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20220206 48 Elm St OPW RFI Response · OPW24/0043
28 June, 2024 Noosa Shire Council PO Box 141 Tewantin QLD 4565 Attention: Morgan Gaffney Dear Morgan, Re: Response to Information Request for Operational Works Application OPW24/0043 for proposed commercial development at 48 Elm Street, Cooroy Please see our response below to the following engineering matters raised in Council’s above mentioned Information Request dated 2 May 2024. Issue – Retaining Structures The proposed layout plan shows up to a 2m finished level difference between the building pad and carparking/footpath area. The location and heights of retaining structures required to stabilise these earthworks is unclear. Information Required 1. Amend the plans to show the location and heights of any proposed retaining structures. Or alternatively, include additional details of proposed stabilisation methods for these areas. Response: We have updated our previously submitted engineering drawings to include locations and details of proposed retaining wall structures. Please see attached drawings for details. It should be noted that the majority of retaining walls will be part of the building structure and will be detailed on structural engineering drawings by others. The maximum height of retaining wall is 1.5m. Please refer to Drawing 020 for further details. Issue – Stormwater The Stormwater Layout Plan nominates Structure A-1 as the bio basin, Structure A-2 as the OSD tank and Structure A-3 as the existing gully pit on Pearl Street, with green arrows assumed to be the indicated direction of pipe flow as highlighted below. The pipe invert levels shown on the Stormwater Drainage Long Section however indicates that stormwater is flowing from the existing gully pit (A-3) to the OSD tank (A-2), and then to the bio basin (A-1). Overflow from the carpark therefore will bypass the OSD tank, and lawful discharge is not achieved. It is also unclear where the bio basin outlet is proposed to connect to, will there be a connection into the existing Council stormwater pipe within the easement?
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Response:
The previously submitted stormwater design has been amended. The following key changes have
been made to the previously submitted design:
The bioretention basin has been deleted from the design. As the site is less than 2,500m2
in area, water quality objectives are not required to be met under the State Planning Policy
(2017).
All downpipes and roofwater will be connected directly to the proposed on site detention
(OSD) tank. All gutters, downpipes and roofwater pipes are to be designed for the 1%
Annual Exceedance Probability (AEP) storm event. These will be documented by the
Hydraulic Consultant for the project.
A kerb has been added along the eastern side of the proposed carpark. This will grade
downhill towards the north. A new grated inlet pit will be constructed within the existing
easement in the north eastern corner of the site over the existing 300mm diameter
stormwater pipe. The proposed kerb will direct runoff from the carpark area to the new
grated inlet pit. An upstand kerb is proposed around the grated inlet to ensure runoff ponds
on field inlet and enters pipe.
It should be noted that runoff from the proposed carpark will bypass the proposed OSD
tank. The OSD tank has been designed so that roofwater is attenuated to a level that
ensures the total peak runoff from the entire developed site is not more than existing
conditions.
We have updated our previously submitted Stormwater Management Plan to reflect the above
amendments to the design. Attached is a copy of the amended Stormwater Management Plan for
review.
Information Required
2. Provide amended stormwater plans clarifying the direction of stormwater flow between the three
structures A1, A2 & A3.
Response:
As discussed above, the proposed stormwater design has been amended. Please refer to attached
updated drawings and Stormwater Management Plan for further details.
3. Clarify the outlet connection for the bio basin.
Response:
As discussed above, the previously proposed bioretention basin has been removed from the
design.
4. Provide finished surface levels for the drainage easement to clarify the proposed direction of
overland flow along the eastern boundary.
Response:
The attached updated design drawings were amended to include additional details. As discussed
above, a kerb has been added along the eastern edge of the proposed carpark instead of the
previously proposed grass swale. This will ensure the carpark drains to the north eastern corner of
the site and ultimately to Pearl Street.
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Trusting the above is to your satisfaction and should further information on engineering matters be
required, please contract this office.
Regards,
Adam Broughton
Principal Civil Engineer
BE (Civil) MIEAust RPEQ
Encl: Civil Design Drawings, Stormwater Management Plan
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