Engineering Response · MCU23/0094

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Information Request
19.     Provide a revised Stormwater Management Plan (SMP) which addresses the following:
        a.      Provide a revised stormwater quantity analysis which identifies changes in peak flows at each of
                the three proposed discharge locations. Where peak flows are still predicated to increase post-
                mitigation, then an analysis of the downstream drainage network is required in order to show that
                the performance of the drainage network will not be detrimentally affected. In this regard:
        i.      Provide an assessment for the downstream drainage network from Point B in the 1%AEP event,
                demonstrating that the QUDM major event criteria are still achieved (refer QUDM s7.4.3);
        ii.     Provide an assessment for discharge point A, demonstrating that the QUDM major/minor event
                criteria are still achieved in Eumundi-Noosa Rd; and
        iii.    Undertake a HGL analysis for the inter-allotment drainage system downstream of discharge Point
                C, showing the AEP when the system currently surcharges and surface flows through the
                downstream lots occur. Show that the frequency of surface flows through these lots will not
                increase post-development.
        b.      Include the predicted effects of climate change by including an assessment for the 1%AEP with
                increased rainfall intensity at year 2100 for each discharge location.
        c.      The engineering plan SW1.2 has details for the outlet from bio-detention basin C which differ from
                those stated in the SWMP and the 1%AEP peak water surface level on the plan is blank. Amend
                and include 1%AEP peak water surface level at year 2100.

Response

Please refer to our revised Stormwater Management Plan attached. The revised proposal has reduced the site’s
discharge points to Headland Drive and the existing inter-allotment drainage network connection in the north east
corner of the site. The revised proposal maintains existing peak flow rates at each discharge point therefore,
assessment of Council’s existing network is not considered necessary.

The Noosa Plan does not provide design criteria for regarding climate change effects to rainfall intensity nor does
it state it as a requirement. Therefore, an assessment of the predicated effects of climate change is unable to be
verified.

Issue: Stormwater Quality

The Stormwater Management Plan (SMP) provided by SCG Consulting Engineers (Reference 33132(SWMP),
Date 19 July 2023) does not include reuse assumptions and bioretention parameters required to demonstrate
compliance with the State Planning Policy and Noosa Plan 2020.
Information Request
24.     Provide and updated SMP which addresses the following issues relating to stormwater quality:
        a.      State all reuse assumptions (rate of reuse and identification of areas to be irrigated from each
                tank)
        b.      State all bioretention parameters (filter media depth, filter media area, extended detention depth,
                hydraulic conductivity etc.) required for each bio-retention bed in Section 6.1.4 of the SMP.

Response

Please refer to our revised Stormwater Management Plan attached, which includes further design assumptions
adopted in the adopted MUSIC Model. As a conservative approach, we have removed the modelling for the
rainwater harvesting tanks. A summary is as follows: -


                                                  Bio-Retention A               Bio-Retention B
                 Filter Media Depth                   400mm                         400mm
                 Filter Media Area                     50m2                          30m2
                 Extended Detention Depth             200mm                         200mm
                 Hydraulic Conductivity               180mm/hr                      180mm/hr




SCG Consulting Engineers                            Page 2 of 5                                   SCG Ref: 33132(L1)

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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)

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service all communal landscaped areas within the development would require approximately 21 kL of stored
water. Therefore we consider the proposal for 24kL adequate for the development.




As a side note, we are of the opinion that the requirement for 3kL of storage per dwelling for a multi level town
house/unit development is not a sustainable design outcome. The total roof area for the development would not
service the volumes suggested resulting in unnecessary infrastructure.

Issue: Erosion and Sediment Control

While it is accepted that detailed erosion and sediment control (ESC) plans can be prepared post-approval and
prior to site works commencing, the plans supplied with the application have significant conceptual issues which
may impact the possibility of acceptable detailed ESC plans being prepared.
Information Request
34.      Provide updated preliminary/conceptual ESC plans which address the following:
         a.       A single Type-D sediment basin is proposed. This is not considered capable of achieving the
                  SPP2017 construction-phase targets and has not been best-practice for some time. A revised
                  strategy is requested incorporating high-efficiency sediment basins.
                  Plans are only provided based on existing levels. Provide additional plans showing controls
                  proposed based on final levels.

Response

The submitted ESC plan is preliminary and conceptual only. Design input is required from the future principal
contractor as to how the site will be managed with regards to construction sequencing and similar matters. On
that basis we request that this information be provided post approval and prior to works commencing.
Issue: Extent of Earthworks
  Council requires additional details of the extent of earthworks proposed to clarify construction impacts and
  address the Acid Sulfate Soils Overlay Code and Earthworks Code.



SCG Consulting Engineers                            Page 4 of 5                                  SCG Ref: 33132(L1)

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  Information Required
  35.    The total cut and fill volumes across the site.
  36.    A post development contour plan.
  37.    The height of all proposed retaining walls.
  38.    Confirm if the extent of cut and fill will be assessable per Figure 5.9.1.1 below.
  Note - that Acid Sulfate Soil reporting will be required if earthwork volumes exceed those specified below.




Response

Please refer revised plans attached to include additional information regarding total cut/fill volumes and retaining
wall heights. No earthworks (including pipe trenches) have been proposed below RL5.0m, therefore in accordance
with the State planning policy 2017 and the above extracted image, Acid Sulphate Soil testing and management
are not assessable.

If you require any further information, please do not hesitate to contact our office.

Yours faithfully


        Digitally
        signed by
        S
        Richardson




TOM HARRNGTON – RPEQ 27698
For and on behalf of
SCG CONSULTING ENGINEERS

Enc   SCG Drawings Numbers: 33132 – CS, CN, OW1.1 thru OW2.2, OW3.1.B thru OW4.2.B, OW4.3,
OW4.4.B, OW4.5, OW5.1.A, OW6.1.B, OW6.2.A, SW1.1.B thru SW1.4.B




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