Glen - REC13/0146 Flood Study BSM 30 April 2014. Email 2 Attachment 2

2.0      INTRODUCTION

This report describes Brisbane Stormwater Management P/L’s (BSM’s) stormwater detention and
flooding assessment of a proposed subdivision of Lot 2 on RP173883 and Lots 135, 136 and 137 on
MCH978, at Elm Street and Lake Macdonald Drive, Cooroy. The site locality and the proposed lot layout
are shown in Figure 1 of this report.

The site is traversed by two tributaries of Six Mile Creek, hereafter described as the eastern and western
tributaries. The catchment areas of the two tributaries are shown in Figure 4, and the tributary floodways
can be seen in more detail in Figure 6.

Some of the increased stormwater runoff from the proposed residential areas is proposed to be mitigated
by conventional detention volumes in two bio-retention basins, but most of the increased runoff is
proposed to be attenuated by an increase in the floodway storage on the site. The increased floodway
storage will be achieved by deliberately “roughening” the floodways (by revegetating the riparian zones,
as required by Council) and by constructing a restrictive footbridge across the eastern tributary to back-up
the flood profiles in that tributary floodway.

Hydraulic model analyses of the interaction between increased site runoff and floodway through-flows
can be skewed by “hydrograph separation”, whereby the urbanisation of a particular site is predicted to
hasten the discharge of the increased site runoff down the floodway before the arrival of the major
floodway flows from the upstream catchment – which can lead to a perception that the peak flow-rates
along the floodway will not be significantly increased, which in turn leads to a perception that little or no
attenuation volume is required. This perception may be valid for a site development that is considered in
isolation, but it becomes invalid when the “equal opportunity” development of all such sites in the
catchment is collectively considered.

To overcome the abovementioned modelling difficulty without resorting to a catchment-wide “equal
opportunity” analysis, this assessment has firstly determined the detention volume that would be required
for a fully-conventional system of dedicated basins on the site’s stormwater outlets. The assessment was
thereafter focussed on achieving at least that volume from the proposed combination of bio-basin
detention and increased floodway volume, while also using a floodway model to verify that the proposed
changes to the floodway profiles on the site would not adversely affect the flood profiles upstream and
downstream of the site.

The development proposal is particularly cognisant of the need to not increase local stormwater
discharges (as distinct from tributary channel through-flows) onto an adjoining property to the north –
namely No 2 Pearsons Road (refer Figure 1). This is proposed to be achieved by diverting part of the
runoff from the relevant sub-catchment, directly across to the western tributary floodway within the site.
This diversion proposal had to be accounted in the floodway modelling process.

The assessment process was therefore structured as follows:

-     Determination of the required detention volume - as described in Section 3 of this report.

-     Devising a diversion mechanism, in liaison with the applicant’s civil consultant (Callaghan & Toth),
      to ensure no increase in stormwater discharges to No 2 Pearson Road – as described in Section 3.

-     Calculation of the runoff rates to the floodways and running them though floodway models of the pre
      and post-development scenarios – as described in Sections 4 to 7.



      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                             Page 4