B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village

trigger levels ‘do not directly provide water quality targets for stormwater discharge or for the selection, sizing
and design of catchment management measures.’

Site specific discharges, therefore, have brief impacts on the quality of water within the river. It is considered
more appropriate to assess impacts by considering pollutant loads, given that the total load, rather than the
concentration, affects the long term water quality in the river. Furthermore, the contribution of the development
to the river inflows is small, because the development comprises only a small part of the river’s wider catchment.
The Mary River’s wider catchment consists of a large proportion of open space and natural bushland, which
contributes low contaminant loads relative to an impervious development site. Therefore, concentrations from
the total catchment are more representative of guideline values for receiving waters.

Healthy Waterways (2006) says that the substantial increase in runoff volume that accompanies urban
development can increase pollutant loads to receiving waters, even if concentrations are not increased.

ANZECC (2000) states that water bodies over large continuous tracts of land are highly disturbed and adjacent
water bodies are not necessarily of better quality than the water body of interest. The water body over a large
tract of land, is in this case the Mary River, while the water body of interest would be Six Mile Creek. Figure
3.1.3 in ANZECC (2000) shows that a water body includes lowland rivers and streams, which are applicable to
this site. In regards to pollutant loads, ANZECC (2000) says that excessive growth of aquatic plants occurs
when there are high concentrations and loads of nutrients. However, biomass is more controlled by the total
mass of nutrients available to growing algae.

It is therefore concluded that load reduction targets are considered as a more appropriate water quality objective.

This report, however, will demonstrate that concentrations have been considered to an acceptable extent.

The following load reduction targets which satisfy water quality objectives (WQOs) of the Healthy Waterways
(2006) Water Sensitive Urban Design – Technical Design Guidelines for South East Queensland (2006) are
listed in Table 6.2. The load reduction targets of Healthy Waterways (2006) have been adopted for the purposes
of this Stormwater Quality Management Strategy. Healthy Waterways (2006) are the regionally adopted
guidelines for load reduction targets. The design objectives are reductions in mean annual loads leaving
developed areas, compared to traditional urban design where stormwater is not treated (Healthy Waterways,
2006).

                                       Table 6.2: Load Reduction Targets*

                                    Pollutant                  Load Reduction Target (%)

                         Total Suspended Solids                             80

                         Total Phosphorous                                  60

                         Total Nitrogen                                     45

                         Gross Pollutants                                   90
                                            * Healthy Waterways (2006).




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