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Dollymount Flood Defence Level
Glossary of flood defence terms.
1. Current (December 2015) estimated 100 year flood level. This is the estimated 100
year flood defence level of 3.03m above average sea level (Malin Head) in the year
2000 plus 0.12m of sea level rise seen in Dublin Bay from 2000 to 2015 giving a total
level of 3.03m + 0.12m = 3.15m Malin Head. This level is 0.1m (100mm) lower than
the 200 year flood level of defence recommended by Dr. Jimmy Murphy in his
recent Independent assessment.
This lower than recommended flood level of protection produces increased flood
risk and it is therefore subject to an assessment of the increased risk posed to
businesses and houses along the Wooden Bridge to Mount Prospect Avenue section
of Clontarf Road and extra emergency measures which may be required, closing off
the main road which may be required from time to time.
2. Future sea level rise estimate. Dr. Jimmy Murphy recommended defence against a
sea level rise of 400mm to the year 2100 in his recent report. He stated that this
could be reduced on a temporary basis in environmentally sensitive areas by up to
200mm. This leaves only 200 mm to protect against this continuing threat. At
current sea level rise rates in Dublin Bay this allowance would only last 25 years,
which would require rebuilding of the wall portions of the cycleway in that time
frame or possibly less.
3. Associated wave height. With a 100 year high tide event there will be very strong
winds and therefore an associated wave action and wave overtopping on the flood
wall. This is estimated by Dr. Murphy in his report as 0.3m or 300mm. There is
significant uncertainty with this height. The very short distance across the lagoon to
Bull Island was taken into account in this small wave height estimate.
4. Freeboard. This is a safety factor to take into account the uncertainty in estimating
the above factors. It is used in all engineering estimates. Rather than giving a new
house a 50:50 chance of collapsing at the end of its design life a significant safety
factor is applied to it. The safety factor here includes especially for uncertainties in
wave height and global warming (which appears to be accelerating worldwide). The
normal range of freeboard is 0.3m to 1m. The minimum freeboard recommended by
the Office of Public Works, the National Competent Authority in Ireland for
implementing the EU Floods Directive, is 0.3m which is Dr. Murphy and all other
internal and external experts recommend for this project.
5. 1 in 100 year flood event. This could happen any year. The estimation of this is
based on the analysis of recorded historical tides occurring in Dublin Bay.
In summary we are proposing a reduction of 100mm under heading number 1, i.e.
giving protection for a one in one hundred year flood event rather than protection
for a one in two hundred year flood event . This is subject to “an assessment of the
increased risk posed to businesses and houses along the Wooden Bridge to Mount
Prospect Avenue section of Clontarf Road and extra emergency measures which
may be required, closing off the main road which may be required from time to
time”, as stated on previous page. It should also be noted for clarity that a one in
one hundred year flood event could in theory occur next week rather than in a
hundred years time.
The proposed reduction of 200mm is being taken off number 2, i.e. future sea level
rise estimate. This means that the flood defence will have to be raised again inside
the next twenty five years.
This reduction of 300mm only applies to the relevant 450 metre stretch between
Mount Prospect Ave and Causeway Road. In effect it means that the wall at its
highest point along this stretch will be reduced to 800mm (2ft8in).
To clear up some confusion it should be noted that when this 300 mm reduction is in
place all motorists travelling in either direction at this location will have a line of sight
view above the wall, thus addressing the most commonly quoted adverse impact of the wall.
A post on Clontarf.ie states that there will be a 75 stretch on the outward lane which would
not have full line of sight. This relates to a 200mm reduction and does not apply to the
proposed 300mm reduction.
I again confirm that work will commence on the non-contentious area i.e. Mount prospect
Ave to Wooden Bridge around 01st April (exact date will depend relevant planning
compliance). I trust the above clarifies the current situation.
Declan Wallace
Director of Traffic

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DCC Dollymount Flood Defence Report

  • 1. Dollymount Flood Defence Level Glossary of flood defence terms. 1. Current (December 2015) estimated 100 year flood level. This is the estimated 100 year flood defence level of 3.03m above average sea level (Malin Head) in the year 2000 plus 0.12m of sea level rise seen in Dublin Bay from 2000 to 2015 giving a total level of 3.03m + 0.12m = 3.15m Malin Head. This level is 0.1m (100mm) lower than the 200 year flood level of defence recommended by Dr. Jimmy Murphy in his recent Independent assessment. This lower than recommended flood level of protection produces increased flood risk and it is therefore subject to an assessment of the increased risk posed to businesses and houses along the Wooden Bridge to Mount Prospect Avenue section of Clontarf Road and extra emergency measures which may be required, closing off the main road which may be required from time to time. 2. Future sea level rise estimate. Dr. Jimmy Murphy recommended defence against a sea level rise of 400mm to the year 2100 in his recent report. He stated that this could be reduced on a temporary basis in environmentally sensitive areas by up to 200mm. This leaves only 200 mm to protect against this continuing threat. At current sea level rise rates in Dublin Bay this allowance would only last 25 years, which would require rebuilding of the wall portions of the cycleway in that time frame or possibly less. 3. Associated wave height. With a 100 year high tide event there will be very strong winds and therefore an associated wave action and wave overtopping on the flood wall. This is estimated by Dr. Murphy in his report as 0.3m or 300mm. There is significant uncertainty with this height. The very short distance across the lagoon to Bull Island was taken into account in this small wave height estimate. 4. Freeboard. This is a safety factor to take into account the uncertainty in estimating the above factors. It is used in all engineering estimates. Rather than giving a new house a 50:50 chance of collapsing at the end of its design life a significant safety factor is applied to it. The safety factor here includes especially for uncertainties in wave height and global warming (which appears to be accelerating worldwide). The normal range of freeboard is 0.3m to 1m. The minimum freeboard recommended by the Office of Public Works, the National Competent Authority in Ireland for implementing the EU Floods Directive, is 0.3m which is Dr. Murphy and all other internal and external experts recommend for this project. 5. 1 in 100 year flood event. This could happen any year. The estimation of this is based on the analysis of recorded historical tides occurring in Dublin Bay.
  • 2. In summary we are proposing a reduction of 100mm under heading number 1, i.e. giving protection for a one in one hundred year flood event rather than protection for a one in two hundred year flood event . This is subject to “an assessment of the increased risk posed to businesses and houses along the Wooden Bridge to Mount Prospect Avenue section of Clontarf Road and extra emergency measures which may be required, closing off the main road which may be required from time to time”, as stated on previous page. It should also be noted for clarity that a one in one hundred year flood event could in theory occur next week rather than in a hundred years time. The proposed reduction of 200mm is being taken off number 2, i.e. future sea level rise estimate. This means that the flood defence will have to be raised again inside the next twenty five years. This reduction of 300mm only applies to the relevant 450 metre stretch between Mount Prospect Ave and Causeway Road. In effect it means that the wall at its highest point along this stretch will be reduced to 800mm (2ft8in). To clear up some confusion it should be noted that when this 300 mm reduction is in place all motorists travelling in either direction at this location will have a line of sight view above the wall, thus addressing the most commonly quoted adverse impact of the wall. A post on Clontarf.ie states that there will be a 75 stretch on the outward lane which would not have full line of sight. This relates to a 200mm reduction and does not apply to the proposed 300mm reduction. I again confirm that work will commence on the non-contentious area i.e. Mount prospect Ave to Wooden Bridge around 01st April (exact date will depend relevant planning compliance). I trust the above clarifies the current situation. Declan Wallace Director of Traffic