Peer Review - OPW22-0319 25-01-2023

25th January 2023
Mark Lewis
Engineering Coordinator
Noosa Council
[email protected]

Dear Mark
RE: peer review of Stormwater management for OPW22/0319 (25 and 27 Toolar St Tewantin)
Please find below a peer review of the proposed Stormwater Management for the above application.
1. Background
The development involves creation of 8 townhouses over 2 existing properties. The existing properties fall towards Toolar street. The higher-order approval is MCU21/0083.
The stormwater management design provided with the MCU was generally compliant, apart from a condition requiring the detention basin sizing to be revised with the OPW.

2. High-Order Condition Compliance

The applicable conditions from the MCU are conditions 29 and 30. These conditions are reproduced below along with a comment on how they have been addressed in the OPW application in RED.

29 	The site must be provided with a stormwater drainage system connecting to a lawful point of discharge.  The works must be undertaken in accordance with an Operational Works approval and the Queensland Urban Drainage Manual, and must include in particular:
7kL of storage for roofwater capture and reuse. The storage tank is to receive only roofwater runoff and is to be provided with a solid lid to prevent the inflow of surface runoff. A backflow prevention device is to be fitted to restrict the backflow of water in the detention tank into the roofwater storage;
Most criteria met apart from absence of backflow prevention device. FIR provided.
Captured roofwater is to be plumbed to communal non-potable fixtures, including bin washdown and landscape irrigation;
Note to this effect shown on civil plan
Stormwater pits receiving surface runoff are to be provided with litter-baskets; and
No litter baskets specified – FIR provided
An underground detention tank is to be provided within the site to ensure site discharges achieve non-worsening for the range of durations relevant to the downstream drainage network.

30 	In conjunction with the development application for Operational Works, revised detention tank design calculations and documentation is to be provided which identifies all assumptions and the methodology applied in deriving the tank design, and which is based on the recommendations of Australian Rainfall and Runoff 2019 and QUDM 2017. Ensemble temporal patterns are to be utilised for the analysis as well as the current rainfall intensities recommended by Bureau of Meteorology. The detention tank is to achieve non-worsening for the range of durations and recurrence intervals relevant to the downstream drainage network.
Not addressed. Detention tank has actually decreased in size by 33% from MCU submission. An FIR is provided which includes the option to use a simplified deemed-to-comply solution.

3. General Comments

Drainage design is generally compliant and low-risk, with stormwater pits and pipe system provided above southern retaining wall to intercept external runoff (even though little/no external catchment seems to be present), and all internal driveways drain to the street.

An underground pipe connection is also now provided from most of the site to the stormwater pit  on the opposite side of the street.

The civil plans do identify that trenching rather than tunnel boring is proposed for this connection – so Council may wish to condition tunnel boring.

Although not non-compliant, the FFLs for the units are at absolute minimums. The FFL and immunity from drainage could be improved through a simple step-up at the entrance from the garages/carports (say 150mm) as is commonly used in this type of development. An FIR is provided.

4. Further Information Request

Issue
Condition 29 of MCU21/0083 has not been fully addressed

Information Request
Provide amended plans which fully address Condition 29 of MCU21/0083 and which specifically:
Include provision of a backflow prevention device on the connection between the detention tank and roofwater tank
Nominate litter baskets on all stormwater pits within the site

Issue
Condition 30 of MCU21/0083 has not been addressed

Information Request
Provide amended plans and supporting calculations which fully address Condition 30 of MCU21/0083. If the applicant does not have the capability to undertake modelling, then a deemed-to-comply approach such as that nominated in Section 4.3 of the Flooding and Stormwater Management Guidelines (SCC, 2020) may be an acceptable alternative. Include calculations for the maximum water level in the detention tank and demonstrate that freeboards are met.

Issue
Freeboard to unit FFL’s

Information Request
Some of the nominated Unit FFLs have minimal elevation above the driveway levels and resultiugn surface stormwater flows. Consider improving the resilience of the units to local flooding by providing a step-up into the units from the carport and garage areas.

Yours Sincerely,

Leon Rowlands
BE Env Eng, RPEQ, CPESC
Director – Switchback Consulting

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