Ltr-20190925-IR Response-Noosa Marketplace_FINAL (1)

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 predeveloped 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.
    41. Demonstrate the modelling has considered the external catchments to the north and west of
        the site.

        To the north
        o Eenie Creek Road formation and Easement T on SP182880.
        o Eenie Creek service road adjacent to the northern property boundary.
        To the west
        o   Commerce Court – through No 3 Commerce Court and Easement G on RP230888.
Response to sub-item 40 and 41:
Prior to this development application there has been no detailed modelling of this site and the
surrounding area. During the development of the base case flood model several issues were
identified. These include, non-compliant drainage outlets, undersized drainage infrastructure and
inundation of local roads during minor events.
Hydraulic modelling has been revised based on ARR2019 and QUDM Section 5.6.1. It should be
noted that QUDM 2017 states to refer to the latest ARR version for guidance regarding initial and
continuing losses.

As detailed in Section 7.4.2 of Appendix H, the modelling of the existing scenario (pre-development)
shows that in all AEP events modelled, Hofmann Drive as well as Eenie Creek Road experience
inundation from flood waters.

In the 2% AEP event, Hofmann Drive is inundated to a maximum depth of 200 mm, increasing to 300
mm in the 1% AEP event at the roundabout on Hofmann Drive near the site access. The current 1200
mm x 300 mm RCBC under the main site access restricts flows along the eastern boundary of the site.
These flows then spill out onto Hofmann Drive, flowing south along Hofmann Drive and finally entering
Rene Street.


 Ltr-20190925-IR Response-Noosa Marketplace_FINAL                                                     74