Plans to build 200,000 homes in East London over the next 20 years should be music to the ears of anyone struggling to find a place to live in our overcrowded capital.

But unless planners are willing to spend billions on new flood defences it is possible that these new homes will be particularly at risk of flooding should rainfall levels increase with climate change.

Many of the developments mooted in London Mayor Boris Johnson’s plan for a City in the East are situated on flood plain, according to the Environment Agency.

On reading through the blueprint flood risk isn’t mentioned once.

As a minimum, the EA estimates that the cost of maintaining the defences until 2035 will be around £1.5 billion, with an additional £1.8 billion needed to repair and upgrade the defences until 2050. Particular bits of marshland could also be set aside to store tide waters.

More ambitiously, the government could fund a new barrier in either Tilbury in Essex or Long Reach in Kent. Such a barrier would be designed to resist the highest surge tides identified by the Met Office’s analysis of how conditions will change this century. The EA estimates a new barrier could cost as much as £7 billion, though that figure could go up if conditions change significantly as the climate changes.

The Agency also say the existing barrier could be converted to include locks, which could open and close more flexibly and extend the life of the defences. The EA report says a decision on a new barrier will have to be made by 2050.

It may turn out that there’s a more pressing reason to spend. The EA says 50 is the maximum number of times the barrier should close each year, and beyond that, the barrier could start to fail.

How high is the likelihood of flooding in this part of London? A look back through history shows the geography of the Lea Valley has made the area prone to inundations for hundreds of years.

In January 1809 the lower River Lea burst its banks in several places following a deluge that dumped two inches of rain in the space of 24 hours. The rain abruptly ended a snowy cold spell that had begun over a month before in the middle of December. From Luke Howard’s readings it is likely that up to half a metre of snow had fallen in the previous weeks in the upper parts of the surrounding countryside with only slight thawing. With the frozen ground unable to absorb any of the rapidly melting snow and rainfall the amount of water flowing downstream must have been immense. Howard takes up the story…

“The River Lea continued rising the whole of the 26th… The various channels by which it intersects this part of the country were united in one current above a mile in width which flowed with great impetuosity and did much damage.”

Howard, his chemical factory located on the banks of the river Lea close to what is now Bow flyover, wrote at length about the event, his account replacing the usual brief notes about daily weather in his book The Climate of London.

He talks of embanked pasture land being “filled to the depth of eight or nine feet” and people driven to their upper rooms relieved by boats plying under the windows.

“The Thames was so full during this time that no tide was perceptible.” It took until February 23rd for things to return to normal.

Miraculously no lives were lost in the flood and cattle “by great exertions” were saved by being kept in their stalls. Howard, saying that the flood could have been far worse, believed a neap tide, strong westerly winds urging water down the Thames and mild weather helped avert a tragedy.

Howard’s statistics of the previous months show that the second half of 1808 were wetter than average – though not especially so. And nothing like the rainfall this area recorded during the winter of 2013/14. From December 1st until February 8th 1809 Howard recorded 130mm of rain, which is about 100mm less than what was recorded during 2013/14.

This fact alone shows just how much the nature of the river has changed in the last 200-odd years thanks to spending on flood defences.

Work to improve the defences was prompted 67 years ago when similar catastrophic flooding came with the thaw that ended the severe winter of 1947, one of the coldest winters in history and an episode remarkably similar to what Howard recorded in 1809.

The Lea Valley, along with many parts of the country, saw some of its worst flooding in a generation. The river burst its banks at several points bringing misery to surrounding communities. Valleys turned into lakes in 40 counties and East Anglia’s fens were a sandbagged inland sea. More than 100,000 properties were damaged and, then as now, heroic battles were fought by the military to keep water-pumping plants and power stations dry.

A marker of that flood, together with a history of flood defences, can be found here. There is also British Pathe footage of another flood between 1910 and 1919 here .

The River Lea Flood Relief Channel, that flows between Ware, Herts, and Stratford, took almost three decades to complete. The channel incorporates existing watercourses, lakes and new channels. Since it was completed in 1976, there have been no major flood events in the Lea Valley, although there have been three occasions when the river system was full virtually to its capacity: in 1987, 1993 and 2000. Since its completion, the level of protection afforded by the structure has declined, so that in some areas it offers 2 per cent protection, and in some, only 5 per cent protection. The EA published a strategic environmental assessment in 2008, which looked at ways to maintain the flood defences in the Lea Valley.

In summing up its ambitions for a new development the Greater London Authority says it is trying to “overturn the historic perception of the east being seen as apart from London, rather than as a part of London”.

Jim Ward, a director of research at property consultancy Savills, said the challenge was “building something that makes people want to live and work there”, he said. “You need to build more than homes, you need to build a place.”

Boris’s record of late has not been good: an estuary airport to replace Heathrow has been dismissed as pie in the sky while his promise for a 24-hour Tube was made without first consulting unions that it was possible.

London’s safety from flooding is an altogether different proposition, both for old and new developments. Boris really needs to get London’s flood defences right before any brick is laid on these desperately-needed new developments.