Tag Archives: weir

What’s a cumec, Daddy?

I don’t know, but here’s a picture of 415 of them flowing through Castleconnell. [Update 13 December: that may be only 405 cumec.]

Castleconnell 20151212 10_resize

415 cumecs on 12 December 2015

Actually, a cumec is one cubic metre of water per second, which is roughly one ton per second, which is a lot of water. And 415 cumec is the amount that, according to the blatts, the ESB is currently letting down the original course of the Shannon from Parteen Villa Weir; the minimum flow in that channel, as seen in summer, is 10 cumec.

Castleconnell 12 December 2015

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Younger trees getting their feet wet

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Who’d have guessed?

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Below the bridge

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Above the bridge

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Stormont

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Pump

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Sandbags

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Sandbag Central

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Sandbags filled here for distribution

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Army engineers

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More equipment arriving

Clonlara 11 December 2015

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Ardnacrusha headrace, said to take 400 cumec

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Errina bridge on the Plassey-Errina Canal

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The stop planks seem to be quite effective …

PEC Clonlara upstream 20151211 01_resize

… as there is little water getting down the canal to Clonlara bridge

Park Canal 9 December 2015

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Depth gauge at Park Lock

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Lock chamber

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Full canal upstream

 

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 a map of the Shannon International River Basin District. As the Shannon RBD site says,

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]. Switch between the modern Street Map and the Historic views here to see what has happened.

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.

Cumec

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.

Floods

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.

Envoi

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.

Hamilton Lock

Victoria (Meelick) and Hamilton Locks (OSI ~1900)

Victoria (Meelick) and Hamilton Locks (OSI ~1900)

Lord Dunkellin: Do you know the Victoria lock at Meelick?

Sir Richard Griffith: I do.

Victoria Lock, Meelick

Victoria Lock, Meelick

Dunkellin: Do you know what is called the Old Cut, the old canal?

Griffith: Yes.

Dunkellin: The Victoria lock is a new work, is it not?

Griffith: It is.

Dunkellin: Should you be surprised to hear that vessels do not use that frequently, but go by the old cut?

Griffith: In times of very high flood I am aware that the canal boats find it advisable and beneficial to go by the Hamilton lock, on the old cut, in preference to the other.

Dunkellin: Prima facie, one would have thought that a new work like the Victoria lock would have the effect of regulating the state of things?

Griffith: It arises from the Counsellers’ Ford, as it is called, above Meelick; it has not been sufficiently excavated, and there is a strong current, and the boats are not able to get up to it in times of high flood.

Dunkellin: Then the boats made use of the old canal instead of the new lock?

Griffith: Under those peculiar circumstances they did.

Evidence of Sir Richard Griffith to the Select Committee on the Shannon River 12 June 1865

My OSI logo and permit number for website

Interesting info from Waterways Ireland

Two interesting PDF documents available on this page:

No mention of Saunderson’s Sheugh, but I suppose dredging of the River Finn is proceeding.

Backtracking the Barrow trackway

Some time ago I put up a page about the Barrow trackway [towing-path]. For some reason, the page disappeared shortly afterwards. I have now recreated it; unless or until it disappears again, it is here.

Waterside Belturbet

Here is a small amount of information about Belturbet and some of its industrial heritage. The photos were taken on a brief visit in July 2011.

Crossing the Barrow

The trackway [towing-path] on the River Barrow changes from the east {left) bank to the west at Leighlinbridge and back again at Graiguecullen/Carlow.

It seems to me that there may have been some difficulties in getting horse-drawn boats from one side of the river to the other and I have found no evidence on how it was done, so here is some speculation instead.

The Deel navigation

The Deel linked the Co Limerick town of Askeaton to the south side of the Shannon Estuary. Here is a page about the navigation and some of its quays. Note that it is a long page with many maps and photos, although they’re all reduced in size to minimise the strain on tinterweb.

The Shannon in winter

Downriver from Shannon Harbour to Dromineer in December 2014. It began as a bright, cold morning.

Shannon Harbour to Dromineer December 2014 01_resize

Leaving Shannon Harbour after icebreaking between the locks

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Flooding to the south-east

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But southward, look …

Shannon Harbour to Dromineer December 2014 06_resize

The Brosna

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Heading for Banagher Bridge 1

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Keeping close to the pontoons

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Heading for Banagher Bridge 2

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Heading for Banagher Bridge 3

There is a YouTube video of the shooting of the bridge here. It seems to start automatically, including sound; I don’t know how to avoid that.

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Looking back at Banagher

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Colours

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Invernisk

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Shannon Grove

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Current

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Scarpering heron

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Colours

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Marker and gauge

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House

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Boats at Meelick

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Meelick weir

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East bank

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Protective boom

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Sluices

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Through Meelick Lock

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One bird

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Many birds

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Reeds

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Architecture

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Munster Harbour

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Delaying Eamon Egan

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Gateway to civilisation

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Connacht Harbour

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Lough Derg: weather has changed

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Journey’s end, Dromineer

 

A little rain goes a long way

Here are some water level readings from Athlone Weir. I’ve taken them from the OPW’s very useful water levels site, where you can monitor levels from the comfort of your own armchair. [If only there were a gauge at Killaloe ….]

At Athlone, staff gauge zero is 35.360m above Poolbeg datum (from 18 Oct 2003). I chopped the bottoms off the first two graphs. The first one shows the level for 35 days to 30 September 2014; I presume that the level of Lough Ree was being reduced to enable it to hold some of the autumn’s rainfall.

Athlone Weir 20140930

Athlone Weir 35 days to 30 September 2014 (truncated)

Here’s the graph for the 35 days to 15 October 2014.

Athlone Weir 20141015

Athlone Weir 35 days to 15 October 2014 (truncated)

Then, in October, the level began to rise again. By 30 October, it was back to about 2.1 metres, roughly the starting point on the first of the graphs above.

Athlone Weir 20141030

Athlone Weir 35 days ro 30 October 2014

And since then it has continued to rise.

Athlone Weir 20141117

Athlone Weir 35 days to 17 November 2014

Met Éireann’s weather summary for October 2014 [PDF] has this chart:

Met Eireann weather Oct 2014

Met Éireann rainfall October 2014

It says:

Rainfall: wet conditions nearly everywhere

Monthly rainfall totals were above-average nearly everywhere with the exception of stations in coastal areas in the Northwest and West and in parts of the Midlands. Percentage of Long-Term Average (LTA) values ranged from 81% at Gurteen to 161% at Johnstown Castle, which recorded it wettest October since 2002.

Fermoy (Moore Park) reported 135% of its LTA with 153.6 mm and its wettest October in 10 years. The wettest days were mainly the 3rd, 5th and 28th, with the month’s highest daily rainfall reported on the 5th at Cork Airport with 38.6 mm, its wettest October day in five years. Mullingar reported its wettest October day in 12 years on the 28th with 26.4 mm. The number of wet days (days with 1 mm or more rainfall) ranged from 12 at Casement Aerodrome to 24 at Valentia Observatory and Claremorris, with Claremorris reporting its highest number of October wet days since 1973.

Use of the OPW charts is licensed under Directive 2003/98/EC [PDF] of the European Parliament and of the Council on the re-use of public sector information. Met Éireann allows use of “the web pages, and the information contained within them, for private and non-commercial purposes, for teaching, and for research […] is allowed subject to the condition that the source of the information is always credited in connection with its use”.