RFI Eng response notice items 4-19 MCU150111

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    (f) Clarification regarding the depth of water in proposed basins during major storm events particularly for
        the central basin (reference is made to Table 3-10 of the SWMP) and any proposed safety measures.

Our Response

Council officers clarified their safety concern was regarding the Central Basin where the combined pond depth and
potential stormwater detention storage being approximately 3.7m.

The following safety measures have been incorporated into the basin design to minimise access to the ponded area:

•   Thick planting and safety bench around the perimeter of the pond to discourage entry into pond;
•   Retaining walls over 1m are to be fenced;
•   There are multiple points around the basin with batter slopes flatter than 1 in 5 to enable easy access out of the basin
    area; and
•   1 in 6 access track provided at the downstream end of the basin.

It is noted that maximum ponding has a low likelihood of occurring dying any year (1% Annual Exceedance probability)
The normal ponding area has measures to limit access or risk (dense planting and shallow benches).

We have prepared a Safety in Design review for the Central Basin, attached, to further demonstrate that safety matters
have been addressed.

Item 8. Remodel with consistent surface type and runoff parameters. Otherwise, provide justification with
previous acceptance by Council Officers for the disparity between the surface type and runoff input parameters.

Our Response

After additional review a minor error was identified that had no consequential impact on the design, the parameters for
catchment G2 are commercial when they should be residential.

The MUSIC model has been updated to reflect the proposed landuse and the proposed site layout Palm Lake Resort
Dwg C0-000-2014 Rev H (Attached)

Item 9 Update the MUSIC model to be consistent with MUSIC Modelling Guidelines. In particular, confirm default
values are used for the serial correlation.

Our Response

We can confirm that stochastic generation was used and that the serial correlation values were as per the MUSIC default
value (0). No action necessary.

Item 10 Revise the MUSIC model without the pond system included as at treatment node. Otherwise,
demonstrate how constituent loads within the waters entering the pond will be at concentrations that are
unlikely to cause an algal or cyanobacteria outbreak.

Our Response

As discussed with Council officer on 13 January, we believe that the standards quoted were not applied correctly. The
MUSIC Modelling Guidelines (Water by Design, 2010) state:

         “MUSIC is generally not suitable for modelling water quality treatment in natural waterways, natural wetlands,
         naturalised channel systems, environmental buffers, and lake and pond systems. These are receiving
         environments and cannot in any case form part of the stormwater treatment train for an urban catchment”.
         (underline added to highlight).