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TfL ‘needs to reassess’ flood risk after stations left underwater

Transport for London (TfL) has been urged to reassess the viability of its flood measures after weekend storms left two of its stations underwater.

Video footage shared on social media showed water from heavy downpours cutting off access at Hackney Wick station and at Pudding Mill Lane station, which is just south of the 2012 Olympic Park. Both stations are situated in the Lee Valley catchment area which saw heavy rains hit on Sunday afternoon.

Barking in East London received 41.3mm of rain in two hours between 3pm and 5pm on Sunday. The London flash flood is the latest in a series of extreme flood events in Europe and Asia. Earlier this month the Chinese city of Zhengzhou was hit by a deluge that saw 624mm of rain fall in a 24-hour period. By comparison the rainfall in Barking on Sunday was 6.6% of the rainfall hitting the Chinese city.

Civic Engineers founding director Stephen O’Malley said that sustainable drainage options will need to be revisited across many projects in London to help deal with future flood events.

He said: “It is remarkable that in the space of a fortnight we’ve had two of these major [flood] episodes. It does highlight how the network is exposed to the flood threat.

“Authorities right across the UK need to be more mindful of the flood risk, there is arguably a myopic approach to these problems [such as flooding] and they are often dealt with one at a time, however as an engineering profession we should be looking at overlapping inter-dependent systems and networks.

“Sadly, that’s not how funding is determined or how design is developed and implemented, which I think is a real missed opportunity.”

While mitigation measures have been put in place to deal with tidal or river flooding at both stations, O’Malley added that flash flooding from surface water - known as pluvial flooding - is a different challenge.

He added: “TfL will have complied with the prevailing regulations [in terms of flooding] at that point in time.

"I think the blind spot has been that when it comes to these extreme events you are not going to be able to ameliorate or accommodate all the flooding nor should you spend the money to accommodate all such extreme events, but you can design infrastructure in a way that it recovers, or can recover faster and does not experience too much damage.”

Pudding Mill Lane DLR station was rebuilt to the south of its previous site to make way for one of Crossrail’s portal tunnels. The rebuilt station was given detailed planning consent by Newham Council in 2011, after outline planning was granted via the 2008 Crossrail Act.

The station lies in an area which is noted as being predominantly in flood zone 3. (A category 3 flood zone is considered to be at risk of a flood event in more than 1 in a 100 years.)

A spokesperson for TfL said that they were looking into whether there were any station-specific drainage issues at Pudding Mill Lane and what action could be taken. TfL were also in contact with Network Rail over issues at Hackney Wick station.

In its Annual Report and Statement of Accounts for the Year Ended 31 March 2021, TfL identifies climate risk as a "growing threat" to its network.

It adds: "The number and intensity of severe weather events is increasing over time and with prolonged periods of hot and cold weather, flash flooding and winter storms. There has been an anecdotal increase in incidents across TfL that have been linked to weather events and the potential of further severe weather events due to climate change could lead to many more incidents.

"Extensive flooding or failure of assets (for example, embankments and failure of flooding controls and defences) have the potential to be catastrophic for London and TfL."

The report adds that TfL's current data collection and monitoring system "do not currently allow for quantitative or comprehensive assessments of the impact of current weather events on our operations".

It adds that current "funding constraints" mean that it is "likely to remain a challenge over the medium term".

The extent of surface water flooding in London and the risk it posed to infrastructure in the capital was laid bare when Whipps Cross hospital was forced to evacuate patients and cancel operations on Sunday when heavy rainfall caused a power outage.

O’Malley notes that until recently a lot of engineering allowed rain to pass as quickly as possible to the next location, however landscapes need to soak up and slow down the flow of water to allow it to dissipate, and not contribute to further flooding downstream.

He said: “Up to very recently the strategy has been to get water flowing as quickly as possible, and accelerate through pipes which have very low friction, in fact they’ve been designed to minimise friction with water, and accelerate the rate at which water passes through the network into some sort of trunk sewer.

“The principles of SUDS [sustainable drainage system] is the complete opposite, you slow water flow down at every contact trace and replicate natural processes. It means that particularly with these short sharp showers when the system is stressed you hold back the water and releasing it into that network at a much slower rate and increasing resilience.

“I think the Thames Tideway tunnel has a role to play in freeing up capacity in the network.” Speaking to the BBC Veronica Edmonds-Brown, senior lecturer in aquatic ecology at the University of Hertfordshire attributed the flooding to a number of reasons: "The first is building on the floodplains of the Thames and River Lea. The second is urbanisation. The more impervious surfaces we install - and we are amid a rapid housing programme at the moment - the worse this situation will get.

"The final reason is that our drainage system is not built for the amount of water it is receiving. Due to budget limitations, local authorities are not able to maintain or improve them."

London Mayor Sadiq Khan added: "We are seeing increasing incidents of extreme weather events linked to climate change. This is not the first time in recent weeks that London has been hit by major flooding.

"Despite having limited powers in the area, it remains a key priority for myself and London's council leaders that more is done urgently to tackle flooding and the other impacts of climate change."

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