The Upper River Bann runs through the heart of Portadown, Co. Armagh towards Lough Neagh. The surrounding area is subjected to fluvial flooding due to the high-water level within the River Bann due to prolonged periods of rainfall. This Leads to extensive flood damage to commercial and non-commercial properties alike, alongside the local infrastructure.
DfI have commissioned Amey Consulting to review, design, manage procurement and construction of the options that has identified 21no. sites equating to C.9.5km of flood defences to protect c.380 properties, both residential and commercial, to a 1% Annual Exceedance Probability (AEP) + Climate Change (CC) standard level of protection against fluvial flooding.
Amey’s multi-disciplinary team are currently progressing through detailed design prior to entering the procurement process, to identify a suitable contractor to construct the scheme.
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Figure 2 - Portadown Flooding, Winter 15/16 Figure 3 - Portadown Flooding, Winter 15/16 |
The challenge
The catchment area of the Portadown and Craigavon area of potential significant flood risk is made up of the River Bann with a catchment greater than 650km2 (Shown in Figure 4) and its main tributary the Cusher also contributing to the overall catchment. These areas extend as far south as Newry, Co. Down and beyond. Amey Consulting undertook a review of the modelling carried out by the previous consultant. The review and subsequent remodelling of the 650km2 flood area to a 1% AEP+CC identified 21no. sites at risk from flooding.
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Figure 4 - Portadown and Craigavon APSFR and Catchment Area |
Our approach
Amey’s review and remodelling of the catchment area to a 1% AEP+CC standard level of protection identified 21 sites both at risk from flooding and meeting protection criteria. Given the significant size of the catchment area and scale of the project, a Project Quality Plan (PQP) was developed to ensure smooth running and timely delivery.
The new model looks at flood damage across the Portadown and Craigavon APSFR, considering the significant size of the river Bann catchment (650km2). It was therefore necessary to devise a Project Quality Plan (PQP) to ensure the smooth and effective delivery of such a vast scheme which is currently approaching the end of detailed design. Amey Provided the following input:
- Scheme Assessment Report (PQP)
- Scheme drawings, specification, and contract documentation.
- Preparation of land plans and documentation for statutory processes.
- Public utilities liaison and negotiation.
- Geotechnical survey, reporting and certification.
- Environmental Statement.
Throughout the project Amey assessed the various hydraulic engineering options in accordance with the Integrated Management systems (IMS) and Quality Plan (QP). Value engineering, risk evaluation and workshops were held throughout to minimise costs and manage risks.
Environmental Considerations
Amey Consulting have produced an Environmental Statement identifying and mitigating against environmental issues arising from the proposals for the flood alleviation scheme, covering the following topics:
- Air Quality
- Cultural Heritage
- Landscaping
- Conservation of Existing Habitats
- Noise Pollution
- Vibrations
- Road Drainage
- Riverine Environment and Habitat
The consultation process for the draft Environmental Statement was completed in spring 2023 (Statutory and Public Consultees) with generally positive feedback from both the statuary and public consultees.
An Arborist’s impact assessment was undertaken on trees across the extents of the Portadown scheme, taking into consideration species, current condition and assessing any potential impact that the proposals may have. The subsequent report allowed considerations to be made in the design to minimise impact on existing trees through safe construction distances and altering lines of defences to avoid existing trees. If the removal of trees cannot be avoided, the intention is to carry out a programme of compensatory planting. Amey Consulting will identify appropriate sites where tree planting could be undertaken in association with the local landowners and public bodies.
A CEEQUAL assessment is also being undertaken on this scheme to ensure sustainability, the environment and social aspects are the focus of the design decisions being made. The CEEQUAL assessment involves multi-disciplinary input for all teams associated with the project (Structures, Environmental, Lands, Geotechnical and Hydraulic/Rivers). This assessment is currently being progressed through detailed design and will be completed after the scheme has been constructed with input from the contractor.
Site information and Surveys
Ground investigations
On the 21no. sites requiring defences; extensive ground investigations were undertaken over the course of two years. Due to the complexity of undertaking GI within flood plain areas, it was essential to allow for seasonal changes and the associated issues within the programme of works. Amey Consulting designed, procured, and supervised the geotechnical investigations (GI) within the study to aid in the design process regarding potential flood alleviation measures (sheet piles, secant piles, reinforced concrete walls, earth embankments, etc.). The proposed ground investigation works involved cable percussion boring alongside in-situ and laboratory testing of the acquired soil samples. Upon completion of the fieldwork, factual reports were produced illustrating the recorded ground information. Figure 5 below shows these works taking place at the Irwin’s Mill Site, located in the centre of Portadown.
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Figure 5 – GI Works Being Undertaken at Irwin’s Mill |
Topographical Survey
Across the extents of the 21no. proposed site locations, an approximate 380no. properties, both residential and commercial are defended by the proposals included within the Flood Alleviation Scheme. Finished floor levels were gathered within the topographical survey illustrated in Figure 6. Likewise, across the extents, ground levels were surveyed, to aid with flood modelling. In addition, topographical survey data was obtained for manholes, gullies and other amenities including street furniture, services, and boundary markers such as fences, walls etc.
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Figure 6 – Finished Floor Levels Within Portadown APSFR |
Service providers
11no. service providers were contacted, through an initial C2 service data request, to obtain maps to identify potential services within the area of proposed works. On review of this information, further C3 requests were sent to the service providers identifying potential conflicts with our design proposals. On receipt of C3 information, Amey facilitated and attended site walkovers with several of the affected providers such as Northern Ireland Water, Northern Ireland Electricity and BT/Openreach to confirm location of amenities and discuss the design proposals. Further site checks were undertaken by service providers independently to obtain exact location of amenities. Amendments were then made to designs to avoid services, and where not possible, agreements with providers to accommodate services have been included in the designs, including alterations, diversions, bridging details such as king posts, and additional ducting to allow for futureproofing and maintenance.
Modelling and Scheme Design
To gain knowledge of the flooding mechanisms affecting the Portadown area, Amey Consulting, in conjunction with Waterco undertook extensive modelling of the catchment area through the use of Infoworks Integrated Catchment Modelling (ICM). The initial modelling used in the 2018 feasibility study considered flooding in a 1% AEP event. The new standard at the time the report was received by Amey Consulting required that an additional 20% was to be included to allow for the effects of climate change. Therefore, the study area was modelled under the conditions of a 1% AEP+CC storm event plus an additional 600mm of freeboard. This resulted in an increase from circa 6.5km to 9.5km of flood defences across the catchment area. Figure 7 below showcases the output produced by the model of current flooding conditions.
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Figure 7 - Existing Flood Model, 1% AEP+CC |
As a result of the simulation, Amey have developed appropriate flood alleviation solutions after consideration of the ground conditions, environmental constraints, existing land use and aesthetics. It was quickly identified that majority of the 21no. sites adversely affected by the current flooding mechanisms were of residential and commercial use, with a total of 380no. properties affected, with limited space available and potential for high footfall within the areas. The proposals put forth culminated an approximate 9.5km of defences to protect the 380no. properties consisting of various defence types as outlined below:
- Steel Sheet Pile Walls
- Secant Pile Walls
- Flood Embankments
- Flood gates
- Localised Landscaping
- Culverting of Existing Open Watercourse
- Flap Valves on Legacy Culverts
The proposed height of flood defences ranges from 0.65m – 3.00m across all 21no. sites.
Following the determination of the appropriate heights required at each site, the flood model was re-run accommodating the proposed solutions. Figure 8 below showcases the output produced by the model of flood conditions once measures have been installed.
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Figure 8 - Revised Flood Model, Including Proposed Defences, 1% AEP+CC
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In order to enhance available recreational space, Amey Consulting are considering the implementation of proposed flood alleviation measures into the surrounding landscape through the use of cladded defences or blended landscaped solutions.3D rendered visuals are being produced to gain an appreciation for how the proposed measures will fit into the surrounding landscape. Figures 9 & 11 below highlights the proposed flood wall at the Bowling Green site and Castle Street located within the centre of the town. See also Figure 10 which shows a raised footpath at the Parkside site, within the People’s Park, a popular walking area within the town.
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Figure 9 – Before and After: Bowling Green 3D Render of Proposed Clad Sheet Pile Wall |
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10 – Before and After: Parkside 3D render of Proposed Raised Footpath |
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Figure 11 – Before and After: Castle Street 3D render |
Economic Modelling
As part of the PQP process an economic assessment was undertaken to determine the overall Cost to Benefit Ration (BCR). As part of this process an overall scheme cost estimate was produced detailing the project risk and optimism bias allowances. The economic modelling programme and Cost Benefit Analysis (COBA) was used to evaluate the proposal and assist in the overall decision-making process.
Business Case
A Business case addendum was drafted as an extension to the 2018 Portadown Economic Appraisal Report completed, providing clarity and explanation for the increase in expenditure, due to inflation, change in construction techniques, increase in sites, and addition of climate change to hydraulic modelling, all required to implement the Preferred Option, when compared to the recommendations in 2018 Portadown Economic Appraisal Report. The Addendum provides clarity on the damage assessment and associated benefit calculations for the scheme and how the effects of climate change have been included in the damage assessment. This was developed under the “Better Business Cases NI” guidance and framework using the Five Case Model 5 Cases Guidance to establish the Case for Change using the most recent modelling and damage assessment, including economic benefits of raising the SoP to 1% AEP + 20% CC, in line with Departmental Guidance.
The Business Case Addendum is currently being drafted and will be submitted to the Department of Finance (DoF) for final approval of the required budget.
Benefits
The successful Implementation of soft/hard flood defences in the 21no. identified sites within Portadown, Co. Armagh will reduce the risk of fluvial flooding to the identified areas at risk through the use of a safe, reliable, and sustainable design. In return reducing the potential of future property damage of the 380no. residential and commercial property owners to a standard level of protection (1% AEP).
Our solution is informed by careful, cutting-edge integrated catchment modelling and remains robust to a 1% AEP +CC standard. Sensitive to the local environment, it also seeks to increase and protect recreational space to the benefit of the local population. Blended landscape solutions and cladded defences will minimise impact on the existing landscape.
Currently in a consultation phase, procurement is scheduled for Winter 2023/Spring 2024 with construction set to begin winter 2024/25 subject to funding.
Imagery
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