Human Settlements Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Professor Peter W. Newton, CSIRO Building, Construction and Engineering, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06747 7
Waste, recycling and reuse (continued)
Urban stormwater (continued)
The urban flood problem in Australia is not large. It is estimated that 1% of the Australian population is at risk from riverine flooding. The annual cost of flood damage can run into millions of dollars - estimated in 1993 to be between $20m and $50m for the Sydney region alone (Dowsett 1994). Table 60 shows the number of dwellings at risk of flooding and the direct actual damage for states and territories due to a once in 100 years flood event. New South Wales and Queensland together have 80% of the national total of flood-prone dwellings (Smith 1998).
|State||Number of buildingsA||Annual average damage
|New South Wales||65 000||24.0
|Western Australia||5 800||1.2
|South Australia||1 500||1.1
|Northern Territory||2 000||0.4
ANumber of buildings as at 1996.
BAverage annual damage at 1996 prices: estimates of damage limited to dwellings liable to flooding from the once in 100-year event.
Source: Smith (1998).
Since the figures in Table 60 estimate the damage of a once in 100 years event only, the average annual damage caused by all flood events will be considerably higher. For example, the average annual flood damage for the whole of Victoria has been estimated to be some $55.7 million (DNRE 1998).
In future, climate change could alter the hydrological regime to one with more intense, more frequent or heavier rains in many regions. Forecast changes in runoff, soil moisture and sea level rises by the year 2030 and beyond are significant enough to require mitigation and adaptation (DNRE 1998). For more detail on climate change and its settlement impacts, see Emerging Issues.
Drought affects human settlements in a number of ways. In regional areas, crop and stock losses result in loss of income for farmers, which will have consequences for supporting townships and businesses. Environmental damage from drought, such as vegetation loss and soil erosion, threatens the sustainability of agricultural enterprises (Bureau of Meteorology 2000). In urban areas, the impact is on water availability and consequent restrictions imposed by water authorities on local populations.
Urbanisation of a catchment generally leads to the lowering of stormwater quality. Urban stormwater contaminants comprise fine particles and dissolved materials (micropollutants), as well as litter and vegetation (gross pollutants). Natural sources of stormwater contaminants are derived from the atmosphere, from organics in the soil profile, and from decaying organic debris (see photograph below). Sources of contamination which can be attributed to human activity include sediment transmission from construction sites, pesticides and fertilisers, litter, faecal matter, vehicle emissions, metal particles from corrosion and abrasion, spills of substances such as oil and paint on land surfaces, and air pollution emissions. After stormwater enters the drainage system, its quality can be lowered further by sewer overflows and the infiltration of poor-quality water leaching from landfill or septic tank sites (Newman and Bishaw 1985, SPCC 1989, O'Loughlin et al. 1992, Chiew et al. 1997).
Stormwater quality varies greatly between locations, although the values quoted in Table 61 provide a guide to the typical range in contaminant concentration observed across Australian urban areas. Values for urban areas worldwide are given for comparison. On average, the level of stormwater contamination observed in Australia is greater than recorded in other parts of the world, and exceeds stormwater quality guidelines such as the Victorian State Environment Protection Policy (SEPP) (Government of Victoria 1988). Although, due to the high variability in quality between locations, stormwater contamination levels are better than guideline levels in some locations, as illustrated in Figure 78.
|Pollutant||Guidelines for urban waterwaysA||Australian data set||World data setB|
|Suspended solids||50 (90)D||141.0
|Biological oxygen demand||n.a.||15.1
A Values are limits for geometric means based on at least five samples in 42 days.
B World data set does not include Australian data.
C Range is taken to be one log standard deviation of the mean.
D Annual median limit with 90% of samples below value in parentheses.
n.a. - not applicable.
Source: Mudgway et al. (1997); Government of Victoria (1988).
Figure 78: Water quality in the Port Jackson catchment, 1998-99.
Source: Sydney Water Corporation (1999).
The levels of suspended solids, total nitrogen, copper, and biological oxygen demand observed in the Australian urban stormwater data are similar to those observed throughout the world. Lower total phosphorus concentrations in Australia may be due to the lower phosphorus content of Australian soils, or the lower contribution from vegetation during periods of suppressed growth caused by water stress (Mudgway et al. 1997). Higher concentrations of zinc and cadmium may be associated with the prevalence of galvanised products such as iron roofs in Australia. Chromium is used in a wide range of products found in the urban environment, including metal alloys, dyes, paints, pesticides and fertilisers. Mudgway et al. (1997) suggested that degradation of painted surfaces caused by stronger sunlight may cause the higher concentrations of chromium in Australian urban stormwater. They also suggested that lower concentrations of nickel could indicate a lower use of nickel plating in comparison to the rest of the world.
The quality of stormwater is often said to be determined primarily by land use, but views differ about the level of importance (Newman and Bishaw 1985). Recent research has shown that the link between land use and stormwater quality in an urban area is less important than commonly assumed (Duncan 1997). The importance of a preceding dry period on urban stormwater runoff quality is also unclear. A number of studies have concluded that the length of the dry period affects the runoff quality (Field and Fan 1981, Stephenson and Green 1988), while others have found its affect to be insignificant (Mance and Harman 1978, Chiew et al. 1997).
Gross pollutants are large pieces of debris, such as litter and vegetation, flushed through urban catchments and stormwater systems (Allison et al. 1997). Monitoring has found that urban areas contribute some 20-40 kg/ha/year (dry mass) of gross pollutants to stormwater systems, which for Melbourne translates into 60 000 tonnes of gross pollutants per year (Allison et al. 1997). Figure 79 illustrates the composition of gross pollutants by mass sampled in Coburg (an inner suburb of Melbourne), clearly showing that vegetation such as leaves, twigs and grass clippings makes up the largest proportion, followed by a range of litter types.
Figure 79: Composition of gross pollutants by mass in Coburg, Melbourne, for a compilation of storm events, 1996. [HS Indicator 2.8]
Source: Allison et al. (1997).