The Shannon floods of November 2009 affected people living close to the river and the waterways infrastructure and boats thereon. I have a page about those aspects on lower Lough Derg and in Castleconnell, Co Limerick.
This page is the second of three about the old (pre-Ardnacrusha) Limerick Navigation, from Killaloe to tidal Limerick. The floods provided an opportunity to see what the water levels on the old navigation might have been like, before Ardnacrusha laid claim to the first 400 cubic metres of water per second.
Water, Ardnacrusha and Parteen Villa Weir
The floods of November 2009 caused high water levels on the River Shannon. The ESB (Electricity Supply Board) controls the water levels on the lower Shannon by using Parteen Villa weir: it can send water down the headrace canal to the Ardnacrusha hydroelectric power station (maximum 400 tons per second) or down the original course of the river through O’Briensbridge and Montpelier, Castleconnell, Plassey, Athlunkard and Corbally.
The capacity of this latter route may have been over 700 tons per second before Ardnacrusha was built, but (a) many houses, and some university buildings, have been constructed near the river since then and (b) the extent of the recent rainfall is such that even the pre-Ardnacrusha capacity of the river may not be enough to take all the water that is coming down the Shannon. In normal times, the original course of the river gets 10 tons per second; Ardnacrusha gets the next 400; anything that is left over is sent down the original course. During the floods the ESB sent more down the river; it timed its evening discharge to coincide with low tide in Limerick. The Irish Independent said that the discharge was 460 tons per second.
This page shows photographs taken in November 2009, with some older photographs for comparison. It covers two sections of the old Limerick Navigation plus one section of the modern navigation:
- the river from Plassey to the head of the Park Canal
- the Park Canal itself (at the bottom of my sketch-map below).
The old navigation used the Plassey–Errina (or Errina–Plassey) Canal, which left the Shannon just downstream of O’Briensbridge and rejoined it on the County Clare side (which I usually call the west side, but might more accurately be called the north bank at that point) opposite Plassey. Before the Shannon Commissioners made their improvements in the 1840s, the towing horses had to be ferried from one side of the river to the other, but the erection of the “Black Bridge” made the crossing much easier.
Just upstream of the Black Bridge is a relatively new bridge that links the two sections of the University of Limerick’s campus.
Here is a photo of the Black Bridge, taken from the new bridge, in poor weather (wind against current) in August 2009. There is still good headroom under the bridge.
And here is the Black Bridge, again seen from the road bridge, on Sunday 22 November 2009. Note the runner on the bridge, which has now been closed off. The water level shown was achieved before the ESB began releasing extra water on 23 November.
There was debris lodged against the bridge.
The water rose further.
On the new bridge, motorists are separated from cyclists and pedestrians, and you can look down the gap between the sections.
Upstream of the new bridge, the lower-lying land was flooded.
There are fishermen’s cottages, and traditional fishing-boats, on the bank between the bridges.
Further down the towing-path, on the east (or, if you insist, south) bank, the university has built new premises for its rowing club. The building need not concern us, but some of the waterside facilities help to show the change in water level.
The “regulating weir” at the World’s End in Castleconnell sets the minimum (summer) water level for the river up through O’Briensbridge/Montpelier as fas as (nowadays) Parteen Villa Weir; it also maintains the depth in the upper stretch of the Plassey—Errina Canal, from where it leave the Shannon to Errina Lock.
Similarly, the regulating weir at Corbally sets the minimum water level for the river up to Plassey; it also maintains the depth in the upper stretch of the Park Canal, from where it leaves the Shannon to the upper lock. You can see the weir in this Google satellite photo, stretching south-east from the tip of what we might call the Corbally peninsula. (You may also be able to see the Park Canal, running just a little south of west, near the bottom of the map.)
And here it is in flood.
The Park Canal: upper lock
The name “Park Canal” is, I think, a recent but useful identifier for the one-mile stretch linking the Abbey River to the Shannon, bypassing the rapids at Corbally. Limerick City Council has tried to restore the canal, including installing some rather silly bollards in the harbour section, but while the canal has not been made navigable it does have good paths along both sides and it is an attractive walking route.
The canal has two locks. At the downstream end, a lock allowed boats to move from the canal into the Abbey River (and vice versa). There is a second lock upstream, about half way along the canal, which is one Irish mile long (there is a milestone where it meets the river at the upstream end).
This photo is taken looking upstream. Note how low the water-level is. Note also the dam in the lock; there are no gates. Just below the dam, on the right-hand side in the previous photo, there is an aperture in the wall above the lock gates.
The aperture is well above the water level, even of the level above the dam. I and others believe that to be the entrance to a covered bywash, enabling water to flow into the lower section of the canal without having to open either the lock gates or the racks or paddles thereon. Here is the exit. The tunnel has been referred to as “the lady’s hole”; the word “syphon” has also been applied to it.
The bywash tunnel is clear but there is a steel plate near the upper end, preventing water from getting through. The plate presumably replaces a sluice mechanism, controlled from above, but the area has been paved and no trace of the mechanism can be seen.
I assume that normal post-Ardnacrusha water levels (ie levels since Ardnacrusha was built) are equivalent to pre-Ardnacrusha minimum or summer levels, held up by Corbally weir. At those levels the bywash would be of little use (even if the dam were not keeping the water from it). However, the floods of November 2009 have created water levels closer to pre-Ardnacrusha levels, and it is clear that the bywash would have been under water.
There is a depth gauge above the bridge. The next photo was taken from a boat in August 2008.
The Park Canal: lower lock
Here is the lock at the entrance from the Abbey River, taken on a misty day in December 2004, not long after new gates had been installed. (I don’t think they have ever been used to let a boat through, despite the elaborate control mechanisms.)
And a photo taken during the floods.
The upper gates on this lock use a patented mechanism, designed by Martin Cullen of Glasgow Caledonian University‘s Centre for the Built Environment (CBE). The gates are designed to withstand water pressure from both sides, so that this lock, for instance, could resist high water from spring tides on the outside of the gates as well as flood water on the inside. You can read about them in the CBE newsletter issue 9 of October 2004 (the link to the PDF is near the bottom of that page). When installed on historic structures, these gates do far less damage than sector (radial) gates like those at Spenceer Dock on the Royal Canal in Dublin. Here are the upper gates in normal conditions, front and rear.
And here are the gates during the floods, held open to allow flood waters out. The first photo is slightly out of focus; sorry about that.
One thing that is clear is that the old Limerick Navigation can cope with much higher water levels than those available since Ardnacrusha was opened.