When the tide's right a considerable difference in water levels can be seen at Bow Locks. On the left is the Lea Navigation, essentially a canal where the water tends to remain at the same height. On the right is Bow Creek, a tidal waterway with a considerable vertical range. This creek empties out twice a day, as we saw in my photos earlier this week, then at high tide rises much closer to the rim of the channel. At the highest spring tides (as blogged here) the water in the creek rises to 4.8m above sea level and can briefly overtop the path. But because tides are predictable the riverbank has been engineered to cope with these occasional maxima, and only when heavy rain and wind conspire is wider flooding a serious risk.
It's much the same on the Thames in central London. Water levels vary from "barely enough water for a pleasureboat" to "ominously lapping the top of the wall", with a potential 7.8m range inbetween. These regular tidal cycles are totally familiar and already built-in, so the worst that normally happens is the slight overtopping of the Custom House walkway downstream of London Bridge. It takes a rush of floodwater coming down the river or a tidal surge coming up to disturb the equilibrium, and only very rarely do bad weather and spring tides coincide to devastating effect. The last fatal inundation in central London was in January 1928, caused when snow melt, heavy rainfall and a storm surge coincided with a spring tide, with February 1953's infamous surge narrowly failing to overtop the river wall.
Central London is now protected by the Thames Barrier and has been since the 1980s. Its gates have been closed for flood defence more than 200 times, most recently on 7th November, as well as countless times as a rehearsal. This ever-ready sentinel provides reassurance for those of us living upstream and should prevent long-term catastrophic damage to basements, tunnels and services that weren't even present in 1928. But global warming will eventually make the barrier less reliably effective, and by 2070 maybe even obsolete. Even a small rise in sea level is expected to cause an increase in closures that'd make the Thames navigationally unreliable, and we can't keep it closed forever.
It's instructive to see what the City of London is doing in response. Earlier this year they published a Riverside Strategy with an eye to the year 2100, having recognised that some of the most valuable infrastructure on the planet needs to be better protected. The plan is to shore up their river defences to withstand the worst the 21st century might deliver by raising walls by up to one metre while still retaining an "outstanding riverside environment". The City has almost bottomless pockets when it comes to flood risk, as they recently proved by restructuring Hampstead Heath's ponds in case of highly unlikely catastrophic dam failure.
At present the Environment Agency requires that riverside flood defences are at least 5.28m above sea level downstream of London Bridge and 5.41m upstream. The City of London's mile of waterfront currently meets these requirements, helping to keep even the highest tide at bay, but they recognise this won't always be the case. Their plan is to raise flood defences to 5.85m by 2065 and to 6.35m by 2100, which is quite a hike. Only two brief stretches currently meet the 2100 target, one at Billingsgate and the other under Southwark Bridge, and the remainder needs remediation before the end of the century, either in two phases or all in one go.
For example the section of river wall between Blackfriars and the Millennium Bridge requires an uplift of 15cm by 2065 and maybe 65cm by 2100. An even greater intervention is required on the Victoria Embankment west of Blackfriars Bridge, potentially an extra metre by the end of the century. An appropriate response could be as simple as raising the top of the wall but the City also wants views of the river to be retained so it might also require raising the pavement. They intend to act in "a pragmatic and timely way", hence the need to start planning early so that any future building works along the riverside bear long term targets in mind. The project's summary webpage has artists' impressions and slidy maps and is well worth a read.
It's not clear what happens halfway along the Embankment where the City merges into Westminster, only that other boroughs will have to act eventually or face increasing risk of inundation. It's one thing hoping a once in a lifetime flood won't come upriver, but quite another to cope with inexorably higher tides twice a day. Scientists estimate that sea level could rise anywhere between 30cm and 100cm by the end of the century, maybe even more, and best not imagine the mess that would make of central London without additional protection. One melted ice sheet (or other tipping point calamity) and the Underground could become the Underwater.
Humanity's been fortunate to evolve through a period of long term sea levelstability. Shorelines have been pretty much consistent over the last 6000 years, a rise only starting to accelerate with the dawn of the Industrial Revolution. But a considerable amount of land's been lost to the sea before, for example when the land bridge between Britain and continental Europe was submerged about 10000 years ago, and a lot more will be lost in the future as temperatures rise. One day, long after we're all dead, the inexorable advance of the waves will smother countless coastal settlements and relegate central London to the sea bed. Until then it's all about how good your defences are, as the City well knows, so best start planning now.
