As tempers rise to match the River Shannon floods, much anger is focused on the Ardnacrusha hydroelectric power station and its associated dams, reservoirs and channels. Although many readers will already have a detailed knowledge of its design and layout, for those who don’t I thought it might be useful to provide a quick summary of the scheme and offer a few rough calculations of my own.
When the Shannon Scheme began operation in 1929, it was the largest hydroelectric station in the world, but perhaps more significantly for the government of the day, it was a huge statement of intent for a newly-independent country and a great gamble, since it cost more than one fifth of the State’s entire annual budget to construct. What’s more, with an initial generating capacity of 35 megawatts, later increased to 85 megawatts with the addition of a newer and more efficient fourth turbine, according to the project’s detractors Ireland would never need so much power. There were even some who condemned the project on the grounds that it amounted to Communism, much as the same people would later attack the Mother and Child Scheme, but that was Ireland for you.
Ninety years later, Ardnacrusha contributes about 2% of the total ESB output– enough for a town the size of Ennis.
It didn’t come without a social cost either, as I outlined in this post. They were hard days, different times and attitudes to workers were at best callous.
Thomas McLaughlin, the man behind the scheme, was 29 years old when his project began. A man with little interest in non-academic pursuits, he had achieved a PhD in engineering and a collection of influential friends from his college days who would become instrumental in persuading the government to invest in the scheme.
McLaughlin wasn’t the first to propose using the lower Shannon for power generation. As early as 1844, Robert Kane was pointing out the opportunity for hydro power presented by the drop in water level between Killaloe and Limerick. Potential energy, quite literally. But it wasn’t until John Chaloner-Smith’s research on river flows was published in 1921 that the hard research became available to design a workable scheme.
McLaughlin’s plan, developed with Siemens, required a dam at Parteen Villa (not to be confused with Parteen), just south of Lough Derg, to impound the lake and turn it into a reservoir.
From the dam, a 12-km canal, the head-race, diverts most of the Shannon’s water to the power station at Ardnacrusha where it falls nearly 30 metres through pipes 6 metres in diameter, the penstocks, to drive the turbines. The water leaving the power station runs along the tail-race, a 2-kilometre channel blasted out of solid rock until, at Corbally, it rejoins the old River Shannon which was reduced to a trickle by the diversion.
Before moving on, I have to acknowledge that the scheme is a magnificent engineering achievement by any standards and even today, almost a century later is still deeply impressive. Nothing like it had ever been seen in Ireland before.
Revealingly, the main contractor, Siemens-Schuckert, came from a country that was being crushed under punitive war reparations imposed by the Treaty of Versailles, and yet that was the reason why Germany was so active in developing hydro-electricity.: France had control of all the German coal-fields Ireland’s infrastructure was so primitive that all the construction equipment had to be shipped from Germany because none existed here.
Everyone will have an opinion on the reasons why Ireland was so primitive compared to England, Wales and Scotland, but it’s hard to escape the conclusion that the London government prior to 1922 had little interest in promoting industrialisation in Ireland, apart from Dublin, Belfast and perhaps Cork.
According to Siemens, they imported the following:
|6||Large multiple bucket dredgers on rails each about 220 tons|
|3||Large bank building machines on rails each about 240 tons|
|27||Smaller dredgers and shovel excavators on rails and caterpillars|
|31||Portable air compressors|
|13||Portable concrete mixers|
|3||Cableways, each 310 m long|
|31||Barges, trugs, launches and pontoons|
In order to construct the power station, it was first necessary for Siemens to build a power station with nine diesel generators producing nearly 3 megawatts. They also built a large engineering shop, a smithy, a joiner’s department in addition to a tool-making shop and a welding shop.
Only when everything was in place could the project begin.
By law, the ESB is contractually obliged to guarantee a flow of only 10 cubic metres per second along the old course of the river, which is just one-fortieth of the volume it abstracts for the power station, and this has led to many different problems, both hydrological and environmental. With such drastically reduced flows, there has been a great amount of encroachment by alluvial forest, with alder, ash and willow clogging the river and causing major blockage when it floods. On top of that, the scheme killed what was once a world-famous salmon fishery and an eel-fishing industry.
Now, a few figures.
The average annual flow of the Shannon is a little over 200 cumecs (cubic metres per second), so the ESB wouldn’t always get its full 400 cumec flow, nor would it wish to. But in a flood it could take that much while leaving the old river bed untouched. If the flow goes above 410 cumec then the next step is to store the flood in Lough Derg, using the Parteen Weir, and releasing the extra water gradually down the old channel. All well and good until we get flows of 800 cumec and more as we have today. That means 400 cumec down the head-race and the other 400 going somewhere else, but the question is where?
It’s often suggested that the ESB could have created storage by lowering the lake in advance of the storm, and that’s true, but no matter by how much the lake is lowered, eventually it will fill up and after that, any excess flow is going straight over the weir and down the old Shannon channel, because it has nowhere else to go. Therefore, the only hope is that they can create enough capacity to store the excess water until the rain subsides and the water stops concentrating in the river. It’s a relatively slow river, and we don’t see it peaking until several days after the rain has stopped.
The lake’s surface area is 130 million square metres. Its average volume is 887 million cubic metres and its average depth is 7.6 metres. This allows us to make a crude estimate of how much storage can be created by reducing the water level during periods when there is no flooding.
The current flow into the lake is 840 cumec. How much storage do you reckon we’ll need?
A half-metre drop would give enough storage to contain a day and a half of flood, after which the water would need to be released down the old Shannon. But will the flood only last a day and a half?
A metre would give three days storage.
1.5 metres would give protection against a flood lasting four and a half days. That’s a five-foot drop, in the old money. Is that acceptable to the people and businesses who rely on Lough Derg? What if the flood lasts five days? A week? Two weeks?
Furthermore, you don’t just lower the level of a water body like Lough Derg overnight. It takes weeks and you’d be doing it in contemplation of an event like Storm Desmond, which does not provide weeks of warning.
What are the other factors?
With the exponential growth in paved, impervious areas around the country — roads, car-parks, driveways, roofs and all the rest of it — rainfall no longer has so much time to percolate through the soil and gradually make its way to the watercourses. With uncontrolled development, widely seen as A Good Thing, rainfall now concentrates in a flash, running straight down the gulleys, into the drains, out to the streams and into the rivers. Before you know it, you have a flood.
There’s the neglect of watercourses, as I mentioned. We could be clearing intrusive alluvial forest and replacing it with forestation on land.
Let’s not even mention climate change, but we all know it’s there and we all know it will lead to more of this.
The important thing to bear in mind is that there’s only a given, fixed amount of storage and you can make it as big as you want, but eventually a storm will come along that will fill it up and then the water will go where the water wants to go. Or to put it another way, the ESB can only delay the flood. It can’t prevent it.
Let’s remember something else. Shannon floods are nothing new. The river below Corbally has the same flow now as it did before the power station was built, though perhaps made worse by the factors I just mentioned. Whatever about upstream at Castleconnell, Montpelier or Plassey, Ardnacrusha is not to blame for flooding in Limerick City. In 1850, a fearful flood filled up all the basements of the houses and covered the quays in some places to a depth of three and four feet. It even threatened to tear the parapets off Mr Nimmo’s fine bridge across the Shannon, which had been replaced at great cost following a similar catastrophe some years earlier.
None of this is intended to get the ESB off the hook, but it seems to me that if we’re to have a rational debate on the issue of flooding, at the very least we should be basing it on some sort of rational facts and figures, and we should be looking at all the factors involved. Of course, I do realise that the figures I have here are very crude, but in the absence of a complete hydrological model, I’m afraid they’re the best I can do for now.