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WORKING FOR YOUR PROJECTS SUCCESS
STORM WATER MANAGEMENT PLAN
79-83 Eumundi Noosa Road & 26 Headland Drive, Noosaville
CLIENT: Capital Prudential
MULTI TOWNHOUSE/UNIT DEVELOPMENT
p: 07 5455 5604
F: 07 5455 5696
e: [email protected]
w: www.scg-engineers.com
PO Box 1661 Noosa Heads Qld 4567
Unit 1/11 Project Ave Noosaville Qld 4566 STRUCTURAL | CIVIL | GEOTECHNICAL
Debuild Pty Ltd Trading as ABN 40 098 481 899
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Page 2
Document Status
Document Type Author Checked by Date Issued Revision
Report Tom Harrington Sean Richardson 19 July 2023
Project Details
Project Name 79-83 Eumundi Noosa Road & 26 Headland Dr,
Noosaville
Client Capital Prudential
Authors Tom Harrington
Checked by Sean Richardson
SCG Consulting Engineer reference 33132(SWMP)
Page 3
Table of Contents
1.0 INTRODUCTION ................................................................................................................................... 3
2.0 EXISTING SITE DESCRIPTION ......................................................................................................... 4
3.0 PROPOSED DEVELOPMENT ............................................................................................................. 5
4.0 EXISTING DRAINAGE.......................................................................................................................... 5
4.1 Existing Drainage and Infrastructure .................................................................................................... 5
4.2 Lawful Point of Discharge ........................................................................................................................... 6
5.0 STORMWATER QUALITY .................................................................................................................. 7
5.1 State Planning Policy and Water by Design Assessment ................................................................. 8
5.2 Construction Phase ....................................................................................................................................... 8
5.2.1 Silt and Sediment Erosion ...................................................................................................................................... 8
5.3 Completed Development............................................................................................................................. 9
5.3.1 Water Quality Objectives ........................................................................................................................................ 9
5.3.2 Sources of Contaminants ..................................................................................................................................... 10
5.3.3 Proposed Treatment Measures ......................................................................................................................... 11
5.3.4 Pollutant Modelling ................................................................................................................................................ 11
6.0 STORMWATER QUANTITY ............................................................................................................ 14
6.1 Drains Analysis ............................................................................................................................................ 14
6.1.1 Existing Catchments .............................................................................................................................................. 15
6.1.2 Proposed Site ............................................................................................................................................................ 16
6.1.3 Peak Flow Comparison ......................................................................................................................................... 16
6.1.4 Peak Flow Mitigation (OSD) ............................................................................................................................... 17
6.2 Proposed Drainage .................................................................................................................................... 19
6.3 Downstream Infrastructure Consideration ...................................................................................... 19
6.4 Climate Change Analysis for 1% AEP Storm Event at Year 2100 .............................................. 21
7.0 FLOODING ........................................................................................................................................... 22
8.0 CONCULSION ...................................................................................................................................... 22
APPENDIX A – Detailed Site Survey ........................................................................................................ 23
APPENDIX B – Site Plan ............................................................................................................................... 27
APPENDIX C – Concept Stormwater Management Plan ................................................................... 28
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1.0 INTRODUCTION
SCG Consulting Engineers have been commissioned by Capital Prudential complete a Stormwater
Management Plan (SWMP) for a residential development at Lots 75-77 RP 71151 & Lot 3 SP 208257,
79-83 Eumundi Noosa Road & 26 Headland Drive, Noosaville.
The proposed development involves the construction of 38 townhouse/apartment dwellings. The
objective of this (SWMP) is to demonstrate that the following measures will be incorporated into the
design and construction of the development:
• Maintain stormwater quality – Protect receiving waters (during and post construction)
• Manage stormwater peak flow rates of runoff and existing flow path characteristics
• Protect against flooding
• Adopt Water Sensitive Urban Design (WSUD) principles throughout
• Implement Stormwater Harvesting and re-use for landscaping.
This report should be read in conjunction with our detailed site stormwater management plans
(Appendix D drawing No. SCG 33132 – SW1.1, SW1.2 & SW1.3 & SW1.4
The proposed development has been assessed against the following guidelines and planning documents:
• Queensland Urban Drainage Manual 2017 (QUDM)
• Water Sensitive Urban Design (WSUD) – Technical Design, Guidelines for South East
Queensland
• Noosa Plan 2020
• AS/NZS 3500.3:2018 – Plumbing and Drainage Part 3 – Stormwater Drainage
• State Planning Policy 2017
• Sunshine Coast Stormwater Management Guidelines VI - 2020.
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2.0 EXISTING SITE DESCRIPTION
The site is described as Lots 75-77 RP 71151 & Lot 3 SP 208257 and is located within Noosa Shire
Council and is approximately 6,318m2 in area. Access to the existing sites is off Eumundi Noosa Road,
which bounds the western property boundaries, refer to Figure 2.1, for a locality plan and Appendix A
for a detailed site survey. Each Lot currently contains a detached residential dwelling located to the
front portions of the site. 71 & 81 Eumundi Road also contains a shed located beyond each dwelling.
The rear of each site is covered by short grass and scattered trees.
The site generally falls from the front boundary on Eumundi Noosa Road (Approx. RL 13.5-14.5 AHD)
in a eastern direction down towards the rear boundary (Approx. RL 7.75-10.5 AHD). Surface grades
for the majority of the site are in the order of 5-7%. A Council stormwater inter-allotment drainage
system runs adjacent to the eastern (rear) boundary of the site which contains a series of pipes, varying
in size and are contained in the downstream properties within an existing easement.
Council infrastructure also runs along Headland Drive to the south of the site.
Figure 2.1 – Locality Plan
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3.0 PROPOSED DEVELOPMENT
A residential development is proposed for the allotment comprising of 38 townhouse/apartment
dwellings.
The proposed development plans are detailed in Blackwood Architecture and Design drawings 202327,
Drawings DA1.01 thru to DA1.19 (refer Appendix B.) The development includes the following
features: -
• 10 visitor and 40 private carparking spaces
• Bin storage and washdown areas
• Dual property driveway access to Eumundi Noosa Road and Headland Drive
• Bio retention/detention basin for stormwater quality and quantity
• Rainwater harvesting tanks for stormwater re-use
• Landscaping areas across the site
• Padmount transformer.
4.0 EXISTING DRAINAGE
4.1 Existing Drainage and Infrastructure
Data obtained from Noosa Council’s Interactive Mapping, accompanied with a detailed site survey
prepared by Bennet & Bennet confirms the following drainage infrastructure: -
• A series of gully pits located on the eastern boundaries (rear) connected to an underground
pipe network within a drainage easement of the downstream properties
• A series of underground pipes varying in size (225mm to 375mm) located within the
drainage easement of the downstream properties
• An underground drainage network in Headland Drive with pipe sizes varying in size (600-
675mm) located in the road reserve
• Existing kerb adaptor connections to Headland Drive.
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Refer to Appendix A for the detailed site survey and Appendix C for Noosa Council Drainage
Infrastructure.
4.2 Lawful Point of Discharge
The lawful point of discharge is described in Section 3.9.1 of QUDM 2017. The ‘Lawful Point of
Discharge Test’ may be helpful in assessing whether a lawful point of discharge exists at a particular
location. As defined in QUDM 2017 the criteria for determining the lawful point of discharge are: -
1. Will the proposed development alter the site’s stormwater discharge characteristics in a
manner that may substantially damage a third party property (see section 3.6).
• If not, then no further steps are required to obtain tenure for a lawful point of
discharge (assuming any previous circumstances and changes were lawful).
• If there is a reasonable risk of such damage, then consider issue (2) or (3).
2. Is the location of the discharge from the development site under the lawful control of the
local government or other statutory authority from whom permission to discharge has
been received? This will include a park, watercourse, drainage or road reserve,
stormwater registered drainage easement, or land held by local government (including
freehold land).
Note: The regulatory authority (in its capacity as land holder) is likely to require
information about the potential impact of the site’s stormwater discharge characteristics
on third party properties (particularly those downstream of the proposed discharge point)
before it will consent to the discharge entering its land.
• If so, then no further steps are required to obtain tenure for a lawful point of
discharge.
• If not, then consider issue (3). A land owner or regulator may require that the
developer obtain an authority to discharge as described in (3) in order for the
stormwater to ultimately flow to a location described in (2).
3. An authority to discharge over affected properties will be necessary. In descending order
of certainty, an authority may be in the form of:
• Dedication of a drainage reserve or park
• A registered easement for stormwater discharge/works
• Written discharge approval.
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Note: Some authorities may not accept a written approval from a current land owner
because a subsequent purchaser will not be bound by the previous owner’s letter or
contract (refer to Section 3.10).
If the proposed development maintains pre-development flow conditions, the lawful point of discharge
for the subject site is the existing inter-allotment stormwater drainage infrastructure within Lots 102,
107, and 108 SP 106085 and Starboard Avenue, east of the site, the Eumundi Noosa Road kerb and
channel and the Headland Drive stormwater drainage within. The proposed lawful point of discharge is
considered to be compliant with the criteria outlined in QUDM 2017 see below: -
1. The proposed development stormwater discharge will not alter the existing site’s
stormwater discharge characteristics.
2. The location of the proposed discharge point is under the lawful control of the local
government.
3. Authority to discharge over affected properties will not be required as the proposed
development will discharge as per existing drainage conditions.
5.0 STORMWATER QUALITY
In the absence of adequate controls, urban development can increase both stormwater runoff volumes
and peak discharge rates. In addition, this urban development has the potential for increased creek
erosion and the stressing of in-stream aquatic ecosystems. The use of stormwater detention or retention
systems aims at reducing these threats by limiting property flooding to acceptable levels and also
filtering stormwater runoff through a vegetated soils media layer, thus reducing the threats caused by
urban development.
The following factors have been identified as potential contributors which can adversely affect
stormwater quality draining from the site:-
• Silt and sediment erosion
• Gross Pollutants
• Hydrocarbon runoff
• Nutrient runoff
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5.1 State Planning Policy and Water by Design Assessment
The proposed development has been assessed against the State Planning Policy 2017 and the Water By
Design (Stormwater Quality Deemed to Comply Solutions – 2010).
The proposed residential development involves a land area greater than the trigger value of 2500m2 for
stormwater quality measures as required in the State Planning Policy 2017 and the Water By Design -
Stormwater Quality Deemed to Comply Solutions - 2010. Therefore, stormwater quality treatment is
warranted for the proposed development.
5.2 Construction Phase
Sediment generated during the construction phase shall be controlled in accordance with a site specific
Erosion and Sediment Control (ESC) Plan and kept on site throughout construction.
5.2.1 Silt and Sediment Erosion
A potential for sediment mobilisation/transportation exists during and beyond the construction of the
proposed development. This can elevate turbidity levels of stormwater runoff.
A RUSLE assessment of the site confirms proposed development is a Low Risk for soil loss (< 150
Tonnes).
Potential Causes
• Stormwater scour and erosion of excavated or stripped surfaces and stockpiled soils.
• Wind borne transportation of sediments.
• Disturbance/breakup of soil profile via vehicular traffic followed by stormwater scour
and erosion.
• Transportation of sediments from site via excavation vehicles.
Mitigating Measures
• Construction of silt fences around the development site located to cut off all runoff.
• Construction of a sediment basin throughout construction at a low point of the
development site.
• Provide 80-100mm crushed rock (no fines) hardstand under all vehicle paths expected
during construction. Generally, this would be provided from the existing crossover and
extended to a distance within the site that would accommodate the maximum delivery
vehicle length expected.
• Provide rumble grates at exit points from the site for passing earthmoving vehicles.
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• Where possible maintain existing landscaping/grasses downslope from construction areas
to act as buffer/silt trap.
• Install temporary downpipes from completed roof structures and ensure runoff passes
through silt fences.
• Minimise concentrating surface flows.
• Place silt barriers around council stormwater pits that the site drains to.
• Cover soil stockpiles with hessian or similar.
5.3 Completed Development
We are proposing to use a combination of active and passive stormwater treatment measures. Active
measures will include bio-retention basins a water storage tanks for re-use and passive measures via
gross pollutant inserts into stormwater pits within paved areas that receive runoff from areas prone to
gross pollutants.
For the proposed development we have proposed incorporate Water Sensitive Urban Design (WSUD)
principles stormwater harvesting has been proposed for stormwater re-use for communal landscaping.
The proposed water re-use tank has been sized in accordance with the Sunshine Coast Council Flooding
and Stormwater Management Guidelines adopting 1kl per 25m2 of communal landscaping area water
tank for stormwater re-use have been incorporated into the proposed development.
5.3.1 Water Quality Objectives
Water quality objectives for South East Queensland are outlined in the State Planning Policy 2017 and
are displayed in Table 5.1. These reductions are achieved by adopting best practice techniques in
accordance with the abovementioned guidelines.
Table 5.1: Minimum Mean Annual Load Reductions
POLLUTANT / ISSUE MINIMAL MEAN ANNUAL LOAD REDUCTIONS
Total Suspended Solids (TSS) 80%
Total Phosphorous (TP) 60%
Total Nitrogen (TN) 45%
Gross Pollutants (GP) 90%
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5.3.2 Sources of Contaminants
Gross Pollutants
Gross pollutants include litter, grit and heavier sediments which can be mobilised from within or from
neighbouring sites.
Potential Causes
• Unsecured refuse areas.
• Un-landscaped areas prone to stormwater scour.
• Insufficient drainage for surface runoff from within the site.
• Wind Bourne transportation.
Mitigating Measures
• Well established refuse areas with ample storage bins. Fenced off where deemed
necessary.
• Provision of ample litter bins across the commercial footprints of the site. Particularly in
any outdoor dining or areas of congregation.
• Promotion of stormwater surface runoff through landscaped areas or similar sediment
buffering strips/traps.
• Provide stormwater pits outside paved areas that receive runoff from areas prone to gross
pollutants with surrounding buffering measures, such as stone pitching.
• Provide gross pollutant inserts into stormwater pits within paved areas that receive runoff
from areas prone to gross pollutants.
Hydrocarbon Runoff:
Hydrocarbons can be mobilised and transported in stormwater runoff from the site. This can contaminate
stormwater runoff compromising Water Quality Objectives.
Potential Causes
• Leaks from parked vehicles within the completed development.
• Washing of vehicles within the completed development.
Mitigating Measures
• Fall driveways and parking areas towards adjacent landscaped strips, vegetated swales
and the bioretention bed.
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Nutrient Runoff:
Nutrients such as Nitrogen and Phosphorus can be mobilised and transported in Stormwater runoff from
the site. This can compromise Water Quality Objectives.
Potential Causes
• Runoff from hard surfaces within the site.
• Detergents used in car washing and similar activities.
• Fertilisers used in propagation of landscaped areas.
• Embodied nutrients contained in silt (topsoil) runoff.
• Embodied nutrients contained in runoff.
Mitigating Measures
• Fall vehicles pavement areas towards landscaping areas and the bioretention bed.
• Implement management plan for landscaping propagation. Use stable organically bound
fertilizers cultivated immediately prior to turf placement. Mulch over areas of exposed
topsoil.
• Construction of silt fences during construction and maintained beyond completion of
landscaping.
• Drain stormwater runoff through the Bio Basin.
• Capture roof runoff and store for re-use.
5.3.3 Proposed Treatment Measures
Following the water quality objectives for the site mentioned above, to ensure that on-site stormwater
management facilities can be economically maintained, it was deemed the most appropriate method for
treatment is the use of a small grass swale and an on-site bio-retention basin. Roof and surface runoff
will be collected by an internal drainage network and directed to the on on-site bio retention basin prior
to the lawful point of discharge.
5.3.4 Pollutant Modelling
The impact of the stormwater quality management strategy of runoff discharged from the site has been
assessed using the Pollutant Export Model, Model for Urban Stormwater Improvement
Conceptualisation (MUSIC).
For our model the proposed treatment train has been modelled: -
1. Roof water from buildings containing units 601-602, 201-204 and the rear portion of the
roofs for units 701-704 and 101-105 will be collected by a 14kL rainwater re-use tank
with overflow to the onsite Bio retention basin located in the north-eastern corner of the
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site, we have labelled this Bio-retention Bed C. The rainwater tank will be re-used for
landscaping irrigation only, to all common areas.
2. Roof water from buildings containing units 401-404, 501-504 and the rear of units 601-
602 will be collected by a 14kL rainwater re-use tank with overflow to the onsite Bio
retention basin located in the South-eastern corner of the site. The rainwater tank will be
re-used for landscaping irrigation only, to all common areas, we have labelled this Bio-
retention Bed B
3. Roof water from the front of units 701-704 and 101-102 of the proposed development
will be collected by the internal roof drainage network to discharge to Eumundi Rd Kerb
& Channel
4. Surface runoff from hardstand and landscaping areas will be collected by various pits and
pipes. The internal drainage network will then discharge into the onsite bio retention
basins for treatment.
The Music Model was set up in accordance with the Water By Design Music Modelling Guidelines for
South East Queensland.
Figure 5.1 displays a print screen of the MUSIC Model Schematic for the proposed development and
Figure 5.2 displays the annual pollutant loads that will be discharged from the proposed development.
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Figure 5.1 – Music Model Schematic
Figure 5.2 – Music Modelling Results
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The modelling results show the pollutant reduction objectives are met for all contaminants listed in Table
5.1. Details of the proposed bio-retention basin are provided in the Stormwater Management Plan
drawing no: SCG 33132 - SW1.1 thru to SW1.2 (Appendix D.)
6.0 STORMWATER QUANTITY
Noosa Shire Council’s Planning Scheme – Noosa Plan 2020, requires post development peak flows
leaving the development site will not exceed the pre-development peak flow rates for the required design
storms.
A Stormwater Quantity Analysis has been undertaken of the existing and developed site conditions in
accordance with the following guidelines.
− QUDM 2017
− Noosa Planning Scheme 2020
− Australian Rainfall and Runoff (ARR 2019)
The following section of this report discusses the proposed developments impact on peak stormwater
discharge from the site and the proposed mitigation measures to ensure a non-worsening effect is
achieved to downstream properties.
6.1 Drains Analysis
To calculate the peak discharges for the site under existing (pre-development) and proposed (post-
development) conditions. Hydraulic modelling has been undertaken for the site using the software
package PC DRAINS, to determine peak flows. We have modelled the site to establish existing and
proposed stormwater drainage characteristics.
Rainfall patterns accompanied with Initial Loss (IL) and Continuing Loss (CL) was sourced from the
Australian Rainfall and Runoff 2019 (ARR 2019) data hub for Noosaville.
Initial Losses (IL) and Continuing Loss (CL) were approximated based on the storm losses from the
ARR Data Hub, Table 6.1 below states the adopted values: -
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Table 6.1 - Adopted (IL) and (CL) Values
AREAS (IL) (CL)
(mm) (mm/hr)
Impervious 1 0
Pervious 27 2.6
Design ARR 2019 storms for Noosaville have been added into DRAINS using data obtained from the
ARR 2019 Data Hub accompanied with IFD rainfall depths from the Bureau of Meteorology (BOM)
website
Times of concentration (Tc) were calculated as defined in QUDM 2017 using a combination of standard
inlet time & overland sheet flow time. Table 6.2 displays the Adopted time of Concentrations.
Table 6.2 – Tc Values
STATE (Tc) (mins)
Pre Development 15
Post Development 6
6.1.1 Existing Catchments
The existing site consists of 4 Lots containing residential dwelling on each, detached sheds & landscaped
ground areas. The existing surface generally falls in a eastern direction towards the rear boundary. Roof
runoff from existing dwellings appears to be a mixture of discharging directly to ground level & via kerb
adaptors to Headland Drive.
The total impervious area has been estimated using the detailed site survey and was found approximately
19% Impervious Area (Fi = 0.19) which equates to approximately 1200m2.
The existing site catchment is as follows.
- Area – 0.6326 Ha
- Tc – 15 mins
- Fraction Impervious (Fi) – 0.19
Peak flow rates for the median design storm for the existing site are given in Table 6.3
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Table 6.3 – Existing Site Peak Flow Rates (L/s)
Storm Event Q1 Q2 Q5 Q10 Q20 Q50 Q100
(1EY) (0.5EY) (0.5EY) (10%AEP) (5%AEP) (2%AEP) (1%AEP)
Existing PFR 111 160 216 251 295 352 391
(PFR – Peak Flow Rate)
6.1.2 Proposed Site
The development consists of the construction of residential townhouse/ unit development along with a
shared access driveway and ground level landscaping/ courtyards & a community pool area. The total
pervious area for the site was calculated to be approximately 1330m2 which equates to a fraction
impervious of 0.79.
The Developed site catchment is as follows.
- Area – 0.6326 Ha
- Tc – 6 mins
- Fraction Impervious (Fi) – 0.79
Peak flow rates for the median design storms for the developed site are given in Table 6.4
Table 6.4 – Developed Site Peak Flow Rates (L/s)
Storm Event Q1 Q2 Q5 Q10 Q20 Q50 Q100
(1EY) (0.5EY) (0.2EY) (10%AEP) (5%AEP) (2%AEP) (1%AEP)
Developed 184 232 286 347 399 468 523
PFR
(PFR – Peak Flow Rate)
6.1.3 Peak Flow Comparison
Table 6.5 indicates the increased peak flow rates for the developed cases.
Table 6.5 – Comparison of Existing Vs Developed Peak Flows (L/s)
AEP Q1 Q2 Q5 Q10 Q20 Q50 Q100
Storm (1EY) (0.5EY) (0.2EY) (10%AEP) (5%AEP) (2%AEP) (1%AEP)
Increase 73 72 70 96 104 116 132
PF
(PF – Peak Flow )
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As shown in Table 6.5 Stormwater peak discharge will increase as a result of the proposed development
due to the increase in impervious area. Therefore, it is proposed to utilise On Site Detention (OSD)
systems to mitigate the peak stormwater discharge rates from the proposed development prior to
discharging to the lawful point of discharge as discussed in section 4.2.
6.1.4 Peak Flow Mitigation (OSD)
To mitigate the peak flow increase from the developed site, it is proposed to include a mixture of
inground detention tanks, located under the proposed driveway and an inground open bio
retention/detention basins.
The site has been divided into sub catchments to divert flows through the two Bio-Retention Beds,
namely Bio-Retention Bed B (BR-B) located in the south east corner of the site and Bio-Retention Bed
C (BR-C), located in the north east corner of the site.
These bio-retention beds will also include storage capabilities to provide OSD. An additional inground
OSD tank has been included to further detain flows with sub-catchment B upstream from BR-B.
We propose for the detention systems to cater for flows up to the Q100 storm event and a limit the peak
flow rates of pre-development levels for all storms up to the Q100 storm event.
The proposed BR-B will have the following properties: -
-
Base Area = 35m2
-
Internal Depth = 0.9m
-
Low Flow Orifice = 225mm diameter
- High Flow Orifice = Bio Bed grated inlet
-
Required Volume = 33m3
-
Provided Volume = 35m3
The proposed BR-C will have the following properties: -
• Base Area = 89m2
• Internal Depth = 0.75m
• Low Flow Orifice = 275mm diameter
• High Flow Orifice = Bio Bed grated inlet
• Required Volume = 54m3
• Provided Volume = 71m3
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The proposed inground detention tank will be located under the driveway fronting units 504/401. It will
have the following properties: -
• Base Area = 30m2
• Internal Depth = 1.1m
• Low Flow Orifice = 150mm diameter
• High Flow Orifice = Bio Bed grated inlet
• Required Volume = 30m3
• Provided Volume = 33m3
Figure 6.1 illustrates the DRAINS Model layout for the site.
Figure 6.1 – Drains Model Layout
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Peak flow rates determined for each 3 cases are outlined in Table 6.6.
Table 6.6 – Comparison of Peak Flows (L/s)
AEP Storm Q1 Q2 Q5 Q10 Q20 Q50 Q100
(1EY) (0.5EY) (0.2EY) (10%AEP) (5%AEP) (2%AEP) (1%AEP)
Existing 110 160 216 251 295 352 391
PFR
Mitigated 105 159 204 248 293 349 386
(PFR – Peak Flow Rate)
As shown in Table 6.6 the proposed OSD facilities have demonstrated the proposed mitigated peak
discharge flow rates will not exceed the total pre-development flows for all design storms, up to and
including the 1% AEP Design Storm.
The proposed stormwater detention system is therefore considered to be adequate for the development.
6.2 Proposed Drainage
The proposed site drainage is detailed in drawings SCG 33132 - SW1.1, SW1.2 & SW1.3 & SW1.4
(Appendix D) and is discussed below.
As defined in the Noosa Plan 2020 stormwater drainage must be designed in accordance with QUDM
2017, therefore the following design storms were adopted: -
1. Minor Drainage System - 10% AEP ARI (Q10 Storm)
2. Major Drainage System - 1% AEP ARI (Q100 Storm)
6.3 Downstream Infrastructure Consideration
To determine if the existing lawful points of discharge for Catchment B is capable to safely convey the
post development mitigated flow we have modelled the existing drainage infrastructure as discussed in
Section 4.1 using the software PC DRAINS. The modelling has demonstrated that all post development
mitigated peak will be below existing flow rates and can be safely contained within the existing 375m2
diameter stormwater line traversing the rear of Lots 75-77 RP 71151 & Lot 3 SP 208257.
Proposed Catchment B will be connected to the existing stormwater drainage infrastructure within
Headland Drive, via a new 375 diameter RCP branch into the site.
This will increase flows within a length of the pipes within Headland Drive.
The below figure 6.2 shows the pipe run in question: -
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A DRAINS model was completed to compare the impact of Catchment B connecting to the existing
drainage.
Survey data and pipe sizing of the infrastructure was obtained. Catchment areas for the pits along the
pipe run were obtained from Council’s mapping system.
They are as follows: -
Pit Number Catch Area (m2)
H5 18900
H4 5700
H3 3400
H2 2800
H1 2100
Hydraulic grade line comparisons were made for existing and proposed states.
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They are as follows: -
Pit Number HGL(Exist) HGL (Prop.) S.L. (m)
H5 6.43 6.46 8.34
H4 4.54 4.57 6.29
H3 3.54 3.76 4.08
H2 2.96 3.13 4.0
H1 1.71 1.78 2.51
The above results confirm pit number H3 as having the greatest increase in the HGL level of 220mm.
However, there is still 320mm of freeboard within the pit which is in accordance with QUDM minimum
freeboard depth of 150mm.
Catchment A will result in a increased discharge to the Eumundi Noosa Road kerb and channel, however
this has been limited to a Q10 flow rate of 25Lt/s at each kerb adaptor location as per Noosa Council
requirements. The net total discharge from the proposed site will remain below pre development flows
for all storms from Q1 up to Q100.
6.4 Climate Change Analysis for 1% AEP Storm Event at Year 2100
A climate change analysis has been undertaken to determine the impact for a 1% AEP storm event at
year 2100. This includes a 20% increase in rainfall intensity to allow for climate change. As part of the
Climate change analysis, runoff flows were increased by a factor of 1.2 to incorporate a 20% increase
to rainfall intensity.
The PC DRAINS Results displayed in Table 6.7 maintain pre-development flows for catchment C as
displayed in Table 6.5. The Hydraulic modelling shows that the water level does not exceed the
emergency weir level and maintains a freeboard of approximately 100mm. A 300mm gap between the
basin and RP boundary with scour protection has been provided where emergency overflow will fall
onto natural ground level prior to sheet flowing across the boundary. Therefore the hydraulic modelling
has demonstrated that pre-development drainage conditions will not worsen for the Downstream
properties outlined in Catchment B for the 1% AEP storm event at year 2100.
Catchment A will result in an increase to pre development flows. However we have assessed the
capacity of the Eumundi Noosa Rd Kerb and channel in accordance with Section 7.4 of QUDM 2017 &
Aust Roads Guide to Road Design Part 5A Section 5.3 using a 3m flow width for the 10 Yr ARI (10%
AEP). Calculations confirm the Kerb and channel can safely convey the proposed post development
SCG Consulting Engineers Page 21 SCG Ref: 33132(SWMP)
Page 23
10% AEP catchment flows in addition to the existing catchment whilst maintaining a 4m wide flood- free trafficable lane on Eumundi Noosa Rd. 7.0 FLOODING Information provided from Noosa Council’s Flood Hazard Overlay Map confirms the site is unaffected by flooding. 8.0 CONCULSION The stormwater management report demonstrates under the detailed plans the proposed residential development at , Lots 1, 2 & 3 on SP208257, 79-83 Eumundi Noosa Rd & 26 Headland Dr, Noosaville meets the quantity and quality stormwater requirements as described in this report. Sediment generated during the construction phase is to be dealt with in accordance with a Silt and Sediment Control plan and is to be kept on-site throughout construction. The proposed development triggers the State Planning Policy 2017 stormwater quality treatment objectives. An on-site bio-retention basin, grassed swales and a water storage tank for re-use has been incorporated into the proposed development to meet the prescribed water quality objectives, prior to discharging to the lawful point of discharge. On-site detention has been provided to maintain existing peak flow rates and to ensure a non-worsening to flooding of downstream properties. On-site detention is proposed to be contained within the bio retention basin and the designed to contain all flows up to a Q100 storm event. The existing stormwater infrastructure is capable of conveying the post development mitigated flow paths for all storms up to a Q100. The subject site is not affected by localised flooding. SEAN RICHARDSON – RPEQ 5835 For and on behalf of SCG CONSULTING ENGINEERS SCG Consulting Engineers Page 22 SCG Ref: 33132(SWMP)
Page 24
APPENDIX A - Detailed Site Survey
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APPENDIX B – Site Plan Blackwood Architects SCG Consulting Engineers Page 27 SCG Ref: 33132(SWMP)
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APPENDIX C – Concept Stormwater Management Plan SCG Consulting Engineers Page 28 SCG Ref: 33132(SWMP)
Page 30
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