Flooding in London
Abstract
London faces a major risk of flooding. London’s floods are caused by a mixture of
river processes, tidal flooding and drains/sewers unable to cope with the volume
of water. This risk is likely to increase in the future as a result of climate change,
rising sea levels and an increase in building in the Thames Gateway. Existing
defences are solid in some parts but elsewhere they struggle to contain the floods.
These defences will be tested even more in the future.
London floods 2000 and 2002
During the flooding of Autumn 2000, about 10,000 properties were flooded in
England and Wales, some on several occasions and for long periods of time. The
Association of British insurers estimated that the cost to insurers was £1.3 billion.
London experienced severe flooding at this time. The defences on the River Roding
at Wanstead and Woodford in north-east London, were overtopped resulting in 230
properties being flooded. There was also flooding of 75 properties at Edmonton in
Enfield and 15 at Teddington in Richmond. Thus in total 320 properties were flooded
in London. Recent flooding in 2002 shows that this is a recurring problem, and
potentially, an increasing one. London’s population is predicted to increase by over
700,000 by the year 2016, with households increasing by 303,000. Floodplain
development and the increase of impermeable surfaces are likely to increase the risk
of flooding in London over that time scale (Figure 1). The consequences of large
scale flooding in London would be alarming. There are 38 underground stations;
eight power stations, 16 hospitals and 400 schools, plus 500,000 properties that
would fill with water.
Figure 1 New commercial and residential developments on the Thames floodplain
Causes
Figure 2 Three main types of flood risk to London
Type
Likelihood of event
Tidal
Low
River
Thames – Low.
Tributaries – Medium to
Likely Costs
Very High
High but localised
Drainage
Source London Assembly
high
High in certain locations
Medium but localised
Tidal, river and drainage flooding are connected.
Tidal flooding
The tidal stretch of the Thames is over 100 km long, stretching from Teddington in
West London to Shoeburyness, just east of Southend. Prior to the construction of
the Thames Barrier 116 km of flood plain was at risk of tidal flooding. Despite the
building of the Barrier the risk from tidal flooding is expected to increase with rising
tide levels (Figure 3). At present, the areas at risk from tidal flooding approximate to
land below the 5 metre contour (Figure 4).
Figure 3 Flooding risk and existing flood barriers
In April 2002 the UK Climate Impacts Programme (UKCIP) published a report called
‘Climate Change Scenarios for the United Kingdom’ which predicted an increase in
both the amount and intensity of winter rainfall, rises in sea-level and more frequent
extremes of sea-level (as experienced during storm surges). There is also a possible
increase in the height, intensity and unpredictability of ‘North Sea surges’, caused by
a meteorological depression funnelling a surge of high water down through the North
Sea.
A gradual sea level rise of 4mm per year is expected as a result of global warming.
In addition, the south-east of England is sinking as a result of isostatic recovery
following the last glacial period. London is a third of a metre lower, in relation to sea
level, than it was at the end of the Second World War. Current Government guidance
recommends an annual allowance for sinking land levels. Combine this allowance
with the rising sea level and a total allowance of 6mm per year should be made.
Figure 4 London’s many commercial and residential properties are at real risk
Flooding from rivers
There are about 90,000 London properties at risk of flooding. Despite London’s flood
defences, in Autumn 2000 properties were flooded by the River Roding and Salmons
Brook (Figure 5).
Figure 5 Indicative Flood Risk Areas
In addition to the Thames, there are a number of tributary rivers, such as the Wandle,
Lea and Beverley Brook, which have had sections built over or culverted. These
canalised and culverted stretches may not cope with high urban run off flows. River
flooding from the Thames and its tributaries will become more frequent as a result of
climate change producing more intense rainfall.
The standard of flood defence from tidal flooding up to 2030 is very high. However,
river defences in London are variable. There are sections of the lower and upper Lee
where the defences protect against a 1 in 70 year flood level. However, there are
also five stretches of defences along the Lee that are assessed, by the Environment
Agency as being in ‘poor’ or ‘very poor’ condition.
Flooding from sewers and drains
London has a combined sewer and water network which takes both foul flow and
surface water runoff. Some 8,000 properties are at risk from drain flooding from the
sewerage system, over 50% of that number being in the Greater London area.
Freak storms in August 2004 left more than 10,000 dead fish floating down the
Thames and brought chaos to the capital's transport system. Three times in August,
tonnes of raw sewage were released into the river: 650,000 tonnes from pumping
stations at Hammersmith and Chelsea; 350,000 tonnes near Battersea Park and
Isleworth, and 500,000 via the River Lea. Oxygen levels in the Thames were reduced
to virtually nil along a stretch from Kew, Brentford and Isleworth.
The storms caused severe disruptions to London's tube and bus services. In most
years there are up to 60 sewage events a year. Thames Water is now discharging
raw sewage into the Thames 60 times a year because the Victorian sewerage
system cannot cope. This includes 10,000 tonnes of solid sewage flushed in from
pipes, including sanitary pads, condoms, hypodermic needles and dead rats, all of
which float up and down the river with the tide.
To deal with the problem, priority has been given to replacing Victorian water mains,
which in parts of central London are leaking so badly less than 50% of the water
pumped into them reaches the taps. There are plans for a Thames Tideway tunnel to
be burrowed 100 metres under the existing riverbed to avoid tube lines and cables,
and take the storm water away from London to sewage works down river. However,
work will not start until 2010, when Londoners would be asked to start paying through
their water bills. With the government seeking to increase the population of London
by 700,000 by 2016, sorting out 1,000 miles of Victorian water mains is a priority.
The Association of British Insurers listed five areas of concern:
1. Many drainage systems are designed to cope only with high frequency, but
low severity flood conditions.
2. New building developments on large areas of land can significantly increase
run-off, resulting in additional problems downstream.
3. Foul flow can backup into buildings, for example through the toilets and baths
directly into houses.
4. Blocked culverts can create local flooding very quickly.
5. Open drains and minor water courses are often blocked by, for example, flytipping or leaves (Figure 6).
Figure 6 Blocked drains - a major cause of flooding in London
Other factors
In certain areas of London the water table is rising. This causes a particular risk of
flooding to the sub-surface infrastructure such as deep basements, car parks, the
Underground network and building foundations. The problem is partly linked to the
decline of manufacturing industries, which had previously used great quantities of
groundwater and kept the water table low. Rising groundwater is one of the main
risks to the flooding of the Underground network.
In addition, there are a number of other factors increasing the risk of flooding and/or
reducing the effectiveness of current flood management plans. These include
 the monitoring of the flood risk
 the responsibility for coordinating flood management
 the role of the emergency services
 population characteristics, such as mobility and poverty.
Monitoring of the flood risk
An important factor in the effective defence of London against river flooding is regular
inspection of rivers and streams for blockages, and of flood defences to ensure the
proper maintenance and repair of defences considered poor. However, the
Environment Agency has limited access to many sections of such watercourses as a
result of private developments, making inspection and maintenance difficult.
Monitoring work is made more difficult by the fact that there is no comprehensive
map available of London’s sewer and drainage system.
Responsibility for co-ordinating flood management
Until recently there was no single body that had a statutory duty to take an overview
of the flooding risk of London. This situation has improved now as The Greater
London Authority (GLA) have a key role in coordinating the many organisations with
an interest in this question, including the Environment Agency, Thames Water, the
Boroughs and Thames Gateway London Partnership.
Role of emergency services
The Fire Brigade lacks a clear statutory duty to respond to floods. At the time of the
Autumn 2000 floods the London Ambulance Service had no equipment to effect the
recovery of injured or ill persons caught up in the flooding. Police vehicles are fitted
with public address equipment, but the speakers are located under the front bumper,
an obvious problem in flood waters.
Population characteristics
Knowledge of flood risk is limited. Up to 50% of those living in flood risk areas in the
United Kingdom are oblivious to the risk and only 10% take any action to prepare.
25% of households choose not to insure their contents, rising to 50% in low income
groups. Some households and businesses may be underinsured. In addition, the
very transient nature of London’s urban population means that the Agency faces a
difficult challenge in achieving continual coverage of flood awareness.
Responding to flood risk
In 2001 the government published a report called PPG25 ‘Development and Flood
Risk’ and the Institution of Civil Engineers published ‘Learning to live with Rivers’.
These papers pointed out that it is impossible to build defences against all flood risk,
and that sometimes the building of flood defences can produce further problems. We
therefore have to live with an element of flood risk.
PPG25 ‘Development and flood risk’
The PPG25 document defines three types of flood zones based on flooding
probability:
 Zone 1 – Little or no risk. Annual probability of flooding: River, tidal and coastal
<0.1%
 Zone 2 – Low to medium risk. Annual probability of flooding: River 0.1-1%, Tidal
and coastal 0.1-0.5%
 Zone 3 – High Risk. Annual probability of flooding: River 1.0% or greater, Tidal
and coastal 0.5% or greater.
It states that constraints on development should not be imposed in areas of little or
no risk. In low to medium risk areas, a flood risk assessment should be provided and
flood resistant construction and warning/evacuation procedures may be required.
High-risk areas are divided into three categories:

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Developed areas that may be suitable for development.
Undeveloped and sparsely developed areas generally not considered suitable for
development.
Functional floodplains suitable for some recreation sport, amenity and
conservation uses.
Funding of flood defences
According to the London Authority Steering Group, there are four main funding
options. These include:




a ‘development charge’, provided for under PPG25, which enables flood defence
costs arising as a result of a new development on a flood plain to be met by
developers.
a ‘connection charge’, payable by all developers on a flood plain to recover
general flood defence costs attributable to an individual development.
a ‘flood plain levy’ through which those occupying the flood plain would contribute
to flood defence costs.
a ‘surface water drainage charge’ based on the “polluter pays principle”, either
based on ability to pay or as a universal charge.
Flooding from the tidal Thames – the Thames Barrier (Figure 7)
The Thames Barrier became operational in October 1982. On average the Barrier
has to close three times per year to prevent flooding. During the most recent flood
season of 2001/02 there were four closures. But in the preceding winter of 2000/01
there were 24 Barrier closures (Figure 8).
The need to close the Thames Barrier and associated defences can cause flooding
downstream. The ponding of the tidal water downstream of the Barrier means that
the downstream tributary rivers have nowhere to go other than over their banks.
Figure 7 The Thames Barrier
Figure 8 Thames barrier closures (to May 2003)
The Thames Barrier gives a high standard of protection to London properties at risk
from Thames tidal flooding. The current standard of flood defences in London means
that there is only a 0.05% chance of properties in the tidal floodplain being flooded.
Defences were constructed with an allowance built in for a sea level rise of 8mm per
year. Although the risk remains small, the risk of tidal flooding in London is estimated
to double by 2030.
From 2030 the chance of flooding rises to a 0.1% risk, and this will continue to rise
unless improvements are made. Preliminary estimates of the cost of providing a
0.1% standard to the year 2100 show that a major investment in the flood defence
infrastructure of the order of £4billion may be required within the next 40 years. By
2030 it is forecast that the Barrier will have to close about 30 times per year to
maintain the standards of tidal defence in the Thames Estuary. With closures this
frequent, shipping would be severely disrupted. This has serious implications for the
plans to revitalise use of the Thames for freight and passenger transport.
Figure 9 Thames Gateway
In the draft London Plan the expansion of London in the Thames Gateway area is
largely in locations that are at risk of tidal flooding. Forecasts of housing growth from
2001 to 2016 suggest 100,000 homes and employment growth of about 300,000 jobs.
The new settlement will have 120,000 homes below sea level. Among the schemes
being considered are raising homes on stilts; building roads on embankments above
the flood level, and sacrificing large areas of Kent and Essex farmland to flood waters
during a tidal surge to prevent London defences being overwhelmed.
Much of the areas identified by Thames Gateway London Partnership (TGLP) for
development have a medium risk of flooding, which will get worse with the effects of
climate change.
The proposals of the TGLP are to create a Thames Park and open space both along
the Thames and tributaries to accommodate flooding events, enhance biodiversity
and provide accessibility to the river for all with recreational opportunity.
Figure 10 In the Thames estuary (Thames Gateway) major industrial activities have
made use of low-lying floodplain sites.
River flood defences
The Institution of Civil Engineers warns against too great a faith in flood defences.
Schemes giving a certain protection today may offer less protection for the same
probability of flood event in the future.
Whilst the risk of tidal flooding from the Thames is significant, the probability of it
happening is low because the Thames Barrier and a number of other defences
including the Barking and Dartford Creek Barriers,provide London with a higher level
of protection than is enjoyed by any other part of the UK.
In the 1970s and 1980s 75% of funds were spent on the Barrier and the remaining
25% on associated defences. It is thought that for the forthcoming upgrading of
defences the proportions will be reversed, with 25% of funds being spent on the
Thames Barrier and the remaining 75% invested elsewhere on the Thames tidal
defence system, including the walls upstream of the Barrier, the downstream
embankments and other major moveable defences.
Vulnerable riverside areas of London upstream of the Barrier, such as Hammersmith,
Chiswick and Twickenham, need to receive adequate flood protection as soon as
possible, as they are especially at risk from flooding when the Barrier is down.
Some 32km of flood defences were built downstream of the Barrier, with bank levels
around 2m higher than has previously existed. The defences also include barriers
such as the 60m high Barking Barrier. Such embankments and associated barriers
downstream of the Thames Barrier also provide a 1 in 1,000 year or 0.1%, standard
of protection.
What are the solutions to drain flooding?
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Decreasing runoff by using soakways and swales rather than drains;
Preventing backup into buildings, by fitting one-way flap valves;
Regularly inspecting culverts CCTV ‘moles’ and regularly clearing blockages
Regular inspection and action is needed to avoid problems related to fly tipping
Using Sustainable Urban Drainage Systems (SUDS). SUDS allows surface water
to soak into the ground and then find its way to rivers at a ‘natural rate’. SUDS is
particularly important for developments such as industrial estates and distribution
warehouses which have traditionally been responsible for considerable volumes of
surface water runoff. SUDS technology includes porous surfaces (for example in
car parks), reed beds, ‘balancing ponds’ and gravel swales.
What are the solutions to flooding in houses?
 Using solid concrete floors on the ground area
 Placing electrical sockets at around one metre above floor level
 Replacing chipboard/MDF kitchen units with PVC units that have a higher flood
resistance
 Installing temporary barriers at gates or doors and windows.
 The use of different types of plaster that have a higher flood resistance e.g. limebased plaster
 Replacing steel cavity wall ties with stainless tell ties
 Waterproofing external walls
Conclusion
There are many causes of flooding in London, mainly river, tidal and drain flooding.
The floods of August 2004 illustrate the seriousness of the hazard. The potential for
flooding is likely to increase in the coming decades as a result of climate change,
along with changes to London’s physical and human geography.
The problem of flooding is compounded by the sheer size of London. It has proved
difficult to map and monitor all parts of the London drainage system. Nevertheless,
there have been many advances in the production of flood risk maps, and this would
be useful as an initial assessment of the drainage and flood protection system.
Protecting London from all types of flooding may prove difficult and expensive, but
there are many people at risk, as well as real estate, infrastructure and businesses.