Scheme Aim
The principle aim of the Portadown FAS feasibility study was to assess whether an economical, environmentally and socially sensitive scheme can be produced which will reduce the impact of potential flooding to properties, infrastructure and businesses from the River Bann and some of its Tributaries.
Scheme Summary
- Estimated Cost: £17.1m
- Benefit Cost Ratio: 2:1
- Length: 9.4 km
- Current proposed Defences:
- Steel sheet piled flood walls,
- Reinforced concrete secant piled flood walls,
- Engineered embankments,
- Ground re-profiling
- Status: Detailed Design
Scheme Background
Portadown has flooded regularly over the last 30-40 years. Significant flood events have been recorded during 1986, 1987, 2009, 2011, 2014 and 2015/16.
In October 2011 out-of-bank flooding occurred in the Portadown area resulting in the flooding of properties and infrastructure. It is estimated that this flood was approximately a 1 in 100 year flood on the River Bann.
Further floods in 2014 and 2015 caused damage to residential properties and businesses across the wider Portadown area.
As part of implementing the European Floods Directive 2007/60/EC (the Floods Directive) Portadown was included on Rivers capital works programme and a feasibility study was completed in 2018.
The study identified that 308 properties in the Portadown area were at risk of fluvial flooding in the event of a 1% annual exceedance probability (AEP) flood. This is the flood that has a 1% chance of occurring or being exceeded in any given year. The main risk of fluvial flooding comes from the River Bann, which flows through Portadown, and the Corcrain River, which is one of its tributaries.
Feasibility Preferred Option
The feasibility study recommended an economically viable preferred option to reduce the impact of any potential flooding in the event of a 1% AEP flood. The option consisted of works at 17 different sites across the town of Portadown and the surrounding area. These works included the construction of 6.5km of defences and a flap valve. The preferred option had an estimated cost of £7.8m, avoiding £15.6m in flood damages, resulting in a benefit cost ratio (BCR) of 2:1.
Scheme Design and Review
In 2019 DfI Rivers commissioned their partner engineering consultants, Amey, to undertake the design of the Portadown FAS and progress the scheme to the works procurement stage.
Following publication of the DfI “Technical Flood Risk Guidance in Relation to Allowances for Climate Change in Northern Ireland” in 2019, a review of the scheme was undertaken by Amey as part of the design process to ensure appropriate consideration of climate change.
Adoption of this guidance resulted in an increase in the design flood levels and subsequently the length, height, number of defences and therefore the estimated cost of the original scheme proposals has increased. The information from the revised modelling has reported that there are 474 residential properties and 100 commercial properties at risk of fluvial flooding in the event of the design flood. The current scheme proposals have an estimated construction cost of £17.1m, avoiding approx. £43.6m in flood damages, resulting in a BCR of 2:1.
Proposed Scheme Description
The current scheme proposes to construct 9.4km of flood defences at 20 sites in the Portadown area.
Scheme Benefits
The negative effects which can be caused by flooding are extensive and are described within the Portadown Feasibility Report.
Using standard flood damage analysis, over the 100 year life of the scheme the level of flood risk is estimated to cause damages valued at approximately £43.6m. This assessment excludes the impact on the local economy as a result of unproductive employment and loss of access.
Environmental Statement and Habitats Regulations Assessment
The Department has determined that the works specified in this scheme are likely to have significant effects on the environment and as such are required to carry out the appropriate assessment. This assessment is currently being progressed by the Department’s consultants (Amey).
Scheme Documents
Key Dates
2018
- Feasibility Report Completed
2019
- Amey Consulting appointed as Principal Designers
- Review of feasibility study commenced
- Ground investigation works commenced
2020
- Application of Departments Technical Flood Risk Guidance in relation to Allowances for Climate Change in NI
- Revised scheme proposals produced
- Detailed Design Stage commenced
2021
- Ground investigations completed
- Detailed structural and geotechnical design commenced
2022
- Notification of Determination That Proposed Flood Alleviation Scheme Is Likely To Have Significant Effects on the Environment