Clontarf Sea Wall - Independent Heights Survey
We have received the following note from John Morrissey from Dollymount Sea Scouts. We STRONGLY recommend that you read this. Further discussion will follow in due course....
SEA WALL DEFENCES – RESULTS OF INDEPENDENT HEIGHTS SURVEY
I have been asked to post my note (sent to Dublin City Councillors) summarising the results of the independent survey of sea wall defence heights. Here it is:
As promised in my e-mail of last Tuesday, 10th November, 11:53am, here is the outcome of the independent survey of heights:
Selected Height Measurements:
All measurements are in metres over Ordnance Datum (“OD”) Malin Head (tolerance +/-25mm):Customs House Steps (open to river) 2.749
Custom House Quay Wall 3.375
IFSC/CHQ Quay 3.315
Seafront Path at Yacht Pub 2.507
Seafront Wall at Yacht Pub 3.218
Clontarf Yacht Club – Top of Slipway 3.137
Bull Bridge 3.514
Bull Bridge – Top Rail 4.508
Bull Wall at Dollymount Scout Den 3.494
Scout Den Plinth 3.171
Scout Den/Royal Dublin Wall 2.548
Bottom of Scout Den Slipway 1.349
Existing Wall at St. Anne’s Pond 3.659
New Wall (excluding capping*) 4.252
Centre of Coast Road at New Wall 3.063
Side of Coast Road at New Wall 2.976
Causeway Berm – Low Point 3.503
Centre of Causeway Road 3.366
* Assuming capping of 15cms, New Wall height would be 4.402 OD. This may be in breach of planning permission, which I understand is 4.250 OD.
(i) The Existing Wall at St. Anne’s Pond was already 910mm higher than the river steps at the Customs House and 344mm higher than the River Liffey wall at the IFSC/CHQ.
(ii) Water can access behind the New Wall via the Bull Bridge and the Causeway at heights, respectively, 888mm and 899mm lower than the New Wall and 145mm and 156mm lower than the Existing Wall.
(iii) The New Wall and Existing Wall are, respectively, 908mm and 165mm higher than the North Bull Wall.
(iv) The wall between the Dollymount Scout Den and Royal Dublin Golf Club has a low point that is 1,854mm lower than the New Wall. As such, all excess tidal flows will have long since escaped across the island and Royal Dublin golf course.
(v) The highest tide experienced historically as per Dublin City Council (“DCC”) is 3.040 OD, some 655mm below the Existing Wall and 1,362mm below the New Wall.
(vi) The Existing Wall is 522mm higher than the top of the slipway at Clontarf Yacht Club.
(vii) The new sea defences at George’s Quay are being built to 4.000 OD.
(viii) DCC has yet to finalize its strategy re city centre River Liffey defences, which are more than 1,000mm and 300mm lower than the New Wall and Existing Wall respectively.
Comments from DCC Consultants
Paul Winfield of Royal Haskoning DHV (consultant to DCC) noted the following:
- “There is an acceptance that different situations require different levels of protection.”
- “Areas of high amenity may be developed to a lower standard with ‘future-proofing’ built in to allow for upgrade.”
There is no evidence that these considerations were incorporated within the engineering solutions. Mr. Winfield also noted that “Berms need to be 300mm higher than walls as they may erode or settle”. As such, the effective Causeway Berm height should be taken as 3.203 OD rather than 3.503 OD.
The Balance between Safety and Amenity
I am an actuary by profession. One of the keys questions that actuaries focus on is the trade-off between certainty and the costs of achieving that certainty.
To be absolutely sure of something, you need enormous safety margins. However, to be 99.9% sure, i.e., if a very small probability of failure can be countenanced, then safety margins can be materially lower.
The point, in connection with the debate over the height of the sea defences, is that if there are one or two relatively minor “statistical outlier” floods within 200 years that do damage to a limited number of houses or businesses, or that require the more vulnerable points to be temporarily “sandbagged”, it is probably worthwhile from a societal perspective incurring those costs if it keeps access to the views and amenity which are such a fundamental part of the identity and character of the area.
Lowest Astronomical Tide (“LAT”) versus OD Malin Head:
Dublin Bay tide tables are based off LAT while construction drawing heights are measured using OD Malin Head. As such, it is essential that the relationship between the two systems of height measurement, and particularly the delta between the “zeroes” on both systems, is fully understood.
It has emerged that there is a significant difference between the Department of the Marine and DCC in this regard. The Department of the Marine uses 2.910 metres and DCC uses 2.510 metres. This difference of approximately 400mm is very material in the context of sea defences. It is essential that this difference is fully explained before final decisions are made on the necessary sea defence heights.
There is a related question as to whether OD Malin Head is a relevant measurement system on which to base East Coast sea defences.
Highest Historical Tides:
The highest tides in Dublin Port are forecast at 4.50m LAT approximately. The actual tides experienced depend on variable items such as (low) air pressure, wave action, wind direction and strength and, over time, changes in sea levels as a result of global warming. The very highest tides require a simultaneous combination of all of these factors, which renders them both very rare and quite predictable events. DCC may have up to a week’s warning of same, which allows it time to consider the use of removable flood barriers in certain areas.
I accept DCC’s estimations that the exceptional tide levels of 1st February 2002 and 3rd January 2014 were level with (i.e., approximately 2.976 OD) and 64mm higher than (approximately 3.040 OD) the Coast Road respectively. It is noteworthy that both of these exceptional tides remained more than 600mm below the Existing Wall.
Flooding Originates from the Land Side not the Sea Side:
There has never been any seaward originating flooding between the Bull Bridge and the Causeway. This only occurs on the Clontarf side of the Bull Bridge.
I note that, in its presentation to Councillors of 5th November 2015, DCC used land-originated flooding photographs on Pages 25 and 26 of its presentation that gave the impression of seaward-generated flooding. All floods in the section between the Bull Bridge and the Causeway have been caused by rain and excess flow in the Nanikin River and/or Santry River. I have since sent to DCC the Exceptional Weather Events report from Met Éireann that explains the inland rather than seaward origin of the most severe flood, which occurred in June 1993.
It is further noteworthy that the 1993 overflowing of the Nanikin River began some distance above the St. Anne’s pond and had already spread widely before it reached the Coast Road, which suggests that the proposed new culverts may be ineffective for the next similar flood. Given the extra wall height, the flood waters will be spread wider and will take significantly longer to drain into the sea, in particular if the floods coincide with high tide, thus increasing traffic disruption and running the risk of damaging commercial and residential property at Mount Prospect.
Global Warming Projections:
According to DCC sources, it is estimated that world sea levels rose by between 100mm and 250mm during the 20th Century. It is further claimed that global figures indicate a current annual increase of 2-4mm/annum. Research is needed to explain the claim of a rise in average tide levels in Dublin Port of approximately 90mm over the past 15 years, i.e., an average of 6mm per annum.
DCC also claims that average tide levels are now 145mm higher than in 1963. This suggests a rise of 55mm over 37 years (1963 to 2000) and then 90mm over 15 years (2000 to 2015). This seems anomalous and requiring of further investigation.
This does not chime with the experience of the Sea Scouts at Dollymount. Our HQ on the Bull Island, where the plinth lies at a height of 3.171 OD, is the only building on the North side of Dublin Bay to be built on the sea side of the sea defences and, as such, is in a very sensitive location. When I chaired the re-building project between 2003 and 2009, we spent considerable time on the issue of height. We settled on a height that is 482mm below the Existing Wall and 1,251mm below the New Wall. For the Sea Scout group, the more pressing issue as regards water levels in the lagoon is its rapid silting rendering its Northern half unusable for boating.
The Bull Island is Still Growing:
DCC has asserted that, with sea level rises, much of the Bull Island will be overtopped and wave action will increase: “The fact is that as sea levels rise, much of the Bull Island will eventually be overtopped and wave action will have implications for the required height of the New Wall.”
This is distinctly at odds with the experience of Dollymount Sea Scouts. There is no evidence whatsoever that the Bull Island is doing anything other than continuing to grow in all directions, and at a rate that is significantly faster than any observed rise in sea levels. The speed of silting is orders of magnitude quicker than any rise in sea levels.
As such, within our lifetimes, in the absence of a replacement of the Causeway Road by a bridge, the entire area between the Bull Bridge and Sutton is likely to be reduced to salt marsh with the attendant risk that the bird sanctuary is threatened. Designing a long-term sea flooding solution needs to take into account that the island itself will have joined the mainland in a relatively shorter timeframe The current plans miss entirely a key issue that has long been evident to locals.
Lessons from the Past:
In “A History of the Port of Dublin” (Gilligan), there is a section dedicated to the building of the North Bull Wall and the subsequent development of the Bull Island. It describes clearly why the North Bull Wall was built the way it was; with a gap where the wooden bridge (Bull Bridge) now stands and the half-tide section from the “Holy Mary” statue out to the North Bull Light. Simply, it was to prevent excess water building up inside the area enclosed by the two breakwaters by allowing it to escape into the bay. The gaps acted as pressure relief for the harbour during extreme tide and weather events. That pressure relief system was effectively sealed shut when the Causeway Road was built. The only way for excess pressure to be released now is for the sea defences to fail within the harbour and for flooding to occur.
Broadening the Decision-Making Process:
It appears that engineers alone are making 200 year decisions and putting in their customary loadings. The fact that sea defences could be built using a modular solution that would allow further height to be added in 30, 50 or 100 years' time, depending on the actual outcome re global warming/sea level rises etc., does not seem to have been considered. New materials, such as synthetic “glass” with the strength of steel, are likely to be invented within these time-frames and existing solutions will become much cheaper as technologies mature. At present, the amenity view and the visual impact on the biosphere are being sacrificed to engineering margin alone. We need the response of other parts of DCC and other interested parties, including residents, to balance out the engineering view.
The Vision of Herbert Simms:
The inspiration of Mr. Herbert Simms, the Dublin City Council architect responsible for the design of the famous Dollymount shelters, should be recognized and his spirit invoked in the consideration of the design of the sea defence plans. It is possible that the solution can be beautiful as well as functional.
The views across to the city and the mountains beyond, and to Howth in the opposite direction, are among the most beautiful in Dublin. From the survey data, it appears prima facie that these views are being sacrificed for no real purpose.
Chairman – Dollymount Sea Scouts Re-building Project