Attachment D - Stormwater and Water Quality report

                                                                                   Callaghan & Toth

         is 50m long and 4.2m wide, with 1 in 4 batters. Swale 4 to treat flows from the grassed
         area in the north of Lot 5

    2. New Grass Swales (Swales 2 & 3) – two new grass swales will collect stormwater flows
       from the new access road and new carpark area and divert the flows to the existing
       biobasin. The new swales are 40m and 45m long and 2.6m wide, with 1 in 3 batters.

    3. Existing Biobasin – the existing biobasin in Lot 7 is 12m x 12m giving a total filter area of
       144m2. It is proposed to treat the runoff from the new 30 space carpark, the turnaround
       and new access road, new pathways and the extra concrete/asphalt required to
       formalise the carpark areas adjacent the existing buildings. As this biobasin was
       constructed in 2006, it is proposed to refurbish the biobasin by replacing the filter media,
       installing an outlet pit and lining the basin. The treated runoff will then be directed to the
       existing stormwater outlet which discharges to Cooroy Creek. The bio-retention
       component will remove the nutrients and retard flow.


4.3      MUSIC Model
4.3.1 Source Nodes

The existing source nodes within the existing development have been maintained as per the
original MUSIC model as industrial nodes for Lots 6 SP161035, Lot 15 SP301607 and Lot 3 & 4
SP173771 (Lot area = 1.5144 ha) with the road reserve of Lower Mill Road (area = 0.346 ha).
The source nodes have been updated to reflect any changes in the MUSIC modelling guideline.

Whilst the current proposed changes to Lot 5 and Lot 7 are defined using split catchment
technique and source nodes as per the 2010 guidelines.

This approach is considered acceptable as the existing industrial lots have already been
approved with the treatment system in place. The proposed changes to the car parking at the
Lower Mill sites do not change the existing industrial uses adjacent.

MUSIC allows for the characteristics of these source nodes to be amended to ensure that
accurate pollution generation and hydraulic characteristics are used to generate stormwater
runoff within the model. Table 4.1 below details the sub-catchments used in the MUSIC model,
land use, percent imperviousness and area of each sub-catchment. The MUSIC Modelling
Guidelines for South-East Queensland – Version 1 December 2010 have been used to set
levels of imperviousness. The site has been related to a “Industrial” land use and pollutant
export parameters detailed in the Guidelines for this land use have been used for each node.




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2001 Lower Mill Carpark Extension SWMP                                          February 2020