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

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village

                          Table 6.8: Concentration Results for Various Scenarios*

                                                               Pollutant Concentration
 Scenario
                                       Total Suspended Solids           Total Phosphorous            Total Nitrogen
                                               (mg/L)                          (mg/L)                    (mg/L)
 Existing Site
                                                25.4                            0.133                      1.2
 (Agricultural Node)
 Developed Site
 (No mitigation measures)                       16.2                            0.153                      1.80
 plus an External Catchment
 Developed Site
 (With mitigation measures)                     8.62                            0.073                      1.15
 plus External Catchment
 WQOs                                             6                             0.050                     0.500
                *Concentrations given are median values and are flow based sub-sample results for flows > 1 L/s

As the site was recently an orchard with fruit trees, a MUSIC model has been run for the site with an Agricultural
source node to represent the site. Table 6.8 also shows that the existing site concentrations are significantly
higher for TSS and TP than from the mitigated developed site. Therefore, it is argued that the proposed
treatment train of SQIDs is effectively treating developed scenario runoff to better quality than runoff from the
existing scenario.

As can be seen in Table 6.8, the proposed SQIDs are effective in reducing concentrations from the developed
scenario with an external catchment by approximately one half for TSS and TP and by approximately one third
for TN. As there is a reduction of concentrations from existing conditions, that the point site discharges are not
downstream ambient water quality levels and the high variability of concentrations off a small catchment, the
concentration outcomes are considered to be within an acceptable level. Net load reduction achieved is high
and concentration levels are low.

It should also be noted that MUSIC has various limitations when modelling concentrations. The default k and C*
variables adopted within MUSIC treatment nodes effectively limit the level of treatment achievable, especially for
TN. These default variables will produce relatively high outflow concentrations irrespective of inflow
concentrations irrespective of inflow concentrations or size of treatment device.

Treatment of stormwater runoff from external site areas of the railway corridor and Cooroy Connection Road
adjacent to the site is not required by Council. Not treating these external areas would be difficult in practice as
they drain through the site and could not practically be diverted around treatment measures. The opportunity to
treat stormwater runoff from these areas has been taken and will benefit the water quality of downstream
systems. Significant achievements have been demonstrated for improvements to water quality runoff.




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