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RFI - Engineering - Stormwater
UNIT DEVELOPMENT
24 KATHARINA STREET AND 10 NATASHA AVENUE
NOOSA HEADS
TECHNICAL NOTE ON STORMWATER MANAGEMENT
Introduction
Tod Consulting have been engaged by Mark Bain Constructions Pty Ltd to prepare preliminary
civil engineering plans for a proposed 8 unit development at the above site. The scope includes
design of the driveway and turning areas, bulk earthworks and preliminary stormwater drainage
design. This technical note focuses on the stormwater drainage design and management.
Stormwater Quantity
The total site area is 2039m2 and currently contains two separate residential unit complexes. The
proposed redevelopment of the site increases the impervious area slightly, in the order of 250m2.
As such it is proposed to include a detention tank under the lower driveway area to mitigate any
increase in peak flows. A detailed assessment and sizing of the detention tank will be conducted
during the detailed design phase, however for the purpose of the preliminary design and MCU
development application we have adopted a tank size based on a Sunshine Coast Council
deemed to comply solution for medium density development – refer SCC fact sheet attached. As
outline in table 2 of this document the detention volume is based on 90m3 per Ha. In this case
based on the total site area of 2039m2 the detention tank would be 18,000 litres. We believe this
is a very conservative approach and the final tank size is likely to be smaller but have
demonstrated that an 18,000 litre tank can fit under the driveway servicing the Katharina Street
units.
Runoff from the unit complex will be discharged to Council’s drainage infrastructure via a new
manhole over the existing drainage pipe in Katharina Street as shown on the preliminary
drawings.
Stormwater Quality
The State Planning Policy 2017 specifically excludes the need for any water quality treatment for
developments under 2500m2, however it is noted the new 2020 Noosa Plan overrides the SPP
and requires water quality treatment to be addressed for developments of all sizes. With
developments under 2500m2 it is often difficult to provide sufficient space for stormwater
treatment devices and this is certainly the case with this development. Notwithstanding, we have
identified a section of the site landscaping that can be constructed as a bio filter area that will treat
run off from 75% of the site (upper catchment area). The upper catchment area is 1500m2 and
based on Healthy Waterways deemed to comply solutions for Stormwater Quality Treatment for
the North Coast region, the filter area would need to be at least 1.7% of the catchment area, or
26m2. Refer deemed to comply solution R5 for appendix D2 of the above document – extract
attached below. We have determined and a filter area in excess of the above area can be
constructed in the garden bed at the rear of the upper catchment, driveway area.
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In addition to this treatment area, we propose to include rainwater re-use on the site by combining
a storage component to the detention tank in the lower catchment of the site. This stored water
can be used to irrigate the landscaped areas of the site. We have nominated a volume of 18,000
litres (in addition to the detention volume) for rainwater storage/re-use.
Whilst the above actions will not strictly meet the performance outcomes listed in PO6 of the
Water Quality and Drainage Code, we believe they are more than adequate for a development of
this size.
Prepared by
Doug Reynolds RPEQ 9072
TOD Consulting
01/04/2022
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On-site Underground Detention Storage Orifice Plate Sizing
for Deemed-to-comply Detention
October 2020
This fact sheet describes the steps that
should be taken to size an orifice plate for
on-site underground detention when
using deemed to comply detention
storage. The standardised orifice plate
design will help to ensure post-
development flows match pre-
development conditions for a range of The outlet pipe to which the orifice plate is
storm events. installed should be sized to convey the 10%
AEP (at 2100) peak flows. This is to allow for
An orifice plate is required to ensure that potential future upgrade of the receiving
peak flows being discharged from an drainage network. Allowance for blockage of
underground storage tank can be attenuated the underground stormwater network and
for a range of storm events. Without an surcharge of the underground storage
orifice plate, outflows would be controlled by should be considered in the siting and
the capacity of the outlet pipe. As the outlet design of the underground detention storage.
pipe will typically be sized to convey the 10% The overland flow path is required to have
Annual Exceedance Probability (AEP) event sufficient capacity so that surrounding floor
(at 2100) peak flows, the post-development levels will not be inundated if there is
discharges will likely exceed pre- complete blockage of the underground
development peak flows for smaller AEPs if stormwater drainage network during the 1 in
an orifice plate or another form of staged 100 AEP (at 2100) event.
discharge control is not installed.
The stormwater drainage upstream of the
Existing drainage networks, particularly in detention storage should be designed so that
older established urban areas, were the 1 in 100 AEP peak flow can be conveyed
generally sized to convey the 39% AEP peak to the underground detention storage.
flows. Therefore discharging at higher than
pre-development rates during smaller storm To the maximum degree practical, the entire
events could cause local flooding issues for site should be drained to the detention
surrounding areas. Control of peak flows storage. Where this is not possible, the
during smaller events is also necessary in equivalent storage for the entire site should
order to reduce the risk of increased scour be provided and the outlet should be
and erosion in receiving waterways. reduced in size to achieve the same overall
site discharge. A maximum of 20% of the
Having an outlet pipe sized to convey the developed portion of the site is allowed to
smaller 39% AEP peak flow, without an bypass the detention storage.
orifice plate, so that outflows would be
restricted for all storm events, would require The depth of the required underground
more detention storage volume than detention storage is flexible in order to allow
proposed by Table 2 of the Flooding and for a range of proprietary detention storage
Stormwater Management Guidelines (SCC, products to be utilised. Pre-treatment of
2018). flows entering the tank should be provided
by a Gross Pollutant Trap (GPT) or gully pit
baskets.
Fact Sheet On-Site Detention Orifice Plate Sizing
Page 4
A removable debris screen, in accordance The orifice plate holes should be
with the Section BN5.5.2(p) of QUDM (2017) spaced so that the:
should be provided upstream of the orifice i. bottom third of the plate can
plate. A sump pit, which houses the outlet convey 5/12 of Q0 when H is
orifice, will also be necessary. The access 2/3 of the tank depth
chamber to the underground storage should
be adequately sized to enable the debris ii. middle third of the plate can
screen to be removed from the tank. convey 1/2 of Q0 when H is
2/3 of the tank depth
The orifice plate should be made from a
corrosion resistant 5mm thick stainless steel. iii. the top third of the plate can
The orifice plate will have numerous holes at convey 1/12 of Q0 when H is
specified levels and diameters cut out of it. 2/3 of the tank depth
The minimum acceptable orifice plate hole Step 4. Multiple holes will be required in
diameter is 25mm. each section of the orifice plate. The
minimum acceptable orifice plate
Orifice Plate Sizing Process hole diameter is 25mm. Check that
The orifice plate should be designed using the proposed number of holes can
the following steps: fit within the available orifice plate
area over the outlet pipe. In order to
Step 1. Size the outlet pipe to which the accommodate the required number
orifice plate will attach so that it has of holes, multiple outlet pipes or a
a diameter half the depth of the box culvert may be required. The
underground tank. Multiple pipes procedure to size the orifice plate
can be used to achieve the required holes will not be modified, the holes
total depth, in order to allow smaller will just be spread over a greater
outlet pipes to be used. area.
Step 2. Determine the current climate pre- Step 5. Check that the outlet pipe/s has
development 10% AEP peak flow sufficient capacity (based on
based on A Review of Simple Peak Colebrook-White equation) to
Flow Estimation Methods for use on convey the 10% AEP (at 2100)
the Sunshine Coast following the peak pre-development flows.
release of ARR 2016 (SCC, 2017).
This will be the target Qo. Step 6. Size the removable debris screen
so that the area (including blocked
Step 3. Size the required orifice plate holes area) is at least 20 times the total
based on the following equation: orifice area. A grid mesh should be
Qo = C A (2gH)1/2 used (e.g. Weldlok A40/203 or
Where: similar).
C = 0.6 (unless otherwise justified) An example of the orifice plate is attached.
A = total orifice area (m2) The proposed outlet design should help to
ensure that post-development peak flows
g = acceleration due to gravity from the site are attenuated to pre-
(m2/s) development levels for all storm events, up
H = depth of water above the to and including the 1 in 100 AEP (at 2100).
centroid of orifice plate hole (m) The depth of water (H) used to size the
orifice plate is limited to two thirds of the
depth of the tank. During major storms,
2 of 3 Fact Sheet On-Site Detention Orifice Plate Sizing
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additional storage above this level will be 0.075 m3/s. 7/85mm orifice
therefore be available. plate holes (or equivalent) are
In some instances, multiple small outlets therefore required in the
may be able to be used instead of an orifice middle third of the orifice
plate. The capacity of the outlet pipes (which plate.
is dependent on factors such as grade and iii. In the top third of the plate,
materials) should be verified to ensure that the average depth of water
they do not become the hydraulic control. above the centroid is 0.3m,
The multiple pipes should drain to a single and it will be required to
pit before connecting to Council’s convey 1/12 of 0.15m3/s, i.e.
underground drainage network. 0.0125 m3/s. 1/105mm orifice
plate hole (or equivalent) is
Orifice Plate Sizing Example therefore required in the top
An example of the orifice plate sizing is third of the orifice plate.
provided below for an example of a 0.5ha Step 4. The orifice plate holes are able to
rural-residential block being developed into a physically fit onto the 600mm
medium density development. The volume of diameter orifice plate, as shown in
storage required is 210m3. The pre- Figure 1, and so multiple outlet
development 10% AEP peak flow is pipes will not be required.
0.15m3/s (current day) and 0.18m3/s (at
2100). The depth of the proposed tank is
1.2m.
Step 1. The outlet pipe will be required to
have a diameter of half of 1.2m, i.e.
600mm.
Step 2. The target Qo is 0.15m3/s.
Step 3. The following orifice plate holes will
be required:
i. The centroid of the bottom
third is at 0.1m. The average Figure 1 Orifice Plate
depth of water above the Set Out Example
centre of the holes at 2/3 of Step 5. The 600mm RCP outlet at a
the tank depth is therefore minimum grade of 0.2% will have a
0.7m. The orifice plate holes capacity of 0.27m3/s, and so will
are required to convey 5/12 of have sufficient capacity to convey
0.15m3/s, i.e. 0.0625 m3/s. the future climate 10% AEP peak
4/95mm holes (or equivalent) flow of 0.18m3/s.
are therefore required in the
Step 6. The minimum debris screen area is
bottom third of the orifice
20 times the total orifice plate hole
plate.
area, i.e 1.53m2.
ii. In the middle third of the
plate, the average depth of
water above the centroid is
0.5m, and it will be required to
convey 1/2 of 0.15m3/s, i.e.
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EXTRACT FROM HEALTHY WATERWAYS DEEMED TO COMPLY SOUTIONS FOR WATER QUALITY [OCR of page images] EXTRACT FROM HEALTHY WATERWAYS DEEMED TO COMPLY SOUTIONS FOR WATER QUALITY Deemed to Comply Solution R5 - utilising rainwater tank(s) and bioretention system(s) Deemed to Comply Solution R5 only applies to residential development greater than 2 dwellings (townhouse style up to 2 storeys) but <12,500m? in site area. REGION South East Queensland - Greater Brisbane - North Coast - West Region - South Coast NOTES Region - Refer to STEP 1 of Selection and Design Process of main document for schematic of climatic regions Rainwater Tank requirements defined by Queensland Development Code MP 4.2 and 4.3. Refer to code to confirm tank sizes. Bioretention system filter media area refers to the functional flat portion of the bioretention system only and does not allow for coarse sediment capture and batters/bunds to surrounding ground levels. The total bioretention footprint area must be defined as part of Selection and Design Process (STEP 7) to account for batters and coarse sediment as well as filter media area. Typically for small bioretention systems (<50m7?), the Total Footprint Area = 3 X Filter Media Area. This should be used as a starting point for the design process and refined provided the proponent develops suitable detailed concept drawings as part of the Development Approval. RAINWATER TANKS Attached dwellings 3kL BIORETENTION SYSTEM FILTER MEDIA AREA (% of catchment area) Detached dwellings SkL 1.1% 1.7% 0.9% 1.3% Location on Public or Private Land - The bioretention systems should remain on private land and be managed by the owners (body corporate). There exists an option to create a small parkland or drainage reserve for the bioretention system which could be handed to the local authority but this would require approval.