Rivers have been forced to flow underground as cities have developed over time, but Sheffield is the latest to join a global movement to recover these lost waterways
Tucked away in a corner of Sheffield's cultural quarter, among the graffiti and red-brick housing blocks and a derelict industrial site, lies a green oasis. Porter Brook is a "pocket park" - a small amphitheatre, sloping down to the banks of the river Porter, where wild trout spawn in spring and students from the technical college picnic and paddle.
Two years ago, the pocket park did not exist. There was just a crumbling car park with the river Porter nowhere to be seen, save for a brief glimpse before it disappeared into a culvert.
"There's no green spaces at all in this part of Sheffield, so the pocket park has been a great success," says Katie Bayliss, a local resident. "Before, I don't think people realised there was a fully flowing river here. You could see a bit of water going into a tunnel, but it looked more like a drain."
Like many UK cities, Sheffield is built around a network of rivers that first attracted settlers about half a millennium ago. Its very name is derived from the old English Scafeld, meaning "a clearing by the river Sheaf", and the city's topography - the valley and the rolling hills of the surrounding Peak District - has been carved out by water over thousands of years.
Not that you would know it now; Sheffield may lie at the intersection of two rivers but its watery roots are largely invisible. The Porter, Sheaf and Don rivers spend most of their urban life underground.
"You'd be hard-pressed to find the Sheaf anywhere in the centre," says Simon Ogden, head of city regeneration at Sheffield city council. "As in many cities, Sheffield's rivers were culverted over in the late 19th century because the water quality was so poor - they were basically sewers."
But the success of uncovering Porter Brook is the first step in a new project to bring back Sheffield's lost urban rivers - to remove the culverts and integrate them into the city again. Known as "daylighting", it is part of a broader global movement towards rediscovering urban rivers in cities worldwide.
The poster child of all daylighting projects is Seoul's Cheonggyecheon stream, completed in 2005. A brainwave of then-mayor and future president Lee Myung-bak, the project removed roads, buildings and virtually anything in its path to create an artificial waterway that joined up with the underground river at a cost of $900m (£615m). The 3.6 mile-long water corridor now acts as a major flood-relief channel and draws more than 60,000 visitors each day, transforming an area of Seoul previously renowned for crime.
In Auckland, more than 180 metres of underground piping and 5,000 cubic metres of natural clay were removed to daylight the Fairburn and Parahiku reserve streams in 2014. It was part of an ambitious scheme to provide both better flood water protections and restore the rivers to a more natural state to support the many migratory fish species in New Zealand. "We have these seasonal species which need these pockets of upstream habitat to live in, and then they go back out to sea and spawn," says Tom Mansell, stormwater project engineer at Auckland council.
"When urban rivers are forced underground into piping, that disrupts their whole behavioural pattern. But by daylighting these rivers and creating a more natural ecology, it makes the rivers more desirable. You then find that people actually care about and want to look after the river a lot more," Mansell says.
More recently in the US, $19m was invested to daylight the Saw Mill river in downtown Yonkers, New York. The aim was to regenerate the area and bring back habitat for a range of species including muskrats and snapping turtles.
"The whole project created this massive green corridor through the city by digging up car parks and wasteland," says Adam Broadhead, who runs the research website daylighting.org.uk. "It didn't please everyone - the construction work caused a lot of disruption to local businesses - but the overall benefit to the economy since then has probably paid off."
When urban rivers are forced underground into piping, that disrupts their whole behavioural pattern
In Zurich, daylighting is actually enshrined in law. Known as the "Bachkonzept" or the "stream concept", urban river restoration has been common practice in Switzerland's largest city for 30 years. Urban rivers have been daylighted and integrated into Zurich in all manner of ways, such as complementing the local architecture.
"They really are the masters of it because they've been doing it for so long," Broadhead says. "They're very clever as a lot of the funding for the work comes from the fact that by restoring these rivers, they're keeping clean water out of the sewage streams, and so they save enough money in reducing their water treatment costs to justify it."
Such has been the success of global daylighting projects and the Porter Brook pocket park, that Sheffield council and several landowners are now hoping to create a series of mini urban parks along the same lines. Most ambitiously, they intend to open up part of the river Sheaf near the site of Sheffield's former medieval castle to create a waterway in the centre surrounded by parkland, flowers, trees and seating for the public. It is one of Britain's most ambitious regeneration projects.
But river daylighting isn't purely for aesthetic reasons. Climate change has been one of the major driving factors for large-scale investment in these projects. Planners hope to utilise the passive cooling provided by rivers to help combat the urban heat island effect, and most of all, offer benefits in terms of flood protection.
"We've been experiencing more flash floods in recent years," says Ogden from Sheffield council. "So we need to keep the water in the river and make more space for it. Culverts act as a kind of choke on the river, so any blockages or sudden increased rainfall forces the water out onto the streets."
Last year, Rochdale successfully restored the river Roch for the first time in 100 years by daylighting it through the main high street. The hope was to transform the town centre by creating a waterfront by the shopping district. While not everyone was convinced, the initiative was forced through by a desire to reduce flood risk.
"A lot of the local shop owners were initially unhappy about it because the high street was closed off for some time," says Penny Stevenson, a resident of Rochdale. "But it's really boosted the attractiveness of the town centre and probably ultimately helped businesses.
"Shortly after they finished opening the river up, we had another flood event - but whereas before the excess water didn't have anywhere to go because of the culvert, this time it was able to drain away naturally, and didn't swamp the town centre."
Daylighting can also be a more practical and cost-effective alternative to many of the UK's Victorian-era culverts that are difficult to maintain. Last year, in central Sheffield, a culvert over the river Porter collapsed, causing part of a car park to completely cave in. Although the council would now like to daylight that stretch of the river, these schemes are not always straightforward.
"I'd probably say that 99% of people are in favour of daylighting schemes," says Sheffield resident Martin McQuaid. "But the other 1% typically see it as a waste of usable land. It would be fantastic to daylight the river Porter in that spot, but to do that would probably require a large compensation package for the landowner of the site. It's the same with the scheme to daylight the Sheaf, the only people grumbling about it are the ones who usually hold a market on that site."
Indeed, not all urban river restoration projects proceed with efficiency. In Wood Green, north London, the river Moselle was daylighted in 2013, slowly meandering through parkland before joining the river Lea in the east. But the resulting stench of human waste revealed the newly restored river as something of a health hazard - misconnections with sewage channels from many of the nearby housing estates choked the river with sewage fungus, a problem that Thames Water and the Environment Agency are still trying to tackle.
"In some ways it's good that the project highlighted an existing problem which had been going on for years, contaminating the river below ground," Broadhead says. "But it's also an important lesson; if you're going to spend large amounts of money on daylighting a river, you need to check the water quality at the same time."