Engineering Response · MCU23/0094

Issue: Bioretention Basin Design

Council has several concerns regarding the design and accessibility of the proposed bioretention basins. It is
acknowledged that these treatment systems will be privately maintained, however design consideration must be
given to eliminate or minimise health, safety and aesthetic hazards in accordance with PO9 of the Water Quality
and Drainage Code.
Specifically, bioretention basins should be designed without walls where possible to avoid adverse safety and
amenity impacts. Provide to Council further information and amended plans.
Stormwater Management Plan amended addressing the following: Information Required
25.     Where possible, reduce the extent of retaining walls within the bioretention basins in favour of landscaped
        batters. Refer to Table 9 of the Bioretention Technical Design Guidelines Version 1.1 (WaterbyDesign)
        for guidance on the recommended extent of walls.
26.     Provide an accurate section through Basin B to Headland Drive (is this basin actually in cut?) and show
        how an acceptable level of landscape amenity and integration will be achieved in this location and how
        safety will be addressed without the need for safety fencing. A block wall on the street frontage will not
        be acceptable in terms of landscape amenity. It is expected that batters (at least on the street side) will
        be required.
27.     Confirm that any saturation will not impact on the structural integrity of any proposed retaining walls. Refer
        to Section 3.3.5.5 of the Bioretention Technical Design Guidelines Version 1.1 (WaterbyDesign) for the
        design of these walls.

Response

Please refer to our revised Stormwater Management plans attached. Further details have now been included to
address safety concerns, this includes the provisions of a gated access and balustrades surrounding the bio-
retention and detention basins. A 1.5m wide access ramp has been provided to bio-retention bed A for
maintenance purposes. The proposal is consistent with the approved and constructed developments at 89-93
(OPW22/0100) and 99-105 (OPW21/0351) Eumundi Noosa Road.

An accurate section of proposed Bio Retention B to the headland drive road frontage has now been included into
the revised plans. We are proposing to remove the existing rock retaining wall along the boundary adjacent to the
road reserve and reinstate a grassed batter up to the front boundary. The proposal will result in a superior
landscape amenity and activation of the sites street frontage.

Structural engineering will include the design of all retaining walls (including potential impacts from saturation)
and will be completed by an RPEQ. Detailed structural engineering will not be completed until building approval
for the development will be sought.

Issue: Roofwater Capture and Re-use

  The Sustainable Building Design Code AO2.1 requires 3kL of roofwater storage per dwelling. A much lower
  figure is proposed.
  Information Request
  1.     Provide an updated SMP which includes water balance modelling for the 2 proposed roofwater storage
         tanks and which justifies the proposed size of the tanks; for example by showing that 80% of the reuse
         demand can be met with the proposed configuration. Include plans showing the areas to be irrigated
         by each tank and state reuse demand assumptions.

Response


The proposed water re-use tanks have been sized in accordance with the Sunshine Coast Council Flooding and
Stormwater Management Guidelines 2020 which suggests 1kL of storage per 25m2 of communal landscaping
area. The site consists of approx. 350m2 of communal landscaped area which would in turn require 14kL of
storage, 24kL has been proposed, which far exceeds this specification.

In addition, Waterwise Queensland – “Efficient irrigation for water conservation: guideline for water efficient urban
gardens & landscape” (attached) outlines irrigation requirements across Queensland (see extracted table below).
Based on a month’s supply using the August median irrigation rates of 15mm/week 1m2. The required storage to



SCG Consulting Engineers                              Page 3 of 5                                   SCG Ref: 33132(L1)