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Attachment 1_20-000713 - Line 5 Tech Memo.docx
Attached to the e-mail Minor Change Application OPW20/0145 - Cover letter - Stormwater Plans - Line 5 Tech Memo - Owner's Consent - DA Form 5.
NOOSA CIVIC ROL
Date: Wednesday, 15 December 2021
To: Stockwell Development Pty Ltd
From: Calibre Professional Services Pty Ltd
SUBJECT: NOOSA CIVIC ROL – LINE 5 OUTLET REDESIGN
1 BACKGROUND
In November 2021 Noosa Council raised concerns in relation to the design of the stormwater outlets at Line 5 and Lot 3 at
Noosa Civic. The concern raised was in relation to the outlet level being lower than the surrounding ground levels, resulting in
the stormwater network not being free draining (holding water).
The purpose of this memo is to outline the design methodology used to resolve the Line 5 and Lot 3 stormwater drainage issue.
This details problems identified with the initial design of both outlets, challenges faced with rectifying the issue, proposed
temporary designs and the proposed ultimate design.
2 DESIGN ISSUE
The outlet headwall structure 1/5 was initially designed at a level approximately 500mm below the natural surface level within an
existing drain to the west of Hoffman Drive. The design nominated works being required within the drain to “chase out” the invert
level for it to free drain. This required tree and vegetation clearing within the drain. Furthermore, the stormwater network
upstream would be holding some water in the case of storm events as the swale was not capable of freely draining large
amounts of flow.
Similarly, the outlet headwall for Lot 3 had a similar issue. The primary concern with this outlet was the need for vegetation
clearing and earthworks in the drain.
Calibre prepared design drawings showing the extent of the swales that would be required to achieve free draining outlets. The
extent of excavation and clearing was not supported by Noosa Council.
At the time that the issue was identified, the majority of the upstream drainage had been constructed. As such there was limited
opportunity to augment the upstream drainage design to raise the outlet/s.
3 PROPOSED RESOLUTION
To address the issue, the following design changes have been implemented:
1. The Line 5 outlet has been relocated to the south at a location where the outlet level (RL2.5m AHD) can be achieved
with minimal works within the drainage reserve
2. The Lot 3 drainage has been connected into the Line 5 drainage via a new pipeline adjacent lot 4, thereby eliminating
the additional outlet into the drainage reserve.
3. The stormwater network downstream of the Lot 3 connection has been upsized to cater for the increased flows in the
network.
4. Grout filling of the line down stream of pit 3/5 (to avoid excavating within the newly constructed road for removal)
5. Realign stormwater line 1/23 (renamed to 1/2) to connect to the new Lot 3 drainage line at manhole structure 3/1.
Due to constraints associated with the outlet level and existing services, the main line downstream of MH 5/1 is required to be
1200 x 600 box culverts. Some minor clearing and shaping will be required at the proposed outlet location. This consists of a
shallow swale of approximately 12m length, which will be constructed in a manner to minimise disturbance as much as practical.
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CONTINUED
4 PROPOSED INTERIM SOLUTION
At the time of this Memo, there is a significant delay in pipe supply. Currently lead times currently sit at between 20 to 32
weeks, primarily due to COVID 19 constraints as well as the recent closure of a major pipe supplier in Qld.
In the short term it is proposed to adopt an interim solution to enable the balance of the project to be completed. An interim
solution utilising HDPE pipes (which are readily available but not accepted by Noosa Council) was investigated but discounted
to avoid sacrificial works and multiple excavations with Hofmann Drive.
The interim design consists of utilising the currently constructed stormwater network – i.e. the original Barlow Shelley design –
and submit an uncompleted works bond to Noosa Council to the value of 150% of the proposed ultimate design configuration.
This approach has the following benefits:
1. It retains the use of the existing network constructed as part of the development works.
2. Avoids the need to excavate the road to install additional redundant pipework.
3. Provides certainty to Noosa Council that the ultimate configuration will be constructed.
It is acknowledged that this approach will result in the drainage network being partially submerged until such time as the
ultimate configuration is construction. This is not considered problematic as the ultimate configuration is expected to be
completed within the maintenance period and the uncompleted works bond shall have sufficient value to clean out the line on
completion.
5 CONCLUSION
The proposed design solution discussed above is considered the most efficient given the site and construction constraints. The
proposed final design mentioned above includes the relocation of the Line 5 outlet headwall structure to a more suitable location
with a lower surface level, thus reducing the area of clearing and earthworks within the drainage reserve. The stormwater line
from structure 2/1 (Lot 3 outlet) to outlet headwall structure 7/1 to be constructed using 1200 x 600 box culverts. This design
solves both stormwater drainage problems in the most effective and efficient manner.
Yours sincerely
Calibre Professional Services Pty Ltd
Shaun Pilcher
Principal Engineer - Urban Development
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