Overview
Newry has a history of flooding and was ranked second in the 20 most Significant Flood Risk Areas (SFRAs) in Northern Ireland. The flood event in November 2014 caused serious damage to residential and commercial properties within the city, specifically to those on Bridge Street. This prompted DfI Rivers to look at options for flood protection.
Amey was commissioned, by DfI Rivers, to review the Newry River and Tributaries Feasibility Study and to progress the Phase 1 proposals through to Detailed Design and Construction. This involved a review of proposed flood defences in the Newry area recommended in an earlier 2016 Feasibility Study.
The key objective throughout, was to reduce the risk to life and the damage to property from flooding. Amey’s scheme was developed to provide flood protection for properties located adjacent to Newry’s tributaries.
Construction was completed in March 2023 and within the pre-construction cost estimation.
The challenge
Amey’s brief was to review and update feasibility proposals, to progress through to detailed design and to assist in the procurement of the construction contractor and manage construction.
On receiving this brief, Amey appointed Waterco as a sub consultant to bring in complementary expertise and undertook a joint review of the existing hydraulic model. The model needed to be updated to revise the design level for a Q100 flood level, or 1% Annual Exceedance Probability event (which is a flood event that has a 1% chance of the event happening every year for the next 100 years). In addition, 20% was added the flows to account for climate change (as per DfI document “Technical Flood Risk Guidance in relation to Allowances for Climate Change in Northern Ireland”, published in Feb 2019) and an allowance for 600mm freeboard was included, as per DfI guidance.
A preliminary design and assessment of options was followed by detailed design of the proposed defences. Significant consideration went into producing a design that is constructable in a predominantly urban area, allowing for the restricted nature of the sites as well as the operational needs of businesses, schools, and leisure centres.
Amey’s approach included an extensive consultation programme with affected landowners including Newry, Mourne and Down District Council (NMDDC). This allowed Amey to agree suitable accesses and develop construction programmes that minimised disruption. Accommodation works were also agreed with affected landowners, including specific reinstatement details.
A tender package for the procurement of the scheme - following an NEC 3 Option A contract - was prepared by the design team. The scheme was advertised for tender in December 2020 and construction works began in August 2021.
Works were finished in advance of the contractual completion date of March 2023.
The full programme of work included upgrading existing defences and constructing new hard defences:
Downshire Stream
- Replacement of the Downshire stream open channel with a U channel (2m wide x 2.7m high, approximately 75m long)
Knox Pebbles Drain
- Construction of sheet piles with concrete pile cap (to a height of 2.15mAOD, approximately 680m long)
- Construction of a sheet pile core flood embankment (to a height of 2.15mAOD, approximately 30m long)
- Construction of clay core flood embankment with Redi-Rock retaining wall (to a height of 2.15mAOD, approximately 350m long)
Craigmore Road
- Replacement of existing inlet structure with a new design in accordance with C786 guidance
Armagh Road Drain
- Replacement of existing 900mm diameter culvert with 1350mm diameter culvert, approximately 990m long
- Construction of 20 catchpit manholes (including three 3m wide silt traps with 1m sumps)
- Construction of headwall on outlet of upgraded culvert.
Benefits
The Amey solution kept disruption to a minimum, benefiting businesses, residents, and schools. This was achieved through a thorough and effective consultation process and good, two-way communication, with stakeholders throughout the design and construction phases. Amey set and met an ambitious target of zero service outages during construction.
A combination of precast and cast insitu sections were used to aid speed of construction and to give flexibility to on-site contractors.
The properties adjacent to the tributaries are now protected up to the Standard of Protection (SoP), 1% AEP with an 20% additional flow to account for climate change, with 600mm included for freeboard. The climate change allowance is as per DfI document “Technical Flood Risk Guidance in relation to Allowances for Climate Change in Northern Ireland”, published in Feb 2019.
The Phase 1 works were finished in March 2023 with snagging walkovers taking place currently to address any defects found.
The works associated with Newry Flood Alleviation Scheme Phase 2 and 3 are currently ongoing.