach levees (earthen embankments) all the way to the Sacramento-San Joaquin delta.

As a safety precaution, the motion urged FERC to have the emergency spillway paved with concrete. This was dismissed on grounds that even if the emergency spillway suffered significant damage, it “would not affect reservoir control or endanger the dam”. Beside that, the money could be put to better use elsewhere.

Those words have come home to haunt both FERC and California’s Department of Water Resources, the dam’s operator, which continued to insist (until disaster struck) that the emergency spillway was up to the job. Unfortunately, the water flowing over the weir gouged channels in the hillside, sending yet more boulders and debris crashing down into the Feather River below. The hillside bedrock turned out to be a lot softer and more porous than thought.

More worrying still, the concrete lip of the 1,700-ft wide weir at the top had not been anchored to the surrounding hillside, but merely placed upon it. As the water poured over the weir, it quickly became apparent that it was eating away at the lip’s base. Suddenly, there was a very real possibility that the weir could be washed away. In that case, a 30-ft wall of water would go racing down the hillside, inundating the cities downstream. That was when the evacuation order went out.

To shore up the weir, construction crews dumped 40 truckloads an hour of rock and concrete slurry beneath the weir, while a fleet of helicopters dropped sandbags and rocks—in case all else failed and the emergency spillway had to be used again. Meanwhile, to take pressure off the weir, the main spillway was opened wide, despite fears that the 300-ft gash in the concrete could spread further up the chute and breach the gates at the top.

Fortunately, despite the crater growing to over 500 feet long and 45 feet deep, the rest of the concrete chute and adjoining hillside managed to cope with the increased flow. Some 48 hours after the emergency began, the water level behind the dam had fallen 30 feet and residents were allowed to return home. With yet more storms on the way, the water level was subsequently lowered to 50 feet below full capacity.

Could the Oroville Dam have failed catastrophically? It was certainly touch and go. Officials were alarmed enough to order 23,000 National Guardsmen to stand by ready in case the spillway collapsed and brought down chunks of the dam itself.

Even in America, such a catastrophe is not unprecedented. The worst such disaster the country has experienced was the failure of the South Fork Dam at Johnstown, Pennsylvania, in 1889, when heavy rains caused the dam to over-top after the spillway failed, claiming the lives of 2,200 people. A history of cost-cutting and shoddy maintenance was blamed for the disaster. Since then, America has seen over two dozen dam failures, including the Saint Francis Dam near Santa Clarita, northwest of Los Angeles, which collapsed in 1928 with a death toll in excess of 400. None, however, compares with the 1975 failure of the Banquiao Reservoir Dam in China that killed an estimated 171,000 people.

The question facing the team of independent experts appointed by FERC in the immediate aftermath of the near-disaster is why, after all these years, did the Oroville Dam's main spillway fail? The destruction of the unpaved emergency spillway was less of a surprise. The only question there was why no proper geological tests of the hillside’s bedrock had been done over the years, especially after concerns were raised in 2005. Core samples would have quickly revealed the dubious porosity of the rock. As far as the failure of the main spillway was concerned, cavitation was clearly the culprit. But what set of circumstances triggered it?

When a torrent of water gushes rapidly over a solid surface, the turbulence causes small bubbles of water vapour to form wherever tiny cracks, ridges and other minor imperfections snag the flow. Though minute, each of these bubbles collapses with an explosive force. Collectively, their effect is like a jack-hammer, hacking away at the surface and prising open any weakness. Only twice in the mighty Hoover Dam’s 80-year history has it used its spillway tunnels, and on both occasions they were damaged by cavitation. The scoured spillways have since been patched with heavy-duty concrete and their surfaces polished mirror-smooth.

The cavitation that afflicted the Oroville Dam’s main spillway could have been caused by shrinkage in the 50-year-old concrete, along with residual roughness stemming from repairs done over the years. Gravity may have taken its toll as well, with a cat's cradle of stress cracks forming within the concrete as the chute tried to slither imperceptibly down the steep slope with its 700-ft drop. The roots of adjacent hillside trees probably helped undermine the concrete chute’s foundations. And the recent heavy rains no doubt aggravated matters.

Apart from carrying out a forensic analysis of both the main and the emergency spillways, the independent investigators will have to make recommendations in the light of their findings. That could well involve replacing the damaged main spillway completely and paving the hillside course of the emergency one. The cost of doing so could easily exceed $200m. Whatever the remedial action, the work will need to be completed before the next rainy season begins in just eight months’ time.