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Outline
Cardiff is the local authority area facing the highest risk of floods in the UK with around 33,000 properties predicted to be threatened by 2050. This means that 17% of properties will be at risk, a rise from 15% in 2020.
A project (Greener Grangetown) was undertaken in 2018 to increase the resilience of the sewer network in the Grangetown area of the city. The drainage network is a combined foul and surface water sewer, resulting in large quantities of rainwater being pumped for treatment in times of heavy rainfall. Through the creation of ‘rain gardens’, around 40,000m³ of surface water has been removed from the public sewer network annually. This is important in that it maintains sufficient headroom and protects the system’s long-term resilience.
This approach and others have been extended to other areas of the city, including the area around the Cardiff City Centre transport interchange and Central Square development. Located in Cardiff city centre, Wood Street was characterised with high vehicular traffic and narrow walkways with entirely impermeable surfaces. As part of the redevelopment of the area, cycleways and wide pavement areas were installed with the route for vehicle traffic reduced. A series of rainwater gardens were installed in the area as part of the traffic changes and to provide greenery and biodiversity as part of a multi-functional space.
Combined, the system removes 6 – 8,000 square meters of water runoff from the area, with the water cleaned before draining into the River Taff. This also increases the resilience of the drainage system in the city and reduces the cost of pumping combined sewage, containing largely rainwater, for treatment.
Analysis of climate hazards
Climate change predictions for the city were used to analyse the hazards. Whilst Cardiff is a coastal city, with potential future threats from sea level rise and storm surges, river flooding is predicted to be the main risk, with the River Taff and River Ely flowing through the city. The greater frequency and intensity of (particularly winter) rainfall puts pressure on the drainage systems and the rivers’ capacity to carry excess water. The River Taff burst its banks in January 2023, flooding sports pitches, but causing no damage. In 2020, Storm Denis caused the Taff to flood in parts of Cardiff and the surrounding areas, causing widespread damage.
Resilience assessment
Resilience was considered by assessing the current climate change predictions and by installing a system that removed surface water from the foul sewer. It was also informed by the performance of the previous scheme. The resilience of the system itself is based on using a highly permeable soil mix, capable of draining 400mm per hour. In suburban areas, a ratio of 1m2 of rain gardens to 10m2 of surface drainage is used, whereas in the city centre, this is reduced to 1m2 to 5m2 due to increased silt loading. Additionally, there is a stone silt trap placed on the inlet. The plants chosen for all gardens are slow-growing and toxin-tolerant.
How are adaptation solutions considered?
The Greener Grangetown scheme helped inform the design of the rain gardens in the city centre., where things that worked well were repeated and other aspects improved upon based on lessons learned.
It was noted that retrofit SUDS (Sustainable Urban Drainage Systems) is significantly different from SUDS installed as part of a development. There is a more highly engineered level beneath the surface, whilst the road and kerb design are different to how contractors normally install it. At a more general level, many of the schemes have been commissioned as part of wider schemes, for example, the Greener Grangetown team was primarily to reduce the pressure on the combined drainage system, the scheme around Wood Street and Central Square focussed on increasing active travel, whilst the uncovering of the Dock Feeder Canal on Churchill Way is part of redevelopment of the area and creation of a waterside seating and café area. What is common to all is that they increase biodiversity, slow down and clean the water flow to the drains and provide greenery in the city.
How is the adaptation strategy implemented in practice?
Cardiff Council endeavours to install retrofit SUDS, such as rain gardens, across the city where viable. This has been hampered by budget cuts and a lack of technical officers working for the council. Maintenance is undertaken around once per year in Spring, where dead plants are removed along with detritus and anything that could form compost. Litter picking in the city is undertaken year-round. Other practical considerations for new gardens are to plant in late autumn or early winter when the plants look at their worst, as this reduces the risk of theft of plants, whereas by spring, the root ball of the plants has spread and they are more secure. Planting young plants in spring could also require considerable watering to prevent die-off if there was a warm spell.
More generally, Wales is the only country in the world where there is a statutory requirement for SUDS, which supports the implementation by the 22 local authorities.
Lessons learned?
The Lessons learnt were that green infrastructure is required if we are going to make our roads more resilient and improve and solidify the operation of drainage systems as climate change takes hold.
Undertaking projects in the city centre can be particularly challenging, given the high numbers of vehicles and pedestrians, the high-profile nature of the work, and the involvement of multiple stakeholders, which requires effective communication.
In residential areas, it is critical to get the buy-in of the residents and to work with them. Where these schemes have been installed, there is widespread support and in some newbuild developments, the houses facing on to balancing ponds and rain gardens have been the ones to be sold first.
