I am making this submission in relation to flood risk, surface water drainage and cumulative hydrological impacts associated with the proposed development. I am also a property owner within The Meadows Estate, which is located within Flood Zone A under the OPW flood mapping. As a resident within an identified flood risk area in close proximity to the proposed development, I have particular concern regarding any development that may increase surface water runoff or place additional pressure on the existing drainage network serving the area.

Given that properties within The Meadows Estate are already identified within Flood Zone A, it is particularly important that any new development within the local drainage catchment is supported by robust hydraulic assessment demonstrating that flood risk will not be increased downstream.

Reliance on the Phase 1 Site Specific Flood Risk Assessment

The planning documentation confirms that the Site Specific Flood Risk Assessment (SSFRA) prepared in May 2024 was originally undertaken in support of the Phase 1 development of 16 residential units, with the scope subsequently extended to include the Phase 2 lands.

The assessment appears to rely primarily on desk-based analysis using existing national datasets, including:

OPW Flood Maps and CFRAM mapping
EPA mapping
Geological Survey Ireland datasets
historical flood records
topographical mapping
policies contained within the Sligo County Development Plan
national climate change rainfall allowances

While this type of desk-based assessment is commonly applied where sites fall outside mapped flood zones and where no watercourses are identified on site, such studies primarily assess flood risk at the site itself and do not necessarily provide detailed hydraulic modelling of downstream drainage capacity or cumulative runoff impacts.

It is therefore unclear whether the SSFRA has been updated or supplemented with hydraulic modelling that specifically considers the combined surface water runoff from both Phase 1 and Phase 2 developments.

Cumulative Surface Water Runoff from Phases 1 and 2

The combined development results in 28 residential units, which will significantly increase impermeable surface cover within the site through:

roofs
driveways
internal roads
paved areas

This increase in impermeable surfaces reduces infiltration and results in a greater volume of surface water entering the drainage system during rainfall events.

The planning documentation indicates that surface water from Phase 2 will discharge into the Phase 1 storm water drainage network, which raises an important design question:

Was the Phase 1 drainage system designed from the outset to accommodate runoff from both phases of development or was it originally designed solely for the Phase 1 scheme?

Without clear hydraulic modelling demonstrating the capacity of the combined system, there remains uncertainty regarding whether the existing network can safely accommodate the additional runoff.

Evidence of Potential Drainage Pressure within the Existing System

The Planning Report itself acknowledges the potential for drainage pressure within the existing system.

Page 10 states:

“The proposed surface water drainage system is similarly designed as a gravity system, discharging to the Phase 1 storm water network. An offline attenuation tank will be provided to accommodate the additional impermeable area associated with Phase 2 and to mitigate surcharging which may occur within the Phase 1 attenuation system during extreme rainfall events.”

The reference to “surcharging” within the Phase 1 attenuation system during extreme rainfall events indicates that the designers already recognise that the existing system may come under pressure during periods of intense rainfall.

While the provision of an additional attenuation tank is noted, it is not clear whether the downstream drainage network, including pipes and outfall infrastructure along Chapel Road, has been assessed to ensure that it can safely accommodate the cumulative discharge from both phases.

Downstream Drainage Network Capacity

The critical issue is not simply whether the development site itself floods, but whether increased runoff could overload downstream drainage infrastructure.

Clarification is therefore requested regarding:

the hydraulic capacity of the Phase 1 storm water drainage network
the capacity of the downstream drainage infrastructure serving Chapel Road
whether hydraulic modelling has been undertaken for combined Phase 1 and Phase 2 runoff scenarios
whether modelling includes climate change rainfall intensity allowances
whether extreme rainfall event scenarios have been assessed

Without this information it cannot be concluded that the proposed development will not contribute to surface water flooding or drainage surcharge elsewhere within the local drainage network.

Downstream Receiving Watercourse

Properties within The Meadows Estate, located in proximity to the proposed development, are already identified within Flood Zone A and are influenced by a small river that runs behind the estate. This watercourse has previously overtopped its banks during periods of heavy rainfall.

In this context, clarification is requested regarding the ultimate discharge point of the surface water drainage system serving the proposed development.

In particular:

Where does the attenuated surface water from the site ultimately discharge?
Does the drainage network connect, either directly or indirectly, to the same catchment or watercourse system serving the river behind The Meadows Estate?
Has the capacity of the receiving watercourse and downstream drainage infrastructure been assessed to ensure that additional runoff from the development will not contribute to increased flood risk downstream?

Given that properties within The Meadows Estate are already located within a mapped Flood Zone A, it is important that any new development within the wider drainage catchment demonstrates that it will not increase flood risk elsewhere, in accordance with the OPW Flood Risk Management Guidelines for Planning Authorities (2009).

Local Groundwater Conditions – Well

There is a known well in the vicinity of the development which discharges water via a pipe near the site. Such groundwater discharge points can indicate local groundwater emergence or elevated groundwater levels and may influence drainage conditions in the surrounding area.

It is not clear from the planning documentation whether:

the presence of this well has been identified or mapped as part of the Site Specific Flood Risk Assessment,
local groundwater conditions or groundwater discharge points were considered within the drainage design, or
potential interactions between groundwater flows and the proposed surface water drainage system have been assessed.

If groundwater discharge points such as wells exist within or adjacent to the development lands, it is important that they are fully considered as part of the hydrological assessment to ensure that development works and drainage alterations do not interfere with natural groundwater flow paths or contribute to localised flooding.

Climate Change and Extreme Rainfall

Given the increasing frequency and intensity of rainfall events associated with climate change, it is important that drainage systems for new developments are designed with adequate resilience.

Clarification is therefore requested as to whether the drainage design and modelling undertaken for this development includes:

climate change rainfall allowances in accordance with national guidance
combined runoff from both development phases
extreme storm event modelling

In Conclusion

While the site itself may not fall within a mapped flood risk zone, the principal concern relates to the cumulative surface water impact and downstream drainage capacity associated with the combined development.

In particular, clarification is required regarding:

whether the Phase 1 drainage infrastructure was designed to accommodate both phases of development
whether downstream drainage infrastructure along Chapel Road has sufficient capacity to accommodate additional flows
whether cumulative runoff from both phases has been fully modelled
whether climate change rainfall allowances have been incorporated into the drainage design
whether local groundwater conditions and hydrological features, including the well and associated groundwater discharge in the vicinity of the site, have been fully considered

It is therefore requested that the planning authority seek further clarification and detailed hydraulic assessment to ensure that the proposed development will not increase flood risk or place additional pressure on the surrounding drainage network.

(Part 8) Proposed Housing Development - 12 no. units at Harte’s Court (Cúirt Uí Airt), Grange Townland, Grange, Co.Sligo. - Aighneachtaí

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