The public are being encouraged to attend an information day from 2pm to 9pm at Cork City Hall, where representatives of the Office of Public Works (OPW) design team and environmental consultants will answer questions about the proposals. While finer details will not emerge until after this latest consultation phase, it is the first chance to see the level of forecasting, warning systems and infrastructural developments planned to help prevent a repeat of the severe damage that has affected homes and businesses over the last five years in particular.
A number of flood events have devastated the city centre since the November 2009 flood which followed the release of record levels of water from the EBS hydroelectric dam at Inniscarra after days of heavy rain. As recently as last February, the city centre was swamped after a combination of heavy rain and high tides, leaving the business community impatient for action on defence systems.
The scheme, for which a preferred option will remain on display for the next week at City Hall, will cover engineering works to be carried out from the Inniscarra dam and along the remaining 10 miles before the River Lee reaches the city and beyond, as well as works planned for Blackpool and Ballyvolane.
“This public information day will give everyone an opportunity to look at the designs so far, and members of the project design and environmental teams will be there on the day to answer any questions. I know the teams have been developing a number of flood defence options aimed at finding a solution to the flooding which we all know has such a devastating effect on the city,” said Simon Harris, minister of state with responsibility for the OPW.
An initial public information day took place a year ago, ahead of the publication of a constraints report identifying environmental issues. But the detailed designs will still have to be undertaken next year and the selection of contractors is not scheduled to start until late 2015, after publication of the outline design and a public exhibition over six weeks, expected to begin next December.
University College Cork, on behalf of its insurers, Aviva, has taken ESB to the High Court over the damage caused to many of its properties in the 2009 flood and could be seeking €19m compensation if the operator of the dams at Inniscarra and Carrigadrohid is found to have a liability.
The Kingsley Hotel recently reopened on the banks of the Lee, about a mile from the city centre, after a major refurbishment to the Carrigrohane Road premises which was destroyed in the same event.
River flood forecasting system being developed
In addition to a tidal forecasting system already in place, a river flood forecasting system is being developed.
The plan is that these systems will use predicted rainfall in the lead up to an event, as well as real-time data as it happens.
This monitoring for potential extreme events will provide an alarm to the ESB a number of days out, whenever a predicted significant rainfall event or a predicted tidal surge event is above a predefined threshold that could result in flooding, without preventative action.
This will allow dam levels to be lowered at predefined spill rates which will not flood property, in preparation for, or anticipation of, the extreme event. It will also allow management of discharges in real time, if required, taking account of tide levels and inflow from the rivers Shournagh and Western Bride which both join the Lee downstream of Inniscarra.
“Most of the time, the dams will continue to operate as they do at present. Continuous monitoring and simulation of predicted rainfall using the new flood forecasting system will allow potentially significant flood events to be detected further in advance,” says the information for the public.
Rather than being used as individual dams, the Inniscarra dam and the Carrigadrohid farther west, or upstream, will be used as a system. This, say the design team, will help the dam levels to be safely drawn down to create storage in advance of any potentially significant event detected.
The creation of washlands, which may be periodically flooded, and downstream defences, should allow for greater discharges without causing flooding of properties. This will require warning sirens of increased advance discharges to be in place for recreational users of the river and the floodplain downstream of Inniscarra dam.
There would need to be direct notification for landowners in washland areas to allow, for example, livestock to be moved. Also in need of warning will be Cork City Council, for the erection of demountable elements if necessary, and emergency response services.
In addition, local authority websites, social media, council text-alert systems, and maybe also radio and television public alerts, would be used to warn of impending floods.
During any rising flood, it is planned that continued increased discharges, will ensure dam storage is retained until it is needed at the peak of the event. Dam discharges would be managed optimally in conjunction with the tidal cycle, but the integrity of the dams will take priority at all times.
“The new rules do not impact on dam safety requirements, and if a flood in excess of the design event occurs, existing dam safety rules will take precedence,” says the OPW information.
Range of proposals to tackle flooding
To help prevent the effects of flooding in the city centre, it is proposed to take a number of measures.
Among them will be the use of a flow-reduction structure on the south channel of the River Lee, which splits west of the city centre and reforms into a single body of water again at the eastern quays.
The plan will be to re-balance flows in the north and south channels, diverting a greater proportion of flow during extreme events to the north channel. Most of the recent city flood events associated with high tides have affected key shopping streets like Oliver Plunkett Street and surrounding areas like the South Mall, which are closer to the south channel. In addition, the preferred options include the following:
- Possibly raising the Cornmarket St footbridge (opened in 2004) and St Vincent’s pedestrian bridge, both crossing the north channel of the river;
- Floodgates at some footbridges and boardwalks;
- Raising the heights of walls on city quays;
- Providing surface water pumps to deal with ‘back of wall drainage’.
Between the Inniscarra dam and the city centre, about 16km, a range of other proposals include:
- Local conveyance improvements and direct defences at Inniscarra bridge to protect against advance discharges from the dams;
- Localised defences or individual property protection at some isolated properties between Inniscarra and city;
- Direct defences such as raised embankments and walls at Curraheen and Glasheen rivers;
- Defence at Inchagaggin.
In Blackpool, plans include installing direct defences from upstream of the Northpoint Business Park to Fitz’s Boreen, about a mile north of the village. Some bridges would be replaced at Fitz’s Boreen and the Dulux paint facility, where a sedimentation area would be located.
In the village itself, a pedestrian bridge would be removed and a road bridge replaced at Orchard Court, while the culvert between there and the Catholic church would be replaced or upgraded.
Remedial works have been taking place locally this summer but other plans include:
- In-filling of open channel section at Blackpool church; cleaning and sealing of existing culvert downstream of the church;
- Realignment of culvert junction of rivers Bride, Glin & Kiln at Madden’s Buildings.
Screens and swales
In Ballyvolane, a northside suburb which was hit badly by the floods in summer 2012, a range of works have been proposed.
An existing trash screen at the upstream end of Mervue Lawn would be replaced with a larger screen design.
In addition, a defined overland flood route would be created to manage residual overland flood risk. This would be achieved by:
-Removal of a boundary wall north-east of Kempton Park;
-Creation of swales (ditch-like depressions to carry or hold floodwaters) in Kempton Park, and east of Leeds Park and Park Court;
-Replacement of a number of manhole covers with slotted gratings.