LATE RFI - SCG 33114 (SWM) Rev A

6.0      STORMWATER QUANTITY

Noosa Shire Council’s Planning Scheme – Noosa Plan 2020, requires post development peak flows
leaving the development site will not exceed the pre-development peak flow rates for the required design
storms.
A Stormwater Quantity Analysis has been undertaken of the existing and developed site conditions in
accordance with the following guidelines.
                       −     QUDM 2017
                       −     Noosa Planning Scheme 2020
                       −     Australian Rainfall and Runoff (ARR 2019)
The following section of this report discusses the proposed developments impact on peak stormwater
discharge from the site and the proposed mitigation measures to ensure a non-worsening effect is
achieved to downstream properties.



6.1      Drains Analysis
To calculate the peak discharges for the site under existing (pre-development) and proposed (post-
development) conditions. Hydraulic modelling has been undertaken for the site using the software
package PC DRAINS, to determine peak flows. We have modelled the site to establish existing and
proposed stormwater drainage characteristics.
Rainfall patterns accompanied with Initial Loss (IL) and Continuing Loss (CL) was sourced from the
Australian Rainfall and Runoff 2019 (ARR 2019) data hub for Cooroy.
Initial Losses (IL) and Continuing Loss (CL) were approximated based on the storm losses from the
ARR Data Hub, Table 6.1 below states the adopted values: -
                                 Table 6.1 - Adopted (IL) and (CL) Values

                                    AREAS               (IL)           (CL)
                                                       (mm)           (mm/hr)
                                   Impervious            1              0
                                    Pervious             32             2.6


Design ARR 2019 storms for Cooroy have been added into DRAINS using data obtained from the ARR
2019 Data Hub accompanied with IFD rainfall depths from the Bureau of Meteorology (BOM) website.


Times of concentration (Tc) were calculated as defined in QUDM 2017 using a combination of standard
inlet time & overland sheet flow time. Table 6.2 displays the Adopted time of Concentrations.
                                          Table 6.2 – Tc Values

                               STATE                       (Tc) (mins)
                           Pre Development                        12
                           Post Development                       6


SCG Consulting Engineers                  Page 11                                SCG Ref: 33114(SWMP)