United Kingdom

Flash floods and Urban flooding United Kingdom

Vulnerabilities - Surface water flooding

Urban flooding can be the result of two different causes: surface water flooding and sewerage flooding.

Surface water flooding occurs when the drainage system cannot cope with the speed that cumulative rainfall hits it and flooding spreads from water collecting on roads and pavements. This type of flooding is becoming more common, which is due to a number of interrelating factors (1):

increased frequency of intense rainstorms. This is expected to worsen in predictions on climate change;
increase in hard surfaces which reduce the time that water takes to hit the drains and increases the amount of water as less is absorbed or evaporates. This is due both to the level of development in London and from the cumulative effect of changes to our houses and gardens;
inadequate maintenance of drains;
inadequate capacity of drains to deal with the increase in run-off.

Much of the intra-urban flooding in the past was flash flooding caused by unusually heavy rainfall combined with inadequate and/or ill-maintained drainage systems. After noting the heavy rain, residents in the areas hit by the summer floods believe that poor drainage was the most significant factor in causing the floods. For example, one in 100 year river defences currently sit alongside one in 30 year drainage systems (2). Examples of surface water flooding are the flooding of Derry City on 17 August 2004 (3) and the flooding in England in the summer of 2007 (4).

The Pitt review (4), a review of the 2007 summer floods in England, concluded that perhaps the most significant feature of these events was the high proportion of surface water flooding compared with flooding from rivers. They stated that currently no organisation is responsible for overseeing and planning for surface water flooding, creating problems which were particularly evident in places like Hull and parts of Sheffield. There are no warnings for this type of flooding, which can occur very rapidly, and people, including the response organisations, were not well prepared.

The floods of 2007 demonstrated that insufficient capacity of drainage systems can play a crucial part in surface water flooding. It is simply not feasible to increase the capacity of the whole sewerage system, but it is possible to introduce changes and investment choices which avoid making problems worse (4). Besides, there is ‘urban creep’: the cumulative impact that paving over front and rear gardens is having on our towns and cities. This can have a significant impact on the natural drainage of surface water, as water that previously soaked into the ground has nowhere to go and can increase the risk of surface water flooding.

Urban expansion over the last 200 years has resulted in the loss of several open rivers within central London such as the Fleet, Tyburn and Effra that now flow underground. A survey of the landscape status of London’s river channels between 1992 and 1996 revealed that 29 per cent were natural, 56 per cent were artificially surfaced, and 15 per cent were culverted (5). This is of consequence, not only for the recreational assets of the City, but also for the rate and volume of runoff following excessive rainfall or snow melt. In fact, a significant proportion of insurance claims are from non-riverine floods arising from intense rainfall events overwhelming urban drainage systems (6). The changes in future rainfall patterns point to an increased likelihood of such flooding by the 2080s.

Vulnerabilities - Sewerage flooding

Urban flooding can be the result of two different causes: surface water flooding and sewerage flooding.

Apart from where sewers break, these floods are normally caused by the overflow of ‘combined sewerage overflows’ which are common across London. These carry both sewerage and surface run-off. Their existing capacity is failing to meet growing levels of run-off as discussed above and the increase of sewage as we use more water in our homes (1). Surges of water through sewers can handicap the operation of sewage treatment works(7). Eighty thousand properties in towns and cities are presently at risk from flooding caused by heavy downpours that overwhelm urban drains (8).

The impacts of climate change on sewage treatment work capacity and size of sewers are regarded by water companies as being probably of greater significance than direct water resource issues. The network of foul sewers is largely 19th or early 20th century and its capacity and state of repair are such that significant investment is required (9). The main issues are seen as:

over-flow of the foul sewer into settlement areas;
overwhelming of sewage treatment works by river flooding, direct run-off or interception of urban storm run-off by foul sewers;
overwhelming of storm sewers by surface runoff intensities greater than design capacities.

Overwhelming of sewage treatment works by river flooding, direct run-off or interception of storm run-off by urban foul sewers is an area of concern particularly in relation both to increased winter precipitation and the potential increased storminess. Whilst many sewage treatments plants are over-sized to accommodate for growth and safety margins there is still potential for plants to be overwhelmed as a result of increased run-off likely though climate change (9).

Some scenarios project an increase of extreme rainfall events (in the summer), others a decrease. Even with fewer events, the polluting potential from the flushing of consented sewer outfalls (CSOs) could still be greater due to a combination of less diluted stronger sewage (due to lower summer infiltration to sewerage systems) and/or lower flows in the receiving water course(s). Wash-off pollutants, accumulated by impermeable surfaces such as roads during extended dry periods, could also adversely affect water quality (10).

In some ways, sewage overflow could be seen as the biggest flood problem in Scotland, simply because floodplain floods have been relatively rare in recent years (11). Indeed, since the Elgin/Moray floods in 1997, all the major flooding events in Scotland have been predominantly drainage related. Research in Scotland (12) has suggested that the number of sewer surcharge events could increase dramatically due to climate change – as much as 20% in Edinburgh by 2080. Scottish Water has already committed an additional ₤270 million over the next four years to upgrade sewers in Scotland.

The annual economic cost of property flooding from sewers in England and Wales is currently estimated to be £270 million. This damage is expected to increase considerably due to increasing rainfall frequency and intensity, limited capacity of ageing sewer infrastructure, and increasing urban density and economic vulnerability to flooding. The figures listed in the table below are in addition to the risk estimates due to flooding from main rivers and the sea.

Estimated annual costs of sewer flooding in England and Wales for four scenarios of economic growth (13) are:

	2002	World markets 2080s	National enterprise 2080s	Global sustainability 2080s	Local stewardship 2080s
Properties at risk of flooding in 1 in 10 year event (thousands)	82	380	340	300	320
Expected annual damage (£ million)	270	7880	5055	1870	740
Uncertainty range on expected annual damage (£ million)	100-500	3500-15,000	2000-10,000	900-3600	350-1400

As more land is covered with buildings, and as climate change increases the threat of severe rainfall events, sewage backup and drainage surcharge floods are becoming increasingly common in Britain. If someone suffers from such events on a regular basis, insurers will be bound to consider that such claims are becoming inevitable and uninsurable. Alternatively they may seek recovery from the responsible water company or authority (21).

The Environment Agency report on the autumn 2000 floods (22) indicated that 14% of the number of properties flooded were flooded by drainage surcharge or sewage overflows. Insurance claims data suggest that the figure could be much higher. For example, it is estimated that 30-50% of flood insurance claims after the autumn 2000 floods related to properties outside any floodplain (21).

The Construction Industry Research and Information Association (CIRIA) estimated in 1998 that 32,500 properties have a one in 10 or one in 20 chance of sewage flooding each year (23).

The impacts of climate change on urban drainage infrastructure have been assessed for a catchment in northwest of England. This was done for two climate change scenarios: a high-end global warming scenario and a scenario of reduced CO₂ emission (the so-called IPCC SRES A1FI and B1 scenarios, respectively). The projections were made for 2070–2099, based on three global circulation models. These results do not give a definite answer of what the future consequences of climate change will be. Changes in rainfall patterns may increase the spilling volume from manholes and surcharge in sewers, which would cause a significant number of properties to be affected by flooding. For the scenario with reduced CO₂ emissions compared with the current emissions, however, a drop in the number of surcharged sewers is predicted, and consequently the number of properties at risk of flooding will decrease as a result of this (24).

Vulnerabilities - London

On 7th August 2002, an inch of rain fell in central London in 30 minutes during the evening “rush hour”, resulting in the closure of 5 mainline railway stations, and considerable disruption. London’s drainage infrastructure is too old and overloaded to cope with such events. More than 50% of drainage and sewage overflow problems in England take place in London. Most of the drainage in Britain is still through combined sewers (a single pipe) (21).

The probability of surface water flooding in London is much higher in comparison to tidal flooding, and will increase for the following reasons (20):

most drainage systems are designed for high frequency, low volume rainfall;
many drains and gullies are poorly maintained and therefore unable to provide even moderate levels of service;
there is a projected increase in winter rainfall and heavy rainfall events;
the permeability of the urban realm has been reduced by developments using impermeable materials and exacerbated by waterlogged clay soils.

Adaptation strategy

Clean Air Partnership (14) recommends to

prepare high resolution topographic mapping to identify high risk areas;implement sustainable urban drainage systems including: permeable pavements, green roofs to increase on-site retention of storm water, increased use of storm water retention ponds, constructed wetlands and swales;
create natural eco-system buffers for vulnerable water bodies, low-lying areas;
expand capacity of storm sewers to manage extreme weather events;
institute land-use planning and zoning to avoid buildings and infrastructure in flood or landslide prone areas;
flood-proof buildings in vulnerable locations.

Urban developments tend to increase flood peaks and decrease the lag time of peaks. This provides a challenge to ensure sustainable urban drainage systems (SUDS) in a climate change context. SUDS options need to be evaluated and designed carefully on a case-by-case basis and may not be suitable in every situation (9). Increased application of Sustainable Urban Drainage Systems (SUDS) and permeable ground surfaces should be considered in urban environments as a means of counteracting potential under-capacity in storm-sewer systems resulting from higher rainfall intensities / frequencies.

For many areas in UK, climate change may result in an 40% increase in rainfall depths in excess of the current values with a subsequent doubling of flood frequency and volume. Following an increase in rainfall will result in storage volumes to prevent internal property flooding needing to increase by more than two fold (15).

Adaptation to climate change could increase costs of responding to intra-urban flooding events by some £17 billion (from £5.1 billion now to £22 billion in 2080) (16).

Strategies to mitigate the impacts of higher peaking floods are diverse and generally require a catchment based approach. In general, the intention of such strategies will be to either increase the capacity of the network to absorb the extra flow, reduce the extra flow total volume through local storage or to delay the movement of flood water through the catchment to decrease the peak flow height (15).

Sustainable Urban Drainage Systems (SUDS) control surface water runoff as close to its origin as possible before it enters a watercourse. This involves moving away from conventional piped systems and toward engineering solutions that mimic natural drainage processes and minimise adverse effects on the environment. SUDS may take the form of infiltration systems whereby water is soaked away into the ground or they may be attenuation systems, which release flows gradually to watercourses or sewers. Both types of system involve storing water underground or in open landscaped basins. These systems will become increasingly important as the effects of climate change are felt, potentially leading to increased rainfall in winter and the potential for flash floods (17).

Crichton (11) states that Sustainable Urban Drainage Systems (SUDS) is intended to imitate natural processes, by using a combination of features such as:

permeable surfaces, to allow water to drain through the surface into filtration material underneath;
Filter strips, for initial filtration of pollutants in surface water, for example oil from car park surfaces;
swales, long shallow channels to slow down the water and allow it to infiltrate into the ground with natural treatment from vegetation;
interception ponds, ponds with reed beds for further infiltration if necessary, for example beside car parks and filling stations;
retention ponds, ponds to store surface water runoff and give it a chance to drain away naturally into groundwater and watercourses;
detention basins, shallow basins, which are normally dry and covered with vegetation, but which can fill up with water in the event of a severe rainstorm, to prevent flooding downstream.

The Pitt Review Team (4) stated that England is already responding to climate change. The 2004 Foresight Future Flooding Study (8) identified that carriageway flooding incidents were expected to increase substantially by 2085 due to a 20 to 30 per cent increase in predicted rainfall. In anticipation of climate change and more frequent and heavier rainfall, drainage standards were reviewed following the severe flooding events of autumn 2000. Since then standards for new works and drainage maintenance renewals have been raised to provide increased capacity for the 20% to 30% increase in rainfall intensities expected up to 2085. Also, a programme of work is in its early stages to identify those structures, such as culverts, which may not function as intended within the frequency and higher levels of rainfall now predicted and experienced.

Domestic and settlement-area contamination frequently occurs following flooding. Whilst consented sewer outfalls (CSOs) permit sewage overflow into rivers during floods, the issue of backing-up still occurs when water levels are high. Individual properties can be protected against sewer back-flooding by the introduction of one-way valves. New-build and re-plumbing schemes may be advised to incorporate such valving where properties are low-lying. However, localised flooding is increasingly caused by run-off excess which is inherently less predictable; the identification of properties at risk is not straightforward. Siting of new CSOs is probably best considered down-stream of settlements to minimize potential contamination (9).

New drainage systems in Britain are still being designed for the two to five year rainfall event, despite climate change projections and the suffering caused by sewage surcharge (11). Sensitivity analysis should be applied to existing combined sewers to ensure that overflows are managed in such a way as to avoid damage to land or property and upgrading should be applied in a rolling programme. Such a programme could include one way flap valves to prevent sewage backup in toilets and baths, and “sewage paths” to direct surcharges into roads and detention tanks or basins rather than gardens and houses (11).

Insurance

The Environment Agency report on the autumn 2000 floods (18) indicated that 14% of the number of properties flooded were flooded by drainage surcharge or sewage overflows. Insurance claims data suggest that the figure could be much higher. For example, it is estimated that 30-50% of flood insurance claims after the autumn 2000 floods related to properties outside any floodplain (11).

The Construction Industry Research and Information Association (CIRIA) estimated in 1998 that 32,500 properties have a one in 10 or one in 20 chance of sewage flooding each year (19).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for the United Kingdom.

Greater London Authority (2005)
Association of British Insurers (2007)
Scotland and Northern Ireland Forum For Environmental Research SNIFFER (2007)
The Pitt Review Team (2008)
Environment Agency (2001b), in: London Climate Change Partnership (2002)
ABI (2002), in: London Climate Change Partnership (2002)
Farrar and Vaze (2000)
Foresight (2004)
C-CLIF and GEMRU (2003)
London Climate Change Partnership (2002)
Crichton (2003)
Futter and Lang (2001), in: Crichton (2003)
Hall et al. (2003)
Clean Air Partnership (2007)
ICF International and RPA (2007)
ICF International and RPA (2007), based on Foresight (2004)
Land Use Consultants, CAG Consultants and SQW Limited (2003a), summarised from Ipswich Drainage and Flood Defence Policy, Ipswich Borough Council (2002)
Environment Agency (2001), in: Crichton (2003)
May et al. (1998), in: Crichton (2003)
Greater London Authority (2010)
Crichton (2005)
Environment Agency (2001)
May et al. (1998), in Crichton (2005)
Abdellatif et al. (2015)