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Document Control Stormwater Management Plan Residential Lot Development at 12 Gympie Street, Tewantin Prepared for Novato Group PTY LTD Reference No. P0198-25-001 4 November 2025
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Document Control
Document Control
Document Stormwater Management Plan
Synergy12d://remote.recorconsulting.com/RECOR/Clients/0198 Novato Group PTY
File Location LTD/P0198-25-001 12 Gympie Street, Tewantin/03 Engineering/01 Civil/Stormwater and
DRAINS files
Project Name 12 Gympie Street, Tewantin
Project Number P0198-25-001
Revision Number 1
Revision History
Revision No. Date Prepared By Reviewed By Approved for Issue By
1 4/11/2025 John McLaughlan Eoin Whitmore Eoin Whitmore
Issue Register
Distribution List Date Issued Number of Copies
RECOR Company Details
Approved by Eoin Whitmore
Address Level 11
269 Wickham Street
Fortitude Valley QLD 4006
Telephone 0451453640 Website www.recor.au
Email [email protected]
Signature RPEQ 34231
The information within this document is and shall remain the property of RECOR.
12 GYMPIE STREET, TEWANTIN
Stormwater Management Plan
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Important Notice Important Notice This document is confidential and has been prepared exclusively to support the town planning application for the proposed development. It is issued in accordance with the Consultancy Agreement between RECOR and Novato Group, under which RECOR was engaged to perform a specific and limited scope of work for Novato Group. The contents of this report are strictly confined to the matters outlined within and are subject to various assumptions, qualifications, and limitations. No implicit applicability to other matters should be inferred. RECOR makes no representation that the scope, assumptions, qualifications, or exclusions within this report will be appropriate for any other purpose, nor does it assert that the report comprehensively addresses all matters of potential relevance to the reader. This report must be read in its entirety, as the executive summary does not serve as a replacement. Any subsequent reports must be considered in conjunction with this document. This report supersedes all prior drafts or interim versions, whether written or verbally presented, preceding its issue date. It has not been and will not be updated to reflect events, transactions, or material changes occurring after its issuance. RECOR assumes no obligation to notify the recipient of any such developments or to revise the report accordingly. Unless expressly agreed otherwise in writing, RECOR disclaims any duty of care or legal responsibility in relation to this report, its associated inquiries, advice, or related work. Furthermore, RECOR does not extend any representation regarding this report to any party other than Novato Group. Any third party who obtains a copy of this report, whether in whole or in part, or engages in discussions regarding its content with RECOR, acknowledges and accepts that they may not rely on this report or any associated information or advice for any purpose. 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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List of Tables and Figures
Table of Contents
Executive Summary ............................................................................................................................. 3
1 Introduction ................................................................................................................................ 4
1.1 Background and Scope .............................................................................................................................. 4
1.2 Stormwater Management Objectives and Design Approach .................................................................. 4
1.3 Supporting Documentation and Reports ................................................................................................... 5
2 Existing Site ................................................................................................................................ 6
2.1 Site Overview .............................................................................................................................................. 6
2.2 Flow calculation using the Rational Method ............................................................................................. 7
2.3 DRAINS model and comparison to Rational Method ................................................................................ 9
3 Proposed Development Works ................................................................................................ 12
3.1 Proposed Site Description........................................................................................................................ 12
3.2 Time of Concentration Inputs into DRAINS ............................................................................................. 13
3.3 Hydrological Modelling Results ................................................................................................................ 14
3.4 Stormwater Detention and Mitigation Strategy ...................................................................................... 15
4 Stormwater Quality .................................................................................................................. 17
4.1 Introduction ................................................................................................................................................17
4.2 Stormwater Quality Modelling (MUSIC) Methodology ............................................................................17
4.3 Stormwater Pollutant Reduction Performance ....................................................................................... 18
4.4 Analysis and Compliance with Water by Design Guidelines .................................................................. 18
4.5 Stormwater Management Implications.................................................................................................... 18
4.6 Erosion and Sediment Control ................................................................................................................. 19
5 Conclusion ................................................................................................................................ 20
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List of Tables and Figures List of Tables Table 1-1 Property Details ................................................................................................................................................ 4 Table 2-1: Rational Method key inputs ............................................................................................................................ 8 Table 2-2: Rational Method flow calculations ................................................................................................................. 8 Table 2-3: DRAINS results for existing catchment ....................................................................................................... 10 Table 2-4: Comparison between DRAINS and the Rational Method ........................................................................... 10 Table 3-1: Comparison of Pre-Development vs. Post-Development Peak Flows ...................................................... 14 Table 3-2: Detention Basin parameters......................................................................................................................... 15 Table 3-3: Mitigated Flow comparison to existing ....................................................................................................... 16 Table 4-1: Pollutant Load Reduction .............................................................................................................................. 18 Table 5-1 Pollutant Load Reduction ............................................................................................................................... 21 List of Figures Figure 2-1: Subject Site Location ..................................................................................................................................... 6 Figure 2-2 Max overland flow path .................................................................................................................................. 7 Figure 2-3: Friend’s equation (QUDM, 2025) .................................................................................................................. 8 Figure 2-4: DRAINS model for existing catchment ......................................................................................................... 9 Figure 2-5: DRAINS catchment inputs ............................................................................................................................. 9 Figure 3-1 Developed Masterplan .................................................................................................................................. 12 Figure 4-1 MUSIC Model ................................................................................................................................................. 17 Appendices Appendix A – DRAINS Results Appendix B – Rational Method Results 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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Executive Summary RECOR Consulting Engineers have been engaged by Novato Group to prepare a Stormwater Management Plan (SWMP) to support a Development Approval Application for a proposed seven-lot residential subdivision at 12 Gympie Street, Tewantin. This SWMP addresses the stormwater management strategy for the proposed development and demonstrates compliance with the requirements of Noosa Shire Council, the Queensland Urban Drainage Manual (QUDM), and Water by Design guidelines. The report outlines measures to ensure the development can be constructed and operated in accordance with all relevant stormwater quantity and quality objectives. Lawful Point of Discharge Stormwater runoff from the development will be directed to a single lawful point of discharge via a stormwater network into Council’s dedicated reserve to the north of the Butler Street Cul de sac. This configuration consolidates site flows and ensures integration with existing downstream infrastructure. The discharge arrangement has been verified through hydraulic modelling and is consistent with Council’s lawful discharge requirements. Stormwater Quantity The hydrological assessment was undertaken using both the Rational Method and DRAINS modelling to establish pre-development baseline conditions and to assess post-development impacts. Results confirmed an increase in peak flow rates underdeveloped conditions due to added impervious surfaces. To mitigate these effects, a bio-retention/detention basin is proposed within the north-eastern corner of the site. Hydrological modelling confirms that post-development discharges remain equal to or below pre-development flows across all assessed Annual Exceedance Probability (AEP) storm events, refer to section 3.3. Stormwater Quality A stormwater quality assessment was completed using MUSICX in accordance with the Water by Design MUSIC Modelling Guidelines. The proposed bio-retention basin (filter area 45 m², filter media depth 0.4 m) forms the primary treatment measure for runoff from impervious surfaces. Section 4.3 summarises the pre-treatment pollutant loads and post-treatment residual loads, demonstrating compliance with the Urban Stormwater Quality Planning Guidelines (2010). The results summarized in Table 4-1 confirm that stormwater discharges from the site will not degrade receiving waterways and that the system satisfies best-practice stormwater quality objectives. Erosion and Sediment Control During the construction phase, ESC measures must be implemented in accordance with the applicable requirements. 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan 3 Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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1 Introduction
1.1 Background and Scope
RECOR has been engaged by Novato Group to prepare a Stormwater Management Plan (SWMP) for the proposed
seven-lot residential development located at 12 Gympie Street, Tewantin.
This report has been prepared to support the Development Approval Application submission to Noosa Shire
Council and any nominated referral agencies. The proposed development’s property details have been summarised
in Table 1-1.
The SWMP addresses potential stormwater impacts resulting from the proposed development and outlines the
necessary mitigation measures to meet both quantity and quality requirements of Noosa Shire Council.
This report presents the hydrological assessment of the proposed development, including any external catchments
that may directly influence peak flows exiting the site. The purpose of the assessment is to ensure that post-
development stormwater discharge does not adversely impact existing flooding or stormwater conveyance
conditions on surrounding properties and that the stormwater management objectives of the QUDM and Noosa
Shire Council requirements are achieved.
Additionally, the assessment evaluates pollutant load reductions to ensure compliance with Water by Design
guidelines and council water quality requirements.
Table 1-1 Property Details
Site Details Parameters
Site Location 12 Gympie Street, Tewantin QLD
Lot Number 42 RP867323
Intended Land Use Reconfigure of Lot for residential housing
Development Area 5897 m2
1.2 Stormwater Management Objectives and Design Approach
The intent of this report is to document the design considerations and outcomes that satisfy the stormwater
management objectives for the proposed development. The strategy has been developed to address the
following key aspects and performance objectives:
• Catchment Definition: Identification of all catchments and sub-catchments discharging through or into
the site.
• Hydrological Assessment: Determination of existing flow conditions for multiple storm events and
assessment of development impacts on surrounding properties.
• Flow Management: Ensuring post-development peak flows do not exceed pre-development flows
across all relevant storm events, up to and including the 1% AEP.
• Lawful Point of Discharge: Identification and utilisation of a lawful point of discharge in accordance with
the Queensland Urban Drainage Manual (QUDM), ensuring no actionable nuisance to downstream
properties or infrastructure.
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• Site Drainage and Conveyance: Development of a preliminary site drainage layout that ensures safe
conveyance of stormwater through the site without adverse impacts on adjacent areas.
• Stormwater Quality Management: Implementation of a bio-retention system to meet or exceed the
pollutant reduction targets outlined in the Urban Stormwater Quality Planning Guidelines (2010).
• Erosion and Sediment Control: Adoption of construction-phase measures consistent with the
International Erosion Control Association (ICEA) – Best Practice Erosion and Sediment Control (2008)
guidelines.
• Sustainable Design: Incorporation of water-sensitive urban design (WSUD) principles to support long-
term sustainability and improved water quality outcomes.
Stormwater management for the proposed development has been designed in accordance with the following
documents:
• Noosa Shire Council Planning Scheme Policy – Stormwater Management
• State Planning Policy (SPP), 2017
• Environmental Protection (Water) Policy, 2009
• Environmental Protection Act 1994
• Queensland Urban Drainage Manual (QUDM) Fourth Edition, 2016
• Water By Design MUSIC Modelling Guidelines, Version 3.0, 2018
• Water by Design – Construction and Establishment Guidelines: Swales, Bioretention Systems and
Wetlands V1.1, 2010
• Australian Rainfall and Runoff Guideline (ARR)
1.3 Supporting Documentation and Reports
The SWMP is supported by technical documentation and reports which provide the basis for the engineering
recommendations and support the DA application, including:
• Civil Design Drawings prepared by RECOR, File No: P0198-25-01_TEWANTIN_OPW_REVA
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2 Existing Site
2.1 Site Overview
The site, located at 12 Gympie Street, is bounded by two residential houses and an open space (RP867318) to the
west, a rural residential property (T16321) to the north, four residential houses (RP867323) to the east, and Gympie
Street along the southern boundary. Properties to the south, west, and east are zoned Low Density Residential,
while the northern adjoining property is zoned Rural Residential.
The site’s topography generally falls towards the north, with surface levels ranging from approximately RL 4.5 m
AHD along the southern boundary to approximately RL 2.5 m AHD in the northern sector. Levels along the Gympie
Street frontage are approximately RL 4.5 m AHD.
Drainage across the site occurs in two primary directions:
• The majority of the site drains towards the northern sector, discharging into the adjoining rural residential
property and
• A minor portion of the site drains towards the south-western corner, discharging into the adjoining
residential property.
At present, no dedicated stormwater infrastructure is provided within the site.
Gympie Street is expected to contain sufficient stormwater infrastructure to capture and divert runoff from the
residential properties to the south, directing it away from the development site. As such, this external area has
not been considered as part of the site’s stormwater catchment for this report.
Figure 2-1: Subject Site Location
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2.2 Flow calculation using the Rational Method
The Rational Method was initially applied as a verification tool to cross-check outputs from the DRAINS
hydrological model, which was subsequently developed to assess both pre- and post-development flows.
The time of concentration (Tc) was determined using Friend’s Equation (refer to Figure 2-1) in accordance with
Table 4.6.4 of QUDM. This approach provides a reliable estimate of the travel time for runoff from the hydraulically
most distant point in the catchment to the outlet.
A maximum flow path length of 92.82 m was adopted, based on survey data collected by DSQ Surveyors Pty Ltd
in 2025 (refer to Figure 2-2).
The resulting Tc of 22 minutes was further validated against QUDM’s Figure 4.4 – Overland Sheet Flow Times,
which provides empirical guidance for various surface and slope conditions. This comparison confirmed the
calculated Tc to be consistent with expected values for the site’s topography and surface characteristics.
To refine the analysis, the following factors were incorporated to ensure accuracy and representativeness of the
stormwater design:
• Surface roughness and vegetation cover, which influence flow resistance and travel time
• Topographic gradients, confirming alignment between flow velocities and assumed conditions
• Site hydraulic connectivity, including potential depressions or ponding areas, verifying the
appropriateness of the adopted Tc.
Figure 2-2 Max overland flow path
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Figure 2-3: Friend’s equation (QUDM, 2025)
The fraction impervious for the site was determined to be 12%, based on a manual measurement of the exact
impervious area to accurately represent existing site conditions in accordance with QUDM methodology. This
measured impervious area was input directly into the model to ensure realistic simulation of runoff generation.
Using QUDM Table 4.5.2, the C₁₀ was determined based on the 1-hour, 10 % AEP rainfall intensity of 74.3 mm/hr.
Applying this methodology, a C₁₀ value of 0.70 was derived, as summarised in Table 2-1.
Table 2-1: Rational Method key inputs
Catchment Area (ha) Tc (min) Actual Tc (min) fy C10
Pre-Development 0.59 22 22 0.12 0.70
To quantify the peak flow rates for various Annual Exceedance Probability (AEP), the Rational Method formula
was applied:
where:
• Q = Peak discharge (m³/s)
• C = Runoff coefficient
• I = Rainfall intensity (mm/hr)
• A = Catchment area (hectares)
The following Table 2-2 presents the calculated pre-development flow rates for each of the assessed storm
events:
Table 2-2: Rational Method flow calculations
AEP % 63.2% 50% 20% 10% 5% 2% 1%
Pre-Development Flow (m³/s) 0.065 0.079 0.119 0.146 0.175 0.220 0.252
These calculations establish the hydrological baseline for the site and form the foundation for further stormwater
management analysis. The results will be used to assess post-development runoff characteristics and determine
the necessary mitigation measures to ensure compliance with the QUDM and Noosa Shire Council stormwater
management requirements.
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2.3 DRAINS model and comparison to Rational Method The DRAINS model was configured using the appropriate Australian Rainfall and Runoff (ARR) parameters specific to the site conditions. Inputs for each catchment were derived from the parameters discussed in previous sections, ensuring consistency with site-specific hydrology and QUDM rural density development criteria. The general configuration of the DRAINS model is shown in Table 2-4 Figure 2-4: DRAINS model for existing catchment The existing catchment was modelled to reflect conditions consistent with rural residential development, as recommended by QUDM. The relevant DRAINS catchment inputs are detailed in Figure 2-5. Figure 2-5: DRAINS catchment inputs 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan 9 Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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The pre-development flow rates for various AEP were calculated using DRAINS. The results are summarized in
Table 2-3.
Table 2-3: DRAINS results for existing catchment
AEP % 63.2% 50% 20% 10% 5% 2% 1%
Pre-Development Flow (m³/s) 0.050 0.066 0.115 0.148 0.177 0.213 0.242
A comparative assessment was conducted to evaluate the variance between DRAINS results and the Rational
Method calculations. The differences between the two methodologies are presented in Table 2-4.
Table 2-4: Comparison between DRAINS and the Rational Method
AEP % Rational Method (m³/s) DRAINS (m³/s) Difference %
63.2% 0.065 0.048 -15%
50% 0.079 0.064 -10%
20% 0.119 0.113 -2%
10% 0.146 0.146 0%
5% 0.175 0.176 0%
2% 0.220 0.212 -2%
1% 0.252 0.242 -2%
The results obtained from the Rational Method and the DRAINS model show differences that are within acceptable
engineering tolerances and align with expectations given the inherent methodological differences between the
two approaches.
• The Rational Method is a simplified empirical approach that assumes uniform rainfall intensity across the
entire catchment for the duration of the storm event. It does not account for hydraulic routing, storage
effects, or time-variant rainfall intensities, which can lead to either under- or over-estimations of peak
flows depending on local site characteristics.
• DRAINS, in contrast, is a hydrologic and hydraulic modelling tool that provides a more detailed, event-
based assessment incorporating:
o Rainfall losses (initial and continuing losses)
o Time-dependent storm profiles.
o Pipe and channel storage effects and
o Flow routing dynamics.
• The Rational Method remains a valid tool for preliminary estimation and cross-checking of peak flows;
however, the DRAINS model, utilising the ILSAX method, provides a more detailed and accurate
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assessment of catchment behaviour. Therefore, DRAINS model results have been adopted for final
design.
• Minor discrepancies, particularly for higher AEP events (20-year and above), indicate that the Rational
Method provides a reasonable approximation for design verification.
• The largest discrepancies are typically observed in low-frequency events (1 to 5 year), where DRAINS
captures additional hydrological complexities such as temporal rainfall variation and surface storage,
which are not represented in the Rational Method.
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3 Proposed Development Works
3.1 Proposed Site Description
The proposed development will comprise a seven-lot residential subdivision accessed via two driveways and
supported by a dedicated on-site stormwater management system.
A key feature of the stormwater management strategy is the inclusion of a bioretention basin located at the north-
eastern corner of the site. This basin has been designed to capture and attenuate stormwater runoff from the
developed catchment prior to discharging via the lawful point of discharge into the Council’s dedicated reserve to
the north of the Butler Street Cul de sac ensuring seamless integration with the downstream network.
In addition to mitigating potential downstream impacts, the proposed system has been designed to achieve
compliance with the Noosa Shire Council Planning Scheme Policy – Stormwater Management, by:
• Consolidating site runoff into a single lawful discharge point
• Attenuating post-development peak flow rates to pre-development levels or better
• Improving stormwater quality prior to discharge, in accordance with QUDM and Council requirements
This integrated approach delivers a robust and sustainable stormwater solution that not only mitigates the
hydrological impacts of the proposed development but also assists in alleviating existing localised flooding issues
experienced within the adjoining retirement village. The layout of the proposed development is illustrated in Figure
3-1.
Figure 3-1 Developed Masterplan
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3.2 Time of Concentration Inputs into DRAINS
The time of concentration (Tc) for each catchment has been determined using a combination of:
1. Roof inlet time (time for runoff to travel from impervious surfaces to the drainage system)
2. Kerb travel time (time for runoff to reach the stormwater pipe network)
3. Pipe flow time (travel time through the underground drainage system)
Each component is broken down below for the catchment, as shown in the provided screenshot.
Catchment
• Total area: 5,897 m² (0.5897 ha)
• Tc components:
1. Roof inlet time Q 5 minutes
Assigned based on standard inlet times from QUDM Table 4.6.2 for urban residential
areas.
2. Kerb travel time Q 0.5 minutes
Assumed minor due to short travel distances along roads and paved areas.
3. Time in pipe flow → 1.5 minutes
Pipe length: 80m
Slope: 1.00%
Manning's n: 0.012
Using Manning’s equation, the travel time is calculated based on hydraulic parameters.
4. Total Time of Concentration (Tc): 7 minutes
Calculation Methodology
• Roof inlet and kerb travel times were estimated using QUDM guidelines and empirical flow path analysis.
• Pipe flow times were determined using Manning’s equation for open channel and pipe flow, referencing
the chart provided in the screenshot for additional validation.
• Catchment delineation and flow paths were confirmed using the subdivision layout and area tool
measurements.
Final Considerations
• A single catchment has been adopted for the entire development, as all overland flows will be fully
intercepted within the site and conveyed to the proposed detention basin.
• These times of concentration will be used in hydrological modelling to determine peak flow rates and
ensure adequate stormwater infrastructure design.
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3.3 Hydrological Modelling Results
Analysis of Unmitigated DRAINS Model Results and Required Mitigation Measures
A review of the unmitigated DRAINS model results indicates a significant increase in peak flows across various
AEP storm events due to the transition from an existing rural setting to a developed urban condition.
Key Observations
1. Increase in Flow Depths and Velocities
• The developed scenario shows higher water levels across all storm events, particularly at
constricted sections of the drainage network.
• Flow velocities have increased, indicating a higher runoff volume due to reduced permeability.
2. Peak Discharge Variations
• Peak discharge rates in the developed scenario have increased substantially, particularly for
higher return period storms (e.g., 1% AEP).
• The most significant changes occur at sections with concentrated overland flow paths,
suggesting increased runoff from new impervious surfaces.
3. Impact of Increased Impervious Area
• The development introduces more impervious surfaces (e.g., roads, driveways, rooftops),
reducing infiltration and increasing direct runoff.
• The loss of natural detention and permeability results in quicker times of concentration and
higher peak flows.
Table 3-1: Comparison of Pre-Development vs. Post-Development Peak Flows
Storm Event (AEP) Pre-Development Peak Flow Post-Development Peak Increase %
(m³/s) Flow (m³/s)
63.2% 0.048 0.163 55%
50% 0.064 0.190 50%
20% 0.113 0.273 41%
10% 0.146 0.328 38%
5% 0.176 0.381 37%
2% 0.212 0.449 36%
1% 0.242 0.500 35%
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Required Mitigation – Installation of a Detention Basin
Due to the projected increase in post-development peak flows, mitigation measures are required to ensure
compliance with Noosa Shire Council stormwater management requirements and to prevent potential
downstream impacts. The most suitable solution is the installation of a detention basin, designed to achieve the
following outcomes:
• Attenuate peak discharge by temporarily storing excess runoff and releasing it at controlled, pre-
development rates
• Control flow velocities, thereby reducing the risk of downstream erosion and infrastructure overloading
and
• Improve water quality by facilitating the settlement of suspended solids and associated pollutants prior
to discharge.
The detention basin will be designed to ensure that post-development peak flows do not exceed pre-
development flow conditions for all relevant design storm events. Further hydrological modelling will be
undertaken to appropriately size the basin and confirm its effectiveness in mitigating increased flow volumes and
velocities.
3.4 Stormwater Detention and Mitigation Strategy
A bioretention basin is proposed on the north-eastern side of the development, as shown in Civil set of plans for
consideration. The basin has been designed to retain and control stormwater flows so that post-development peak
discharges remain at or below pre-development levels. The design provides attenuation across all assessed AEP
storm events, consistent with the guidelines of the QUDM. Refer to Table 3-2 for basin design parameters.
As outlined in preceding sections, most of the site’s natural drainage flows northward. Under ideal conditions, the
preferred location for the bioretention basin would be along the northern boundary, enabling full capture of the
developed catchment and discharge to the site’s natural overland flow path. However, a sewer easement occupies
this boundary, precluding the installation of major stormwater infrastructure. Consequently, the basin has been
positioned in the north-eastern sector, enabling discharge to Council’s dedicated reserve to the north of the Butler
Street Cul de sac.
Flows have been modelled to assuming the entire site drains to the northeast, into the Butler Street road reserve.
This modelling approach provides a clear and consistent comparison between pre- and post-development
scenarios and demonstrates anticipated improvements to local drainage conditions.
Table 3-2: Detention Basin parameters
Parameter Value
1% AEP Storage Volume 289 m³
1% AEP Water Depth 0.8 m
Lower Discharge Control 150 mm orifice at 2.65 m AHD
Secondary Discharge Control 100 mm orifice at 2.85 m AHD
Third Discharge Control 600 x 600 mm field inlet at ODD at 3.05 m AHD
Wier Level 3.45 m AHD
Basin Excavated Level 1.65 m
Top of Filter Media Level 2.35 m
Top of Retaining Wall Level 3.65 m
Outlet Pipe Size 275 mm orifice into 375 mm pipe
Outlet Pipe Invert Level 1.710 m
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The basin outlet will discharge via a new stormwater pipe connection into Council’s dedicated reserve to the
north of the Butler Street Cul de sac.
Stormwater Mitigation Results
A detailed analysis of the mitigated peak flows, as summarised in Table 3-3, confirms that the proposed
bioretention basin successfully reduces post-development discharges across all assessed storm events, ensuring
compliance with QUDM requirements and mitigating potential downstream impacts.
Table 3-3: Mitigated Flow comparison to existing
Storm Event Pre-Development Peak Flow Post-Mitigation Peak Flow Reduction %
(m³/s) (m³/s)
63% (1-Year AEP) 0.048 0.033 19%
50% (2-Year AEP) 0.064 0.037 27%
20% (5-Year AEP) 0.113 0.097 8%
10% (10-Year AEP) 0.146 0.119 10%
5% (20-Year AEP) 0.176 0.151 8%
2% (50-Year AEP) 0.212 0.194 4%
1% (100-Year AEP) 0.242 0.231 2%
Key Observations
The results of the hydrological modelling demonstrate that the proposed detention basin provides effective
attenuation of post-development peak flows, particularly for the more frequent rainfall events.
As summarised in Table 3-3, the following peak flow reductions were achieved:
• 1 – Year AEP approximately 19% reduction
• 2 - Year AEP – approximately 27% reduction
• 5 - Year AEP – approximately 8% reduction
• 10 - Year AEP – approximately 10% reduction
• 20 - Year AEP – approximately 8% reduction
• 50 - Year AEP – approximately 4% reduction.
• 100 - Year AEP – approximately 2% reduction.
These results indicate that the basin has been optimised to effectively manage frequent, smaller storm events,
which are most critical for protecting downstream properties and infrastructure, while allowing major flood events
to pass through the system without increased risk.
This outcome reflects the intent of the stormwater management strategy:
• To provide a robust reduction in nuisance flooding and frequent storm impacts
• To ensure safe conveyance of major flows consistent with pre-development drainage conditions and
• To maintain compliance with both QUDM and Noosa Shire Council stormwater management objectives.
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4 Stormwater Quality
4.1 Introduction
A stormwater quality assessment was undertaken for the proposed development at Tewantin using MUSICX
(Model for Urban Stormwater Improvement Conceptualisation), in accordance with the Water by Design MUSIC
Modelling Guidelines. The model was designed to evaluate the pollutant load reductions achieved through the
proposed treatment train, ensuring compliance with the Urban Stormwater Quality Planning Guidelines 2010.
The stormwater treatment system consists of a bio-retention basin located on the northeastern side of the
development, sized to effectively mitigate pollutant loads generated from impervious surfaces. The filter area of
the basin is set at 45 m², with a filter media depth of 0.4 m, which ensures sufficient treatment capacity for the
anticipated runoff volumes.
4.2 Stormwater Quality Modelling (MUSIC) Methodology
Stormwater quality modelling for the site was conducted using the ‘Model for Urban Stormwater Improvement
Conceptualisation’ (MUSIC) Version 6.3. The model was built to assess the effectiveness of the proposed bio-
retention basin and to ensure that stormwater quality meets the development’s Water Quality Objectives. A
diagrammatic layout of the MUSIC model is presented in Figure 2-1 below.
The MUSICX model was configured in accordance with the Water by Design MUSIC Modelling Guidelines,
incorporating the following parameters:
• Catchment Delineation: Defined based on site hydrology and impervious areas.
• Source Nodes: Representing runoff from urbanized catchments with a mix of pervious and impervious
surfaces.
• Treatment Node: A bio-retention basin serving as the primary stormwater treatment measure.
Figure 4-1 MUSIC Model
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4.3 Stormwater Pollutant Reduction Performance
The following Table 4-1 presents the pre-treatment pollutant loads, residual loads post-treatment, and the
percentage reductions achieved through implementation of the bio-retention system.
Table 4-1: Pollutant Load Reduction
Parameter Pre-Treatment Load Post-Treatment Residual Load Reduction
(kg/yr or ML/yr) (kg/yr or ML/yr) %
Total Suspended Solids (kg/yr) 1273.00 50.60 96.03%
Total Phosphorus (kg/yr) 2.15 0.24 88.66%
Total Nitrogen (kg/yr) 12.34 4.70 61.95%
Gross Pollutants (kg/yr) 29.71 0.00 100.00%
4.4 Analysis and Compliance with Water by Design Guidelines
The Urban Stormwater Quality Planning Guidelines 2010 set out the minimum pollutant reduction targets for
Southeast Queensland developments as follows:
• Total Suspended Solids (TSS): 80% reduction
• Total Phosphorus (TP): 60% reduction
• Total Nitrogen (TN): 45% reduction
• Gross Pollutants: 90% reduction
The results of the MUSICX model confirm that the proposed stormwater treatment system meets or exceeds
these regulatory targets, ensuring compliance:
• TSS reduction of 96.03% (exceeding the 80% requirement)
• TP reduction of 88.66% (exceeding the 60% requirement)
• TN reduction of 61.95% (exceeding the 45% requirement)
• Gross Pollutant reduction of 100% (exceeding the 90% requirement)
4.5 Stormwater Management Implications
The results indicate that the bio-retention basin effectively treats stormwater runoff, significantly reducing
pollutant loads before discharge. The high-performance efficiency of the system ensures that the development
achieves full compliance with stormwater quality regulations while protecting receiving waterways and
maintaining sustainable urban drainage practices.
By implementing this stormwater treatment strategy, the proposed development at 12 Gympie Street ensures:
• Optimal sediment and pollutant retention through the bio-retention basin.
• Hydrological neutrality, with controlled flow rates post-development.
• Alignment with best-practice water-sensitive urban design (WSUD) principles.
This outcome demonstrates a clear commitment to sustainable urban development and reinforces the site’s
long-term environmental performance and regulatory compliance.
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4.6 Erosion and Sediment Control
An Erosion and Sediment Control Plan (ESCP) will be developed and submitted as part of the Operational Works
application. The ESCP will outline the specific measures to be implemented during the construction phase to
minimise erosion and prevent sediment-laden runoff from leaving the site or impacting downstream water
quality.
The ESCP will be prepared in accordance with the IECA – Best Practice Erosion and Sediment Control Manual
(2008) and the requirements of the Environmental Protection Act 1994.
Typical ESC measures to be adopted during construction may include:
• Installation of sediment fences along downstream boundaries and around stockpile areas.
• Stabilised construction entry/exit points incorporating shakedown grids.
• Temporary diversion drains and bunds to direct runoff away from exposed areas.
• Sediment barriers at stormwater inlet pits.
• Dust suppression measures, including watering and covering of stockpiles.
• Progressive stabilisation and rehabilitation of batters and disturbed areas.
The principal contractor will be responsible for implementing and maintaining ESC measures in accordance with
the approved ESCP, including routine inspections and necessary maintenance throughout the construction
period.
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5 Conclusion
The Stormwater Management Plan (SWMP) for the proposed seven lot subdivision at 12 Gympie Street, Tewantin,
has been developed to ensure compliance with the requirements of the Noosa Shire Council, QUDM, and Water
by Design guidelines. Through comprehensive hydrological analysis and modelling, the proposed stormwater
management strategy effectively mitigates potential impacts arising from the development.
1. Hydrological and Hydraulic Considerations
The assessment of the existing site conditions confirmed that stormwater overland flow is primarily directed
towards the north following the natural grade of the site, discharging into the adjoining lot.
Pre-development peak flow rates were estimated using both the Rational Method and DRAINS modelling, providing
a benchmark for post-development assessments. A comparative analysis demonstrated that DRAINS provided a
more detailed hydrologic and hydraulic representation, capturing storage effects and flow routing dynamics not
accounted for in the Rational Method.
2. Stormwater Quantity Management
The post-development scenario introduced increased impervious areas, resulting in elevated runoff volumes and
peak flow rates across multiple storm events. To address these impacts, a bioretention basin was incorporated
into the stormwater management strategy. The hydrological modelling confirmed that the proposed bioretention
system effectively mitigates peak discharges, ensuring that post-development flows remain equal to or lower than
pre-development levels across all assessed storm events.
The basin was designed with the following key specifications:
• Storage Volume: 289 m³ for the 1% AEP event
• Excavated Level: 1.65 m AHD
• Top of Retaining Walls: 3.65 m AHD
• Outlet Pipe Size: 225 mm orifice into a 375 mm pipe, discharging Council’s dedicated reserve to the
north of the Butler Street Cul de sac
The tailwater levels were appropriately set in accordance with QUDM Table 8.3.1, considering mean high water
springs (MHWS) and highest astronomical tide (HAT) conditions, ensuring resilience against potential tidal
influences.
3. Stormwater Quality Management
To ensure compliance with the Water by Design MUSIC Modelling Guidelines, a bio-retention basin was adopted
as the primary treatment measure for stormwater quality management. The MUSICX model was configured to
assess treatment effectiveness in removing pollutants generated from the development’s impervious surfaces.
Table 5-1 presents the pre-treatment pollutant loads, post-treatment residual loads, and the overall percentage
reductions achieved through the proposed stormwater treatment system.
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Table 5-1 Pollutant Load Reduction
Pollutant Pre-Treatment Load Post-Treatment Residual Load Reduction
(kg/yr) (kg/yr) %
Total Suspended Solids (TSS) 1273.00 50.60 96.03%
Total Phosphorus (TP) 2.15 0.24 88.66%
Total Nitrogen (TN) 12.34 4.70 61.95%
Gross Pollutants 29.71 0.00 100.00%
The results confirm that pollutant reduction targets have been achieved, exceeding the minimum thresholds
outlined in the Urban Stormwater Quality Planning Guidelines (2010):
• Total Suspended Solids (TSS) – 80% reduction
• Total Phosphorus (TP) – 60% reduction
• Total Nitrogen (TN) – 45% reduction
• Gross Pollutants – 90% reduction
These outcomes demonstrate that the stormwater discharges from the site will not degrade receiving waterways
and that the proposed treatment system achieves full compliance with best-practice stormwater quality
objectives.
4. Regulatory Compliance and Sustainable Outcomes
The proposed stormwater management measures align with QUDM, Noosa Shire Council requirements, and
Water by Design best-practice principles, ensuring:
• No adverse flooding impacts on surrounding properties.
• Effective peak flow attenuation via detention basin design.
• Significant pollutant load reductions, safeguarding downstream water quality.
• A hydraulically neutral post-development condition.
The proposed stormwater strategy will support the development application by demonstrating that the site’s
stormwater impacts have been adequately mitigated. Further detailed design and operational planning will
ensure successful implementation, maintaining a balance between development objectives and environmental
sustainability.
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5. Final Recommendations
To ensure the successful implementation and long-term performance of the stormwater management system,
the following recommendations are made:
1. Final engineering design verification to confirm basin and pipe network functionality under all
design storm scenarios.
2. Ongoing monitoring and maintenance of bio-retention systems to sustain treatment
performance.
3. Regular review of stormwater infrastructure performance post-development to ensure
compliance with local authority standards.
By incorporating these stormwater management measures, the proposed development will effectively balance
growth with sustainable water management, ensuring long-term resilience against stormwater-related
challenges.
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Appendix A – DRAINS Results 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan 23 Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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Ratuts tor mechan steer cotcal 1%. AEP enaembies seg Ful Unitwady hydraulic medal a fone ST ~ ecegggnn, arsats tor macdin steer mn crtical $7. ALP enaemtias ing Fd Unetwaty ty diaute: model forme sag “,
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Retults for median soem a cotcal IL AEP ememeres weg Fill Lnateaty Syd aul motel ang ful Unstenty Wyorade motet Tinta nies teatime QQ ST ee -, wa” uO Tt) om = 3” . cee guy om nO OR
Page 29
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Appendix B – Rational Method Results 12 GYMPIE STREET, TEWANTIN Stormwater Management Plan 24 Client Reference: P0198-25-001 4 November 2025 Prepared for Novato Group
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REC’ -R Ration Method Results (Catchment Area (m2) ActualTc 5897 22 5897 7 Annual Exceedance Probability (AEP) Duration 63.20% 50%# 20%* 10% 5% 2% 1% 1 min 170 193 261 306 349 403 443| 2 min 144 164 227 270 312 369 413] 3 min 136 154 212 252 290 341 381 4 min 129 147 201 237 272 318 353] 5 min 123 140 191 225 257 299 330 7 min 113 128 174 204 232 268 295] 10 min 101 114 154 180 204 235 257| 15 min 85.6 96.8 131 152 173 198 217| 74.7 84.5 114 133 151 174 191 71.2 80.6 109 127 145 167 183 66.6 75.4 102 119 136 157 172 60.3 68.3 92.7 109 124 143 158 47.7 54.1 74 87.3 100 117 129 40 45.5 62.8 74.3 85.6 100 ily 31.1 35.5 49.5 59 68.4 81 90.7 25.9 29.7 41.8 50.1 58.3 69.4 78.1 3 hour 20.1 23.2 33 39.8 46.6 55.9 63.1 4.5 hour 15.6 18.1 26.1 31.7 37.3 45 51 6 hour 13.1 15.3 22.1 27 31.9 38.6 43.9) 9 hour 10.3 12 17.6 ZS 25.6 31 35.4 12 hour 8.64 10.1 14.9 18.3 21.8 26.6 30.4 18 hour 6.75 7.91 iil s7/ 14.5 17.4 21.3 24.4 24 hour 5.63 6.61 9.86 12.2 14.7 18 20.8 30 hour 4.88 Bre) 8.56 10.6 12.8 15.8 18.3 36 hour 4.32 5.08 7.61 9.47 11.4 14.2 16.4 48 hour 325 4.17 6.25 7.82 9.47 11.8 Pz 72 hour 2.65 3.1 4.66 5.86 7.13 8.97 10.5 96 hour 2.13 2.49 3.74 4.7 5.75 7.26 8.52) 120 hour 1.78 2.08 3.13 3.94 4.82 6.09 7.16 144 hour 1.54 1.8 2.7 3.4 4.15 S225) 6.14 168 hour 1.36 1.59 2.38 2.99 3.64 4.58 5.35
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www.RECOR.au
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