24 November 2017

Snow-Vember! Could the Thames Be About to Freeze Over?

The predictions are being made because the central Pacific Ocean has shifted to a “La Niña” condition, affecting weather around the world. La Niña increases the chances of a “blocking pattern” in the neighbourhood of the UK: an area of high pressure which sits still and – in winter – makes the weather colder.

In previous very cold winters the Thames has actually frozen over, preventing water traffic from moving. During the twentieth century this would not have had much effect, as hardly anybody travelled by river then: but nowadays millions of people once again make journeys on the Thames. A new River Bus pier has just opened at Battersea Power Station, in fact.

Could the Thames be about to freeze over, as it did fairly regularly in the past? Some of the previous freezes produced ice so thick and long-lasting that Frost Fairs were held on the river. These were almost temporary cities on the ice with tents, shops, bars, large fires over which whole oxen were roasted, casinos complete with early roulette tables, dancing, music, skittles and many other amusements. The ice was thick enough during the Frost Fair of 1814 that an elephant was able to walk across the river alongside Blackfriars Bridge. Printing presses operated on the ice, producing mementoes for visitors.

The Frost Fairs were largely run by those whose business was dependent on the river, which in those days was the city’s main transport artery. Just as the London watermen and lightermen – the taxi and van drivers of their time – would charge for river travel in normal times, they also charged people to use the Thames when it was frozen. Watermen would set up planks at the river’s edge by which people could walk on or off the ice, and collect a small fee for using them. If there were no obstacles to be overcome they might create some, making trenches in the ice and snow and putting their temporary toll-bridges over these. Others temporarily deprived of their livelihood might set up trading stalls and tents on the ice, sometimes using their boats, oars and sails as building materials.

The effects went far beyond the professional boatmen: as nearly everything then was shipped by water, business and trade of all sorts was paralysed by the ice and prices soared. Many Londoners would have struggled to afford heating fuel or even food to eat at such times. At this distance in history the Frost Fairs present a picturesque spectacle, but for many they represented a fairly desperate attempt to stay alive through the economic damage and hardship caused by the ice. Running an enterprise on the frozen river was a dangerous undertaking: many people, stalls and booths vanished during Frost Fairs over the centuries.

Humans made the Thames ice-free

The Frost Fair of 1814 was the last one, and the river has not frozen over in central London since then. Many researchers would suggest that this is due to the ending of the “Little Ice Age” of especially cold weather which seems to have occurred over some hundreds of years prior to the 19th century. There is much argument over the exact nature and causes of the Little Ice Age, as this affects the underpinnings of today’s climate-change debate. Fortunately there’s no need to get involved with that here: in the case of the Thames it seems safe to say that non-climate factors were much more important in making the river ice-free today. Humans definitely stopped the Thames icing over – they just didn’t do it by causing carbon emissions.

What humans did do, in the year 1831, was replace the 600-year-old medieval London Bridge, whose narrow arches and obstructive foundations had been a major cause of the river icing over. Slabs of ice forming in the relatively slow-flowing river would get jammed easily against the broad piers of the old bridge, more would then pile up against them, and so a solid raft of ice would quickly build up.

The historical record confirms this picture, as the ice of the Frost Fairs was not a smooth sheet of the sort you might find on a naturally frozen river or pond. According to contemporary accounts the ice presented “a rude and terrible appearance”; “as if there had been a violent storm and it had froze the waves just as they were beating against one another”.

It wasn’t waves that had been frozen in position, but slabs and floes of ice, grinding together in the river’s tidal flow, held back by the narrow arches of the old London Bridge. This is also reflected in many old pictures of the Frost Fairs, these being probably the ones executed by artists who had actually witnessed the scenes they were depicting.

The old London Bridge is also thought to have had the effect of holding the flood tide and its salty sea water back, making the river upstream fresher and thus more easily frozen. This too is very credible, as a five foot drop in water level from one side of the bridge to the other was reported when the tide was flowing fast.

The replacement London Bridge of 1831 with its much wider arches caused none of these problems and it had the added benefit of being much better and wider, able to carry much more traffic. It was also nice to look at: its designer, John Rennie, was famous for his bridges. When this new London Bridge had to be replaced due to subsidence in the 1970s its exterior stonework was actually purchased intact and shipped to Arizona, where it was reassembled onto a specially made concrete structure and so came into service once again.

The removal of the medieval London Bridge undoubtedly had a major effect on the river, but humans did more to make the Thames ice-free, again without having this as their main intention. As London’s population grew in the mid-19th century, the river Thames gradually became a giant open sewer. Cholera outbreaks were rife, and the smell was awful. Following the ‘Great Stink’ of 1858, the famous engineer Joseph Bazalgette was assigned to clean up the river. To do so he narrowed it significantly, building the Victoria, Albert and Chelsea embankments to carry the new low-level riverside sewers which captured the effluent flowing into the Thames. The river now flowed faster through its narrower channel, and it became still more difficult for ice to form.

There were still very cold winters, regardless of the apparent ending of the Little Ice Age in the 19th century, but unsurprisingly there were no more Frost Fairs. Even during the terribly severe winter of 1895, in which temperatures reached -20°C, the Thames in central London did not ice over completely, although large ice floes did form and river traffic was badly impeded. Other severe winters since then, including those of 1947 and 1963, have seen parts of the Thames freeze over completely to the point where people could walk or ride bicycles on the ice, but this has always occurred well upriver, out of town. The London river has remained ice free.

Today, then, while it would be unwise to assert that the Thames will absolutely never again freeze over in central London, it is safe to say that it’s extremely unlikely in the foreseeable future – no matter what happens to the climate. New infrastructure, in this case Rennie’s London Bridge and Bazalgette’s embankments, has had the effect of making the Thames a more reliable transport route: which is good news for the millions of Londoners and visitors to the city who are nowadays flocking back onto the river. It’s good news for people going to and from Battersea and Chelsea, as they can keep using the new Battersea Power Station pier no matter how cold the weather may get.

New infrastructure continues to appear today, and it’s not limited to river transport. As Battersea Power Station is transformed from an empty industrial site into a new town centre it will bring with it a new Underground station – the tunnels linking this to the existing network have just been completed. London’s growing population has finally overwhelmed even Bazalgette’s mighty sewers, and Battersea Power Station is one of the key sites for construction of the new Thames Tideway super-sewer which will solve this problem. The Power Station will also be home to a small but efficient power station and a large heat store, able to supply clean affordable energy not only to the development itself but also the surrounding neighbourhood.

Battersea Power Station, like the ice-free Thames and the millions of Londoners who use it, benefits from the work of Rennie, Bazalgette and other great builders of the past. The Power Station project is also creating infrastructure that will benefit the city around it – and provide underpinnings for the builders of the future in its turn.