Attachment D - Stormwater and Water Quality report

                                                                                Callaghan & Toth


4.0 WATER QUALITY MANAGEMENT PLAN
4.1      Background Lower Mill Site Stormwater Management

The Lower Mill site was developed by Council in 2005 and resulted in the creation of industrial
lots and community use lots.

The lot reconfiguration involved a stormwater management plan and MUSIC model which
included the new industrial lots i.e. Lots 3, 4 & 6 SP173771, Lot 15 SP301607, new part
industrial/part community use Lot 5 SP213104, community use Lot 7 SP173771 and Lot 2
RP47020.

MUSIC version 2.01 was used to determine suitable treatment nodes for the development and
the model determined the following treatment system:
      1. GPTs in each gully pit.
      2. Grass Swales will treat stormwater runoff from roads, carparks and footpaths.
      3. Bio-Retention Systems – one adjacent the bridge on Mary River Road and one in an
          easement in Lot 7.
           The Isolated Bio-Retention Basin designed to have an extended detention depth
              of 0.4m and consist of a 0.8m deep sandy loam layer underlaid with a 0.2m deep
              coarse sand layer.
           The Bio-Retention Pods adjacent roads and carparks designed to have a
              maximum extended detention depth of 0.3m and consist of a 0.3m deep sandy
              loam layer underlaid with a 0.25m deep coarse sand layer.

In addition to this original development, Lot 5 SP213104 was developed in 2006 with the
addition of the Cooroy Camphor Laurel Club shed and associated carparking. A MUSIC model
was prepared for this proposed development and it was determined that the roofwater would be
treated by a rainwater tank which would flow to a bioretention basin (S.A. = 12m2) which would
also treat the carpark areas. It has been noted during site inspections that this rainwater tank
and bioretention basin have not been installed.

4.2      Proposed On-Site Stormwater Treatment Process

The site i.e. Lot 5 and Lot 7 showing the new works was divided into MUSIC model sub-
catchments as shown on Plan No. 2001-01. The previously developed site i.e. the lot
reconfiguration that developed the industrial lots, have also been included in the MUSIC model.
These catchments have been modelled as per the original MUSIC model and the proposed
treatment train for those catchments have not been altered, as it is considered that these works
were completed as per that design and approval conditions.

The new hardstand areas will be mostly diverted to existing treatment systems, particularly the
biobasin (Biobasin 1 – BB1) located on Lot 7 SP173771.

The proposed treatment train for the new hardstand areas for the site is also illustrated on Plan
No. 2001-01 and Fig 4.1 (MUSIC model layout) and listed below:

    1. Existing Grass Swales (Swale 1 and 4) – Swale 1 to treat overland flow from the carpark
       adjacent the buildings (on Lot 5) prior to flowing into one of the new grass swales which
       directs the flow to the biobasin. The existing swales will be reinstated during works and



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2001 Lower Mill Carpark Extension SWMP                                       February 2020