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Appendix 4 - Engineering Report (00012423)
Attached to the e-mail MCU15/0053 Application - 30-32 Doonella Street Tewantin (3 email of of 3) - Engineering Report.
Unit Development 30-32 Doonella Street, Tewantin T 1. INTRODUCTION Our client, Serina Pty Ltd wishes to redevelop the site at 30-32 Doonella Street, Tewantin into a multi dwelling unit complex. At a pre-lodgement meeting with Council a number of engineering issues were raised by Council staff. The purpose of this report is to address these issues and to provide guidance on the redesign of the concept plans to be lodged with Council in support of a Material Change of Use Application. The site covers two lots with a total area of 2359 m2 and is described as lot 32 on RP 893427 and lot 903 on T1631. 2. PROPOSED DEVELOPMENT It is proposed to construct a multi-level unit building with basement car park and access off Doonella Street. Council have indicated that the basement level must be free draining (i.e. a pump out system will not be allowed). Council records indicate there is a drainage pipe from the car park behind the site running through lot 32 on RP 893427 and this needs to be investigated. It is anticipated that stormwater detention and water quality treatment will be required as part of the development works and preliminary sizing of these features has been undertaken by our sub- consultant Aquaintel. 3. SERVICES INFRASTRUCTURE The subject site is currently connected to existing water and sewer infrastructure located in Doonella Street. These existing services are considered suitable for the proposed development. As the site is located on the high side of Doonella Street stormwater from the site currently drains to the existing kerb and channel and conveyed to existing drainage pits in the street. A site investigation has revealed that: The interlot drainage pipe shown on Council GIS system passing through the development site does not exist. The drainage pit in the car park behind the site drains towards Sidoni Street, not Doonella Street. There is a drainage pit in Doonella Street on the low side of the development site (not shown on Council’s GIS) which has an invert level of approximately 1000mm below the road pavement. Based on the contours on Council’s GIS J/N 13672 June 2015 Rev C Page 1 of 2
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Page 2
Unit Development 30-32
Doonella Street, Tewantin
T
the road level at this location is approximately RL 4.0, so the drain invert is
approximately RL 3.0. This is low enough to enable a basement car park at
RL 4.6 (as shown on building concept plan) to be drained by a gravity
stormwater system to this pit. Note that we recommend that survey be
undertaken to confirm the above drainage and pavement levels.
Please refer to the attached marked up aerial photo showing the above details.
4. STORMWATER MANAGEMENT
Please refer to the attached report from Aquaintel which provides preliminary sizing
of bioretention areas and detention volumes.
Prepared by
Doug Reynolds
RPEQ 9072
Tod Consulting Pty Ltd
9 June 2015
J/N 13672 June 2015
Rev C Page 2 of 2
Page 3
No Pipe in this
location
Stormwater
Pit in street
at low side of
property -
Invert level
approx
RL3.0
Page 4
Doug Reynolds
TOD Consulting
by email
10 June 2015
30-32 DOONELLA STREET, TEWANTIN – PRELIMINARY STORMWATER MANAGMENT
Dear Doug,
AquaIntel have been commissioned by TOD Consulting on behalf of Serina Pty Ltd to undertake a preliminary
investigation into the stormwater management options of their proposed multi-unit development at 30-32
Doonella Street, Tewantin. The purpose of this investigation is to identify the preliminary sizing of water quality
treatment and detention device that might be required to be implemented in order to support the application.
Preliminary advice is also sort as to if the site is likely to be impacted by flooding.
The Pollution Load Reduction Targets, non-worsening and flooding criteria as outlined in the Division 20 – Water
Sensitive Urban Design Code and Division 18—Transport, Roads and Drainage Code of the Noosa Shire Council’s
(NSC) “The Noosa Plan”.
SITE AND CATCHMENT
The proposed site is located at 30-32 Doonella Street, Tewantin. The contributing catchment is constrained to
the site only with no major external catchment identified that influences the site. The carpark and buildings to
the rear of the site drain to Sidoni Lane and Sidoni Street. The carpark to the east of the site drains to Doonella
Street. The site has a catchment area of 2539m2 and grades from the rear to the front (to Doonella Street).
m: 0400 436 391 t: +7 5414 7185
e: [email protected]
w: www.aquaintel.com.au
Page 5
DESIGN OBJECTIVES
This preliminary assessment has included both the water quality, quantity and flooding aspects. The design
objectives applied are:
1. Stormwater Quality Objectives (Load Based Reduction Targets)
Total Suspended Solids – 80%
Total Phosphorus – 60%
Total Nitrogen – 45%
Gross Pollutants – 90%
2. Stormwater Quantity Objectives
Non-worsening of peak flows from the development site. That is, the peak post-development flows
should not be greater than the peak pre-development flows.
3. Flooding
Site and buildings to be located outside the 100 year AEP flood extent.
STORMWATER QUALITY
Preliminary stormwater quality modelling has been undertaken using MUSIC version 5. The site has been
represented as a lumped urban source node with an impervious area of 70%. As rainwater tanks are no longer
mandatory, they were not included as part of the treatment train. To achieve the objectives the entire site was
directed to an end-of-line bioretention system. Preliminary calculations estimate that a bioretention area of
approximately 35m2 will be required to service the site. This represents approximately 1.4% of the site.
Bioretention is a conventional treatment measure that generally forms part of a landscape feature on the site.
Alternative methods, such as StormFilters, do exist which function to treat stormwater via cartridge filters and
are located in a stormwater manhole. If usable space on the site is required to be maximised these systems can
provide a solution for a similar magnitude of cost. It is noted that unlike a bioretention system where capital
cost is upfront, for the cartridge filter system option there is no upfront capital cost and instead the body
corporate would be required to enter into a lease arrangement for the upkeep and maintenance of the system.
STORMWATER QUANTITY
Preliminary detention calculations were undertaken to determine the volume of storage that would be required
to ensure that non-worsening of peak flows results from the development. The existing site has an impervious
area of approximately 20%, based on the houses currently located there. As a result of the development this
impervious area increases to 70%. As part of the calculations, no external catchments were assumed to affect
the site. . Preliminary calculations estimate that a detention volume of approximately 30m3 will be required to
ensure non-worsening of flows is achieved from the site.
As the site drains to the front of the site, a lawful point of discharge is achieved to Doonella Street.
m: 0400 436 391 t: +7 5414 7185
e: [email protected]
w: www.aquaintel.com.au
Page 6
FLOODING
According to Council’s contour data on the site, elevations range between 4.5 and 8.5m AHD. Council’s flood
overlay mapping (refer purple shading below) shows that the site is located outside of the estimated 100 year
AEP flood extent and the site would not be subject to the Flood Management Code. A Council flood search
could be undertaken to ascertain flood heights in the adjacent Doonella Lake.
I trust this preliminary assessment of stormwater management requirements for your development meets your
needs. Feel free to contact me should you wish to discuss any of the content in this letter.
Yours faithfully,
Brendon Bolt
Senior Water Resources Engineer
AquaIntel Pty Ltd
m: 0400 436 391 t: +7 5414 7185
e: [email protected]
w: www.aquaintel.com.au