Geography Reference
In-Depth Information
9
Physical problems of the urban
environment
Ian Douglas
Nature is widespread in cities. Only a small part
of an urban area is completely paved and roofed.
In European and North American cities, many
suburban areas are dominated by green vegetation.
Urban areas often have greater biodiversity and
wildlife protection than adjacent intensively
cropped farming country, 3000 different species
having been recorded in a single suburban garden
in Leicester, England.
Land prices and policies encouraging house
building on existing urban land lead to denser
occupation of urban land. Gardens are partially
changed to impermeable, paved surfaces
modifying natural air, water, materials and energy
flows. Relatively little of the urban area is
completely covered by roofs or paved surfaces and
thus totally impervious. Satellite imagery showed
only 0.9 km
2
of the Bolton urban area in England
to be entirely impervious, 6.5 km
2
was 82 per cent
impervious, while 25 km
2
was 45 per cent
impervious (Adi 1990).
THE NATURE OF PHYSICAL
ENVIRONMENTAL PROBLEMS IN
CITIES
Cities are where people have most transformed
nature, replacing vegetation with roofed and paved
surfaces, burying stream channels, creating indoor
climates, and making huge artificial transfers of
energy water and materials. Expanding cities
transform hydrological relationships, changing the
magnitude and frequency of flooding. Rising land
prices often mean that homes are built on
relatively unstable slopes or on the floodplains of
rivers. The poor, especially in dense squatter
settlements in third world cities, often have no
choice but to occupy hazardous sites on steep
slopes, close to rivers, or near polluting factories.
All too often, their settlements are vulnerable to
road collapse, water pipe breakages and sewer
failures and to floods, landslides or subsidence.
Two aspects of this vulnerability are of special
significance to geographers: the differing
vulnerability of social groups and communities
within the city; and the way in which expanding
cities increase in vulnerability through time as they
spread across more hazardous sites and occupy
more unstable terrain. Knowledge of urban
hydrology and urban geomorphology is not only
a key to good urban planning but should also be
available to every house purchaser. The home
builder or buyer should 'know the ground being
built upon'.
URBAN VULNERABILITY TO
CLIMATIC EXTREMES
Cities have little direct impact on the global
radiation balance, but the internal urban climate
produced by the absorption and subsequent
reradiation of heat from the surfaces of the built
enrivonment, and by the emission of artificial heat
through combustion create an urban heat-island
