Wednesday, December 30, 2015

Water under the bridge

One winter in the late 1800s, there was no shortage of firewood in Kilcullen, writes Brian Byrne.

That's because the timber scaffolding for a railway bridge under construction near Brannockstown was swept away by a Liffey flood, and much of the wood salvaged from the banks downstream found its way into the fuel sheds of homes in Kilcullen.

The flood was just one of some fairly regular such events through the Liffey valley on record. One in 1779, which swept away three houses in Celbridge. Another in 1802 destroyed the bridge at Celbridge. The largest Liffey flood ever recorded happened in December 1954, which caused major water damage in both Leixlip and Lucan.

I was prompted to look at these matters as the Liffey in Kilcullen today surged over part of the new walkway on the old weir, in a particularly brown and muddy fashion. That happened last year too, and probably has been frequent but unnoticed over many wet years before we started walking on it thanks to the work commissioned by the Bridge Camphill community in the making of the farm and nature trail.

There is local lore that floods were reasonably regular in the square area of the town prior to 1940, with particular references to 1896, 1903, and 1918 — our local heritage historian Mary Orford sends a note about these in 'The Schools Collection' item from Kilcullen Convent School, information from 'Mr Ffrench, Mrs Kearney, Mr O'Neill, and Betty Byrne'. But the nature of the river prior to the building of the Poulaphuca Dam was uncontrolled and heavy rains in the West Wicklow mountains would likely swell the flow to damaging levels, especially as the meander section between Ballymore and here would get constricted into the valley on which Kilcullen was established and grew.

When we look this morning at pictures of flooded Inistioge, Graignamanagh and Enniscorthy, we in Kilcullen should thank our stars that the Poulaphuca and Golden Falls area was considered the most suitable location for Ireland's second hydro-electric scheme by the ESB. Charlie Talbot reminds me that the final section of the Poulaphuca Dam was put into place in March of 1940, and from that time onwards the Liffey has been under control, and its flow carefully managed. In national power generation terms it is no longer as significant as it used to be, but in conjunction with Turlough Hill, it provides a small but useful contribution to 'topping up' power needs, especially close to Dublin. And it is a sustainable, local source of power. Probably though, today the more significant element of the scheme is the reservoir that supplies a substantial part of Dublin's water supply.

For us in Kilcullen, it has meant that the previously very vulnerable parts of the town don't often suffer flood problems in periods of heavy rains and storm conditions. In conjunction with the ESB dam at Leixlip, it has also saved that town and neighbouring Lucan, and some of the Dublin suburbs including Palmerstown and its neighbouring locations from fairly regular devastations.

After Hurricane Charlie resulted in bad flooding in parts of Dublin in 1986 (which this writer covered at the time as a news broadcaster in RTE), especially the catchment area of the Dodder, there was a study modelled to see what would have been the result if the Liffey dams were not in place. The experts deduced that the flow on the river at Leixlip would have been twice that of the previous record in 1954, and a number of Dublin suburbs would have suffered serious flooding. What that might have meant for Kilcullen was never detailed, but the study did note that the rainfall on the upper catchment of the river was 'considerably more severe' than on the parts of the river below it, so it would likely have been disastrous for the lower village.

The Liffey flood in 2000 was the next serious such event. Over the two days 5/6 November, an estimated 420 m3 of rainwater per second entered the Poulaphuca Reservoir at the peak of the rainfall, and it reached its highest water level in 50 years. In order to maintain a safe level, a 24-hour controlled discharge was undertaken by the ESB, but only after the peak inflow was reached and the bulk of the storm's water had been absorbed by the lake. An analysis of that event showed that the 55 m3 discharge at Ballymore via the generating station would have been 425 m3 otherwise. Downtown Kilcullen would certainly have suffered significantly. And in Dublin itself, they reckoned that the peak floods in affected suburbs would have exceeded the 1954 levels.

We have had floods since the dam was built. In 1993 the Liffey burst its banks here and the Bank of Ireland was among the buildings affected, particularly its basement. There was also flooding in the lower town area in August 2008 during a severe rainstorm, with the most affected businesses being Fallons and the White Horse Inn. In that event, the problem wasn't the river, but blocked drains, and the flooding subsided after the Fire Service cleared them. In 2009, following a warning to all communities along the Liffey from the ESB, there was some excess water through the Mill Stream, but no major damage.

How the hydro-electric project otherwise impacts on Kilcullen can be gauged by the fact that during the annual Liffey Descent canoe race, the ESB releases 30 m3 of water per second to provide a flood for the event. For those who prefer to work in weight, that's 30 tonnes per second. An awful lot of water under Kilcullen bridge.

And here's a thing, while we're on figures. If you wonder where all that water comes from, consider the 50mm rainfall they were forecasting for the Wicklow area during storm Frank. That's 50m litres, or 50,000 tonnes of water per square km falling on the lake alone, never mind what flows in off the surrounding hills. And with the area of the lakes at 20 square kms, that's 1,000m litres, or 1m tonnes, falling just on the lake over the last 24 hours or so.

Imagine if that, and more, was barreling unrestricted through Kilcullen this morning? It'd be more than a somewhat muddy Liffey we'd be watching from the bridge. If we still had a bridge ...

NOTE: The original article was updated with information provided by Mary Orford, which we greatly appreciate.