It is often said that if the Clyde dam ruptured, the resulting torrent would bypass the town of Clyde allowing sufficient time for the residents to leave before the waters arrived. This is an official myth.
What would cause a dam burst?
Obviously, a large quake along the Alpine Fault could wrench the Cairnmuir-Dunstan faultline laterally a few metres or even several metres. This fault is some 3 kilometres above the dam. Gerald Lensen, one of New Zealand's leading geotechnical scientists when the dam was being built, maintained that this fault movement would tend to open up the secondary River Channel Fault which branches off the main fault and runs directly under the dam. This opening movement is described as “tensional," and alarmingly the “slip-joint” is designed for lateral movement, not tensional. Regardless of this argument, a large earthquake has the potential to rupture the dam. Earthquakes are the main cause of concrete dam failures.
Earthquake Generated Wave:
Just as earthquakes cause ocean tsunamis, they can also cause wave events on inland waters. The Alpine Fault is moving laterally while one side tilts upward and the other side is subducted. The Clyde “slip-joint” is designed to cope with a maximum of 1 metre of vertical movement, and two metres of lateral movement. If such a 1 metre vertical movement occurred in a strong earthquake event, the bed of the reservoir on the upward side of the fault would be abruptly lifted. It has been calculated that a 1 metre lift would move 23 million cubic metres of water, generating a swift and destructive wave. It would carry debris from the sides of the gorge that when combined with the force of the wave, could do catastrophic damage to the dam. Such an earthquake-induced wave would at least overtop the dam with devastating consequences.
Landslide Generated Wave:
Any large landslide in the Cromwell Gorge could cause a wave capable of overtopping the dam. The likely cause of such a landslide is heavy rain or an earthquake. The wave would travel in both directions, towards the dam, and towards Cromwell, and could be powerful enough to do considerable damage. For example, if the massive Nine Mile slide collapsed into the reservoir it could easily block the gorge creating a fast-moving and devastating wave. It is possible that the dam could survive a wave strike reasonably intact, though the Clyde dam is not the stronger arch design. Even if the dam wasn't breached, the overtopping wave could still cause a major disaster. The wave could be 100 metres high and such a wave would travel at around 160-240 kilometres per hour, devastating a large area below the dam.
Earthquake, Landslide, Wave:
In the event of a large earthquake, any one or all three of the above scenarios could occur. For example, an earthquake could rupture the dam without causing a significant wave, or both a rupture and a wave could occur. Also, an earthquake might do minimal damage to the dam, but cause a large landslide generating a powerful overtopping wave, which might rupture the dam or flow over it, causing a disaster either way. Worst of all, is the combination of an earthquake rupturing the dam, followed by a wave induced by the same earthquake, and also landslides caused by that earthquake, in turn causing more waves.
So what would really happen in an overtopping or dam burst event?
It is ironic that the “slip-joint,” hailed as an innovation to mitigate earthquake damage, could itself become a weakness if it failed to work as designed. If the wedge pulled apart during an earthquake or was damaged sufficiently to cause a breach, the resulting leak would be under immense pressure, and what would happen then is open to speculation. If the flow increased, how that could be stopped from rapidly deteriorating into a catastrophic dam burst is unknown.
The dam was built in two “halves” either side of the 2 metre wide join. The left side (facing), furthest from Clyde, was built by the Ministry of Works, while the right side (facing), closest to Clyde, was built by Zublin-Williamson. During construction the German contractors (98% of the so-called Zublin-Williamson “joint venture”) were found to be rushing the preparation of the concrete batches, pouring them so fast that “honeycombing” was occurring. The supervising contractor, the Ministry of Works, repeatedly asked for sub-standard poured blocks to be drilled out and re-poured. This has led to a widely held view that the right side of the dam has weaker block work than the left side.
In January of 1990, Electrocorp released a report of their findings following a computer modelling exercise using a US software programme called ‘Dambrk.’ The software was developed to determine the extent of damage in the event of a dam being overtopped or breached. The results depended entirely on the data input, and in this case Electrocorp entered the scenario of a 20% outflow of the reservoir, said to be the equivalent of a maximum flood. They expected this water to be released through the three “blocks” nearest the right side abutment, presumably the weakest part of the dam.
Whether a rupture occurred through the “slip-joint” or on the right side, in both parts or elsewhere, the sudden release of water would be phenomenal. The water would blast through the dam at 160-240 km per hour, scouring everything as it went, carrying ever more debris as it pulverised everything in its path. Rocks and boulders, trees and buildings, cars and people, would all be swept away in the disaster.
An overtopping landslide wave would have less pressure but would still travel almost as fast as water rupturing through the dam. If the dam survived the wave strike, the wave pouring over the dam could still scour down the block work or scour into an abutment. The water would blast through any weakness. Such a wave would not necessarily travel directly down the gorge, but could wash from side to side, and therefore could strike the dam initially on either side.
According to Electrocorp, however, the waters issuing from an overtopping or dam burst event would take a full 6 minutes to reach Clyde, even though this is longer than it takes for the river to cover the same distance in normal flow. They said that the Clyde Bridge would be washed away, and that a “gentle swell” would go down the river and reach Alexandra in 1.5 hours, where the river would be some 12.5 metres above normal at the Alexandra Bridge, supposedly equivalent to a 1000 year flood. The report said that there would be some flooding in Alexandra, and more serious flooding in the Manuherikia area, before the water would flow down the Roxburgh Gorge.
Electrocorp’s version of events was unbelievably benign. Strangely, they considered their ‘Dambrk’ report to be insufficient for the purpose of Civil Defence planning. This alone, suggests that the report was too unreliable, too deficient in realistic input data, too fictional, to be taken seriously. In short, they belittled what is a gravely serious issue, trying to make it palatable to the public.
What would happen to the Roxburgh Dam?
Remember, all that water has to go somewhere, and a wall of water and debris travelling at 160-240 kms per hour doesn’t give much warning, or allow much preparation for the coming disaster. The main form of mitigation at the Roxburgh dam, Electrocorp said, would be lowering the level of the reservoir. The Roxburgh dam, they said, if it was discharging and generating to capacity, could lower the Roxburgh reservoir 45cms in 6 hours.
It is difficult to imagine that this would be enough to contain the water surging down the gorge. The narrowness of the upper gorge would certainly restrict but also speed up the flow, and at the lower end of the gorge where it turns in an ‘S’ shape and opens out around McKenzie’s Beach, the wave would diminish, but the dam would still be faced with more water than it could safely spill. If the initial surge left the dam intact, the rising reservoir would soon overtop the dam and there would be no way to prevent widespread, catastrophic flooding, and probably a major dam failure.
We are supposing, of course, that if the cause of the devastation at the Clyde dam was an earthquake, that the Roxburgh dam was spared, and did not also breach or receive an overtopping wave at the same time. But either way, the Roxburgh dam is unlikely to survive.
Millions of tonnes of silt would be drawn down and spread out over the Teviot Valley. The surge of water, silt and debris would be partially restricted at Dumbarton Rock, but would nevertheless continue towards the sea, destroying and burying everything in its path. At Balclutha, the speed of the flood would be slowing and the level of silt and debris would be reduced to perhaps a few metres, and yet the waters would still inundate the town within minutes, flowing out across Inch Clutha into the Pacific.
Is there a dam disaster like this on record?
The Cromwell Gorge has been compared to a valley in northern Italy, where a dam was completed on a tributary of the Piave River in 1961. The 262 metre high Vajont arch dam was regarded as an engineering triumph. The people living below the dam were assured that the dam was safe. The sides of the gorge above the dam became unstable when the reservoir was partly filled. The reservoir was repeatedly raised and lowered as the landslide areas were monitored. Engineers and officials were reluctant to admit there was a serious threat to the dam.
On October 9, 1963, at approximately 10.35pm, heavy rain caused a 260 million cubic metre landslide into the reservoir, moving at up to 110 kms per hour. The wave that overtopped the dam was 100 metres high. It reportedly advanced down the valley with incredible speed, preceded by an atmospheric shock-wave. It soon engulfed the towns of Longarone, Pirago, Rivalta, Villanova and Faè, destroying everything in its path, killing 1,450 people. The torrent then swept into smaller villages in the territory of Ert e Casso and into the village of Codissago.
Almost 2,000 people (some sources report 1,909) perished in the Vajont dam disaster. The devastated region was described afterwards as a “mud-covered coffin.” Remarkably, only part of the dam, the top of the right side, was damaged, demonstrating that arch dams provide excellent resistance to wave events, albeit a disaster can still occur. It was later shown that geological investigations had been deficient.
It is sobering to realise that such a deadly disaster could occur as the result of an overtopping event. Surely, a dam breach in a non-arch dam would prove even more devastating because a much greater volume of reservoir water would pass through such a ruined dam.
Money and pride first, safety last?
When it comes to admitting the possible extent of the devastation following a dam disaster at Clyde or Roxburgh, there is a noticeable paucity of official information. When the 1999 flood caused serious damage to Alexandra, because of the silted up river bed at the top end of the Roxburgh reservoir, a hue and cry went out to Contact Energy, the dam owners, to fix the problem. Reluctantly, they offered up limited compensation and some remedial flood protection work. They readily exploit the river, but when their activities cause damage, they are difficult to hold to account. This is indicative of a “head in the sand” approach to dam safety issues (perhaps that should be “head in the silt”). Profit drives any business, and dams are built and managed with a degree of conquering arrogance that never really understands that rivers, and tectonic plates, always have the last say.
The Clyde dam is a monument to engineers and politicians. Few among them would admit that they have built a potential disaster. That pill is too bitter to swallow. But the landslides in the Cromwell Gorge are still feeling the impetus of gravity. The rain still falls, sometimes in thunderstorms. Earthquakes still happen, and the “big one” along the Alpine Fault is overdue. When the Earth or the sky rumbles, let the dam builders explain how safe their dams are, and ask yourself ~ why should they be able to risk your town, or your life?
An independent review into the safety of the Clyde and Roxburgh dams is urgently needed. Grave mistakes have been made, and it’s time to face up to the potential consequences of dam failures on the Clutha. Of course, it usually takes a tragedy to kick start such a process.
If you are standing in the main street of Clyde when a wave overtops the dam, don’t wait 6 minutes for it to “gently” arrive. You will probably have a few seconds …