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Introduction
This review of the wall height of the Dollymount promenade was carried out to assess if the current
crest level of 4.25m ODM (Ordnance Datum Malin Head) is appropriate for this location. A number
of documents relevant to the promenade design and the determination of extreme values for the
Dublin Bay area were assessed in this review. A listing of all documents, communications,
presentations etc used in this review are listed at the end of this report.
The basis of design of the wall was to protect against a two hundred year event in relation to water
levels and winds in Dublin Bay and to provide a level of safety against anticipated sea level rise. The
final height of the wall is made up of 4 components as shown in figure 1 [7]. The initial designed
level was 4.6m ODM however this was reduced to lessen the visual impact. The resultant wall height
of 4.25m ODM is considered an interim solution which may, depending on the rate of sea level rise,
need to be increased to the original designed values in the future. In relation to the existing
defences the new wall height ranges from being equal in height to being at a maximum 0.85m higher
than existing defences. The impact of the increased height is minimal to pedestrians and cyclists
using the promenade and slightly restricts views of the lagoon from the footpath on the St Annes
Park side. It does however restrict views in places from standard cars travelling along the road.
The primary issue of public concern is in relation to the height of the wall, that it is not in line with
other flood defences in Dublin and it obscures views of Bull Island. The analysis carried out in this
review only considers the appropriateness of the design decisions made in terms of the wall height
and examines other options to provide the same level of protection. No comment is made about
any other flood scheme in the Dublin area as that is not part of the current remit.
Figure 1 Designed wall Configuration [7]
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Expert Reviewer Profile
Dr Jimmy Murphy is a senior Research Engineer in the MaREI Centre in University College Cork and
has over 20 years experience in marine projects in relation to the design and evaluation of coastal
and harbour structures. This work involves the use of such tools as physical model testing,
numerical modelling and field measurements to optimise the layout and design of marine structures.
He lectures in the School of Engineering in UCC on the subjects of Environmental Hydrodynamics
and Harbour and Coastal Engineering. In addition he supervises a number of PhD and masters
students and has a number of publications in the areas of coastal engineering and ocean energy.
Site visit.
A site visit took place on Friday 18th
December 2015 where in the presence of Mr. Gerard O’Connell
from Dublin City Council (DCC) the promenade scheme and the wall design were explained. Where
access was available we examined the wall and drove along the section of road most affected by its
construction. We also visited the Bull Island biosphere via both the causeway and wooden bridge
and generally discussed flood defence levels in the local area and the impacts of previous extreme
events. This visit clarified the issues in relation to the visual impacts and the flood defence
requirement.
Public Input
I have reviewed officially submitted documents from the public ([15] and [16]) as well as kept in
touch with media reports ([18], [19] and [20]) while working on this review. This has highlighted to
me the importance of this issue to the local community and the connection they have with the
physical environment. Thus I feel that I am aware of the significant public concerns in relation to the
flood wall although many issues raised are outside the remit of my review (as outlined in Appendix
1), which focuses on the engineering justification for the current wall height. After submission of
this draft report it is planned that I meet with DCC and public groups and subsequently finalise the
document.
Design Assessment
This section considers each of the elements that determine the wall height. I should state to begin
with that even though all the relevant information was available, I did not find a clear consistent
design document and the information provided, as used to determine the wall height, sometimes
differed between documents, i.e. [4] and [7].
Extreme Water Levels
The extreme water levels are the most dominant component in terms of determining the wall
height. In Figure 1 this has a value of 3.2m ODM (in this report I will only use Ordnance Datum Malin
as the reference for the vertical datum) which represents the 200 year return condition. Extreme
water levels are made up of a combination of astronomical and meteorological (surge) components,
the former can be predicted well in advance whilst the latter is dependent on the weather. If the
astronomical component is first considered it can be seen from Figure 2 (column labelled mODM)
that the design water level for the wall is 1.21m higher than the highest astronomical tide (HAT).
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This indicates that there is a high surge component to the water levels in Dublin Bay and this is
borne out by the number of extreme water levels in recent years. The 2002 event had a water level
of 2.97m ODM twhich was the highest ever recorded water level up to that point whilst the tide of
3rd January 2014 reached a recorded level of 3.014m ODM at Alexandra basin in the Docklands with
unconfirmed information of a level of , 3.047m ODM at Dollymount. A study undertaken by Royal
Haskoning [9] after the extreme event of 2002 showed that the 200 year event had a value of 3.13m
ODM which differs slightly from the value indicated in Figure 1. A re‐evaluation of extreme levels in
2015 [5] using the most up to date data showed that the magnitudes were largely unchanged.
However when taking account of sea level rise in the period between 2003‐2015 and more accurate
tide level data there is an adjustment of the 200 year water level to 3.25m ODM. This is stated as
follows in [5];
The residual mean water level of the tidal analysis gives an indication of the changes in
Mean Sea Level (MSL) that have occurred over the years. Based on the additional tide
gauge data, an increase of 0.13m was observed in the last 13 years (Svasek Hydraulics
2015)
Although the analysis has shown that to date in Dublin the extreme water levels are stable there is a
risk that they could change due to climate influences. Changes in storm duration, intensity and
frequency as well as sea level rise can contribute to surge levels increasing and this has been
highlighted by the IPCC. The text below is taken from [14].
In summary, dynamical and statistical methods on regional scales show that it is very likely
that there will be an increase in the occurrence of future sea level extremes in some regions by
2100, with a likely increase in the early 21st century. The combined effects of MSL rise and
changes in storminess will affect future extremes. There is high confidence that extremes will
increase with MSL rise yet there is low confidence in region‐specific projections in storminess
and storm
Gregory [13] also concludes that
It is very likely that there will be a significant increase in the occurrence of future sea level
extremes.
It is not suggested that any change be made to the extreme values due to possible future changes
but in terms of the wall height the freeboard allowance can incorporate this risk.
The 200 year design criterion for coastal flood defences is standard for Ireland and I am satisfied that
the data available and methods of analysis used were satisfactory for determining the level to be
used for the flood defence wall. The most relevant value to use in this case would be 3.25m ODM
and not the 3.13m ODM or the 3.2m ODM values that are indicated in some documents ([4] and [7]).
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Figure 3 Sea Level Rise Trends [13]
Wave Conditions
The analysis of wave conditions for the determination of design conditions was not fully clear so I
carried out a separate analysis which will be presented in this report. The Roughan O Donovan
report [4] indicate that given the restricted nature of the fetch (distance over which the wind blows)
and the shelter provided by local topography it is likely that the 1 year return significant wave height
(Hs) values along the Dollymount frontage to be of the order of 0.47m with a potential maximum
wave height of 0.8m . My assessment would be that such wave magnitudes are very high in relation
to the length of the fetch and may not consider that direction of propagation as this has a significant
influence on how the waves interact with the flood defence. For instance the highest waves are
likely to be generated along the longest fetch which is orientated in a circa SW‐NE direction. Waves
propagating in this direction will be confined by the wall but will not break or reflect from it. This is
different from waves generated from easterly winds which are directly incident on the wave wall but
are likely to be much reduced in height due to the shorter fetch. The analysis to clarify the wave
conditions incident on the wall used two different sets of empirical formulae for determining the Hs
and Tp (peak wave period). The first method is as outlined in the Coastal Engineering Manual whilst
the second formulation was developed by Donegan and Walsh.
Input information includes wind speed, fetch length and water depths and which this information
the design wave conditions on the wave wall can be is determined. In this analysis three wind
directions were considered as can be seen from figure 4‐6. The fetch lengths used in each case are
shown in these figures and in the analysis the average water depth over the fetch for a 3.66m ODM
water level was chosen as 3.0m. Also I used the same wind speeds for all directions as used by [4]
even though there are slight variations that I considered not to be significant. I should note that
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Minimum Freeboard
Freeboard is required to provide a level of safety in the design particularly when there is uncertainty
regarding input values that were used. The application of safety factors is standard engineering
practice so it is justifiable to use them for the wall design. In this case the main uncertainty relates
to the determination of the design wave conditions and their interaction with the wall. Examples of
uncertainties are provided below;
Given that the wind speed used to generate the wave conditions is the average hourly value
and the short length of this fetch would indicate that the 10 minute wind speed would be
more appropriate to use. Therefore wave conditions are expected to be slightly higher than
indicated.
The exact nature of wave interaction with the wall is dependent on a number of factors that
cannot be fully quantified. In this case field monitoring should be carried out and also
physical modelling testing in order to better understand what is happening.
Uncertainty regarding extreme water levels as previously outlined
I consider a value of 0.3m freeboard as used in the design to be appropriate. Generally in exposed
coastal locations higher freeboards are required mainly to cater for wave action. Given that wave
conditions are benign in the lagoon and the type of wave interaction with the wall this level of
freeboard is acceptable.
Recommended Wall Level
The overall assessment of the wall height is made up of the following components
Extreme water level (200 yr) 3.25m
Sea Level Rise 0.4m
Wave Action 0.3m
Freeboard 0.3m
Total Height Required 4.25m ODM
Therefore I consider the selected wall height of 4.25m ODM to be appropriate in terms of the
criteria set out for the design. It is interesting that even though the components that makeup the
wall height in this review differ from the design the overall value remains the same.
Alternative Wall Solutions
The previous analysis indicates that the currently designed wall height is justified based on the
information on various analyses of the extreme conditions and the design criteria that was used.
This section outlines possible alternative solutions that would not have the same visual impact but
would in general provide the same level of protection from flooding. It would be the intention that
such solutions if implemented will be applied along a 450m section of the road where the maximum
increase in wall height is 0.69m. The introduction of these options is not a recommendation for their
use as each would require full design and planning.
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Demountable barriers
Demountable barriers are a well proven solution and have been successfully used in a number of
flood schemes in Ireland. There are a number of different types of solutions and a design study
would be required to determine which would be most appropriate for this site and how it could be
incorporated into the existing structure. These barriers would need new planning permission and
the costs would be relatively high as they would need to include modifying the existing wall,
purchasing the barriers and providing a storage location. Also their erection would require
significant resources during a flood event from DCC as this scheme would double Dublin’s total
length of such barriers.
Glass Walls
Constructing all or part of the wall using glass panels has been suggested as a potential option to
preserving the views from the road. To date glass walls have been used primarily in riverine and
sheltered estuarine locations and there are examples in Germany, UK and Ireland with the
Waterford City Flood Alleviation Scheme the most commonly referenced site in Ireland. There is no
doubt that glass panels could be designed to work for the Dollymount site even though there is
limited knowledge as to how they would behaviour when subject to regular wave action. Although
this should not really be an issue as, if only the top 0.5m of the wall was glass then wave action at
this level would be rare. There are other factors that should be taken into consideration in relation
to glass walls and these include
Cost – The overall cost of glass panel system can wary depending on the particular
application but figures up to €5k per linear meter have been suggested.
Maintenance – They would require regular cleaning
Possible Vandalism in the form of breaking and graffiti
Environmental impacts – it is possible that reflections and glare from the glass may have a
negative impact on this sensitive costal ecosystem and this would need to be investigated.
Figure 7 Glass Floodwall in Waterford City [10]
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Increase Road Levels
This relatively simple solution would involve raising the road level such that the views from the road
would be at least partially restored. Given an average eye height for car drivers as being 1.1m then
the road level would need to be above 3.15m ODM to enable views beyond the wall. DCC state that
this option has been already assessed at an area Committee briefing and it was estimated that it
would cost of the order of €0.5m which was considered to be impractical.
Water Level Control
This solution would involve controlling the level of water entering the Dollymount lagoon by
constructing a barrier type structure at the Dollymount bridge location. This would be activated
during extreme events and would prevent waters from reaching high levels in the lagoon and so
reduce the required height of the wall. Such a scheme would be expensive, require planning and
potentially be disruptive to the existing natural environment so is unlikely to be a feasible
alternative.
Modify Design Criteria
An important point relates to the use of the 200 year condition in the design process. This is a very
extreme value and a level that has never been recorded in Dublin Bay in over 90 years of records. I
queried DCC regarding the use of this return period in this case. For instance the 100 year extreme
water level is 0.1m lower and if used would have a positive impact in terms of the visual aspects.
DCC responded by stating that their advice from the OPW is that coastal flood defence designs as
undertaken by them should be to the 200 year return period and this is in line with all recent
schemes designed in Dublin. Their view is that damages from tidal flooding are generally
significantly worse than river flooding for a number of reasons including associated wave action, salt
water content, ground water contamination, overloading of combined sewer networks, etc.
A 200 year design event could occur due to a number of different combination of events – the
design case chosen used the 200 year water level with the 1 year wave height as this was found to
give the most severe loading on the structure with respect to flood risk. But other cases studied
included the 10 year water level with 10 year wave condition and the 50 year wave with 1 year
water level [4]. These obviously would have a lesser impact in terms of the height of the wall but as
is standard engineering design practice the most severe condition was chosen. My view is that the
water level should be the dominant criteria as wave conditions are relatively benign at this site. The
Roughan O Donovan report [4] does say that the design horizon of the wall is 2100 and from which
sea level rise adjustment values were determined. Given that this is the case it should be assessed
whether a 100 year design condition could be used instead of the 200 year.
The other option for modifying the design criteria is in relation to the sea level rise allowance. Given
that measured water levels as recent as 2014 were above 3.0m ODM there may be more scope in
reducing the 0.4m allowance then changing the 200yr extreme level design. Therefore if the SLR
element of the wall height was reduced by between 0.1‐0.2m and DCC carried out frequent reviews
based on extreme water levels and sea level rise rates then this could provide a solution.
Crest Re‐design
It was stated previously that waves will not consistently overtop the wall during a storm event but it
will occur occasionally. The current crest design is not optimum for minimising overtopping and
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could be modified. Figure 8 below shows the type of crest configuration that is often used in coastal
structures to reduce overtopping volumes and it may be possible to shape the capping piece of the
Dollymount wall in this manner. This would improve the performance of the wall against wave
action and could delay the ultimate planned increase of the wall to 4.6m ODM.
Figure 8 Example of Crest Detail for vertical wall [12]
Conclusion and Recommendation
The analysis that I have carried out show that the current wall height (4.25m ODM) is justified based
on the design criteria used even though the components that make up this height differ slightly from
indicated values.
This still leaves the issue with regards to the loss of visual amenity and in this review I have
suggested a number of solutions. The majority of solutions considered have significant implications
in terms of costs, planning requirements and environmental effects and would be unlikely to resolve
the immediate issue. Therefore the recommendation that I would make is that DCC review the
design criteria and in particular the SLR allowance included in the design. My suggestion is that a
value in the range of 0.2‐0.3m be used (instead of 0.4) which would mean that by current mid range
SLR scenarios the wall height should still be sufficient to provide flood protect for at least 50 years.
This proposed adjustment of the wall height should only be applied at locations where the visual
amenity is most affected as agreed between DCC and local groups. If this solution is implemented
then DCC would need to frequently review both extreme water levels and sea level rise rates and
have a plan in place for increasing the wall height to ensure that there is a sufficient level of flood
protection.
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Documents and correspondence reviewed
I have listed below all documentation that was made available to me by DCC plus additional items
that I used as part of the review.
[1] Dollymount Promenade and Flood Protection Project ‐ Historic Evolution and Geomorphologic
Assessment (2009)
[2] Dollymount Promenade and Flood Protection Project; Historic Evolution and Geomorphologic
Assessment, Roughan O'Donovan / Dublin City Council, 21 September 2009, Final Report
[3] Dollymount Promenade and Flood Protection Project; Hydrodynamic Modelling Report,
September 2009, Final Report, 9T3615
[4] Dollymount Promenade and Flood Protection Project ‐ Coastal and Flood Risk Engineering Design
Criteria Report, Roughan O'Donovan / Dublin City Council, September 2009, Final Report, 9T3615
[5] Clontarf ‐ Task1 ‐ Wave Transformation Report, Royal Haskoning, 27 August 2015, RDCR001D01,
[6] Dollymount Flood Alleviation Project, Briefing Notes, Prepared by Gerard O Connell, 29th
Oct
2015
[7] S2S Cycleway & Footway Interim Works (Bull Road to Causeway Road), Presentation to Elected
Members, 11th November 2015
[8] Greater Dublin Strategic Drainage Study, Regional Drainage Policies – Volume 5 Climate Change,
March 2005
[9] Dublin Coastal Flooding Protection Project, Final Report Volume 1 ‐ Main Report, Royal
Haskoning, April 2005
[10] Waterford City Flood Alleviation Scheme, Gavin O’Donovan, BE CEng MIEI, Associate, RPS
Consulting Engineers Ltd, (Paper first presented to Engineers Ireland on 12th Feb 2014)
[11] Google Maps
[12] EurOtop; Wave Overtopping of Sea Defences and Related Structures: Assessment Manual, Jan
2007, SSN 0452‐7739, ISBN 978‐3‐8042‐1064‐6, http://www.overtopping‐manual.com/eurotop.pdf
[13] Projections of sea level rise, Presentation by Jonathan Gregory, Lead author, Chapter 13, Sea
level change IPCC, https://www.ipcc.ch/pdf/unfccc/cop19/3_gregory13sbsta.pdf
[14] IPCC Report (2013) Chapter 13 Sea Level Change,
https://www.ipcc.ch/pdf/assessment.../WG1AR5_Chapter13_FINAL.pdf
[15] Morrissey Family Letter to Dublin City Council ‐ 29th October 2015
[16] Follow up Note to Dublin City Councillors re Sea Protection ‐ Height Survey (John Morrissey)
[17] Frequently Asked Questions_Version 2_30th October 2015
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Appendix 1 – Review Brief
Final Draft
Re: Dollymount Flood Wall – External Expert Brief.
Following unanimous approval of a motion by Dublin City Councillors at an
emergency meeting of the Council on Wednesday 11th
November last (copy
attached) it has been decided to procure a external expert to independently
assess the height of the partially constructed and proposed new sea wall along
Clontarf Road and James Larkin Road between the Wooden Bridge and the
Causeway to Bull Island, Clontarf, Dublin 3.
Representations from members of the public who drive past this new wall
contend that it obscures their views of Bull Island thus requesting this review.
Attached are frequently asked questions and links to the City Councils web-site
which outline the history of the project so far and some of the main issues
queried by local and regional residents.
The brief for this external expert includes the following:
1. Site visit.
Meeting with Environmental Monitoring Group.
2. Review previous reports which include this section of flood defence and
the latest relevant national/international best practice documents on
climate change with a view to verifying and/or establishing relevant
design criteria. This shall include but not be limited to:-
Greater Dublin Strategic Drainage Study (GDSDS) Climate Change
Policy Document No.5 adopted by Dublin City Council.
Dublin Coastal Flood Protection Project.
SAFER Project.
Clontarf Promenade Wave Modelling Projects.
Dublin Bay tide gauge records with particular attention to Sea level
rise noted between 2000-2014.
An Bord Pleanala Decisions & Inspector’s Reports for this stretch of
wall.
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Part 8 approval by Dublin City Council for this cycleway, watermain
and flood wall.
National/International best practice on guidelines and
recommendations on current and future flood defence heights.
Eastern Region CFRAMS reports and draft CFRAM floodmaps.
Irish Coastal Flood Protection Strategy (ICFPP) – OPW.
Review of international best practice on design of coastal
defences/flood defences and climate change.
Other reports/analysis which the expert deems necessary.
3. Analysis and recommendation on the four main constituents which
make up the height of this flood wall.
Static tide design level (appropriate return period and associated
level),
global warming element (temporal allowance and appropriate sea
level rise allowance(s) for same)
associated wave defence component (based on wave climate in lagoon
and overtopping of flood wall),
freeboard allowance (if any required).
4. Assessment of the suitability of a glass wall option for the highest
portions of the proposed new wall.
5. Draft report in 3 weeks with final report in 4 weeks. The report will
outline all reports and studies read by the expert, his/her conclusions
from them and his/her analysis and recommendation in respect of the
height of the coastal flood defence wall required for this section of
Dublin City coastline, i.e. adequacy of the existing design or proposed
new design level.
