LATE RFI Attachment 6 - Stormwater Management Plan

4. STORMWATER MANAGAMENT
A holistic site approach has been adopted for the design of the stormwater, stormwater
harvesting and on-site retention use (via the car washing process), stormwater quality treatment
and site stormwater discharge.
A Commercial Car Wash use is a rare use that can use a lot of mains water if not designed
correctly. However, it is because of this water demand that the Car Wash use also has the ability
to make great use of any captured rainwater (ie: harvested and retained stormwater) and draw
down on this stored water quickly which maximises the chances of the underground retention
tanks being able to store the design storm event volume effectively with a small chance of
overflow even for the largest of storm events.
The overall site will have the stormwater comply with the following requirements:
   1. Noosa Shire Council- Noosa Plan 2020 requirements and standard drawings,
   2. Queensland Urban Drainage Manual (QDUM),
   3. Department of Transport and Main Roads Road Drainage manual,
   4. AS/NZS 3500.3 – Plumbing and Drainage, Part 3: Stormwater Drainage.
Refer to the attached plan, Sh: C1, Revision B- which depicts the Preliminary Stormwater Layout
Plan. This plan shows the locations of the proposed down pipes from roofed areas, their
connection into the underground storage retention tanks, all driveway stormwater pits and
associated pipework, the water quality treatment devices and the final LPD connection into the
existing stormwater pipe via a new Manhole in accordance with Council’s standard details.

4.1. PRE/POST DEVELOPED SITE RUN-OFF CO-EFFICIENTS
The existing overall subject site has an overall site area of 2,014 M2.
The overall area of the developed site will be reduced by a 2.0 M width along the full northern
boundary due to the requested road widening on Diamond Lane. This area of 80.4 M2 shall be
deducted from the effective site area.
There is a 2.0M wide footpath area between the Super Bays and the adjoining property (Bowls
Club) to the east of the site which totals 96.6 M2. This area shall also be deducted from the
effective site area.
This yields an effective overall site area of 1,837 M2 that shall be designed for.
The pre-existing motel, cabins and bitumen impermeable areas occupied 1,414 M2 of the
effective site area (ie: only the impermeable areas inside the effective site area have been
included). This yields an existing overall site impermeable coverage of 77%. This results in a pre-
existing site run-off co-efficient of 0.85 over the effective area of 1,837 M2.
The post-developed site has a total of 845 M2 of roofed area being directly connected into the 2
No. 20,000 L underground Retention tanks for re-use onsite. These tanks have been oversized
(the calculated capacity has been doubled from 20,000L to 40,000L which will also cater for
future Climate Change effects) to cater for the 1% EAP design storm event, these tanks will not
contribute to the overall site stormwater discharge. Hence, this roofed area has been removed
from the overall site run-off co-efficient calculation. Other areas include; non-roofed landscaped
areas total of 228 M2, and driveway/paved non-roofed impermeable areas total 764 M2. This
results in a post-development site run-off co-efficient of 0.46 over the effective area of 1,837 M2.
This fact alone assures that the post-developed site will have a greatly reduced overall site
stormwater volume and peak flow discharge than the pre-developed site regardless of the storm
event and storm duration.

  -6-        TMC Building Design Group