Clontarf Seafront

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

(horganjones.jane@gmail.com/086-837-5219) or my colleague Aodhán Ó Ríordáin

TD (aodhan.oriordain@oireachtas.ie/01-8574020) if you have any queries on this or

any other issue.

Kind regards,

Cllr Jane Horgan-Jones

Notes from meeting with DCC, 5th November 2015, Members’ Room, City Hall.

Flood Risk

- 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:

o Standard tide height

o 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.

o 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.

o “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.

o 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.

Funding

- 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.

Height figures

- 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.

1) 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.

2) 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.

Clontarf Flood Defences Update - November 2015