The £35 million scheme protecting Cardiff’s coastline from the impacts of climate change

29 August 2025

Lessons from an in-person event co-hosted by CIWEM’s Welsh Branch and the Institution of Civil Engineers

Coastal flooding and erosion present significant challenges worldwide, and Cardiff is no exception. Situated on the west bank of the Severn Estuary, the Cardiff coastline is perpetually shaped by a range of coastal forces including erosion from wind and waves and rising sea levels.

At a joint in-person event hosted by the CIWEM Welsh Branch and the Institution of Civil Engineers (ICE) at the Cardiff University Trevithick Building, Anne-Marie Moon (JBA) and Jonathan King (JBA) presented on the Cardiff Flood and Coastal Erosion Risk Management Scheme. This is a Cardiff City Council scheme funded though the Welsh Government’s Coastal Risk Management Programme. It aims to improve the protection along the southeastern Cardiff coastline along the Rhymney River estuary, in line with a ‘hold the line’ policy of safeguarding vulnerable residential areas, industries and infrastructure.

The Rhymney River, known for its dynamic nature, is extremely active, trying to reestablish its original meandering course following historical modifications for highways and railway design. Much of this coastline is rapidly eroding and previously reclaimed land, and landfill material is being lost into the Severn Estuary. Although existing defences are in place, hydraulic modelling outlined how these were failing or too low, expected to fail in the short-term and predicted to be overtopped due to climate change-induced sea level rise.

This vulnerability makes it a high priority area for protection by Cardiff City Council, with the following at risk of present day and/or future coastal flooding: 2,326 residential properties, 204 non-residential properties, a Traveller site, essential industries and highways infrastructure.

Further complications were identified during the project due to buried utilities along Rover Way, a solar farm cable beneath the river, and both national and international designations along the coast (including being a site of special scientific interest (SSSI), special area of conservation (SAC), special protection area (SPA) and Ramsar site). These factors influenced the scheme’s design, requiring careful consideration and multiple revisions to address these challenges whilst ensuring effective flood protection.

Striking the balance between development and environment

Over a seven-year period, from 2016 to 2023, the scheme involved detailed asset inspections, hydraulic modelling of baseline and future scenarios, and detailed planning and consenting applications. This included an environmental impact assessment, habitat regulations assessment (HRA), marine licence and flood risk activity permit.

Due to the outcome of the HRA concluding adverse impacts, the project was given ‘imperative reasons of overriding public importance (IROPI) status, justifying the requirement for the scheme and ensuring sufficient compensatory measures to mitigate potential environmental degradation. This process is rarely invoked – the fact that it was here highlights the fine balance between development needs and environmental conservation.

The final design, which takes climate change into account, provides protection against extreme events with a chance of occurring as low as just 0.5 per cent in any one year, with the protection having a design lifetime of 100 years. To the west of the Rhymney River at Rover Way, the scheme proposes putting in rock armour revetments (large boulders piled together against the coastline) and the raising of existing embankments – construction of these is currently underway. These adaptations work with the unstable ground conditions and existing eroded material whilst avoiding disruptions to important highways infrastructure on Rover Way.

Along the west bank of the Rhymney River, a combination of embankment raising, sheet piling walls (long, vertical sheets that are driven into the ground side-by-side to form a continuous barrier), and hard revetments are proposed. Due to the instability of the existing riverbanks and the strong forces the water exerts on the riverbed and banks during flood events, the designs strategically placed a rock amour revetment in front of the sheet pile walls, reducing the risk of bank slips.

Natural flood management (NFM) was also integrated into this element of the design, with mud added to the hard revetments to encourage the enhancement and rehabilitation of intertidal habitats.

Sustainability from the start

A standout feature throughout the presentation was the scheme’s commitment to sustainability, particularly through the integration of local materials in all feasible aspects of the design. The rock armour revetments use rock locally sourced from South Wales quarries. Along the coastline, the rock armour revetments required an underlayer, and so the suitable existing eroded material from the beach was re-used for this purpose.

Further east, the designs of the embankment in front of the sheet pile wall have incorporated mud sourced locally from the Severn Estuary, providing a regenerative and sustainable method of creating the additional intertidal habitats. This approach both enhances ecological value and demonstrates a dedication to environmentally responsible design.

Though the Cardiff Coastal Defence Scheme has progressed significantly, it remains ongoing. This approach was intentional: as the northernmost extent of the scheme isn’t at risk until 50 years' time, the decision was taken to phase the construction works. Within the next 50 years, additional defences may be necessary to provide protection from rising sea levels and coastal erosion.

Continuous monitoring of coastal processes and defence integrity and performance was emphasised as an essential component of the project to assess the scheme’s success in both the early stages and post-completion.

Deepened understanding

As an early career professional and graduate CIWEM member, attending the event was an invaluable experience. Having researched coastal defences and NFM for my master’s dissertation, I found that learning about this large-scale scheme deepened my understanding of the numerous stages and different components vital to successful project delivery. It offered a fantastic opportunity to network with a diverse group of professionals, spanning both CIWEM and ICE backgrounds, as well as other early career professionals and experienced industry leaders.

I am excited to apply the skills I gained from this event to future coastal defence projects that I am involved in and look forward to seeing how the scheme progresses in the future.

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Natalie Hacker is a graduate engineer at Arcadis