Why is the Tawe Full of Bubbles? 🫧

Good morning, Swansea!

If you've ever peered down over the Sail Bridge or wandered along the river near SA1 on a still morning, you might have noticed areas of the water doing something weird. Beneath the calm surface of the Tawe, there's a gentle fizz, like a giant glass of tonic water.

It isn't a trick of the light - Swansea's river is literally being bubbled. And the reason why takes us back to the early 1990s, when a bold engineering project changed the shape of the city… but also gave it a problem no one quite expected.

Catch you on Sunday!

The Best Laid Plans

The reason for the bubbles is all down to the Tawe Barrage at the edge of the marina. Before the barrage, the lower Tawe was tidal. At low tide, mudflats stretched out, wrecks lay exposed, and the river looked more like a builder's skip than a marina. Not exactly the image Swansea wanted for its shiny new waterfront development.

So, in 1992, the council dropped £18 million on the Tawe Barrage - a concrete wall to seal off the mouth of the river. The plan was simple (in theory): hold the water at a steady level, make the marina look permanently gorgeous, and turn the old docklands into somewhere people might actually want to live.

And it worked brilliantly. At least for about two years.

When Good Plans Go Bad

By 1994, water quality experts were having quiet nervous breakdowns because salt water was flowing over the top barrage at high tide. Instead of mixing with the fresh water, the denser salt water was sinking to the bottom of the river and staying there like an oxygen-sucking blanket - endangering the life of fish and other wildlife.

"The design of Swansea Barrage is probably itself to blame," admitted National Rivers Authority officials. "Fresh and sea water do not mix… so the problem will not go away on its own."

The headlines started getting brutal. "Stink Crisis at Barrage" screamed the Evening Post in June 1994, warning that the project could cause thousands of fish to die if oxygen levels in the river were not fixed - they had already dropped to dangerous levels on a number of occasions.

Panic Stations

Emergency measures kicked in - sluicing, partial draining, even pumps - anything to keep the fish from floating belly-up in front of the tourists. Eddie Ramsden, Swansea's director of environmental health, was doing his best to sound optimistic: "Last week dissolved oxygen… read between 0.06 and 0.51 milligrams per litre. Now… it's 3.4 to 6 milligrams. No dead or distressed fish have been seen."

Which roughly translates to: "We've gone from catastrophically bad to merely terrible, and at least the fish aren't at imminent risk of suffocation."

The Lightbulb Moment

By the late 1990s, plans were in place to sort the problem once and for all. On 21 January 1999, the Evening Post reported, "Swansea Council admitted today that part of the £18 million Tawe Barrage is wrong. Ratepayers face a £250,000 bill to oxygenate the brackish water…"

The fix? Turn the Tawe into a kind of giant SodaStream. Engineers laid 80 diffusers along several kilometres of the riverbed - like oversized air stones in a fish tank. Compressed air is pumped through them, sending steady streams of bubbles up to the surface.

Those bubbles churn the water as they rise, blending the heavy saltwater trapped at the bottom with the fresher layer above. That mixing restores oxygen to the river and helps flush the stale saltwater back out to sea.

In short, it mimics the natural ebb and flow that the barrage had cut off - keeping fish alive, the water fresh - and Swansea’s river permanently fizzing.

The system worked so well that when Cardiff Bay's new barrage hit exactly the same problem, the same system was installed there too.

The Fizz Continues

More than two decades later, the aeration system is still hard at work. It might look odd to visitors who wonder why our river appears to be gently carbonated, but those bubbles are what keep the Tawe’s wildlife thriving.

So next time you see the Tawe doing its impression of a massive glass of Perrier, give it a nod of appreciation. It's not every city that can claim their river runs on bubble power.

Catch you on Sunday!

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Sources

• History Points: The Tawe Barrage, Swansea

• South Wales Evening Post — “Stink crisis at barrage” (14 June 1994)

• South Wales Evening Post — “Concerns on barrage over troubled waters” (20 June 1994)

• South Wales Evening Post — “Barrage rain heaven sent” (22 June 1994)

• South Wales Evening Post — “Oxygen level at barrage is out of danger” (16 June 1994)

• South Wales Evening Post — “Poison water fears remain” (1 June 1996)

• South Wales Evening Post — “Breath of air” (21 January 1999)

• BBC News, “Spring clean for river” (19 November 1999)

• Wikipedia: Swansea Barrage 

• Geograph: Construction of the Tawe Barrage

• Kayak Crew: Tawe River Kayak Ynys Tawe to Swansea

• ISS Flowthrough: Experts in Diffuser Based Aeration for Guaranteed Results