Attachment 1_20-000713 - Line 5 Tech Memo.docx

                                                                                                                 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.




File Ref: 20-000713                                                                                                       Page 1 of 2