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HomeMy WebLinkAbout2660129 - F.I. Received Doc. - Flood Risk assessment report Main Street, Castleblayney, Co. Monaghan, Ireland www.cspringle.com Phone: (042) 9746492 Fax: (042) 9746511 info@cspringle.com Xisting Stage/Revision: Author: Checked: Issued: Marcus Dancey Eamonn McMullen 30th June 2026 Site Specific Flood Risk Assessment Aughnamullen GFC - Proposed Extension to Playing Pitches Cooltrimegish & Tullynamalra Castleblayney Co. Monaghan. TITLE: CLIENT: LOCATION: RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 2 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 1. Introduction CS Pringle has been commissioned to undertake a site-specific flood risk assessment in relation to the proposed extension of the existing playing pitches at Aughnamullen GFC. The proposed development comprises the extension of the existing grass playing pitches together with associated earthworks. The finished playing surfaces will remain grassed greenfield surfaces and, accordingly, the development is not expected to materially alter the hydrological response of the site or significantly increase runoff from the developed area. The principal flood risk consideration is therefore the potential displacement of existing temporary pluvial flood storage arising from localised raising of ground levels rather than any significant increase in runoff generated by the development itself. The purpose of this assessment is to determine the flood risk to the proposed development and to assess whether the proposed earthworks would materially reduce the available flood storage within the catchment, alter existing flow paths or transfer flood risk to adjacent lands or downstream receptors. The assessment has been undertaken using LiDAR topographical data, IH124 catchment runoff estimation procedures and Design Rainfall derived from Met Éireann Depth-Duration-Frequency (DDF) data for the Castleblayney area. Sensitivity testing has also been undertaken using reasonable variations in the principal hydrological assumptions. The predicted design flood level was found to vary only marginally, demonstrating that the conclusions of this assessment are robust and not unduly sensitive to any individual modelling assumption. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 3 of 15 Rev: Flood Risk Assessment Aughnamullen GFC Figure 1 – Overall site location and the low-lying basin to the west of the development. Location A indicates the point at which the watercourse leaves the basin. The blue shaded area indicates the approximate extent of the natural pluvial flood storage area considered in this assessment. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 4 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 2. Existing Site The existing sports complex occupies higher ground within the catchment. The proposed extension lies immediately west of the existing pitches on falling ground which ultimately drains towards a depression or basin the natural low point of which is shown at Location A. Inspection of the LiDAR survey together with OS mapping indicates that this low area functions as temporary surface storage during significant rainfall events. A defined watercourse drains the catchment towards the natural outlet at Location A. The adjoining lower ground is thought to function as a natural pluvial flood storage area during significant rainfall events, temporarily storing runoff generated within the catchment before it is conveyed downstream by the existing watercourse. The proposed development has therefore been assessed to ensure that neither this flood storage function nor the existing drainage path is adversely affected. Figure 2 – Proposed Pitch Extension Layout. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 5 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 3. Catchment Definition A review of the surrounding topography indicates that the contributing catchment draining towards the proposed development extends to approximately 0.254 km² (25.4 hectares) The catchment boundary was determined using the OSI 1:50,000 Discovery Series mapping and OSI 1:2500 mapping. The outlet controlling discharge from the basin is located at Point A where the contours naturally converge. Figure 3 – Catchment Plan. Catchment Plan identifying catchment boundary and outlet location. The catchment extent was based on the 1:50,000 Discovery Series mapping and 1:2500 mapping. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 6 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 4. Design Methodology The assessment has been undertaken using site-specific topographical and hydrological analysis of the contributing catchment. This provides an assessment of the flood behaviour of the site based upon its physical characteristics rather than solely upon indicative flood mapping. The assessment comprises:  estimation of the design flood using the IH124 methodology;  estimation of runoff volumes using Design Rainfall (DDF) data derived from Met Éireann for the Castleblayney area;  determination of existing flood storage volume from LiDAR contour information; and  comparison of the required temporary flood storage with the available natural storage within the catchment. The objective of the assessment is to determine the highest water level likely to occur during the design event and thereby identify the level below which existing flood storage should not be displaced. It should be noted that the proposed development comprises an extension of the existing grass playing pitches and that the finished surfaces will remain greenfield in nature. Consequently, no significant increase in runoff from the developed area has been assumed as a result of the proposal. The assessment therefore concentrates on preserving the existing temporary pluvial flood storage within the catchment and maintaining the existing drainage regime. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 7 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 5. Design Flow Peak runoff has been estimated using the IH124 method for catchments below 50 hectares. Parameters adopted include:  Catchment Area = 25.4ha  SAAR = 1032mm  Growth Factor = 1.96  Climate Change Allowance = 20%  Resulting peak flow (Q100 + Climate Change): 0.171m³/s A sensitivity assessment has also been undertaken using alternative WRAP classifications. A WRAP 2 classification has been adopted as being representative of the catchment. Although portions of the catchment become waterlogged during wetter periods, the presence of the extensive low-lying basin provides significant natural attenuation and temporary storage, resulting in a relatively subdued hydrological response rather than the rapid runoff characteristics associated with more responsive catchments. 6. Storage Assessment LiDAR contour mapping was used to determine the plan area enclosed by each 100 mm contour interval. The storage volume between successive contours was calculated using the prismoidal relationship V= h/3*(A1+A2+(A1*A2)^0.5) where  V = volume m³  h = contour interval m  A₁ = lower contour area m²  A₂ = upper contour area m² This enables the cumulative storage available within the basin to be determined for any water level. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 8 of 15 Rev: Flood Risk Assessment Aughnamullen GFC Figure 4 – Storage Calculation Spreadsheet. 7. Runoff Volume Assessment Design rainfall depths were obtained from the DDF Tables obtained for Castleblayney area from MET Eireann. Runoff volumes were estimated for a range of storm durations. Allowance was made for: Effective impermeable proportion of the catchment. Although rainfall falls across the entire catchment, only a proportion contributes directly to rapid surface runoff reaching the storage basin during the design event. The remainder is lost through infiltration, interception, depression storage and other natural attenuation processes. An effective impermeable proportion (varying from 0.15 to 0.25) was therefore adopted to represent the percentage of the catchment contributing direct runoff to the basin during the storm event. Design rainfall. Rainfall depths for a range of storm durations were obtained from the Met Éireann Depth- Duration-Frequency (DDF) data for the Castleblayney area. These values were used to determine the rainfall volume falling on the catchment for each storm duration considered. Climate change. The estimated peak discharge from the catchment was increased by the adopted climate change allowance (1.2) to reflect the anticipated increase in rainfall intensity over the design life of the development. Simultaneous discharge from the basin outlet while the basin is filling. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 9 of 15 Rev: Flood Risk Assessment Aughnamullen GFC As runoff enters the natural pluvial storage basin, water continues to discharge through the existing outlet at Location A. The storage requirement was therefore taken as the net volume accumulated within the basin after allowing for the volume discharged through the outlet during the storm event. This reflects the actual behaviour of the catchment, where inflow and outflow occur simultaneously rather than assuming the basin fills completely before any discharge takes place.  Effective impermeable proportion of the catchment. Although rainfall falls across the entire catchment, only a proportion contributes directly to rapid surface runoff reaching the storage basin during the design event. The remainder is lost through infiltration, interception, depression storage and other natural attenuation processes. An effective impermeable proportion (varying from 0.15 to 0.25) was therefore adopted to represent the percentage of the catchment contributing direct runoff to the basin during the storm event.  Climate change. The estimated peak discharge from the catchment was increased by the adopted climate change allowance (1.2) to reflect the anticipated increase in rainfall intensity over the design life of the development.  Simultaneous discharge from the basin outlet while the basin is filling. As runoff enters the natural storage basin, water continues to discharge through the existing outlet at Location A. The storage requirement was therefore taken as the net volume accumulated within the basin after allowing for the volume discharged through the outlet during the storm event. This reflects the actual behaviour of the catchment, where inflow and outflow occur simultaneously rather than assuming that the basin fills before any discharge takes place. The outlet discharge was represented by an average discharge over the storm duration, expressed as a proportion (0.4) of the calculated peak discharge. This provides a realistic estimate of the volume leaving the basin while remaining conservative for the purposes of flood storage assessment. The analysis demonstrated that the critical storage requirement occurs for a storm duration of approximately 4 hours, producing a required temporary storage volume of 2,509 m³. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 10 of 15 Rev: Flood Risk Assessment Aughnamullen GFC Figure 5 – Storage Comparison Spreadsheet. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 11 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 8. Comparison of Available Storage Comparison of the required storage with the calculated basin storage indicates: Required Storage – Approximately 2,500m³ Available Storage at RL 157.21mOD – Approximately 2,510m³ As a check on the robustness of the assessment, sensitivity testing was undertaken using alternative effective impermeable proportions together with alternative WRAP classifications. The resulting predicted flood levels varied only between approximately RL 157.13mOD and RL 157.28mOD, demonstrating that the assessment is not unduly sensitive to reasonable variations in the principal hydrological assumptions. Adoption of RL 157.30mOD therefore provides an additional engineering margin above the calculated minimum storage level. 9. Assessment of Flood Risk 9.1 Flood Risk to the Proposed Development The proposed extension comprises outdoor playing pitches and associated earthworks. The assessment indicates that the predicted maximum flood level within the contributing catchment during the 1% AEP event including an allowance for climate change lies below RL 157.30mOD. The finished pitch levels and associated earthworks have been arranged such that development is located above this level. Consequently, the proposed development is not expected to be subject to flooding during the design event. Where local embankment toes approach the adopted flood storage level (shown X to Y), large toe retaining stones are to be incorporated to prevent encroachment of the embankment below RL 157.30mOD. Accordingly, the development itself is not considered to be at risk of flooding. 9.2 Impact on Flood Storage and Third Parties The principal flood risk consideration associated with the proposal is not flooding of the development itself but the possibility that raising ground levels could reduce available flood storage and transfer flood risk elsewhere. The storage analysis demonstrates that the available storage below RL 157.30mOD exceeds the predicted temporary storage requirement for the 100 year plus 1.2CC flood event. The proposed earthworks have therefore been arranged so that flood storage below this level is maintained. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 12 of 15 Rev: Flood Risk Assessment Aughnamullen GFC Accordingly, the proposed development is not expected to increase flood levels or transfer flood risk to adjoining lands or downstream receptors. 9.3 Flood Storage Preservation The proposed earthworks have been developed specifically to preserve the available pluvial flood storage identified by this assessment. The calculated design flood level of approximately RL 157.3mOD has therefore been adopted as the controlling level for the earthworks design. The majority of the proposed embankment remains outside the area below RL 157.3mOD. At one isolated location (between points X and Y shown on Figure 6), the natural embankment toe would otherwise marginally extend below this level. At this location the toe of the embankment will be locally retained using a short length of stone retaining detail so that the placed fill remains behind the adopted RL 157.3mOD contour. Accordingly, the proposed works consisting of toe stones retaining the toe of the embankment have been specifically detailed to avoid any material displacement of the calculated flood storage volume. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 13 of 15 Rev: Flood Risk Assessment Aughnamullen GFC Figure 6 – Proposed Earthworks and RL 157.3mOD Flood Storage Limit. Green is 158.0mOD, Red is 157.5mOD and light blue 157.0mOD 9.3 Existing Flow Paths Review of the LiDAR survey indicates that runoff naturally collects within the low-lying basin before discharging through the existing outlet at Location A. The proposed development does not obstruct this outlet nor alter the principal overland drainage route. Consequently, existing flow paths are maintained, and no adverse hydraulic impacts are anticipated. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 14 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 9.4 Residual Risk Residual risks are limited to events exceeding the design standard or partial blockage of the existing watercourse at the catchment outlet. These risks are inherent within the existing catchment and are not materially increased by the proposed development. The proposed earthworks preserve both the existing temporary pluvial flood storage and the established drainage regime. Accordingly, the residual flood risks associated with the proposed development are considered capable of being managed to an acceptable level. RECEIVED: 01/07/2026 ____________________________________________________________________________________________ Page 15 of 15 Rev: Flood Risk Assessment Aughnamullen GFC 10. Conclusions This Site-Specific Flood Risk Assessment has been prepared in accordance with the requirements of Policy ILP 1 and Section 15.22.8 of the Monaghan County Development Plan. The assessment has considered the hydrological characteristics of the catchment using IH124 runoff estimation, Met Éireann DDF rainfall data, LiDAR-derived topographical information and site-specific flood storage calculations. The assessment concludes that:  The proposed extension of the playing pitches is not considered to be at unacceptable risk of flooding during the 1% AEP event including an allowance for climate change.  The proposed earthworks have been designed to preserve the existing temporary pluvial flood storage within the catchment by maintaining all placed material above the adopted flood storage level of RL 157.30mOD. Accordingly, the proposal will not reduce flood storage or exacerbate flooding within the immediate vicinity or elsewhere.  The proposed development does not obstruct or impede the existing watercourse or the principal overland flow paths within the catchment. The existing drainage regime is maintained.  Residual flood risks associated with events exceeding the design standard or blockage of the natural outlet remain comparable with the existing situation and can be managed to an acceptable level. It is therefore concluded that the proposed development complies with the flood risk objectives of Policy ILP 1 and Section 15.22.8 of the Development Plan and will not result in the creation or transfer of flood risk either within the site or to adjacent lands or downstream receptors. Chartered Engineer for C S Pringle RECEIVED: 01/07/2026