ATTCH 3 - IR Response - Erosion, Coastal Processes and Stormwater (Dr Trevor Johnson)

Kenlynn Pty Ltd Report Number 620.12166-R01

Castaways Beach 3 November 2017
Stormwater Management Plan Version v0.1
Page 10

 

e Saturated Hydraulic Conductivity = 180 mm/h
e Exfiltration Rate = 180 mm/h
e Filter Depth = 0.5 metres

Default values were adopted for the TN Content of Filter Media (800 mg/kg) and the Orthophosphate
Content of the Filter Media (55 mg/kg).

The adopted infiltration rate of 180 mm/h is significantly lower than the value used for the hydrologic
analysis (as discussed in Section 2.4). Thus, the MUSIC analysis is considered to be conservative.

It is noted that the size of the bioretention basin is large compared to the size of the contributing
catchment, being 100/1000, or 10% of the catchment. In comparison, a typical bioretention basin
which complies with the load reduction targets for Queensland is 1.5% of the contributing catchment
area. Thus, the proposed bioretention basins are in the order of 6.7 times their required size.

The results of the MUSIC model analysis are shown in Table 2.

Table 2 MUSIC Model Results

 

 

 

 

 

Pollutant Source Load Residual Load Reduction Water Quality
(kglyr) (kg/yr) Achieved Objective
Total Suspended Solids 192 0.3 99.8% 80%
Total Phosphorus 0.38 0.00098 99.7% 60%
Total Nitrogen 22.8 0.00761 99.6% 45%
Gross Pollutant 22.8 0 400% 90%

 

These results demonstrate that the bioretention basins provide acceptable outcomes in relation to the
water quality treatment of each proposed allotment, and hence addresses Item 9 of Council's
Information Request.

SLR Consulting Australia Pty Ltd