Reference Documents - MCU23-0142

The modelling results show the pollutant reduction objectives are met for all contaminants listed in
Table 5.1. Details of the proposed bio-retention basin arrangement are provided in the Concept
Stormwater Management Plan drawing No.: SCG 33094- CS.1 (Appendix D).




6.0      STORMWATER QUANTITY:
   Noosa Council requires post development peak flows leaving the development site will not exceed
   the pre-development peak flow rates (non worsening) for the required design storms. However, we
   don’t believe on-site detention should be required for the proposed development site for the
   following reasons.

   1. As a general rule, stormwater detention is less likely to be required at the bottom one-third of
      the catchment and therefore by detaining stormwater flows it may allow peak flows from the
      site to coincide with the wider catchment peak flows and therefore result in an adverse impact
      on the existing drainage infrastructure.



   2. The site discharges directly into the ocean/ river reserve where it would generally be undesirable
      to have detention where it may allow peak flows from the site to coincide with the wider
      catchment peak;



   3. The proposed residential development fits within the existing zoning of fraction impervious as
      per the Noosa Plan



   4. Flooding at the discharge point is controlled by river/ storm tide flooding.



However with the above noted Noosa Council have insisted that on-site stormwater detention is
provided for the proposed development. It is noted the proposed development site was never intended
or zoned as a greenfield site (0% impervious) and therefore we are of the opinion that adopting a fraction
impervious of 0% will result in excessive infrastructure to achieve detention requirements for non-
worsening. As such, as a conservative approach we have adopted a fraction impervious of 40% for the
existing site.

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 on peak
flows. However as noted above as the site is located in the lower third of the catchment, detaining flow
rates may have the potential for the discharge mitigated peak flows to coincide with that of the wider
catchment resulting in an adverse effect.



 SCG Consulting engineers                            11                      SCG Ref: 33094(SWM).Rev A