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Crazy paving

September 2005

Why are gardens disappearing and what is the impact on run-off and flood risk?

Crazy paving

Two-thirds of London’s gardens have been paved over for parking and convenience, according to a new report. Leaving rainwater unable to soak into the ground, changes in urban land use are being blamed for recent failures of the sewer system.

In early August 2004, heavy rainfall in London overwhelmed the Victorian sewer system, leading to a massive discharge of around one million tonnes of untreated sewage into the Thames, killing 10,000 fish (The Guardian, 05 August 2004). Now a report commissioned by the London Assembly claims that the fashion for covering over gardens with concrete, paving and timber decking is responsible. According to Darren Johnson, chair of the Environment Committee, “people want to park their cars, they think 'minimalist' gardens are sophisticated or they just don't like gardening. Whatever the reasons, the results are the same. The more paved surfaces there are, the less rainfall is soaked into the ground and the more London's sewerage system struggles to cope.” (The Guardian, 03 September 2005)

More than a third of London’s green space, and one-fifth of the city’s total land area, is made up not of parks, woodland or farms, but private gardens. Together, London’s private gardens cover an area just slightly smaller than the combined size of the inner London boroughs - 319 square kilometres (or 123 square miles). An estimated two-thirds of London’s trees are growing in domestic gardens. This huge area of land - around 22 times the size of Hyde park – is progressively being transformed from grass cover into concrete and hard timber decking.

Why do gardens matter?

This loss of green space is having an impact on urban biodiversity, the attractiveness of the urban landscape, and on flood risk in London. The new report explains that “because of the size of the area of land they cover, private gardens are a crucial component of London’s ecosystem and, perhaps most significantly, in the city’s ability to absorb rainfall. Whatever rain is not absorbed by the ground will run off into underground drains, putting additional pressure on our already creaking Victorian sewerage and drainage system - as we saw last summer, when one million tonnes of raw sewage were discharged into the Thames after heavy rainfall.”

How has London hydrology changed?

Urbanisation radically alters the flows of the hydrological cycle by speeding up the rate at which water can reach the main river channel, greatly reducing lag-time and increasing peak discharge for the storm hydrograph. This happens because urban design makes previously permeable soil surfaces impermeable, thereby increasing surface run-off and decreasing the effectiveness of soil storage. Ground is covered by impermeable hard surfaces such as concrete or paving slabs. As a result, rainfall cannot infiltrate and quickly runs over the directly into London’s drains. The underground drainage system is not supposed to empty drain water into the Thames, but does in fact need to do so during very heavy storms, when too much water arrives in the sewers at the same time.

The city’s sewers were designed by Joseph Bazalgette in the 1850s to carry a combination of sewage and rainfall. During times of very heavy rainfall, he knew that the drainage pipes would overflow and the contents - both rainwater and sewage – would be discharged into the Thames.

However, such overflow conditions were considered very rare in Bazalgette’s day because the city’s gardens – covering one-third of its total area – provided a safety valve by allowing rainwater to seep away naturally, infiltrating downwards into the soil and percolating into the porous rocks beneath. According to the Royal Horticultural Society, an average suburban garden on a typical rainy day will absorb about 10 litres of rainwater a minute, representing around 10 per cent of the water that will fall in a storm. The loss of this vital water store in recent years now makes sewer failure far more likely.

Why are gardens disappearing?

If gardens play such an important role regulating the flow of water, then why are people paving over them? There are several reasons including:

  • Avoidance of public transport Over-congestion, cancellations and security concerns (after the London bombings) all have an upward impact on car ownership.
  • Lack of availability of on-street parking Much of inner London was built before cars were invented and lacks proper parking facilities. For example, the Royal Borough of Kensington and Chelsea has about 40,000 roadside parking permit holders but only 27,000 spaces!
  • The convenience of low-maintenance gardens Lawns and garden plants require constant care, especially in summer, whereas concrete does not!
  • The fashion for minimalist design Recent years have seen much greater use being made of paving and decking. The report points the finger at the public’s new-found love for TV house and garden “makeover” programmes.
  • Increasing house values Paving over a lawn to create a parking space may increase the value of a house. Potential buyers can see that they will be able to park their cars in front of their homes, for both convenience and security.

What are the problems with paving over front gardens?

This table summarises the problems and the detrimental effects on the environment and to local communities that can be caused by the 'hard surfacing' of front gardens.

Aspects of hard surfacing which cause problem(s)

Problem caused

End result (s)


Increased rain water run-off increased fluctuations in amount of water going into storm drains and thence to local streams and rivers

· Increased risk of flooding, especially flash flooding.

· Erosion and damage to riverbanks and hence to their habitats.

· Increased pressure on storm drains.

· Increased pressure on sewers, even leading to forced release of sewerage into rivers.

· Localised flooding of streets, pavements, nearby properties.

Increased rain water run-off picking up oil and heavy metals from hard surfaces beside and close to roads, plus pesticides, herbicides and other chemicals used in gardens

· Increased levels of pollution of local watercourses detrimental effects on water quality and on wildlife.

· Polluted rivers and streams, unattractive environment, less use for leisure, increased risk of vandalism.

Reduced amount of rainwater percolating through soil

· Reduction in water purification and removal of pollutants from ground water by soil percolation processes.

· Soil drying out, building subsidence.

Artificial and hard

Absorb more solar heat

· Increases the local temperature, 'heat island' effect, exacerbating predicted effects of global warming, including:

· worse air quality;

· increased energy consumption for air conditioning and cooling;

· proliferation of microbes and diseases-bearing organisms e.g. malaria mosquitoes.

Absorb less noise

· Increased noise from traffic and other sources, especially for people living at ground floor level.

Don’t absorb dust

· Increased air pollution (particulates).

Don’t absorb dirt and spills

· Dirtier environment è unappealing to community.

Use for parking

Increased numbers of crossovers in pavement

· Increased corrugation of pavements makes walking more difficult, especially for those with disabilities, the elderly and those with small children, pushing buggies. Also more difficult for two people to walk and talk together side by side.

Vehicles being driven across and reversed across the pavement

· Risk to pedestrians, especially children.

Loss of visibility – parked vehicles are higher and more solid than garden vegetation

Parked vehicles also may overhang pavement

· More dangerous pedestrian environment especially for children (whom we are trying to encourage to walk to school etc.)

Net addition to car parking spaces (often)

· Contributes to generating greater volumes of traffic, contrary to Government policies to reduce traffic.

Loss of on-street parking (created by presence of crossovers)

· Reduces the control that authorities have over parking.

· Adverse effect on neighbour relations because a crossover effectively reserves that part of the road for sole use by the dwelling with the crossover.

· Fewer cars parked on the road creates a wider road with improved visibility, encourages traffic to speed.

Loss of vegetation

Reduced CO2 absorption

· Contribution to global warming (see above for predicted effects).

Loss of habitat for wildlife (both above and below ground level)

· Adverse effect on plant and animal life (i.e. biodiversity).

Loss of street trees removed to accommodate pavement crossovers

· Adverse effect on levels of air pollution due to loss of absorption of pollutants è adverse effects on human health, contribution to global warming.

· More tension and conflict between neighbours (US research).

Loss of grass verges removed to accommodate pavement crossovers

· Adverse effect on plant and animal life; contribution to global warming; increased run-off etc. as above; loss of aesthetic appeal.

Loss of gardening activity

· Less opportunity for informal contact with neighbours.

Changed appearance/


Replacement of soft green areas and trees with cars and hard unattractive surfaces

· Reduced aesthetic appeal, character, visual appearance and attractiveness of the urban environment and its traditional architecture.

· Reduction in community cohesion.

· Reduced house prices when whole street has been converted.

Loss of boundary structures (hedges, fencing etc.)

No barriers to wind

· Increased levels of dust and hence air pollution (particulates).

Loss of demarcation

· Adverse effect on neighbour relations, community in general.

The above table has been compiled from a variety of sources, including various research reports, feedback from Ealing’s LA21 Pollution and Public Health Project Group and Natural Environment and Biodiversity Project Group; feedback from Environment Coordinators in other London boroughs, and miscellaneous other sources and feedback.

What can be done?

Possible alternatives to paving slabs and concrete include greater use of gravel as a surfacing material, or ensuring more people install vertical drainage channels when they pave over their gardens. Such measures allow the ground surface to remain porous, allowing rainfall to penetrate the ground below thereby relieving the pressure on underground drains. The London Assembly now hopes to raise awareness of the importance of natural surfaces for the overall health of the city. Members are calling for the Mayor to launch a campaign to raise public awareness of the impact of paved gardens, not just on the water cycle but also on animal and bird biodiversity.

Writing about run-off at AS-level

1. Explain the human causes of river flooding (6 marks)

Urbanisation makes previously permeable soil surfaces impermeable. Surfaces like concrete and tarmac increase surface run-off and decrease the effectiveness of soil storage. The more ground that is covered by impermeable hard surfaces such as concrete or paving slabs, the less rainfall will soak into the ground and the more will run into drains. Also, these drains are effectively extending the river channel network, allowing water to reach the main channel faster. There will also be less transpiration without vegetation, which is a means of transmitting water back into the atmosphere without increasing run-off. All of this results in more water arriving quickly into a river channel during heavy rainfall, thereby increasing flood risk. Widespread flooding in the UK in 2000 (e.g. along the River Severn) was certainly made worse by floodplain building.

2. How does human activity modify flows and stores of the hydrological cycle? (4 marks)

Flows: concrete surfaces are hard and impermeable, decreasing infiltration and throughflow but increasing overland flow. A loss of vegetation reduces transpiration rates, while inputs of rain can possibly increase, due to pollution and dust particles providing condensation nuclei for rainfall.

Stores: interception stores are lost, while soil and groundwater stores can no longer be easily recharged as rain cannot infiltrate anymore. However, flood alleviation schemes can provide new artificial stores such as detention reservoirs or spreading grounds.

3. How do land use changes modify physical environment? (10 marks)

For this longer piece of writing, focus your answer on the following themes:

  • Recreational spaces (gardens) are instead being turned into parking spaces This impacts upon the hydrology of the Thames valley
  • Your answer could also look at how urban biodiversity and food-webs are effected
  • Consideration could also be given to the idea of landscape and aesthetics

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