Further details on Report

The Downpatrick Flood Alleviation Scheme Feasibility Report (May 2025) provides a comprehensive assessment of flood risk in Downpatrick, following severe flooding in November 2023. Here’s a concise summary of its findings and recommendations:

🔍 Purpose and Context

• Commissioned by DfI Rivers following extensive flooding in Downpatrick on 1–2 November 2023.
• Cause: heavy rainfall over two weeks, high tides restricting Quoile River discharge, and backflow into urban drains.
• Aimed to identify viable flood alleviation measures for a 1% AEP (design flood) event with climate change allowance.

🌍 Study Area

• Focused on the Quoile River and its urban tributaries: Plank Drain, Market Street Drain, Town Drain, Saul Street Stream, and Strangford Road Stream.
• Includes the Quoile Barrier (North) at Hare Island, a key tidal control structure.

📊 Methodology

• Surveys: walkovers, topographical, bathymetric, environmental, geotechnical, and hydraulic modelling.
• Historical flood records and stakeholder feedback included.
• 14 flood mitigation options assessed (Options A–N).

✅ Shortlisted Options

Two technically and economically viable options were identified:

Option F – Town Defences Only

• Raised defences and pump upgrades within town.
• Protects 108 properties (29 residential, 79 commercial).
• BCR: 1.01 (just above viability threshold).
• Requires demountable defence at railway – adds risk.

Option N – Combined Town and River Defences (Recommended)

• Integrates Option F measures + defences near the Quoile River.
• Protects 106 properties (28 residential, 78 commercial).
• BCR: 1.15 – higher benefit-cost ratio.
• No demountable defences needed – lower operational risk.

🔍 Not Progressed

Options like dredging the Quoile River, modifying or removing barriers, upstream storage, and natural flood management were ruled out due to:

• Minimal reduction in flood risk.
• High costs or major environmental disruption.
• Infeasibility of scale (e.g., 5.85 million m³ storage required).

💷 Costs & Economics

• Option F: £26.98m total PV cost.
• Option N: £22.53m total PV cost (lower due to simpler defences).
• Option N achieves greater benefits at lower cost.

🧮 Carbon and Environmental Considerations

• Option F has a slightly lower carbon footprint.
• Option N avoids demountable infrastructure and is preferred overall.
• Both options face archaeological and ecological sensitivities requiring careful design and further liaison with HED and NIEA.

📌 Conclusion & Recommendations

• Preferred Option: Option N – Combined Flood Defences.
• Viable technically, economically (BCR > 1), and operationally.
• Subject to risks that must be investigated further before detailed design:
  • Independent risk and cost reviews.
  • Ground and embankment investigations.
  • Liaison with HED and NI Water.