Statement from Cllr. Jane Horgan Jones & TD Aodhan O' Riordain
Earlier this week, we reached out to our public representatives asking whether they would like to contribute a public statement to loveclontarf.ie regarding the current issue of the Sea Wall. The following is a joint statement from Cllr. Jane Horgan-Jones and Labour TD Aodhán O' Ríordáin;
Many residents of Clontarf and the surrounding areas have contacted me about this issue over the past fortnight. I undertook to provide a comprehensive update following the meeting between Dublin City Council and local public representatives.
This meeting took place yesterday and my notes are below. I have simply reported what was said by engineers at this stage. It is important that residents get as much detail as possible from the meeting to facilitate our discussions.
I would welcome your feedback on the below and look forward to hearing from you. A special meeting of Dublin City Council is due to take place next Wednesday 11th November to discuss the matter.
As always please do not hesitate to contact myself (email@example.com/086-837-5219) or my colleague Aodhán Ó Ríordáin TD (firstname.lastname@example.org/01-8574020) if you have any queries on this or any other issue.
Cllr Jane Horgan-Jones
Notes from meeting with DCC, 5th November 2015, Members’ Room, City Hall.
- Engineers stated that while there are no properties along this particular section of the Coast Road adjacent to St. Anne’s Park, flooding starting here can build up and head towards properties. They stated it would not remain confined to that area.
- OPW maps being circulated by some local representatives only take account of static tidal flooding risk and do not include wave overtopping data, nor global warming risk.
- Since 1930, the average tide level has risen by about 200mm. This means we are at approximately ten times the tidal flood risk that existed in the 1930s.
- As an example of the effect of even minimal wave overtopping, engineers stated that if 100mm (4 inches) of water comes over the sea wall between the Wooden Bridge and the green area before the Causeway Road then that is the equivalent of 20 times the peak flow of the Naniken River.
Components of Flood Design Height
- Engineers told us that there were four main elements to the design:
- Standard tide height
- Associated wave height (this has been reduced in this instance because of the presence of Bull Island). The height has been calculated at 0.25m instead of the usual 0.75m stipulated by the OPW. 0.75 is standard for the east coast of Ireland but the presence of Bull Island means we can reduce this. Otherwise, the wall would be higher than it is now.
- Global warming element which is 500mm. 90-100mm of this has already disappeared in the last 15 years. This is the absolute minimum range as recommended by the OPW, though the thinking now is that 700mm is more likely to be the appropriate height. This means that global warming appears to be happening faster than previously thought.
- “Freeboard” element, or safety margin of 300mm which is the standard safety margin to account for errors in the other areas (for example, if global warming happens faster than anticipated or there were local anomalies). If we were building an embankment rather than a wall, this would have to be 500mm instead of 300mm as an embankment will settle into the ground over time and become lower.
- 4.25 ODM (from sea level) is the resulting height, which is consistent with other flood defences and thus is the minimum height stipulated by the OPW regulations.
Glass Wall/”The Waterford Option”
- Engineers told us that the first and most important difference between the Clontarf and Waterford coastlines is that while Waterford is on an estuary, we are on a bay.
- The wave element is therefore particular to this area. Wave action here is obviously reduced significantly because of Bull Island, but not eliminated completely.
- A glass wall will not be able to take wave impact in the same way that a wall does.
- If any one part of a glass wall is smashed due to wave action or vandalism, then the entire flood defence becomes ineffective. If the defence goes in one spot then the effect is like links going in a chain – and it is not worth much along the rest of the way.
- The other element is cost. Even if a glass wall was appropriate, it would double (or more) the cost of the whole scheme (current contract €6.6m).
- Putting a glass section at the top of the wall would involve rebuilding the sea wall in its entirety, with significant cost implications – it is not possible from an engineering point of view to put it on top without going down and rebuilding the L wall beneath the surface.
- Other locations where glass panels have been used are experiencing vandalism problems, i.e. Grand Canal Quay.
Combination of Projects
Engineers advised us that the two projects (S2S and flood defences) were being built together because elements of the existing wall were in such poor condition that the whole scheme worked together – the new wall had to be built and strengthened before the cycleway and footpath could be constructed, so that is why both are being done at the same time.
- NTA 51%
- DCC 36% (the Flood Defence Works element)
- Irish Water 13% ( the watermain, which is part of the North City Arterial Watermain
- Total cost of project is €6.6m.
- The OPW are not funding this project directly but it is still a requirement that the flood defence element complies with their regulations.
- The worst section in terms of height has already been completed.
- This is opposite St. Anne’s park.
- The highest point of this section is 1.138 metres from the finished footpath level for pedestrians and cyclists (taking into account a 150mm kerb height). View for pedestrians and cyclists on the sea side of the roadway is not obstructed.
- The highest point from the road for motorists is 1.138 metres plus 120-150mm, depending on which part of the road a car is on. This includes the capping stone.
- The capping stone at this section is to be 3 inches.
- The total length of this section is 460 metres. Driving at the speed limit, it would take 33 seconds to pass this section.
- DCC have agreed to put indicators (string lines) along the rest of the route to clearly indicate final intended heights.
What happens if:
1) The project is abandoned and the wall removed?
2) The wall is reduced in height, below current plans?
Councillors asked engineers to comment on both these hypothetical scenarios.
- If the project is abandoned then firstly the site will have to be made safe. The contractor would also have to be paid what they are entitled to. That figure would have to go through a detailed determination process but the overall value of the contract is €6.6m.
- The contractual implications of reducing the height would be that DCC would have to pay the contractor to do this as the highest section has already been built. Funding would have to be found for this. As the wall would then be built below the minimum standards as mandated by the OPW, DCC would be liable if any property owner was subsequently flooded. This is because DCC would have deliberately build flood defences and then removed them to below the recommended level. This would also likely apply in scenario 1) above. Aside from the safety implications, reducing the wall would be a substantial cost that has not been budgeted for. Cutting the wall might also damage its overall flood resilience.
Other measures being considered
- DCC have agreed to do a feasibility and costing analysis to consider whether it is possible to raise the height of the road carriageway opposite St. Anne’s Park where the wall is at its highest point, to mitigate the loss of sea views. However, they caution that this option is likely to have road safety implications. Their research will be presented to Councillors at the December Area Committee meeting.
- The road level is already being raised by 20-40mm in this area.
- DCC will further consider the installation of an additional pedestrian crossing to facilitate pedestrians.