F - Traffic Response and Revised Traffic Impact Assessment (17.09.19)

Stormwater quality and quantity
Issue – Stormwater Quality
The litter baskets and rainwater tanks are supported, however, the proprietary devices are
not. The rainwater tanks have a high reuse demand which presumably is based on irrigating
all of the landscaped areas of the site and possibly the retained covenant vegetation. This
may not be appropriate and further details should be provided on their assumptions.
Information Required
39.   Provide further information and/or a revised design and model as necessary to address
      the following items:
      a.    Provide justification for the assumed annual reuse demands for each rainwater
            tank and provide estimates of the source reliability (i.e. how much of the demand
            will need to be met by mains back-up supply). In general, use of irrigation water
            for landscaping is not encouraged and low-water-usage native plants should be
            used in accordance with Specific Outcome O3 of the Water Sensitive Design
            Code. Water reuse should be focused on areas of water demand which cannot be
            addressed through changes in design/planting – such as for toilet flushing.
      b.    The use of proprietary underground devices for nutrient removal is in conflict with
            O7 of the Water Sensitive Design Code. Provide a revised stormwater treatment
            train and revised landscape design for the car parking areas which utilizes
            vegetated treatment strategies to achieve multiple benefits.
      c.    Provide details of the assumed removal rates for the litter baskets and/or the
            MUSIC model for review.
Issue - Hydrology

The pre-development hydrologic model uses relatively low initial loss values, particularly for
the rarer AEP events. The IWMP acknowledges that the values differ from those provided by
ARR Datahub but states that the values adopted are conservative. While the values (low initial
loss) would be conservative if the purpose was to undertake flood mapping or estimate design
flood levels, this approach is not considered conservative for the stated purpose of determining
hydrologic and flood impacts of the proposal. Low initial loss values increase pre-development
peak flows and peak flood levels which in turn can underestimate hydrologic and flood impacts
and required flood detention storage volumes. This issue is highlighted by the predicted
RAFTS flows being up to 63% greater than the rational method estimates for the pre-
developed case.
The subject site is subject to local overland flowpath flooding from external catchments located
to the North and West of the subject site which are of the order of 7Ha in size. These
catchments are conveyed through the site as overland flow in a South-Easterly direction and
ultimately discharge under Hofmann Drive to an existing conservation area.
The proposal has the potential to worsen flooding conditions upstream of the site by
blocking/modifying this overland flowpath and has the potential to worsen downstream
flooding by increasing peak flowrates downstream of the site by increased runoff peaks and
volume (due to increased imperviousness of the site).
Information Required
40.   Either provide further justification or revised initial loss values for the pre-development
      pervious areas. Reference should be made to ARR2016 and the ARR Datahub and the
      discussion in QUDM Section 5.6.1. Hydraulic modelling and impact assessments should
      also be revised accordingly.




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