HomeMy WebLinkAbout2660011 - F.I. Received Doc. - Flood Risk Assessment
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Stage/Revision: Author: Checked: Issued:
Marcus Dancey Eamonn McMullen 4th June 2026
Flood Risk Assessment and Site Use Constraints
Padraig & Elaine Ward
Derrynaloobinagh, Meeting House Lane, Ballybay, Co
Monaghan
TITLE:
CLIENT:
LOCATION:
RECEIVED: 08/06/2026
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Page 2 of 9
Rev:
Preliminary Flood Risk Assessment Padraig & Elaine Ward
Re:– Preliminary Flood Risk Assessment and Site Use Constraints
at Derrynaloobinagh, Meeting House Lane, Ballybay, Co Monaghan
for Padraig & Elaine Ward
Introduction
This report has been prepared at the request of our client Padraig and Elaine Ward in support of
a planning retention application relating to existing buildings and yard areas located at
Derrynaloobinagh, Meeting House Lane, Ballybay, Co. Monaghan.
The purpose of this assessment is to:
review available flood risk information relating to the site.
consider predicted flood levels affecting the existing development.
compare estimated flood levels with surveyed floor levels.
assess the compatibility of the existing and proposed use with the identified flood risk.
identify appropriate flood resilience and operational measures.
This assessment has been prepared as a preliminary flood risk assessment using available OPW
CFRAM flood mapping and associated model output data together with site survey information.
No independent hydraulic modelling has been undertaken as part of this assessment.
Site Description
The subject site comprises an existing developed yard and associated buildings located at,
Derrynaloobinagh, Meeting House Lane, Ballybay, Co. Monaghan.
The site is located adjacent to low-lying lands associated with the local floodplain and Lough
Major lake system on the Dromore River system and portions of the site fall within areas
identified by OPW flood mapping as being susceptible to flooding during significant flood events.
The existing development comprises hardstanding yard areas together with existing buildings. RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
Site map shows the site outlined in red and location of the buildings relative to the river. The
floor level of the building is 79.735mOD. RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
Topographical Survey and Existing Floor Levels and site levels
Ground levels and finished floor levels within the site were established by topographical survey
and compared against predicted flood levels derived from OPW flood mapping and associated
model outputs.
Available Flood Risk Information
Flood risk information for the site has been obtained from the OPW CFRAM flood mapping and
associated node data for the area. RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
OPW Flood Extent Mapping with Relevant Node Identified as 3602M00035
The OPW flood mapping indicates that portions of the site lie within the predicted flood extents
associated with both the 1% AEP and 0.1% AEP flood events.
Associated OPW model outputs provide predicted flows and flood levels at a number of relevant
model nodes within the floodplain system.
Assessment Table Showing OPW node reference 3602M00035, with 10% AEP level of 79.16mOD
and 18.36m³/s flow, 1% AEP level of 79.77mOD 32.18m³/s flow and the 0.1% AEP level of
80.69mOD 55.50m³/s flow.
RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
Methodology
The OPW model outputs provide predicted flows and water levels for the 10% AEP, 1% AEP and
0.1% AEP flood events for this location.
For the purposes of this assessment, climate change impacts were considered by applying a
Mid-Range Future Scenario (MRFS) flow factor of 1.2 to the OPW 1% AEP flow. A High-End
Future Scenario (HEFS) factor was also reviewed for comparison purposes.
A fitted relationship derived from the OPW flow-frequency outputs was then used to estimate
the equivalent present-day return period associated with the climate-adjusted flow. A second
fitted relationship between return period and flood level was subsequently used to estimate the
corresponding flood level.
This approach allows the OPW model outputs to be used to derive indicative climate-change-
adjusted flood levels without undertaking fresh hydraulic modelling. The derived levels should
therefore be regarded as planning-level estimates based on interpretation of the OPW model
outputs rather than independently modelled design flood levels. Given the level of study
undertaken by the OPW within Ballybay, the model outputs are considered appropriate for
planning-level assessment purposes.
Estimated Flood Levels
The following table summarises the relevant OPW model outputs together with the estimated
climate-change-adjusted flood levels and comparison against existing finished floor levels.
Table gives the MRFS and the HEFS levels of water and height above the floor level of the
estimated flood level for a 100-year event with climate change taken into account. The 100-year
flow is multiplied by 1.2 or 1.3 to get the climate change adjusted flow. This is then used to
obtain the return period for that flow. Knowing the return period for that flow we can obtain
the level to which the flooding is likely to come. This is compared against the existing floor level
and the depth of water likely for each scenario.
RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
The OPW model outputs indicate predicted flood levels in the order of approximately 79.77mOD
for the present-day 1% AEP event and approximately 80.69mOD for the 0.1% AEP event.
Application of climate change flow factors results in estimated flood levels generally
intermediate between the present-day 1% AEP and 0.1% AEP model outputs, namely
approximately 80.07mOD and 80.21mOD for the MRFS and HEFS respectively.
The estimated climate-change-adjusted flood levels exceed the existing finished floor level of
the building by approximately 0.33m and 0.47m for the MRFS and HEFS respectively.
Flood Risk and Site Use
The mechanism of flooding at this location is not rapid flow across the site, but rather gradual
rise in water levels associated with the wider floodplain and lake system. Due to the scale of the
lake basin and floodplain storage area, inundation occurs relatively slowly, allowing time for
operational response measures to be implemented.
In practical terms, this means that flooding of portions of the site may occur during significant
flood events and there is no proportionate engineering solution available that would fully
exclude floodwaters from the site without introducing wider impacts or requiring
disproportionate works.
Accordingly, the approach proposed is not a flood-defence approach, but rather a flood-resilient
approach whereby:
periodic inundation of portions of the site is acknowledged
building construction and site operation are adapted to accommodate flooding
no increase in flood risk arises elsewhere
no pollution risk arises during flood events
The existing buildings and yard construction are generally consistent with a flood-resilient
approach, comprising predominantly concrete construction and hardstanding capable of wash-
down and reinstatement following inundation.
However, the suitability of the site is dependent not only on the physical resilience of the
buildings, but also on the management and operation of the site during flood conditions.
In particular:
potentially contaminating materials should not be stored at low level within areas
susceptible to flooding
oils, fuels, lubricants and similar materials should be stored above predicted flood levels
and within suitable bunded arrangements RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
electrical installations and vulnerable equipment should be located above predicted flood
levels where practicable
vulnerable stored items should be capable of removal or protection during significant
flood events
It is proposed that storage systems within the buildings incorporate raised lower storage levels
such that the bottom pallet or equipment storage level is located approximately 350mm to
500mm above finished floor level. This would place vulnerable or potentially contaminating
materials above the estimated climate-change-adjusted flood levels derived within this
assessment. Such an approach allows the buildings themselves to remain flood-resilient while
reducing the risk of damage, contamination or loss arising from periodic inundation.
Residual Risks
Residual flood risk remains associated with:
uncertainty inherent in flood modelling and interpolation of flood levels. This is partly
mitigated through consideration of the High-End Future Scenario and by maintaining
vulnerable items above approximately 500mm above finished floor level.
future climate variability. This is partly mitigated through consideration of the High-End
Future Scenario and by maintaining vulnerable items above approximately 500mm above
finished floor level.
potential blockage or reduced conveyance within the wider floodplain and drainage
system. Given the size of the Dromore river between Ballybay and Cootehill this is not
considered to be a significant residual risk.
operational management of the site during flood conditions. This is mitigated by a good
warning system and management plan being implemented to ensure that no vulnerable
items are threatened and to minimise the risk of contamination of flood waters.
Conclusion
Based on the available OPW flood mapping, associated model outputs and surveyed floor levels,
the site is located within an area potentially affected by flooding during significant flood events.
Indicative climate-change-adjusted flood levels have been estimated through interpretation of
the OPW model outputs and comparison with surveyed floor levels. RECEIVED: 08/06/2026
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Preliminary Flood Risk Assessment Padraig & Elaine Ward
The proposed approach is based on flood resilience rather than flood exclusion, acknowledging
that periodic inundation may occur while ensuring that building construction, site layout and
operational management are compatible with the identified flood risk.
Subject to appropriate operational controls and flood-resilient measures, including the storage
of vulnerable or potentially contaminating materials above predicted flood levels, the existing
development may be considered capable of continued use without giving rise to unacceptable
flood risk or pollution risk elsewhere.
Chartered Engineer for C S Pringle
RECEIVED: 08/06/2026