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Introduction
Urban Climate
Urban soils
Waterproof ground surfaces
Water quality
Impact of urban development
on streams
Flooding and other concerns
Air pollution
Noise
Introduction
Kedron Brook flows through a largely
urbanised catchment. Here human activities and land use dominate and
affect all aspects of the physical and biotic environment. Human
populations are currently growing at a faster rate than ever before.
Population growth in the Kedron Brook catchment is no exception.
The ecological processes occurring in an
urban catchment differ from natural and rural areas. Resources for
wildlife are often limited. There are many hazards (pollution of all
types, motor vehicles and human interference etc).
Natural areas in an urban landscape are
fragmented; that is, small remnants are isolated from one another.
Development of land in the Kedron Brook
catchment has almost reached its limits. Only in the upper catchment is
further housing development occurring.
Habitat within the older suburbs has
stabilised and in recent years, ecological qualities have been improved
through the activities of Bushcare groups.
Consequences of human land use on an urban
catchment that apply to Kedron Brook are outlined below.
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Urban
Climate
The climate in urban areas is
often 3-4ºC warmer than surrounding rural or natural areas.
This arises from
- high absorption of heat by building
materials and paved surfaces
- loss of shady tree and other vegetation
cover on the ground surface
- reduction of moisture reaching (and
therefore cooling) air due to transpiration
Urban temperature changes can be important
in temperature sensitive species of plants and animals. Temperature may
stimulate flowering in some plants and affect growth rates of plants
and invertebrates.
In urban areas wind speeds are reduced due
to building roughness. However, wind tunnels can form when the wind
direction aligns with straight roadways or other built features.
When compared with surrounding rural land,
urban areas tend to have
- a lower relative humidity
- more chance of fog
- and a mini “greenhouse effect” on the
immediate surroundings (warmer and elevated carbon-dioxide)
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Urban soils
Urban soils are modified from natural soils
due to the action of
- construction and related disturbance
- mixing and movement of soils within the
landscape
Often in developed areas, the rich topsoil
has been removed or mixed with subsoils during construction. Even when
topsoils is stockpiled and respread on the land it takes time for a
functional soil profile to develop.
Urban soils are often compacted. This leads
to poor drainage and limited aeration (oxygen) in the soil. Plants and
soil micro-organisms require oxygen within the soil to grow and
function properly.
Nutrients become depleted in urban soils.
Leaf litter and other organic debris are important contributors to
nutrient addition in natural soils. When there is no vegetation cover,
there is little replenishment of the nutrients leached by rainfall from
the soil profile.
It is difficult for plants to establish in
many urban soils due to
- the low soil oxygen levels
- inadequate drainage
- low nutrient levels
These conditions are also harmful to soil
micro-organisms.
Micro-organisms in the soil have an
important role in the breaking down of organic material to the
component nutrients that are used by plants.
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Waterproof ground
surfaces
Development results in construction of many
surfaces (roads, roof tops, concrete and parking areas). These form a
barrier to rainfall penetrating the soil. These surfaces are described
as impervious.
The high number of impervious surfaces has a
major impact on the hydrology (water flows) within the urban catchment.
- less water reaches the watertable leading
to a decline in water table levels over time
large areas of impervious surfaces lead to an increase in water run
off; this can increase erosion, both on cleared land and within
drainage channels
- stream flow is more variable in urban
areas. As the watertable is not recharged, there is less percolation of
water from the watertable into the stream during dry periods.
- Increased runoff increases flow during
rainfall events in urban areas
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Water
quality
Water flowing from urban areas
often contains particles and pollutants that impact on waterways. These
are described in more detail in Stream characteristics.
These include
- toxic materials
- additional nutrients (commonly nitrogen
and phosphorous)
- sediments
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Impact of urban
development on streams.
General impacts of urban development on
streams include:
- muddier streambeds due to fine sediment
entering the stream
- increased turbidity (cloudiness) of the
water. This is a consequence of suspended
- sediments coming from erosion of exposed
soil surfaces.
- increased erosion of the streambank. This
is a consequence of more water moving through the waterway at a higher
velocity.
- streambank abrasion increase due to
suspended sediments
- pollution in the form of toxic heavy
metals, fertilisers and pesticides
- toxic heavy metals such as lead, cadmium
and mercury. These pollutants can come from tyre wear and vehicle
emissions as well as commercial or industrial operations.
fertilisers. Excess nutrients in waterways often arise from intensively
managed parklands, gardens, golf courses, bowling greens, sports fields
and animal husbandry activities. The increase in nitrogen and
phosphorous can lead to algal blooms.
pesticide. These chemicals can accumulate in the food chain and affect
species far removed from the source.
Water temperatures in urban streams are
higher than that of natural counterparts due to:
- heat which is absorbed by impervious
surfaces and transferred to runoff water
- removal of riparian vegetation which
allows more light to reach and warm water in the stream
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Flooding
and other concerns
To quickly remove the excess
runoff in developed areas streams are often channelised.
This practice was more widespread in the
past and involves:
- removal of the riparian vegetation
- straightening of the stream
- concreting of the channel
Examples of channelisation are evident in
different sections of Kedron Brook.
A 4 km section of the Kedron Brook
downstream from the Webster Road Bridge was channelised following the
1974 floods.
The lower reaches of Kedron Brook were
channelised into Schultz Canal during the redevelopment of the Brisbane
airport in the 1980’s.
Concrete channels were constructed on some
of the tributaries of Kedron Brook including Sandy Creek and the Colac
Street channel.
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Air
pollution
Visible particles of dust,
smoke and exhaust materials from vehicles and other forms of fossil
fuel consumption concentrate around urban areas. Eventually, many of
these particles settle. They are washed by rainfall into the stormwater
systems and eventually reach local streams.
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Noise
Urban areas have a background of noise. The
effect on wildlife is poorly known. Some species will thrive despite
noise, such as the raptors that roost and nest in the vicinity of the
Brisbane Airport. Birds and other animals that rely on acoustic
communication may be more disturbed if the noise prevents communication
for finding mates or food.
Wildlife can also be responsible for
uncomfortable levels of noise to humans. Gatherings of crows cause
early morning disturbances when calling.
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