During the flooding of Limerick City last weekend it is important to remember that areas affected, such as St. Mary’s Park, were flooded by river water rather than saline water from the estuary. The high spring tides, exacerbated by low air pressure and storm force winds, effectively blocked the conveyance of river water to the sea. The water in the river backed up and, with nowhere to go, it flooded low-lying areas of the city.
Essentially, the incoming tide acted like a dam, and any water flowing down the river backed up and out, causing extensive flooding. Could the releases of water from the Shannon dams (Ardnacrusha and Parteen Weir) have been temporarily curtailed as a flood protection measure for Limerick City? Would this have prevented or reduced the flooding that parts of Limerick City suffered? Floods like this happen on all rivers occasionally, but the River Shannon is a regulated river with a hydroelectric scheme so you have to ask if there are better ways to manage water releases from this scheme, particularly at times of very high flood risk?
A representative of the ESB at Ardnacrusha confirmed to us by ‘phone yesterday that flows on the Old River Shannon were reduced significantly recently after being at 220 cubic meters per second (cumecs) since Christmas. Discharge from Parteen Weir was then increased again to 220 cumecs last week. It is understood that discharge rates from Parteen Weir were increased again yesterday. Ardnacrusha has been releasing 350 cumecs constantly; including during the high tide when the flooding occurred last weekend. This is our understanding of the facts, and these were confirmed to us by an ESB official at Ardnacrusha yesterday. The Old River Shannon Research Group (ORSRG) have asked ESB to provide the discharge details to us in writing and invited the ESB to engage in discussions regarding same.
Stopping water releases at Ardnacrusha over a critical four hour period would have prevented the release of a total of over 5 million cubic meters of water entering into a waterbody that was effectively dammed by the incoming tide
ESB confirmed that they acted within their legal obligations and current operational management protocols – and we never at any time doubted that this was the case. However, if things were done differently, and if there was a different management protocol for management of water on the Lower River Shannon in place, would this have made a difference in terms of the extent of last weekend’s flooding? We are calling for an independent review of the water management operation protocols with a view to minimising flood risk and improving the ecology of the Old River Shannon. Limerick city councillors Diarmuid Scully and Michael Hourigan recently passed a motion calling for ESB “not to cause downstream flooding in Limerick city and county by releasing excessive amounts of water at Ardnacrusha”. However, as we understand from talking to ESB yesterday they continued to release water at both Ardnacrusha and Parteen during the high tide period.
At the time of the flooding last weekend, the Mulkear River was also in flood, and there was a further 220 cubic meters of water per second (cumecs) being spilled from Parteen Weir – a flow that had been recently increased by more than 300% (as we understand) from a temporary reduction. This along with other inputs from the smaller tributaries ensured that the Old River Shannon was in full flood. However, it is understood that Ardnacrusha Hydroelectric station was also discharging at this time. This was adding a further 350 cumecs into the Lower River Shannon. All of this water was running into a dead end due to the incoming spring tide, made higher due to strong winds and low air pressure. Unprecedented and ‘catastrophic’ Limerick floods were then inevitable, and did occur.
Although it is certain that ESB were operating within their legal obligations and current agreed protocols, it is clear that an independent investigation into the circumstances that lead to this flooding would be of benefit and could inform a new management approach for the river. Such an investigation should also be undertaken to inform the design and requirement for any new flood defences for Limerick City. This approach just makes good sense. This article is not intended to criticise any individual or organisation, we are just calling for a detailed independent investigation of the circumstances that lead to the serious recent flooding. It is quite possible that there would have been no difference to the outcome, no matter what was done. However, this has not been demonstrated and it is clearly in the public interest to have the circumstances that lead to the flooding of their homes and businesses examined in detail.
It is understood that 350 cumecs of water was being released from Ardnacrusha during this recent flood event. If the release of water from Ardnacrusha had been stopped for the hours immediately before and after high tide, would this have made any difference to the significance of the flooding that occurred? To put things in context, if we stopped releasing water at Ardnacrusha for 2 hours before and 2 hours after the peak of the high tide, what effect would this have had on (1) water levels in Lough Derg and (2) the volume of water released into the Lower River Shannon which had, due to the incoming tide, nowhere to go. We have prepared the following simple basic analysis to put the water balance into perspective.
To stop generation at Ardnacrusha for 4 hours would result in Lough Derg / Parteen reservoir rising by approximately 5cm. This is based on Lough Derg having a surface area of 120 km2 (according to Shannon CFRAMS). To cause a rise of 1mm in the water level of Lough Derg, approximately 120,000 cumecs of water would have to be retained. This would be the equivalent of the amount of water passing through Ardnacrusha at 350 cumecs for almost 6 minutes. Although this volume may sound substantial, it translates to a small rise in water level when spread over the large area of Lough Derg. To have held back water in Lough Derg for 4 hours would have caused the lake to be 5cm higher than if generation had continued. The effects of having a slightly higher water level in Lough Derg (5cm) would have been unlikely to have caused significant effects. However adding over 5 million cubic meters of water to the Lower River Shannon over this critical four hour period was potentially likely to have contributed significantly to the ‘catastrophic’ flooding.
Stopping water releases at Ardnacrusha over a critical four hour period would have prevented the release of a total of over 5 million cubic meters of water entering into a waterbody that was effectively dammed by the incoming tide. This volume of water is as large as what it would take to have the footprint of the playing pitch at Thomond Park Stadium filled with water to a height of 500m, or more than four times the height of the Spire of Dublin. Spread out over Lough Derg this would have only resulted in a rise of less than 5cm across the entire surface of the lake, and this would hardly have had the potential to cause significant damage.
So would it have been better to have temporarily retained all this water in Lough Derg, causing it to rise an insignificant 5cm, or have it released into in the environs of Limerick City with no way for it to flow to the sea? At the end of the day it was this water that ended up in St. Mary’s Park and other flooded areas of Limerick City. Likewise, could the releases of water from Parteen weir have been temporarily curtailed as a further flood protection measure for Limerick City? Would this have helped prevent / reduce the seriousness of the flooding that occured?
Spread out over Lough Derg this would have only resulted in a rise of less than 5cm across the entire surface of the lake, and this would hardly have had the potential to cause significant damage.
It is possible that nothing could have been done and we are in no way saying that everyone involved in the management of the Lower River Shannon did not do their best in these quite exceptional circumstances. However, there is a complete absence of scientific information regarding the event. As with counts of the numbers of salmon that actually make it through the ESB dams, hydrometric data from Ardnacrusha and Parteen is not freely available and made public. We also note that the calculations above are very general, are likely to have wide error margins, and we would welcome any corrections. We accept that they are ‘ballpark’ estimations only! This flood event should be assessed using a formal hydrological model.
What is in no doubt however is that a full review of water management in the Lower River Shannon is urgently required, both for flood risk management and maintenance of good ecological status. Campaigning for a review of water management on the Lower River Shannon is a core aim of the ORSRG.
Other recent articles that we have written that are relevant to this discussion are as follows:-
- Limerick City Flooding: Did ESB water magement have a role here?
- A sustainable compensation flow is needed for the old River Shannon
- Growing awareness of need for more imaginative flood schemes
The last article above is of particular interest to us here, and we were very disappointed that during his visit this week to Limerick City and other flood stricken areas, OPW Minister Brian Hayes only discussed engineering solutions in relation to 250 ‘flood hotspots’ nationally.
Please note that we also tweet regularly on flooding and water related issues at the twitter account @EcofactEcology.
A review of water management on the Lower River Shannon, in the context of an overall management plan for the waterbody, is urgently required. Campaigning for the delivery of a plan to allow the sustainable management of this waterbody is a key aim of the Old River Shannon Research Group.