Appendix 4 - Engineering Report (00012423)

DESIGN OBJECTIVES

This preliminary assessment has included both the water quality, quantity and flooding aspects. The design
objectives applied are:

    1. Stormwater Quality Objectives (Load Based Reduction Targets)
           Total Suspended Solids – 80%
           Total Phosphorus – 60%
           Total Nitrogen – 45%
           Gross Pollutants – 90%
    2. Stormwater Quantity Objectives
           Non-worsening of peak flows from the development site. That is, the peak post-development flows
            should not be greater than the peak pre-development flows.

    3. Flooding
           Site and buildings to be located outside the 100 year AEP flood extent.

STORMWATER QUALITY

Preliminary stormwater quality modelling has been undertaken using MUSIC version 5. The site has been
represented as a lumped urban source node with an impervious area of 70%. As rainwater tanks are no longer
mandatory, they were not included as part of the treatment train. To achieve the objectives the entire site was
directed to an end-of-line bioretention system. Preliminary calculations estimate that a bioretention area of
approximately 35m2 will be required to service the site. This represents approximately 1.4% of the site.

Bioretention is a conventional treatment measure that generally forms part of a landscape feature on the site.
Alternative methods, such as StormFilters, do exist which function to treat stormwater via cartridge filters and
are located in a stormwater manhole. If usable space on the site is required to be maximised these systems can
provide a solution for a similar magnitude of cost. It is noted that unlike a bioretention system where capital
cost is upfront, for the cartridge filter system option there is no upfront capital cost and instead the body
corporate would be required to enter into a lease arrangement for the upkeep and maintenance of the system.

STORMWATER QUANTITY

Preliminary detention calculations were undertaken to determine the volume of storage that would be required
to ensure that non-worsening of peak flows results from the development. The existing site has an impervious
area of approximately 20%, based on the houses currently located there. As a result of the development this
impervious area increases to 70%. As part of the calculations, no external catchments were assumed to affect
the site. . Preliminary calculations estimate that a detention volume of approximately 30m3 will be required to
ensure non-worsening of flows is achieved from the site.

As the site drains to the front of the site, a lawful point of discharge is achieved to Doonella Street.



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