Photos: Jake Chapman
I’m stood in the middle of the Portway on a cold March morning, taking pictures of the flooded concrete canopy, beneath the Clifton suspension bridge, in the hailing rain. “I’m going to be late for work. The Portway’s flooded,” a group message from my colleague that morning read. It was my day off, so I cycled over, past the commuters gawping at the water lapping at the bumpers of the cars in the Cumberland Basin, to see it for myself.
The night before, when a high spring tide hit the 8.6 metre mark in the harbour, 2.56 metres above its usual range, water had gushed over the river wall, flooding the road. It flooded again on the next high tide, the following morning. With sewers and drains overflowing since February, when the rainfall was 257% greater than the month’s 30-year average, the water simply had nowhere to go.
I watched as the tide ebbed, and the Portway slowly drained. For me, it’s the first time I’d seen flooding in Bristol; for many, it’s becoming a much too familiar sight.
Today, 27,000 homes and businesses in Bristol are at risk of tidal and surface water flooding (water unable to flow into the ground or the sewer system, remaining on the surface), and as the effects of climate change become more severe, so too will Bristol’s flooding. Patrick Goodey is the Flood Risk Manager for the council’s Local Flood Risk Management Authority (LFRMA). He says that his team has to look “100 years in the future and manage that flood risk accordingly, so it’s important that we sort a strategy as soon as we can to ensure the growth of the city can continue whilst managing those risks of flooding.”
The LFRMA’s Local Flood Risk Management Strategy, released in February 2018 and due to be revised later this year, sets out Bristol’s game plan for mitigating the impact of tidal and surface water flooding.
Fluvial (river) flooding is also mentioned; however, it’s been far less of a concern than tidal and surface water since Bristol built its last large-scale engineering solution to flooding. In July of 1968, 13 centimetres of rain fell during a 12-hour period, flooding 800 properties, washing or severely damaging many bridges, and killing seven people. To prevent such a catastrophe from happening again, three large tunnels were bored to intercept, divert, and discharge flood waters, from rivers like the Frome and the Malago, into the River Avon, massively reducing the stress on sewers and drains after heavy rainfall. The interceptors have succeeded in protecting Bristol for over 50 years, though their performance will dwindle under the predicted impacts of climate change, as they become increasingly flood-stressed.
Today, 1,000 properties in the city centre, or more specifically, the low-lying parts of Bristol around the Floating Harbour, are at risk of tidal flooding. The LFRMA’s strategy predicts that, due to sea level rise, that number could rise to a total of 3,700 existing properties at risk by 2115 (the MetOffice predicts that Cardiff’s sea level will rise up to 1.13m by 2100), costing Bristol an estimated £1.6 billion in damages and a £4 billion loss to the local economy.
The remaining 26,000 properties are endangered by surface water flooding, making Bristol one of the UK’s top ten Flood Risk Areas for surface water flooding. Dundry Hills, Ashton, Southmead, Bamfield Road are the worst affected areas. The frequency of this type of flooding is set to increase dramatically over the next 50 years: in a high-emission scenario, the MetOffice forecasts that winters will be up to 35% wetter, and even though they could be up to 47% drier by 2070, summers will experience more short-lived, high-intensity showers- the kind that cause flooding.
We need natural solutions to Bristol’s flooding woes
In 2013, the council and the Environment Agency conducted a study on tidal and fluvial flood risk assessments in Bristol, called the Central Area Flood Risk Assessment (CAFRA). Short- and medium-term solutions to tidal flooding included heightening the ‘low points’ around the Floating Harbour as well as constructing flood defence walls, like the Cumberland Road flood wall; however, in the long-term, the study found that the city’s best solution will be a rising tidal barrier, similar to London’s Thames Barrier.
A rising tidal barrier would not only protect against high spring tides but also against storm surges – when stormy weather out at sea ‘pushes’ a tide further inland than usual – a phenomenon likely to cause more severe flooding as sea levels rise. Bristol experienced a storm surge of 80 cm in 2012: a small rise, yet enough to cause minor flooding along the Avon. That being said, Goodey tells me that Bristol is still a long way off getting a rising tidal barrier. “Sea level rise isn’t going to happen as a sudden thing,” he says. “It’s going to be quite gradual, so it’s making sure that we have a robust plan to mitigate that.”
Tidal flooding is confined to a relatively small area of the city and can be controlled by managing one source – the Avon. Surface water, on the other hand, is much, much more difficult to master. Surface water affects large, disparate areas of the city, and its source – clouds – is unmanageable. Reducing the risk of surface water flooding thus requires much more intricate, elaborate solutions than simply erecting walls or building barriers.
One of those solutions is sustainable urban drainage, such as green roofs, a key environmental aim of Mayor Marvin Rees’ One City Plan for 2050: “Sustainable urban drainage will span the city and reduce likelihood of localised flooding during wet weather,” it reads. It’s a goal that harmonises with the LFRMA’s future approach, too.
“We will be looking to revise our local flood strategy to follow in the ethos of the One City Plan but also the Environment Agency’s National Strategy, in that it will have a lot more of a resilient, green infrastructure, nature-based solutions approach” says Goodey.
‘Urban creep’, the replacement of water-absorbing, natural spaces with impermeable, man-made infrastructure, is a significant adverse factor in Bristol’s risk of surface water flooding. Sustainable urban drainage will not only reduce the risk of surface water flooding but also provide other benefits, like improving the water quality of Bristol’s rivers, waterways and harbour, as well as increasing the city’s biodiversity. These just happen to be two other goals set out by The One City Plan for 2050.
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“Green infrastructure and sustainable drainage tick so many boxes that it’s more a case that schemes and developments should be thinking of them anyway, and building them into their projects and programmes because it’s the policy direction that the city is taking at the moment,” Goodey tells me. It’s a win-win for the city, though Goodey admits that influencing developers can be a bit of a ‘carrot and stick’ operation.
While Bristol can be shielded from tidal flooding by artificial walls and barriers, it’s a diverse range of natural, nature-based solutions the city needs to alleviate its risk of surface water flooding. If Bristol wants to protect its growth and economy for the next 100 years, as well as provide cleaner water, increased wildlife, cleaner air, and improved health and well-being, it’s a method that must be adopted by developers, whether they like it or not.