Environmental Conditions & Pressures
Trends in Growth in Urban Areas
Technically, urban growth means an increase in the population in urban areas.
However, it is often taken to mean an increase in the physical size of urban
centres. The interaction between urban population growth and the physical growth
of urban centres depends on urban density, which is discussed below. Urban
growth is often used as an indicator of the impact of urbanisation on the
environment because of the potential for increases in the amount of land
required for urban areas or in the total environmental impact of activities such
as waste production that take place within urban centres.
Urbanisation converts land at the rural-urban fringe to uses such as residential
development. It may affect land which is regarded as an environmental asset,
such as land with agricultural or mineral potential. Land at the rural-urban
fringe is also used for activities that service the adjacent urban area, such as
waste disposal or gravel extraction.
The degree of urbanisation refers to the proportion of the population in urban
as opposed to rural areas. An understanding of the degree of urbanisation can
help to clarify whether urban growth is related to a shift in the population
from rural to urban areas or whether it is simply related to population growth
in the state as a whole.
The environmental implications of a high degree of urbanisation are somewhat
mixed. While it may imply that the impact of population growth can be more
easily managed, it also implies that a greater proportion of the population is
exposed to the local effects of urbanisation such as air pollution.
Urban Density
Urban density is a general term which can mean either the number of dwellings or
the number of people per hectare of urban land. Dwelling density can be
calculated in a number of different ways depending on which particular parcels
of urban land are used in the calculation. The calculation of site density
includes all land required for urban development to produce dwelling densities.
Gross density generally excludes land for regional facilities, but includes
residential lots, local open space, local roads, neighbourhood shopping centres
and primary schools. Net density generally excludes all non-residential uses but
includes collector and local roads. Site density will show the lowest number of
dwellings per hectare and net density will show the highest.
A comparison between gross density and site density will indicate the amount of
land used for regional facilities such as regional open space and arterial
roads. A comparison between gross density and net density indicates the amount
of land used for non-regional, non-residential uses such as local open space.
The site density gives an overall indication of the efficiency of land use for
urban purposes. This information is important in understanding the environmental
implications of urban growth because it may indicate the trade-off between
public and private land in cities. This, in turn, affects urban environmental
management.
The link between urban population growth and urban physical growth is urban
density. Population density and dwelling density are linked by household
formation and size. The rate of household formation and the average household
size are influenced by economic, social and demographic factors. Hence, the
number of dwellings required by a certain population and the number of people in
dwellings varies with conditions. This means that a certain dwelling density can
support a range of population densities. In fact, a density of 15 dwellings per
hectare can mean anything from 20 to 50 people per hectare, depending on the
type of household. There are clear trends in household size and formation. Two
of the primary contributors to the decline in household size are an aging
population and a decline in birth rate.
As a result, population densities should be considered in conjunction with
dwelling densities to make the link between urban population growth, the use of
urban land and its environmental impacts.
There are balances and trade-offs in environmental impacts from different
density of development. Urban consolidation - which the Department of Urban
Affairs and Planning defines as a process of increasing the density of dwellings
or population, or both - can reduce the amount of land converted from rural to
urban uses and decrease energy use and air emissions, including greenhouse
gases, by reducing the need for individual motor vehicle transport. Higher
densities at transport and employment nodes can promote the switch away from
high motor vehicle use. On the other hand, lower urban densities can offer lower
indoor ambient noise levels, high levels of stormwater percolation, better solar
access and opportunities for tree planting and domestic waste disposal; and
highlight opportunities for a greater degree of self-sufficiency, for example,
vegetable gardens.
Information from urban density indicators can, firstly, examine the extent to
which urban consolidation policies are meeting the initial goal of increasing
dwelling and population densities. Secondly, the information provided by the
comparison of different dwelling densities can help identify which land uses are
being affected by urban consolidation policies, and whether the changes are
environmentally beneficial. If further research is undertaken, information from
the indicators can help to show the package of environmental costs and benefits
associated with different population and dwelling densities and land uses.
Answer the following:
1 Explain what is meant by the term urban density.
2 What are the environmental implications for continued urban growth in
a busy city?