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RFI -21-10-13 - 20169 - RFI Response
Our Ref: 20169 Council Ref: MCU20/0150.01 Wednesday, 13 October 2021 Noosa Shire Council PO Box 141 Tewantin QLD 4565 Attention: Patrick Murphy RE: Information Request Response – Bioretention Basin Design, Rainwater Tanks, Flow Concentration and Climate Change Proposed Townhouse Development at 101 & 105 Eumundi Noosa Road, Noosaville In reply to the information request issued by Noosa Shire Council on 20 September 2021, we provide the following information and response: Item 1: Demonstrate how PO7, AO7.1, PO9 and AO9.2 of the Water Quality and Drainage Code will be achieved. In particular, how visual integration, safety and maintenance access are achieved in the proposed design. Response: Refer to the updated Stormwater Management Details (20169-SK12-D and 20169- SK13-D) attached to the updated Site Based Stormwater Management Plan (20169-SBSMP-Rev 4) which incorporates a 1 in 4 maintenance access ramp for each bio-retention/detention basin for maintenance staff to access the base of each basin. In accordance with Table 13 of the Bioretention Basin Technical Design Guidelines by Water by Design, catchments less than 2ha do not require a sediment forebay. However, scour protection has been provided at the inlets into the basins. It is anticipated that a safety fence will be installed around the perimeter of each basin and a lockable gate located at the maintenance access ramp to prevent public access. In addition, education signage will be installed at each basin location to inform the residences of the function each basin provides. Refer to the landscape architectural drawings for the proposed landscaping design surrounding the basin areas. Item 2: Update the SWMP to state all assumptions regarding the rainwater tank and reuse, including a plan showing communal landscaped areas intended to be irrigated. This is required so that hydraulic services plans can accurately reflect the SWMP intent. Response: Refer to section 6.3.2 of the attached updated Site Based Stormwater Management Plan (20169-SBSMP-Rev 4) outlining the landscaping areas of the site that the rainwater tank is intended to irrigate. D:\JOBS\20-Jobs\20169 - 101 & 105 Eumundi Noosa Road, Noosaville\Civil\Correspondence\Letters\21-10-13 - 20169 - RFI Response.doc
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Page 2
MORGAN CONSULTING
E NGINEERS P TY LTD
20169
Page 2
Item 3: Investigate and confirm the existing discharge patterns from the development
site in the North-Eastern corner of the proposed development and demonstrate
that the design will not result in any changes in flow patterns or discharge onto
additional land which is not inundated by stormwater runoff from the site in the
existing case.
Response: The existing drainage catchment and flows have been demonstrated on the
updated Concept Stormwater Catchment Plan (20169-SK20-E) for the north
eastern corner of the site. As shown in the updated Stormwater Management
Details – Sheet 2 (20169-SK13-D) all flows up to Q100 are contained within the
basin. The high flow weir has been designed as the top of the proposed pit
structure within the basin. The proposed weir overflow over the retaining wall
across the boundary will be in emergency cases only. Therefore, all flows up to
Q100 is not expected to inundate downstream properties.
The wall of the basin has also been set back 300mm from the downstream
boundary to ensure that any overflow has an opportunity to return to existing
conditions prior to crossing the boundary.
Furthermore, a site investigation reveals that the existing earth bund does not
exist and it is more so a local depression surrounding the existing stormwater
pit. Therefore, it is expected that any stormwater runoff (likely in major storm
events) exceeding the capacity of the existing pit will likely surcharge and sheet
flow across boundaries of Lot 79 and 80, meaning that the proposed basin will
improve drainage conditions across the boundary as a result of withholding flows
up to Q100 storm events.
Item 4: Investigate and confirm the existing discharge patterns from the development
site in the South-Eastern corner of the proposed development and demonstrate
that the design will not result in any changes in flow patterns or discharge onto
additional land which is not inundated by stormwater runoff from the site in the
existing case. Confirm the weir design is able to be realised within normal
construction tolerances.
Response: The existing drainage catchment and flows have been demonstrated on the
updated Concept Stormwater Catchment Plan (20169-SK20-E) for the south
eastern corner of the site. As shown in the updated Stormwater Management
Details – Sheet 1 (20169-SK12-D) all flows up to Q100 are contained within the
basin. The high flow weir has been designed as the top of the proposed pit
structure within the basin. Therefore, the proposed weir overflow over the
retaining wall across the boundary will be in emergency cases only. It is
anticipated that the weir design is able to be constructed within normal
construction tolerances. Flows will not inundate properties that are not
currently inundated.
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MORGAN CONSULTING
E NGINEERS P TY LTD
20169
Page 3
The wall of the basin has also been set back 300mm from the downstream
boundary to ensure that any overflow has an opportunity to return to existing
conditions prior to crossing the boundary.
Item 5: Provide an updated SWMP which includes stormwater detention modelling and
updated drainage designs which includes allowance for the 1%AEP event at end of
planning period 2100.
Response: Refer to section 5.4.5 of the attached updated Site Based Stormwater
Management Plan (20169-SBSMP-Rev 4) for the considerations in regards to the
allowance for the 1%AEP event at end of planning period 2100.
Please contact me if you have any further queries on (07) 3369 8411.
Yours faithfully,
MORGAN CONSULTING ENGINEERS PTY LTD
Henry Morgan
Director