The battle to control flooding on the River Shannon
As communities continue to struggle after Storm Desmond, experts look for answers
Storm Desmond has come and gone, but is anything but forgotten. Communities up and down the river Shannon continue to struggle; thousands of acres are still underwater, with trees and hedgerows peering out of the floods.
The recriminations have flowed as quickly as water: the Government should have spent more on defences, the ESB should not have let so much water through Parteen, building on flood plains should not have been allowed.
The one question that everyone asks is: when are we going to end the flooding? Why can we not build walls or dredge riverbeds or install pumps to hold back the Shannon once and for all?
Unfortunately, there will be no quick fix to end the battle with the river.
“I don’t think there are solutions or optimal solutions from an engineering point of view,” says Dr Conor Murphy a researcher in the Irish Climate Analysis and Research Units (Icarus) in the department of geography at Maynooth University.
“We have to move beyond the technical responses,” he believes.
“Managing the consequences is where we are at.”
Unfortunately, the Shannon is a river that by its very nature wants to flood, says Don Moore, former president of the Irish Academy of Engineers.
“It is not what rivers do, it is what the Shannon does,” he says.
“Nothing has been done to end flooding because it is not possible to do anything significant.
“You can protect towns along the river with hard engineering. You can protect populated sections, but you can’t protect farms that are on the flood plain of the Shannon.”
Slowly flows the Shannon
Then over the next 30km it falls about 30 metres. It is also very water-rich, given that it drains a fifth of the island of Ireland.
Its three big lakes, loughs Allen, Ree and Derg, hold millions of cubic metres of water and the depths can be controlled by a series of weirs.
The Shannon navigation supports massive amounts of tourism and given the placid waters and consistent depths, it is ideal for boats and cruising.
Critics argue that Waterways Ireland is only trying to keep things good for the tourists.
Farmers argue that they have been abandoned and no one is listening to them.
The trouble starts when the big winter storms roll in, such as Desmond or the infamous 2009 storm, which can bring unmanageable quantities of water in short periods that simply overwhelm the Shannon’s ability to carry it.
We know the result: rivers bursting their banks, water spilling into urban areas and washing out over farmland.
And yet the river is only doing what comes naturally.
If there were no towns or roads to disturb things, the water would flow down its natural channel. This channel would be the size it needs to contain the flow going through it in normal circumstances. An equilibrium would be reached.
“A river will do what it wants to do and will revert over time to a natural dynamic,” explains O’Sullivan, who is also on the water and environment group within Engineers Ireland.
There is also a natural “sediment dynamic” that depends on how much silt the river carries and at what speed it deposits it in the channel.
If a storm comes, water levels rise until it flushes over the river’s banks and spreads out over its natural flood plain.
“Typical rivers expect to come over a bank once every two or three years; that is what rivers do, they flood,” says Prof Michael Bruen, director of the Dooge Centre at UCD.
“The question is how high will the water rise and how far will the water reach?”
The flood plain is a way for the river to spread the water load.
These plains eventually tend to direct the water back into the main channel, so they are a real part of the river system.
Now start to add human construction, villages on the side of the river, towns straddling it and developments such as shopping centres, industrial parks and agricultural cultivation on the flood plain.
Most of the time there is peaceful coexistence, but flooding prompts hard engineering responses as communities seek to keep the river under control.
Bad planning brings its own curse.
“Carrick-on-Shannon has a problem, parts are built on the flood plain,” says Murphy. Often, it is the “new builds” that suffer most, like the cinema and shops in Carrick.
“These were decisions taken over the past 10 to 15 years that have increased the public’s exposure to hazard,” he believes, raising concerns that doctors’ surgeries could be cut off, or ambulances blocked during flooding.
The general demand in places such as Athlone is for hard engineering: the use of concrete and steel to hold the river at arm’s reach, increasing the amount of water the channel can carry.
If done, the dredged channel is less likely to top its banks. Higher riverbank walls help, too, even if they risk much-loved views.
Further back, raising embankments can reduce risk, too.
Yet what might be good for Athlone might spell ruin for communities elsewhere. More water in the channel threatens bigger floods downstream.
Holding back water raises the risk for communities upstream, Moore said.
The IFA accepts that the river will flood. “We need to put the channel back into the condition it was 50 years ago,” he says.
“This would require dredging and the spoil could be put into spent bogs that are only a couple of hundred metres away,” he says.
Water levels in the Shannon’s three big lakes – Lough Derg, Lough Ree and Lough Allen – should be lowered so they could hold back water during flooding episodes.
Like others, the IFA has called for a single river authority, rather the overlapping, often contradictory roles held by the Office of Public Works, Waterways Ireland, ESB, National Parks and Wildlife Service and others. “Unless we have this we will continue to have flooding,” says Turley.
Dredging seems straightforward, but benefits are limited. “You can increase capacity but long term that solution could be eroded,” O’Sullivan says, since slower-running water in a deepened channel will deposit more sediment.
Ardnacrusha, or rather its associated weir at Parteen, was in the wars during Desmond, but Tom Browne, the company’s asset assurance engineering manager, believes Parteen helps, not hurts.
For the IFA’s Turley, the 90-year old Ardnacrusha dam is a working museum-piece, irrelevant to Ireland’s power needs.
Yet the weir and the 15km-long canal that brings water to the turbines stabilise the lower Shannon.
The weir marks a “fork” in the Shannon channel. Opening the weir puts more water into the river channel. Closing it diverts water into the canal.
The power station’s turbines need large volumes of water, but they cannot take more than 400 cubic metres a second.
The weir can be opened or closed, but when water flows are low, it will send no less than 10 or 15 cubic metres per second down the river to keep it from stagnating.
The weir and canal pose no problems until storms come, when river flows can reach 800 cubic metres a second, double the canal’s capacity.
So anything above 400 cubic metres a second must be sent over the weir and down the old Shannon.
During the floods, the ESB increased volumes through the weir into the Shannon until it reached a maximum of 440 cubic metres per second – causing flooding below the weir and at Athlunkard bridge, says Browne.
Opening the weir caused flooding, but closing it would not have helped much since the water would have flowed over it once it was high enough.
Disputing charges that the ESB was at fault, Browne says closing down Ardnacrusha would not help.
One way, or the other, water must exit on its way to the estuary.
Hard engineering solutions may not be high enough, deep enough or wide enough to control storms made more powerful by climate change.
“What we have is a situation that is getting worse. We have climate change coming down the tracks and it has the potential to make things worse through higher rainfall,” Bruen says.
“What is remarkable about the 2009 and 2015 storms was the spatial extent,” Murphy added.
“And these storms are supposed to increase in strength due to climate change.”
A new approach is required; one that accepts, reluctantly, that worse flooding will occur, but accepts, too, that engineer cannot fix every problem: “Structural changes are not particularly sustainable,” says O’Sullivan.
“A better approach is a mix between structural measures and non-structural,” he goes on.
“We have to rethink how we deal with flooding. Eliminating or preventing floods is no longer sustainable.”
Engineers believe that better forecasting and flood modelling could help more warning.
Currently, four or five days’ warning of winter storms are usual, followed by peak flooding four or five days after that.
Better predictions could limit damage, but investment is necessary, Bruen argues.
“The situation now is the Shannon is managed by structures that are old and sections of the channel have silted in. It is time to look again at the system as a whole to see if we can integrate climate change and the ability to manage through forecasting.”
Experience from earlier storms has improved co-operation between State agencies. Storm Desmond, meanwhile, led the Government to give €3.5 million to the OPW and Met Éireann for flood forecasting.
However, there are some DIY approaches that could help, O’Sullivan says. Houses on flood plains could be rewired to have cabling coming down from the ceiling rather than up from the floor.
Doors that seal against water could help and front gates that can block water. One-way valves on toilets could stop sewage returning with the flood.
Flooding cannot be eliminated, but Ireland can cope better. However, limited funds mean that towns at risk will always have to take their place in the queue.
And sometimes, small communities will find themselves under water.