Tag Archives: Ardnacrusha

Why the Shannon floods

From the search terms used, it seems that many people are visiting this site with questions about Parteen Villa Weir, water levels, Shannon floods and so on. They are not this site’s primary focus, but some non-technical information might be of interest.

The best place to start is with this quotation from a former web page of the Shannon International River Basin District:

The Shannon International River Basin District is the largest in Ireland at more than 18,000 km2 in area. It covers the natural drainage basin of the Shannon river itself, stretching from the source of the River Shannon in the Cuilcagh mountains in Counties Cavan and Fermanagh to the tip of the Dingle peninsula in north Kerry. It also includes coastal parts of Kerry and Clare which drain to the sea. It flows through 18 local authority areas and is also an international RBD as a small portion of County Fermanagh in Northern Ireland drains underground to the Shannon Pot.

The district is about one fifth of the area of the island, one quarter the area of the state. Rain that falls on that area of land ends up in the Shannon (or in a few small rivers in Clare and Kerry that flow to the sea). Some goes to the Shannon estuary or its tributaries; most flows into the non-tidal Shannon, which means the river upstream of Limerick.

The nature of the Shannon

Ireland has been described as saucer-like, with a high rim and a low flat centre. It’s not entirely true, but there certainly is a very large central plain, and the Shannon flows down through the middle of that. And, because the land it flows through is flat, the river falls very little.

In 113 miles from Leitrim to Killaloe, the Shannon falls just over 30 feet; the navigation channel needs only five locks. [By way of contrast, the Thames has 45 locks over 135 miles; the Trent has 12 locks over 42 miles.] So extra rainwater allows the Shannon to spread out, covering a much wider area, and it takes time for that water to drain away downstream. But many of the rivers that flow into the Shannon have been subjected to drainage schemes, so they can get rid of their flood waters quickly … into the Shannon.

Why don’t they open [or close] the weirs?

There are some weirs on the Shannon, designed to keep a minimum depth in the river for navigation; there are also some natural obstacles that hold water back. But once the level has risen high enough, water simply flows over the top of the weir, and there is nothing useful anybody can do — apart, of course, from farmers’ representatives and politicians, who can always make use of a photo opportunity.

The bottleneck

Almost all the water that enters the non-tidal Shannon will eventually flow through Killaloe, the town at the southern end of Lough Derg [it’s on the west bank, in Co Clare; the east bank is Ballina, in Co Tipperary].

As James Robinson Kilroe wrote in 1907,

[…] we have the formidable barrier at Killaloe, naturally damming up a considerable depth of water in Lough Derg, and the river falling away southward by a series of rapids which correspond with drops in the canal, south of O’Briensbridge […], along an alternative course, possibly one used by a branch of the Shannon.

The diagrams with that article are worth a look.

In the twelve Irish (fifteen statute) miles between Killaloe and the tidewater at Limerick, the river falls about 100 feet: more than three times its fall from Leitrim to Killaloe. In the nineteenth century, the water level at Killaloe used to change by about eleven feet between summer and winter — even without storms.  The old Limerick Navigation, including the canal Kilroe mentioned, could drain only a small amount of water (which could put the navigation out of action); the rest went down the river’s original course through the Falls of Doonass.

The relief channel

Nowadays, the Falls of Doonass are a shadow of their former selves, and the water level through O’Briensbridge, Castleconnell and Plassey is much below its previous levels. I suspect that the older, larger trees along the river show the original level, with the newer, smaller trees having grown since the 1920s.

The cause was the construction of a relief drainage channel in the 1920s. This channel is controlled by a weir at Parteen Villa [not to be confused with Parteen].

Actually, of course, it’s not a relief drainage channel. The weir [sometimes referred to as the Hydro Dam] controls the flow of water to the original course of the Shannon [the right-hand or eastern channel, which gets the first 10 cubic metres of water per second] and the headrace for the hydroelectric power station at Ardnacrusha [the left-hand or western channel, which gets the next 400 cubic metres of water per second, 100 for each of its turbines]. The power station was built to use that 100-foot fall of the Shannon, concentrated between Killaloe and Limerick, to generate electricity.

But one effect of the construction of Ardnacrusha was to provide a channel, the power station headrace, capable of taking [at least] 400 cubic metres of water per second away from the original river channel, thus reducing the likelihood of flooding.


Water experts talk about cumecs: a cumec is a flow of one cubic metre, or 1000 litres, of water per second. And a cubic metre of water weighs about one [metric] tonne, which is roughly the same as an imperial ton. So one cumec is one ton of water per second, which is a lot.

It was said, on 8 December 2015, that the ESB, using Parteen Villa Weir, had released 315 cumec down the original course of the Shannon on the previous day and had increased that to 375 cumec. If the Ardnacrusha headrace was getting 400 cumec, then the amount of water being discharged from Lough Derg and the upper Shannon had doubled.


As far as I can see, the Shannon has always flooded. The 2009 floods affected some nineteenth century houses, which I guess would have been flooded even worse before Ardnacrusha was constructed. However, I suspect that more houses have been built on the flood plain since then. But I don’t see that there is any way to prevent Shannon floods.


There is a good article in the Irish Times of 9 December 2015; it will no doubt disappear behind a paywall at some stage.

Here is an ESB infographic about the Shannon.

Shannon water levels 8 December 2015

North to south (more or less)

Floods 20151208 Shannonbridge 01_resize

Shannonbridge upstream

Floods 20151208 Shannonbridge 07_resize

Shannonbridge downstream

Floods 20151208 Shannon Harbour 04_resize

Shannon Harbour: 36th lock

Floods 20151208 Shannon Harbour 06_resize

Shannon Harbour: below the 36th

Floods 20151208 Shannon Harbour 16_resize

Shannon Harbour: road to Banagher closed

Floods 20151208 Banagher 03_resize

Banagher: the harbour above the bridge

Floods 20151208 Banagher 05_resize

Banagher: the harbour’s sole inhabitant

Floods 20151208 Banagher 09_resize

Banagher: work goes on

Floods 20151208 Portumna bridge 01_resize

Portumna Bridge: Hawthorn moving

Floods 20151208 Portumna bridge 03_resize

Portumna Bridge

Floods 20151208 Portumna bridge 02_resize

Below Portumna Bridge

Floods 20151208 Portumna bridge 10_resize

Above Portumna Bridge

Floods 20151208 Portumna bridge 12_resize

Portumna Bridge: Waterways Ireland yard

Floods 20151208 Mountshannon 01_resize


Floods 20151208 Mountshannon 04_resize

Mountshannon: the main quay

Floods 20151208 Scarriff 01_resize

Scarriff: the river in flood

Floods 20151208 Scarriff 02_resize

Scarriff: the river flowing on to the road to the harbour

Floods 20151208 Scarriff 06_resize

Scarriff: sandbags blocking the road …

Floods 20151208 Scarriff 04_resize

… to the Waterways Ireland Shannon HQ. Anyone in the building must have waded there

Floods 20151208 Tuamgraney 01_resize


Floods 20151208 Killaloe 16_resize

Killaloe: the flash lock

Floods 20151208 Killaloe 26_resize

Killaloe bridge from downstream

Floods 20151208 O'Briensbridge 02_resize


Floods 20151208 O'Briensbridge 05_resize

Water level with the quay at O’Briensbridge

Floods 20151208 O'Briensbridge 10_resize

Flooded fields at O’Briensbridge

O’Briensbridge is on the original course of the Shannon, downstream of Parteen Villa Weir, which controls how much water goes via the original course and how much goes to the hydroelectric power station at Ardnacrusha.

Normally, the original course gets the first 10 cubic metres per second (10 cumec, they say) of water and Ardnacrusha gets the next 400, 100 for each of its four turbines. In floods, any excess is sent down the original course, through O’Briensbridge, Castleconnell and Plassey. One newspaper today said that, on Monday 7 December 2015, 315 cumec had been sent down the original course and, on Tuesday 8 December, 375 cumec.

The water levels are still below the peak achieved in November 2009, but there is more to come: as the Shannon drains a very large amount of Ireland, and as it is falls very little in its upper reaches, it takes a long time for the runoff to reach Killaloe and Parteen Villa. It may be that the ESB, which controls Ardnacrusha and Parteen Villa, is now running down the level of Lough Derg to make room for the water that has yet to arrive from the upper Shannon.


Hard sums on Lough Derg

According to the Clare Champion, a Clare county councillor called Pat Hayes, who is a member of Fianna Fáil [an excitable lot, Fianna Fáil], is boycotting something or other for some reason that is not clear to me [and, to be honest, is probably entirely unimportant]. Mr Hayes thinks that water from Lough Derg should be sent to the Atlantic, where it is wasted, rather than to Dublin, where it might be used, and the newspaper cites the River Shannon Protection Alliance as estimating that

… up to 350 million litres of water could be taken from Lough Derg by 2030.

The River Shannon Protection Alliance itself doesn’t agree with those figures. It says:

The central principal and immediate purpose of the organisation is to prevent the proposal of Dublin City Council to abstract in excess of 350 million litres of water on a daily basis from Lough Ree on the river Shannon, and to oppose any action that may be harmful to the well being of the river Shannon system. Since then, the abstraction options have been considered and the current recommended proposal is to abstract upwards 500 million litres of water from Lough Derg and store it in a depleted bog hole to be developed by Bord na Móna at Garryhinch bog, (near Portarlington) where the water will then be treated and pumped on to Dublin.

Eek. That’s a bignum: a lot of litres. Let’s all panic.

On the other hand, 500 000 000 litres is 500 000 cubic metres. Each of Ardnacrusha’s four turbines uses 100 cubic metres per second. So the amount of water to be sent to Dublin every day is less than Ardnacrusha uses in 21 minutes.

If the Alliance wants to save the Shannon, shouldn’t it be trying to get Ardnacrusha closed down first?

Please don’t look at these photos

I regret to say that I have published, on these pages, several photos of the Shannon, Parteen Villa Weir, Ardnacrusha hydroelectric power station and its headrace and tailrace canals.

The storage basin between the road bridge over the River Shannon at Killaloe and the weir and canal intake at Parteen, including the right and left embankments constructed to form the said storage basin, together with the land outside and along the said right embankment delimited and separated from the adjoining land by post and wire fencing and also the land outside and, along the said left embankment delimited by the left bank of the Kilmastulla River Diversion.

Parteen Villa Weir from the embankment upstream (2008)

Parteen Villa Weir from the embankment upstream (2008)

The flooded area above Parteen Villa Weir

The flooded area above Parteen Villa Weir

The weir and canal intake, the embankments constructed to form abutments to the said intakes, the syphon under the said canal intake, and adjoining lands inside and bounded by post and wire fencing.

Parteen Villa Weir

Parteen Villa Weir

Parteen Villa Weir from upstream

Parteen Villa Weir from upstream

The six sluices controlling discharges down the old course of the river

The six sluices controlling discharges down the old course of the river

The head race between the canal intake and the power station including the right and left embankments constructed to form the said head race, together with the land outside and along the said embankments delimited and separated from the adjoining land by post and wire fencing, and also the road bridges over and the syphons and culverts under the said race.

The headrace from the bridge at Clonlara 20 November 2009

The headrace from the bridge at Clonlara 20 November 2009

The headrace from the bridge at O'Briensbridge 22 November 2009

The headrace from the bridge at O’Briensbridge 22 November 2009

The power station, the intake to the said power station, the locks and all adjoining buildings and land within the area around the said power station, all of which are delimited and separated from the adjoining land by post and wire fencing.

The upper chamber at Ardnacrusha lock

The upper chamber at Ardnacrusha lock

Ardnacrusha: looking up at the top chamber from the bottom

Ardnacrusha: looking up at the top chamber from the bottom

The penstocks that feed the turbines at Ardnacrusha

The penstocks that feed the turbines at Ardnacrusha

Ardnacrusha power station from the headrace

Ardnacrusha power station from the headrace

The fishpass

The fishpass

The tail race from the power station to the River Shannon, the branch railway running along the said tail race, and the land on either side of the said tail race, all of which are delimited and separated from the adjoining land by post and wire fencing.



And I have lots more photos … here and here and here, which I ask readers not to look at either.

You see the thing is, Your Honour, Sir, I didn’t know. I didn’t realise that, under Statutory Instrument 73 of 1935 Shannon Electricity Works (Declaration of Prohibited Place) Order 1935, the places as described are prohibited places under paragraph (d) of Section 3 of the Official Secrets Act 1911, as amended by the Official Secrets Act 1920, and as adapted by or under the Adaptation of Enactments Act 1922 (No 2 of 1922). That’s because

information with respect thereto, or the destruction or obstruction thereof, or interference therewith, would be useful to an enemy.

Apparently, under those acts, giving anyone information about a prohibited place is a felony, punishable by imprisonment for up to fourteen years.

Wikipedia says that, in Ireland, those acts were repealed by the Official Secrets Act 1963, but was the statutory instrument repealed? I don’t know, but I’ve written to the Department of Justice to ask.

In the meantime, please don’t look at the photos, especially if you’re a Foreign Agent: a term that, under the 1963 act,

includes any person who is or has been or is reasonably suspected of being or having been employed by a foreign power either directly or indirectly for the purpose of committing an act (whether within or outside the State) prejudicial to the safety or preservation of the State, or who has or is reasonably suspected of having (whether within or outside the State) committed or attempted to commit any such act.

I wonder whether that includes the European Central Bank.

Irish GIS data

Not good enough, it seems: the Ardnacrusha headrace and tailrace are missing.

Salmon at Ardnacrusha

Might a salmon cannon work?

Eel update

Dr William O’Connor says that no elver traps are operating on the River Shannon.

Pathé on Shannon

Shannon floods 1959 1

Shannon floods 1959 2


The Carrick-on-Suir creamery chimney (Shannon Scheme electrification)

Building the headrace


Draining Lough Derg

The ESB is currently letting more water down the old course of the Shannon, from Parteen Villa Weir through O’Briensbridge, Castleconnell and the Falls of Doonass. This channel gets the first 10 cubic metres per second from the Shannon; the next 400 go through Ardnacrusha and anything left over is sent down the old course.

The result is to help to reduce the water level on Lough Derg while raising it on the old course.

The footbridge in Castleconnell at normal summer level in 2002

The footbridge in Castleconnell at normal summer level in 2002

The footbridge on 1 January 2014

The footbridge on 1 January 2014

Before Ardnacrusha was built, the old channel took the entire flow of the Shannon, so it can take more than it has now.

The footbridge in the floods of November 2009

The footbridge in the floods of November 2009

The level is still below that of 2009, when the land around the old channel flooded in several places. But much land is waterlogged: I saw yesterday that the upper reaches of the Nore, the Barrow and other rivers were in flood. And more rain is forecast.

Wouldn’t it be nice if some of that could be sent to Dublin instead? I see that some folk claim (on what looks like a website that hasn’t been updated for a while) that the evil Dublin folk want to extract 350 million litres of water from the Shannon every day; the original idea was to take it from Lough Ree but now it seems that Lough Derg is the preferred source.

Now 350 million litres sounds like a lot, but it’s 350 000 cubic metres per day, 14 583.3 per hour, 243.05 per minute, 4.05 per second, which is less than 1% of normal flow through the two channels draining Lough Derg. There’s a lot more at the moment, and the good citizens of Dublin are welcome to come down and fill their buckets. I suspect that Clare TD Michael McNamara has got things out of proportion.

Addendum: 350 million litres per day, over a lake whose area is 130 square kilometres, would lower the level of the lake (if my calculations are correct) by 2.69 millimetres. If no water entered the lake, the level would be down 983 mm after a year, ignoring evaporation and other abstractions and assuming that the Shannon and other tributaries no longer flowed in and that there was no rain.


A press release just in from Waterways Ireland:

Waterways Ireland Change Advice on Travel into Limerick

Waterways Ireland is changing the advice on travel from Ardnascrusha into the Limerick Navigation due to a new service available from the ESB which provides real time information on the output from the turbines.

Boaters are advised not to travel on the Ardnacrusha to Limerick Navigation if the output from Ardnacrusha is above 20 Megawatts. The previous advice advised boaters not to travel if one turbine was operating; 1 turbine is equivalent to 20 megawatts.

The new service offers real time information on the megawatt output of Ardnacrusha by phone (tel 087-6477229) and is available 24hrs a day. Boaters are advised to contact the ESB phoneline for information on the output when making travel plans. In the event of a line failure in the real time information number, boaters can call the main Ardnacrusha number 087-9970131 for the megawatt information as well as any other information required for travel through Ardnacrusha.

This new information service is an improvement in safety for mariners who travel on the Ardnacrusha to Limerick Navigation.

Update 2018: that was in 2013. Much better information is now available here.