DfI report identifies potential significant flood risk in Derry
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DfI’s second cycle Flood Risk Management Plan 2021-2027 focuses on the prevention of flooding, protection of individuals, communities and the environment and preparedness arrangements for flood warning and emergency response.
The analysis concludes that buildings and infrastructure in Derry are most at risk from surface water flooding and from inundations arising from small water courses and streams in the city.
The risk of fluvial flooding from the River Foyle itself is deemed to be limited, as is tidal flooding from the lough.
The report states that the River Foyle ‘does not constitute a major flood risk to Londonderry itself because the city is situated in the wider, tidal reaches of this river’.
However, significant flooding from urban tributaries throughout the city and from the River Faughan to the east is a bigger risk as was disastrously demonstrated during the major floods of August 2017.
The report notes how a Northern Ireland Flood Risk Assessment (NIFRA) in 2018 identified that Derry had the second highest level of flood risk amongst the twelve Areas of Potential Significant Flood Risk (APSFR) in the north.
It also highlights that the annual average cost of surface water (pluvial) flooding dwarfs that of both fluvial and tidal flooding combined.
“Predicted annual average damages (AAD) for surface water amounts to just under £8million, second highest is tidal sources reaching approximately £450,000.
“The AAD from fluvial sources are lowest at approximately £170,000,” the study observes of the overall cost of flooding in the Derry area.
The management plan deals with each of the flooding types in turn and shows that the cityside of Derry appears to be particularly at risk.
“The main areas of the city identified as being at surface water flood risk are the low-lying areas behind the quays on the west bank of the Foyle such as: Foyle Street, Strand Road and Rossville Street,” the report states.
Recent significant flooding incidents in the city adversely affected these areas and their residents and businesses.
“These areas were impacted during the notable event of July 2015. There were also occurrences of surface water flooding across the city during August 2017 in the Springtown and Galliagh areas.”
According to the report infrastructure at risk from surface water flood events up to 0.1 % Annual Exceedance Probability (AEP) (1 in 1000 year) in Derry includes ‘care homes, police stations, schools, electricity substations, environmental heritage sites and built heritage sites’.
While the risk of fluvial flooding from the River Foyle itself is considered to be low, flooding from smaller rivers is an issue, the authors maintain.
“There are several urban watercourses running through the city, and the main areas at risk of fluvial flooding on the west bank of the Foyle lie near the Pennyburn Stream and Creggan Burn.
“The main areas at risk on the east bank are near the Woodburn Park Stream, Burnagibbagh [Tullyally] and Ardnabrocky Drain [Drumahoe].”
Again the infrastructure at risk includes electricity substations, environmental designated sites and built heritage sites.
Tidal flooding is equally considered to be less of a problem than surface water inundations in the short to medium term.
But it is still a potential hazard with the report noting that a major flooding event at Coolkeeragh could lead to a ‘significant infrastructure failure’.
“Lough Foyle is a complex tidal estuary with a narrow entrance and many irregular features, including numerous mud flats.
“Londonderry has no formal sea defences, however, banks/quays act as a form of tidal defence that provides limited protection to low-lying areas of the city.
“These could overtop in more severe flood events, although tidal flood risk is much lower overall than combined fluvial and surface water flood risk and the standard of protection they offer is unknown.
“Further studies are being undertaken at Coolkeeragh Power Station on the coast, where flooding could lead to significant infrastructure failure.
“At Coolkeeragh, the railway embankment acts as a form of sea defence, although it is not recognised or managed as a formal flood defence.”
GP surgeries, a police station, electricity substations and listed buildings are among the infrastructure identified as being at potential risk from tidal flooding from the lough.
The new plan outlines existing flood defences that are in place in the city district.
These include the raised linear parkland along Foyle Road; the sea defences at Black Brae, Longfield, Ballykelly and Myroe on the south bank of Lough Foyle; various agricultural embankments throughout the river catchment; engineered river flood defences protecting the village of Eglinton; a sheetpiled river flood defence on the left bank of the River Faughan at Three Mile House (Drumahoe); and an embankment at Newbuildings which defends properties from inundation from the River Foyle.
Works undertaken by DfI Rivers to address drainage and flooding issues in Derry have also included ‘culvert works at Shantallow, the Springtown Tributary Extension, and Coshquin Burn Extension’.
Meanwhile, NI Water completed two capital schemes during Price Control 15, its 2015-21 investment period. These were the Sheelin Park Flood Alleviation scheme (£65,000) and the Foyle Springs Flood Alleviation scheme (£385,000).
Seven protective measurers were identified in DfI’s first cycle Flood Risk Management Plan 2015-2021 and these continue to progress.
They include Flood Alleviation schemes for smaller watercourses at Ardnabrocky, Ardnabrocky Drain (to protect 10 residential and commercial properties), Springtown, Pennyburn Stream (55 commercial properties), Lower Tullyally, Burnagibbagh (90 residential and commercial properties), Waterside, Woodburn Park Stream (120 residential and commercial properties) and Creggan, Creggan Burn (20 residential and commercial properties). Feasibility studies have been completed for all of these. However, no viable scheme has been identified for Ardnabrocky and it appears this will not proceed.
A feasibility study for a Foyle Coastal Study has also been completed while a Foyle Coastal Emergency Response Plan has been completed and tested. The new Flood Risk Management Plan cycle 2021-2027 proposes moving to a detailed design phase for each of the flood alleviation schemes at Creggan, Pennyburn, Burnagibbagh and Woodburn and proceeding to construction if they prove viable.
More generally, DfI Rivers is ‘working with Derry City & Strabane District Council Planning Authority to update flood risk policy for the Local Development Plan’ and this should be completed by 2025.
Elsewhere ‘NI Water will produce an Enhanced Drainage Area Plan (DAP) for Londonderry commencing in 2021 with a 2 year programme that will provide a better understanding of NI Water storm capacity, opportunities for storm water separation and set out actions to mitigate integrated flooding issues’ by 2024.
The new plan also proposes completing design work for the Foyle Coastal Flood Alleviation scheme and if it proves viable DfI Rivers will seek funding to proceed to construction by 2024.
Finally, the Regional Community Resilience Group will ‘finalise establishment of the Altnagelvin/Drumahoe community group to increase the resilience of the community to flooding’ and ‘progress engagement in further areas of Londonderry, as appropriate or as required, to increase community resilience to flooding’.