Geography Reference
In-Depth Information
smog from the intense morning concentration
around the central business districts to the
suburbs further inland, carried by the sea breeze,
and then a seaward migration again in the
evening when the land cools and the sea stays
relatively warm.
Similar air pollution problems prevail in many
equatorial cities, even though frequent rains
usually wash pollutants out of the atmosphere.
Vehicular emissions account for more than three-
quarters of the air pollutants in and around Kuala
Lumpur, Malaysia.
During August 1991, the suspended
particulates content of the air increased by 13
per cent per day from 120 µg m
3
on 16 August
to over 400 µg m
3
on 27 August, then dropping
rapidly when the weather changed (Samah
1992). However, in 1997, haze over Kuala
Lumpur was associated with forest fires in
Sumatra and Borneo, the dust particles from
which had been carried across the Malacca
Straits and the South China Sea. Causes of
pollution events may thus be local or regional,
and probably the worst conditions arise when
there is a combination of the two. These haze
episodes tend to produce or aggravate
respiratory and eye problems. People susceptible
to related diseases were warned by the Health
Department to stay indoors. Much work
remains to be done on the possible relationship
between conditions such as asthma and air
pollution. In Britain, severe air pollution is
believed to aggravate existing respiratory
diseases, which are probably caused by allergens
in the indoor environment.
that will yield water directly to drains and artificial
drainage channels. In North American and
Australian cities, about 33 per cent of industrial
and commercial areas and 25 per cent of
residential areas are impervious (Nouh 1986).
Urbanisation modifies the hydrological cycle in
four main ways:
1
increase in storm runoff;
2
reduced infiltration to groundwater aquifers;
3
changes in water quality;
4
changes in the hydraulic amenities of streams.
The growth of cities affects the flow of small
streams in two ways:
•
an increasing percentage of the surface
becomes impervious to infiltration as it is
covered by buildings, driveways, pavements
and parking lots;
•
the introduction of storm sewers brings storm
runoff from paved and roofed areas directly to
stream channels for discharge. Runoff travel
time to streams is shortened, while the
impervious area increases runoff volume. These
two changes in combination reduce the time
from peak rainfall to peak stream flow (the lag
time) and raise the peak storm stream flow.
Many rapidly expanding suburban
communities are finding that low-lying,
formerly flood-free, residential areas now
experience periodic flooding as a result of
upstream urbanisation. The size of small,
frequent floods is increased many times by
these processes, but large, rare floods that cause
serious damage usually result from conditions
that saturate entire catchment areas and are
little, if at all, affected by urban land uses (Hollis
1975).
WATER PROBLEMS IN CITIES
Urban floods can be classified into four broad
categories:
A city has a dual hydrological system: the people-
modified natural hydrological cycle of rainfall,
runoff and river discharge; and the artificial water
supply and waste water disposal system. The nature
of the urban surface is particularly important in
the disposal of rainwater and snow falling on a
city. Drainage systems have to take account of the
percentage of the surface that is impermeable and
Localised flash floods:
usually produced by short-
duration, extremely intense thunder-storms,
which produce rapid runoff of large volumes of
water, which exceed the capacity of small stream
channels. This causes localised flooding of roads
and houses and in urban areas is often related to
