Currumbin Creek Catchment
Hydrological Study
April 2014
1
Title: Currumbin Creek Catchment Hydrological Study
Author:
Study for: City Planning Branch
Planning and Environment Directorate
File Reference: WF50/44/-
TRACKS #43749416-v1
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Executive Summary
The Natural Hazards (NH) team of the City Planning Branch is undertaken a comprehensive hydrological study of the Currumbin Creek catchment. The Council of the City of Gold Coast (Council) commissioned WRM Water and Environment Pty Ltd (WRM) to undertake a study to review and update its hydrological models to a consistent standard in December 2007. WRM assessed all aspects of model development, calibration, estimation of design discharges and provided a set of recommendations (12.10 ) including a recommendation to update all hydrological models to a consistent standard. Furthermore Monte Carlo methodologies have since become available and it was considered necessary to contrast these methods with the standard Design Event Approach (DEA). This study addresses the WRM recommendation and includes Monte Carlo methodologies for comparative analysis.
The main objective of this study is to develop a hydrological model for the Currumbin Creek catchment using the URBS modelling software, calibrate and verify the model against historical flood data, verify the modelling results against Flood Frequency Analysis (FFA) and Joint Probability Analysis (JPA). Finally, estimate the design flood discharges for events ranging from 2 years Average Recurrence Interval (ARI) to the Probable Maximum Precipitation Design Flood (PMPDF) and document all the works to a consistent standard.
In this study, the URBS model for the Currumbin Creek catchment was developed using the current land uses, topographic data sets and best available industry standard modelling approaches. The hydrological study undertaken by Council’s NH team has been reviewed by WRM, Council’s Peer Review Group (PRG) and Don Carroll Project Management.
Calibration and verification data for 22 historical flood events between 1954 and 2013 were sourced for this study from the Bureau of Meteorology (BoM). From the available data, four events (January 2013, January 2008, June 2005 and February 2004) were selected for calibration and another four events (November 2004, February 1990, April 1972 and February 1972) were selected for verification. The selections of calibration and verification events were based on quality of recorded data sets.
The calibration attempted to match the predicted and recorded flood peaks, volumes, shapes and timing of the hydrographs. A single set of model parameters were adopted for all calibration, verification and design events. Rainfall losses were adjusted to achieve the best possible hydrograph shapes and flood volumes. A uniform initial loss and a continuing loss rates were calibrated for each event. Table below shows the set of model parameters adopted for all calibration, verification and design event simulations:
Parameter Adopted Value α (Channel lag) 0.14 (Catchment lag) 1.8 m (Catchment non-linearity) 0.65 N (Muskingum non-linearity) 1 F (Fraction of sub-catchment forested) F*0.5
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Very good agreement was achieved between modelled and rated discharges from recorded levels at Nicolls Bridge Gauging Station (GS) for all calibration events. For the purpose of this report rated discharges from recorded levels are labelled as recorded discharges. The table below shows the modelled and recorded peak discharge at Nicolls Bridge GS for all calibration events.
Peak Discharge @ Nicolls Br GS (m3/s) Flood Event Modelled Recorded January 2013 164 155 January 2008 390 392 June 2005 259 251 February 2004 86 88
The calibrated URBS model was then used to estimate the design flood discharges throughout the Currumbin Creek catchment using the Design Event Approach (DEA). The design rainfall data including Intensity Frequency Duration (IFD), Temporal Patterns (TP), Areal Reduction Factor (ARF), rainfall spatial distribution and design rainfall losses adopted in this study were based on the recommendations made by WRM, Council’s Peer Review Group (Table 15) and Don Carroll Project Management.
A Flood Frequency Analysis (FFA) was undertaken for the rated annual peak discharge at Nicolls Bridge GS. Forty three (43) years of recorded data were available at the time of this study. The methodology recommended in Book 4, Section 2 of Australian Rainfall and Runoff (1987) was used to fit a Log-Pearson Type III distribution for an annual series of rated peak flood discharges for this location.
A Monte Carlo based Joint Probability Analysis (JPA) was also undertaken using two methodologies, viz. the Total Probability Theorem Monte Carlo (TPT MC) methodology and the Cooperative Research Centre – Catchment Hydrology Monte Carlo (CRC-CH MC) methodology.
The design peak flood discharges throughout the Currumbin Creek catchment has been estimated using the DEA, FFA, TPT MC and CRC–CH MC methodologies. The peak design discharges for different ARIs at Nicolls Br GS estimated by these methods are shown in the table below:
ARI Design Peak Discharge @ Nicolls Br GS (m3/s) (Year) DEA FFA TPT MC CRC-CH MC 2 107 101 131 123 5 170 167 185 175 10 220 214 235 220 20 281 266 280 272 50 353 343 343 340 100 413 410 398 385 200 464 486 458 446
The comparison table shows very good agreements among the DEA, FFA, TPT MC and CRC- CH MC estimates. Consequently the design hydrographs predicted in this study using the Design Event Approach are considered robust and will provide the most appropriate input to the
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hydraulic models that will be used for flood planning and flood management studies in the Currumbin Creek catchment.
The Table below shows the final peak design discharges at different locations within Currumbin Creek catchment for the 2 year ARI to PMP design flood events. It is of note that the design peak discharges estimated in the above comparison table are based on adopted ARF for the catchment area only upstream of Nicolls Bridge GS. However the design discharges estimated in the table below are based on adopted ARF for the total catchment area1.
ARI Design Peak Discharge (m3/s) (Year) Camberra GS Nicolls Br GS Catchment Outlet 2 84 105 152 5 136 167 239 10 176 217 306 20 226 278 386 50 284 348 498 100 331 409 593 200 372 458 656 500 427 526 752 1000 537 656 951 2000 592 721 1042 PMPDF 1268 1548 2050
1 Adoption of an ARF for the total catchment is appropriate for deriving inflows for a hydraulic model of the lower Currumbin catchment.
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Table of Contents
Executive Summary ...... 3 1. Introduction ...... 9 1.1 Background ...... 9 1.2 Study Objective and Scope ...... 10 1.3 Previous Studies ...... 10 1.3.1 WBM (1995) ...... 10 1.3.2 GCCC (2001)...... 10 1.4 Limitation Statement ...... 11 1.5 Acknowledgement ...... 11 2. Catchment Description ...... 12 2.1 General ...... 12 2.2 Land Use ...... 13 3. Methodology ...... 14 4. Available Data ...... 16 4.1 Topographic Data ...... 16 4.2 Land Use Data ...... 16 4.3 Rainfall Data ...... 16 4.4 Gauge Height Data ...... 19 4.5 Rating Tables ...... 19 4.5.1 Nicolls Bridge...... 20 4.5.2 Camberra...... 21 5. Model Development ...... 22 5.1 Model Description ...... 22 5.2 Model Configuration ...... 23 5.2.1 Land Use ...... 23 6. Model Calibration and Verification ...... 27 6.1 Selection of Calibration and Verification Events ...... 27 6.2 Calibration Methodology ...... 29 6.3 Assignment of Rainfalls and Temporal Patterns ...... 29 6.4 Adopted Model Parameters ...... 29 6.5 Initial and Continuing Losses ...... 30 6.6 Calibration Results ...... 30 6.6.1 General Comments ...... 30 6.6.2 January 2013 Event...... 31 6.6.3 January 2008 Event...... 31 6.6.4 June 2005 Event...... 31 6.6.5 February 2004 Event ...... 31 6.7 Verification Results ...... 32
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6.7.1 November 2004 Event ...... 32 6.7.2 February 1990 Event ...... 32 6.7.3 April 1972 Event ...... 32 6.7.4 February 1972 Event ...... 32 7. Flood Frequency Analysis ...... 33 7.1 Method of Analysis ...... 33 7.2 Available data ...... 33 7.3 Annual Peak Discharge Analysis ...... 33 8. Design Flood Estimation ...... 35 8.1 Methodology ...... 35 8.2 Frequent to Large Design Events (up to and including 100 Year ARI) ...... 38 8.2.1 Rainfall Depth Estimation ...... 38 8.2.2 Areal Reduction Factors ...... 38 8.2.3 Temporal Patterns ...... 39 8.2.4 Spatial Distribution ...... 42 8.2.5 Rainfall Losses ...... 42 8.2.6 Design Peak Discharges ...... 42 8.3 Rare Design Events (200 to 2000 Year ARI) ...... 43 8.3.1 Rainfall Depth Estimation ...... 43 8.3.2 Areal Reduction Factors ...... 43 8.3.3 Temporal Patterns ...... 43 8.3.4 Spatial Distribution ...... 46 8.3.5 Losses ...... 46 8.3.6 Design Discharges ...... 46 8.4 Extreme Design Events - Probable Maximum Precipitation Design Flood (PMPDF) ...... 47 8.4.1 Rainfall Depth Estimation ...... 47 8.4.2 Areal Reduction Factors ...... 47 8.4.3 Temporal Patterns ...... 47 8.4.4 Spatial Distribution ...... 47 8.4.5 Losses ...... 47 8.4.6 Design Discharges ...... 47 8.5 Joint Probability Approach ...... 48 8.5.1 TPT ...... 49 8.5.2 CRC-CH ...... 50 9. Comparison ...... 51 10. Conclusion ...... 53 11. Recommendation ...... 54 12. Reference ...... 55 13. Appendix A: URBS Model Sub-catchment Areas and Land Uses ...... 57 14. Appendix B: URBS Catchment Definition File ...... 58
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15. Appendix C: Calibration and Verification Plot ...... 60
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1. Introduction
1.1 Background
Over recent years, the Council of the City of Gold Coast developed numerous hydrological models of its catchments and waterways. These models were developed by Council’s staff and various consultants, using a range of different approaches and assumptions. The standard of these models, with respect to their configuration, calibration, use for design discharge estimation and documentation vary significantly.
To provide a consistent basis for floodplain management and local government planning, the Council commissioned WRM Water & Environment (WRM) in December 2007 to undertake a major study to review and update its hydrological models to a consistent standard of methodology and documentation. Coomera, Nerang, Logan-Albert, Pimpama, Worongary, Mudgeeraba, Loder, Biggera, Tallebudgera and Currumbin catchment’s hydrological models were included in this study.
A comprehensive review of data, previous hydrological models and associated reports for the above 10 catchments covering the City were undertaken prior to the commencement of model updates. This review assessed all aspects of model development, calibration and use for the estimation of design discharges. Based on the review, a set of recommendations were provided to update the 10 models in a consistent manner across the city area using the latest data and modelling approaches. Details of the model review and its findings are given in the Section 12.10 of this report.
Based on WRM recommendations, Council upgraded the hydrological model for Currumbin Creek catchment using the URBS modelling software in August 2009. The model was calibrated against three historical events (January 2008, June 2005 and February 2004) and verified against four historical events (November 2004, February 1990, April 1972 and February 1972). A Flood Frequency Analysis (FFA) was undertaken at Nicolls Bridge Gauging (GS) for 38 years of recorded data. The calibrated model was then used to estimate the design discharges from 2 years Average Recurrence Interval (ARI) to Probable Maximum Precipitation Design Flood (PMPDF).
This model was reviewed by WRM and Council’s Peer Review Group (PRG) in 2009/10. The PRG included Erwin Weinmann from Monash University and Dr Bill Weeks from the Department of Transport and Main Roads.
The upgraded model was again reviewed by Don Carroll in 2013/14. At the time of review the following modelling tasks were undertaken as per Don Carroll Project Management recommendations:
The January 2013event should be included in the calibration dataset Review the rating table at Nicolls Bridge GS Model is calibrated for January 2013 (additional historical event) Review the fraction of sub-catchment forested factor F Redo the FFA for 43 years (additional 5 years) of recorded data at Nicolls GS Extend the modelling to include Monte Carlo methodologies to address the Queensland Floods Commission of Inquiry Recommendation.
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This report describes the development of URBS hydrological model, calibration, FFA, Monte Carlo simulation and design event simulation for the Currumbin Creek catchment.
1.2 Study Objective and Scope
The main objective of this study is to develop a hydrological model for Currumbin Creek using the URBS model as the preferred city wide common modelling software platform. The model should be calibrated and verified against available data, and fully documented to a consistent standard. Once this objective is achieved, the calibrated model is to be used to estimate the design flood discharges using a consistent and agreed methodology. The scope of work for the Currumbin Creek catchment modelling study is as follows:
Review the existing models and data; Update the existing model to a standard consistent with other updated models; Review and update model calibration and verification; Undertake an FFA at Nicolls Bridge GS Undertake Monte Carlo simulations, Estimate the design discharges and extreme event discharges at key locations throughout the catchment using current industry standard methodologies and Document the adopted methodology, tasks and results to a consistent manner.
1.3 Previous Studies
A number of hydrological modelling studies have been previously carried out for the Currumbin Creek catchment. Brief descriptions of the most relevant of these studies are presented below:
1.3.1 WBM (1995) WBM Oceanics Australia undertook a detailed flood study for Currumbin Creek catchment in 1995 for Gold Coast City Council. As part of the study, WBM developed a XP-RAFTS hydrological model for the catchment and calibrated the model for February 1972, January 1974, March 1987, April 1988, and February 1990 flood events. A FFA was carried out on rated peak annual discharges for Nicolls Bridge GS (146012A). The calibrated model was then used to estimate the design discharges at different locations within the catchment for the 5, 10, 20, 50 and 100 year ARI events.
1.3.2 GCCC (2001) A hydrological study of Currumbin Creek catchment was undertaken by Gold Coast City Council (GCCC) in 2001. In this study, a URBS hydrological model was developed and calibrated for March 1999, December 1991, February 1990, April 1988, May 1987, April 1984, June 1983, February 1976, January 1974, April 1972 and February 1972 flood events. An FFA was also carried out for thirty years of recorded data (1970 – 1999) for the Nicolls Bridge GS (146012A). The calibrated model was then used to estimate the design discharges for the 5, 10, 20, 50, 100, 200 and 500 year ARI at key locations within the catchment.
The Designated Flood Level (DFL) adopted in the Current Planning Scheme 2003 version 1.2 amended in November 2011 is based on this study for Currumbin Creek catchment.
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1.4 Limitation Statement
The following limitations apply in the preparation of this report:
This report is prepared based on available data and information at the time of this study. The analysis and approach adopted in this study is specifically prepared for internal use only. Use of this report by any external entity is prohibited unless a written approval is obtained from the City of Gold Coast. The result of this study is accurate only for its intended purposes.
1.5 Acknowledgement
The assistance provided by the Bureau of Meteorology, and in particular, for this study is gratefully acknowledged. The Bureau provided their URBS hydrological models used for flood forecasting purposes and all the historical rainfall and stream flow data used in this study for model calibration and verification.
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2. Catchment Description
2.1 General
The Currumbin Creek catchment, which is one of the major catchments in Gold Coast, covers approximately 51.7 km2. It is bounded by Mount Cougal and Mount Tomewin Heights to the west, Piggabeen and Cobaki Creek catchments (NSW) to the south, the Pacific Ocean to the east and Tallebudgera Creek catchment to the north. The upper reaches of the creek are quite steep with a narrow channel and are covered by forested areas. The middle reaches are flatter but remain covered by forested areas. The lower reaches are urbanised and have flat floodplains. There are no significant storages like dams and detention basins in this catchment. Figure 1 is a locality map of Currumbin Creek catchment.
Figure 1 - Locality Map, Currumbin Creek Catchment
The total length of Currumbin creek is approximately 26.5 km. The average channel slope upstream of Camberra is about 0.82%. The average channel slope decreases rapidly in the lower reaches. Table 1 below shows the catchment area, channel length and average channel
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slope at four key locations along the creek. The average channel slope has been calculated using the equal area slope method (QUDM, 1994 - Figure 5.05.4).
Table 1 – Currumbin Creek Catchment Characteristics
Upstream Upstream Upstream Key Location Catchment Area Channel Length Average Channel 2 (km ) (km) Slope (%)
Camberra GS 23.2 12.6 0.815
Nicolls Bridge GS 30.6 16.7 0.618
Pacific Highway 49.1 25.1 0.328
Catchment Outlet 51.7 26.5 0.298
2.2 Land Use
Approximately 77% of Currumbin Creek catchment is rural, mostly forested with some pasture areas and rural residential development. The remainder of the catchment is urban, comprising residential, commercial and industrial areas. Land use data are discussed in sections 4.2 and 5.2 .
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3. Methodology
The hydrologic modelling of the Currumbin Creek catchment is undertaken using an approach and methodology consistent with the other catchments in the Gold Coast City area. The study adopted a systematic approach and consisted of the following steps:
Comprehensive review of the existing model and data - the specific tasks included: Review all previous studies, Review the existing URBS model, Review catchment and sub-catchments boundaries using the latest DTM and drainage network data, Review Nicolls Bridge and Camberra GS rating curves, Review available rainfall and stream gauging data and Review existing land use data.
Model construction - the specific tasks included: Update the URBS model configuration, Update Nicolls Bridge and Camberra rating curves, Generate catchment (network) file and assigning appropriate output locations and calibration points and Update model to reflect current land use in the catchment.
Model calibration and verification – the specific tasks included: Select calibration and verification events, Process rainfall and stream flow data for the selected calibration and verification events, Undertake rainfall analysis of all selected events to create sub-catchment specific rainfall sequences to generate rainfall definition files for URBS model and Calibrate/verify the URBS model results against historical recorded data.
Flood Frequency Analysis – the specific tasks included: Analyse the available annual peak height data at Nicolls Bridge GS Convert recorded peak level to discharge using the adopted rating curve Undertake the FFA to fit a Log-Pearson Type III distribution for an annual series of rated peak flood discharge at this location.
Monte Carlo framework – the specific tasks included: Setup and run the model for Monte Carlo Total Probability Theorem (TPT) and Setup and run the model for Monte Carlo Cooperative Research Centre for Catchment Hydrology (CRC-CH).
Design discharge estimation – the specific tasks included: Estimate the design rainfalls and loss rates for storm events ranging from 2 year ARI to PMPDF, Undertake the design event model runs for storm durations from 30 minutes to 72 hours for all ARIs and PMPDF, Undertake Flood Frequency Analyses (FFA) at Nicolls Bridge GS, Reconcile design event results based on FFA, Estimate the design discharges at key locations throughout the catchment for flood events ranging from 2 year ARI to PMPDF, Verify the design discharges against Monte Carlo simulation results and Finalise the design discharges.
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Prepare the study report – the specific tasks included: Document the adopted methodology, tasks and results.
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4. Available Data
4.1 Topographic Data
A 5m grid Digital Terrain Model (DTM) which covers most of the City area is available for this study. Unfortunately, these data do not cover the upper reaches of the catchment which is outside the City boundary. Topographic data for the upper reaches are available from 5m and 10m contour maps held by the Council. In addition, a digital drainage network layer is available from the Department of Natural Resources and Mines (DNRM) for better definition of the drainage paths within the catchment.
4.2 Land Use Data
The land use data used for the catchment are based on the latest GIS layers available from Council’s Infrastructure Planning Coordination Unit. The GIS layers contain Land Use, Domain and Local Area Plan data for the City of Gold Coast. Aerial photos and cadastre data are also used to supplement the above GIS data.
4.3 Rainfall Data
Rainfall data used in this study are provided by the Bureau of Meteorology (BoM). BoM data have been sourced from a variety of rainfall stations and data types including pluvio and daily stations. Table 2 shows the rainfall stations for which available data are supplied by BoM. The BoM flood warning map for rainfall and river height stations is shown in Figure 2.
The following is of note with regard to the rainfall stations:
The rainfall station coverage within the Currumbin Creek catchment is very poor. There are only one pluvio and one daily station within the catchment. Stations indicated by the letters TM or ALERTs are automatic stations. TM stations are connected to the public telephone network and polled regularly by computer during periods of rain. ALERT stations communicate by radio and report every 1mm of rain to the local base station and the Flood Warning Centre in BoM. Daily stations are manual and report the rainfall amount received in 24 hours to 9:00am each day.
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Table 2 – Rainfall Data Availability for Currumbin Creek Catchment Station No Station Name Station Type Station Owner 040599 Camberra TM DNRM
040717 Coolangatta TM DNRM
040717 Coolangatta AWS ALERT BoM
540320 Coplicks Bridge ALERT City of Gold Coast
040609 Elanora Water Treatment Plant TM DNRM
040524 Little Nerang Dam TM DNRM
540054 Little Nerang Dam ALERT City of Gold Coast
540353 Mt Nimmel ALERT City of Gold Coast
040550 Natural Bridge TM DNRM
2220 Nerang Dam TM DNRM
2265 Oyster Creek ALERT City of Gold Coast
540252 Oyster Creek ALERT City of Gold Coast
040607 Springbrook ALERT City of Gold Coast
040700 Springbrook TM DNRM
2205 Springbrook ALERT City of Gold Coast
040192 Springbrook Forestry TM DNRM
040848 Springbrook Lower ALERT City of Gold Coast
7001 Springbrook TM DNRM
040750 Springbrook TM DNRM
040196 Tallebudgera TM City of Gold Coast
540366 Tallebudgera Creek Dam ALERT City of Gold Coast
540356 Tallebudgera Creek Rd ALERT City of Gold Coast
540577 Tallebudgera Creek Mouth ALERT City of Gold Coast
040899 Tallowood TM DNRM
58067 Tomewin (Border Gate) TM DNRM 540354 Tomewin ALERT City of Gold Coast 058150 Upper Crystal Creek ALERT City of Gold Coast
540400 Upper Springbrook ALERT City of Gold Coast
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Figure 2 – Location of Rainfall and River Height Stations in South Coast (source BoM)
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4.4 Gauge Height Data
Currently one ALERT and one TM Gauging Stations (GS) operate downstream of Nicolls Bridge in the Currumbin Creek catchment. Another station was operational at Camberra between 1926 and 1983. Table 3 and Figure 3 show the detail of Nicolls Bridge and Camberra GS.
Table 3 – Gauge Height Data Availability for Currumbin Creek Catchment
Max
Rating Gauged Catchme Station Period of Operation Table Height Station nt Area No. From To Source (m) Name (km2) 146003A Camberra 1 23.2 28/10/1926 01/10/1954 BoM/DNRM - 146003B Camberra 2 23.2 01/10/1954 28/04/1983 BoM/DNRM 2.10 m Nicolls 146012A 30.6 20/02/1970 10/02/2002 BoM/DNRM 2.39 m Bridge TM Nicolls 540070 30.6 10/02/2002 - BoM/DNRM - Bridge AL
Figure 3 – Location of Gauging Stations in Currumbin Creek Catchment
4.5 Rating Tables
The rating tables for Nicolls Bridge and Camberra Gauging Stations are available from the Department of Natural Resources and Mines (DNRM) and BoM. The available rating tables for those stations are discussed below.
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4.5.1 Nicolls Bridge Figure 4 shows the adopted and available rating tables for Nicolls Bridge GS. DNRM updated the rating table at this location in different times. During the previous study in 2009 the DNRM Table 1, Table 7 and BoM tables were available. Table 1 covers the higher discharges and water levels but does not match well with the gauged discharge points. Table 7 matches well with the gauged data. A combination of Table 1 (for higher discharges) and Table7 (for the gauged range of discharges) was adopted in 2009 study. Recently DNRM updated the rating to Table 15. As part of 2013 reviews all available rating tables are analysed thoroughly and tested against the calibration and verification events. Results show significant improvement in calibration and verifications using the rating Table 15 over other tables. Consequently the DNRM rating Table 15 has been adopted for this study.
Figure 4 – Available and Adopted Rating Tables at Nicolls Bridge GS (146012A)
Figure 5 shows the comparison plot of rating Table 15 and gauge data. Gauged data matched quite well with the Table 15.
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Figure 5: Comparison Plot of rating Table 15 and Gauge data (source DNRM)
4.5.2 Camberra The Camberra station is located 4.1km upstream of the Nicolls Bridge. The available historical data for the Camberra station is limited to two events in February and April 1972. BoM and DNRM have adopted similar rating curves for the Camberra station. The maximum gauged water level at this station is only 2.1m (13.6m3/s on 09/06/1958). Figure 6 shows the available and adopted ratings curve for Camberra. The BoM curve has been adopted because it appears to match better with the gauged data for this station.
Figure 6: Available and Adopted Rating Curves for Camberra GS (146003B)
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5. Model Development
5.1 Model Description
URBS is a networked (i.e. sub-catchment based) runoff-routing model that estimates flood hydrographs by routing rainfall excess through a module representing the catchment storage. In URBS, the storages are arranged to represent the drainage network of the catchment. The distributed nature of storage within the catchment is represented by a separate series of concentrated storages for the main stream and for major tributaries to provide a degree of physical realism. The storages in the model are generally non-linear, but linear storages can be used.
The model provides a number of options for conceptualising the rainfall-runoff process. Rainfall excess is first estimated from rainfall data using one of several available techniques (i.e. loss models) before applied to the runoff-routing component of the model to compute the surface runoff hydrograph. Base flow, if significant, is estimated separately and added to the surface runoff hydrograph to provide the total catchment hydrograph. The model can easily incorporate the effects of land use change, construction of reservoirs, changes to channel characteristics and other changes in the catchment.
The model provides different options for runoff routing. The user is given the option of lump the catchment runoff and channel flow components into a single routing component or modelling them as separate routing components. The latter option (i.e. the ‘split’ model) was adopted for this study.
In the split model the rainfall excess for each sub-catchment is first determined by subtracting losses from the rainfall hyetograph. The rainfall excess is then routed through conceptual catchment storage to determine the local runoff hydrograph for the sub-catchment. The storage - discharge relationship for catchment routing is:
2 A (1 F) m Scatch Q (1 U ) 2
Where 3 Scatch is the catchment storage (m h/s); is the catchment lag parameter; A is the area of sub-catchment (km2); U is the fraction urbanisation of sub-catchment; F is the fraction of sub-catchment forested; and m is the catchment non-linearity parameter.
In the above equation, β is determined during model calibration and is a global parameter. The local runoff hydrograph is then combined with runoff from the upstream sub-catchment and routed through channel storage to obtain the outflow hydrograph for the sub-catchment. Channel routing is based on the non-linear Muskingum model. The channel routing storage- discharge relationship is given by:
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n * L n Schnl f ( x Qu ( 1 x ) Qd ) S c Where 3 Schnl is the channel storage (m h/s); α is channel lag parameter f is reach length factor; L is reach length (km);
Sc is channel slope (m/m); 3 QU is inflow at upstream end of reach includes catchment inflow (m /s); 3 Qd is outflow at downstream end of the channel reach (m /s); x is Muskingum translation parameter; n is Muskingum non-linearity parameter (exponent); and n is Manning's 'n' or channel roughness.
In the above equation α and f are the principal calibration parameters. Note also that α is a global parameter, whereas f can be varied for each channel reach. URBS allows the user to select one of several standard loss models. The two primary loss models are initial and continuing loss model and the proportional loss model. The initial and continuing loss model was adopted for Currumbin Creek Catchment Hydrological Study. This model assumes that there is an initial loss of ‘il’ mm before any rainfall becomes runoff. After this, a continuing loss rate of ‘cl’ mm/hr is applied to the rainfall, subject to the limit of the soil infiltration capacity (IFmax). The loss rates can be specified ‘globally’ to the entire catchment or ‘individually’ to each sub-catchment.
Full details of the URBS model and its features are given in the URBS User Manual (Carroll, 2012).
5.2 Model Configuration
5.2.1 Land Use Table 4 shows the major land use categories of the Currumbin Creek catchment corresponding to different land classifications as adopted by the City of Gold Coast.
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Table 4 – Currumbin Creek Land Use Categories
URBS Model Land Use Classification Gold Coast City Council Classifications
Forest UF (Forested) o o Forest/Grassland o Grassland _ Urban/Suburban o Grazing o Open _ Ground o Recreation (Facilities & Sub/Urban UR (Rural Land) Parks) o Rural _ Residential o Tourism _ Recreation Park o Vacant _ Land o Waste _ Disposal o Access _ Restriction Strip o Commercial o Constructed Waterway _ Lake o Industrial o Marina a UH (High Density Urban) o Residential Choice o Tourism _ Accommodation o Transport (Rail, Road & Paved Areas) o Utilities _ Infrastructure o Water o Wetlands UM (Medium Density Urban) o Detached Dwelling Park Living UL (Low Density Urban) o o Tourism _ Caravan Park o Highly Disturbed _ Under UL/UM/UH b Development o Urban Residential a - Roads are included in this category b - Appropriate classification selected based on the aerial photos and site inspections
Out of five different land uses, four relate to the amount of urbanisation and forestation in the catchment, affecting both imperviousness (decrease in losses) and the routing characteristics.
Figure 7 shows the sub-catchment delineation of Currumbin Creek catchment for URBS. The model consists of 21 sub-catchments.
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Table 5 shows the sub-catchment areas and the land uses.
Figure 7 – Currumbin Creek URBS Model – Sub-catchment Delineation
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Table 5 – Currumbin Creek Sub-catchment Areas and Land Uses
Sub- Sub- catchment catchment UL UM UH UR UF 2 ID Area (km ) (km2) (km2) (km2) (km2) (km2) 1 2.22 0 0 0.053 0 2.167 2 1.75 0 0 0.027 0 1.723 3 3.62 0 0 0.171 1.099 2.349 4 1.95 0 0 0.092 1.342 0.516 5 4.39 0 0 0.235 0.108 4.047 6 4.61 0.001 0 0.243 3.179 1.186 7 2.30 0 0 0.152 1.251 0.897 8 2.32 0 0 0.226 1.806 0.288 9 3.99 0.006 0 0.328 3.518 0.138 10 0.96 0 0 0.074 0.568 0.318 11 2.53 0.014 0 0.140 1.886 0.491 12 3.64 0.004 0 0.462 2.194 0.980 13 1.31 0 0.038 0.215 0.950 0.107 14 1.33 0 0 0.393 0.785 0.152 15 1.02 0.002 0 0.049 0.527 0.442 16 1.00 0.006 0 0.037 0.023 0.935 17 1.93 0 0 0.097 1.833 0.000 18 3.36 0 1.019 1.309 0.709 0.324 19 2.23 0 0 2.044 0.175 0.011 20 2.65 0 0 2.384 0.137 0.129 21 2.58 0 0 2.089 0.459 0.033
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6. Model Calibration and Verification
6.1 Selection of Calibration and Verification Events
Currumbin Creek catchment rainfall and stream height data for 22 historical flood events going back to 1954 were available from BoM. However, the quality, quantity and coverage of the available data vary significantly for these events. Data coverage for older events was poor and considered unsatisfactory for good calibration. Therefore, the model calibration and verification events were selected as much as possible from the most recent flood events. Selected calibration and verification events are shown in Table 6. These events cover a wide range of floods.
Table 6 – Selected Calibration and Verification Events Event Calibration Verification
January 2013
January 2008
June 2005
November 2004
February 2004
February 1990
April 1972
February 1972
Table 7 and Table 8 show the available rainfall and stream flow data for the selected calibration and verification events.
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Table 7 – Available Rainfall Data for Calibration and Verification
Station Feb Apr Feb Feb Nov Jun Jan Jan Station Name No 1972 1972 1990 2004 2004 2005 2008 2013
040439 Alpine Panorama 040599 Camberra 040717 Coolangatta 040717 Coolangatta AWS 540320 Coplicks Bridge 040609 Elanora Treatment Plant 040524 Little Nerang Dam 540054 Little Nerang Dam AL 540353 Mt Nimmel ALERT 540252 Oyster Creek AL 040607 Springbrook 040700 Springbrook (Q House) 040534 Springbrook 2205 Springbrook ALERT 040192 Springbrook Forestry 040848 Springbrook Lower AL 040607 Springbrook Road 7001 Springbrook TM 040750 Springbrook TM 040196 Tallebudgera 540497 Upper Tallebudgera AL 540366 Talle Ck Dam AL 540356 Tallebudgera Ck Rd AL 040899 Tallowood 58067 Tomewin (Border Gate) 540354 Tomewin ALERT 58150 Upper Crystal Creek 540400 Upper Springbrook AL
Table 8 – Available Stream Flow Data for Calibration and Verification
Station Station Name Feb-72 Apr-72 Feb-90 Feb-04 Nov-04 Jun-05 Jan-08 Jan-13
146003B Camberra 146012A Nicolls Bridge TM 540070 Nicolls Bridge AL
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6.2 Calibration Methodology
The emphasis of the model calibration is achieving the best possible fit between the modelled and recorded discharge hydrographs at key stations along the Currumbin Creek for the selected calibration events. For these stations, the calibration attempted to match the modelled and recorded flood peaks and volumes, and also the shape of the hydrographs. The calibrated model is then verified by comparing the model predictions against the discharge hydrographs recorded at various gauging stations for the selected verification events. A single set of global parameters (α, β, m and n) are adopted for all calibration events. In addition, uniform initial and continuing losses (IL and CL) were applied for the whole catchment. The model parameters were adjusted to achieve the best calibration (i.e. achieve best timing and hydrograph shape) across all events. Initial and continuing losses were adjusted to match the commencement of the rising limb of the hydrograph and the hydrograph volume respectively.
6.3 Assignment of Rainfalls and Temporal Patterns
Rainfall depth and temporal pattern for each sub-catchment were generated from the available pluviograph and daily rainfall data using an inverse distance squared method using the nearest four rainfall stations to the sub-catchment centroid. This method ensures that all of the available data are used and that the most appropriate rainfall temporal pattern is assigned to each sub- catchment. Table 9 shows the weighted average rainfall at the selected locations for different events. Table 9 – Weighted Average Rainfall at Selected Location Catchment Camberra Nicolls Bridge Event Outlet (mm) (mm) (mm)
January 2013 545 458 234 January 2008 330 266 213 June 2005 527 561 557 November 2004 326 341 345 February 2004 211 197 159 February 1990 316 259 167 April 1972* 324 324 324 February 1972* 968 968 968 *Rainfall data are available from only one station
6.4 Adopted Model Parameters
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Table 10 shows the global catchment and channel parameters adopted for the Currumbin Creek catchment for all calibration and verification events.
Table 10 – Adopted Parameters for Calibration and Verification Parameter Adopted Value α (Channel Lag) 0.14 (Catchment Lag) 1.8 m (Catchment non-linearity) 0.65 n 1 F F*0.5
6.5 Initial and Continuing Losses
Table 11 shows the adopted initial and continuing losses for each of the calibration and verification events. The adopted initial losses vary significantly (10 – 120 mm) but are consistent for calibration and verification events. The adopted continuing losses (0.1 – 7 mm/hr) appear to fall into two ranges. Two events are in the 0.1-1.0 mm/hr range and the remaining six events have a continuing loss rate of 2.5 to 7.0 mm/hr. The Table also shows that there is a strong positive correlation between the initial loss and continuing loss.
Table 11 – Adopted Initial and Continues Losses Initial Loss Continues Loss Event IL (mm) CL (mm/hr) January 2013 100 7.0 January 2008 10 0.1 June 2005 80 5.0 November 2004 80 5.0 February 2004 100 6.0 February 1990 80 5.0 April 1972 120 2.5 February 1972 25 1.0
6.6 Calibration Results
6.6.1 General Comments Good calibration was achieved for the Currumbin Creek catchment. A single set of model parameters are adopted for all four calibration events. The quality of available rainfall data for all events was good. Table 12 shows the comparison of modelled and recorded peak discharges at the Nicolls Bridge GS for different events.
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Table 12 – Modelled and Recorded Peak Discharge at Nicolls Bridge GS Peak Discharge (m3/s) Event Modelled Recorded January 2013 164 155 January 2008 390 392 June 2005 259 251 February 2004 86 88
Calibration results for individual events are discussed below:
6.6.2 January 2013 Event Intense rainfall occurred between 22 January 2013 and 29 January 2013 due to Ex-Tropical Cyclone Oswald. The Upper Springbrook gauging station recorded 744 mm in 24 hours to 9 am on the 28 January 2013 which is the highest 24 hour rainfall total in Australia. Recorded rainfall data from Upper Springbrook, Tomewin, Tallebudgera Creek Dam, Coplicks Bridge, Oyster Creek and Burleigh Water ALERTs are used for this calibration.
Table 12 above and Figure 10 in Appendix C: Calibration and Verification Plot show the comparison of the modelled and recorded discharges at Nicolls Bridge GS. The plot shows very good match between modelled and recorded discharges in terms of shape, peak, timing and volume of the hydrograph.
6.6.3 January 2008 Event Heavy rainfall occurred overnight on 04 January 2008 in the Gold Coast hinterland area. Recorded rainfall data from Upper Springbrook, Lower Springbrook, Tomewin, Tallebudgera Creek Dam, Tallebudgera Creek Road, Coplicks Bridge and Oyster Creek ALERTs are used for this calibration.
Table 12 above and Figure 11 in Appendix C: Calibration and Verification Plot show the comparison of the modelled and recorded discharges at Nicolls Bridge GS. The calibration plot shows very good match between modelled and recorded discharges in terms of shape, peak, timing and volume of the hydrograph.
6.6.4 June 2005 Event Intense rainfall burst commenced in Gold Coast from 27 June 2005 to 30 June 2005. Heaviest rainfall occurred on the 29th overnight and the following day morning. Recorded rainfall from Lower Springbrook, Mount Nimmel, Tomewin, Tallebudgera Creek Dam, Tallebudgera Creek Road, Coplicks Bridge and Oyster Creek ALERTs are used for this calibration.
Table 12 above and Figure 12 in Appendix C: Calibration and Verification Plot show the comparison between modelled and recorded discharges at Nicolls Bridge GS. Very good calibration is achieved for this event.
6.6.5 February 2004 Event Recorded rainfall from Lower Springbrook, Mount Nimmel, Tomewin, Tallebudgera Creek Road, Coplicks Bridge and Oyster Creek ALERTs are used for this calibration.
Table 12 above and Figure 13 in Appendix C: Calibration and Verification Plot show the comparison between modelled and recorded discharges at Nicolls Bridge GS. Very good agreement between modelled and recorded discharges is achieved for this event.
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6.7 Verification Results
The calibrated URBS model was then tested using the verification events viz. November 2004, February 1990, April 1972 and February 1972 flood events using the same global parameters as for the calibration events. Table 13 shows the modelled and recorded peak discharges at Camberra and Nicolls Bridge gauging stations for the verification events. Note that recorded data at the Camberra station are available only for the April 1972 and February 1972 events.
Table 13: Modelled and Recorded Peak Discharge at Camberra and Nicolls Bridge GS
Camberra Nicolls Bridge Peak Discharge Peak Discharge Event (m3/s) (m3/s) Modelled Recorded Modelled Recorded November 2004 N/A N/A 121 128 February 1990 N/A N/A 207 186 April 1972 181 265 228 220 February 1972 258 246 345 244
6.7.1 November 2004 Event Table 13 above and Figure 14 in Appendix C: Calibration and Verification Plot show the comparison between modelled and recorded discharge at Nicolls Bridge GS for the November 2004 event. Good match between modelled and recorded discharges is achieved for this event.
6.7.2 February 1990 Event Table 13 above and Figure 15 in Appendix C: Calibration and Verification Plot show the comparison between modelled and recorded discharge at Nicolls Bridge GS for February 1990 event. The shapes of the hydrographs agree well however the peak, the timing of the peak and volume does not agree quite well. This is attributed to unrepresentative rainfall input to the model for this event.
6.7.3 April 1972 Event Table 13 above and Figure 16 & Figure 17 in Appendix C: Calibration and Verification Plot show a comparison of modelled and recorded discharges at Camberra and Nicolls Bridge GS stations for April 1972 event. The model under predicts the peak at Camberra and very close match at Nicolls Bridge. Overall the calibration is considered satisfactory with respect to the shape and timing of the hydrographs.
6.7.4 February 1972 Event Table 13 above and Figure 18 & Figure 19 in Appendix C: Calibration and Verification Plot show the comparison between the modelled and recorded discharges at Camberra and Nicolls Bridge GSs for February 1972 event. The agreement between modelled and recorded discharge hydrograph at Camberra GS is very good. At Nicolls Bridge the agreements between modelled and recorded hydrographs are good in terms of shape and timing of the hydrographs however the modelled peak is higher than the recorded peak. The recorded peak at Nicolls Bridge is very similar to Camberra and it suggests that the recorded discharge at Nicolls Bridge may not be accurate. Overall the verification is considered satisfactory.
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7. Flood Frequency Analysis
7.1 Method of Analysis
Sufficient recorded annual peak discharge data within the Currumbin Creek catchment is available at the Nicolls Bridge Gauging Station (GS-1460012A). Therefore, design flood discharges in the Currumbin Creek at Nicolls Bridge GS were estimated by flood frequency analysis (FFA) using all available data. The methodology recommended in Book 4, Section 2 of Australian Rainfall and Runoff (1987) was used to fit a Log-Pearson Type III distribution to an annual series of recorded peak flood discharge at this location.
7.2 Available data
Annual peak gauge heights at the Nicolls Bridge GS were available from DNRM for the period 1970 to 2013 (43 years). The recorded annual peak gauge heights were then converted to annual peak discharges using the rating table adopted for this study (Section 4.5.1 ).
7.3 Annual Peak Discharge Analysis
Figure 8 and
Table 14 show the flood frequency distribution and the FFA estimated design peak discharges respectively at the Nicolls Bridge gauging station.
Figure 8: Flood Frequency Analysis @ Nicolls Bridge Gauging Station
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Table 14: FFA Estimated Peak Design Discharge @ Nicolls Bridge GS
FFA Estimated Peak Discharge (m3/s) 95% 5% ARI Confidence Fitted Confidence (Year) Limit Value Limit 2 91 101 113 5 146 167 190 10 182 214 253 20 212 266 333 50 244 343 482 100 262 410 640 200 275 486 857
The FFA estimated design discharges in this study are compared to other methodologies and other studies in Section 9 of this report.
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8. Design Flood Estimation
The calibrated URBS model was used to estimate the design flood discharges throughout the Currumbin Creek catchment using the design rainfall data described in Section 8.2 , 8.3 and 8.4 . Design flood discharges were estimated for a range of storm durations including 0.5, 1.0, 1.5, 3.0, 4.5, 6.0, 9.0, 12.0, 18.0, 24.0, 36.0, 48.0 and 72 hours for the 2, 5, 10, 20, 50, 100, 200, 500, 1000 and 2000 year ARI events and for the Probable Maximum Precipitation Design Flood (PMPDF).
8.1 Methodology
WRM Water & Environment has undertaken a review of available design rainfall and associated procedures to determine the design rainfall data (IFD, Temporal Patterns, Areal Reduction Factors, Rainfall Spatial Distributions and Design Rainfall Losses) that should be used for the City of Gold Coast area (12.13 ). The recommended methodology is summarized in Table 15 and adopted in this study.
It should be noted that this methodology was later modified for other catchments under investigation where it was found that design flow estimates between the 100 year and the 500 year ARI were anomalous due to IFD interpolation and temporal pattern issues. No such anomalies were found with this study and thus the original methodology as recommended by WRM and reviewed by Council’s PRG was adopted.
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Table 15: Summary of Recommended Methodology for Design Event Analysis Design Flood ARI Range Available Parameter Comment Recommendation (Years) Sources/Methods
Rainfall depth ≤100 ARR 1987 Industry standard approach.
AWE 1998 recommended for Gold Coast catchments. AWE 1992
recommended for Logan River catchment. Both methods use standard AWE 1998 a / AWE 1992 Uses same methodology as ARR 1987 with additional data. methodology with a longer period of recorded data.
Based on analysis of daily data. Adopted for Hinze Dam hydrology (HDA 2007) for events from 10 to CRCFORGE 2000 year ARI.
BoM Pilot Study Data was provided by BoM for investigation of Hinze Dam hydrology, but is no longer available.
ARR 1987 Industry standard approach. >100 - 500
AWE 1998 and AWE 1992 recommended for Gold Coast and Logan River AWE 1998 a / AWE 1992 Uses same methodology as ARR 1987 with additional data. catchments respectively for durations < 24 hours.
Based on analysis of daily data. Adopted for Hinze Dam hydrology (HDA 2007) for events from 10 to Recommended for durations ≥ 24 hours. Smoothen the transition between CRCFORGE 2000 year ARI. the two methods (i.e. between 12 and 24 hour depths).
Based on analysis of daily data. Adopted for Hinze Dam hydrology (HDA 2007) for events from 10 to Recommended. However, short duration rainfalls (< 24 hrs) subject to >500 - 2000 CRCFORGE 2000 year ARI. substantial uncertainty. Detailed investigation may be warranted where this ARI range is of critical interest.
Interpolate between No explicit methodology is available to estimate rainfall depths for events of this magnitude. Section 2000 - < PMP Recommended. CRCFORGE & PMP 3.6.3 of ARR 1999 provides a methodology for interpolation. methods.
PMPDF GSDM (≤ 6 hours) Industry standard approach. Recommended. GTSMR (> 6 hours)
Areal Reduction Factors <2000 ARR 1987 Based on United States data.
CRC ARF Derived from regional data for durations ≥ 24 hours. Recommended. Adopt 24 hour duration ARFs for durations less than 24 hours. Verify using flood frequency analysis where possible.
Interpolate between Interpolate as recommended by ARR 1999 Section 3.6 using CRCFORGE and PMP rainfalls which are 2000 - PMP GSDM (≤ 6 hours) Industry standard approach. Recommended. GTSMR (> 6 hours) Industry standard approach. Recommended for Logan River catchment. Temporal Pattern ≤100 ARR 1987 Uses same methodology as ARR 1987 with additional data. Alternative patterns derived for ARI > 30 Use of filtered AWE (2000) patterns recommended for Gold Coast years (but only recommended for sensitivity analysis). AWE 2000 patterns have been filtered by WRM catchments. Note these patterns do not cover the Logan River catchment. AWE 2000 to eliminate sub-duration inconsistencies. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 36 of 65 Design Flood ARI Range Available Parameter Comment Recommendation (Years) Sources/Methods UWS 2006 Uses same methodology as ARR 1987 & AWE 2000 with additional data. Recommended. Use 24 hour patterns for 12 and 18 hour events and use >100 - GSDM (≤ 6 hours) Recommended. Use 24 hour patterns for 12 and 18 hour events and use PMP Industry standard approach. GTSMR (> 6 hours) 6 hour pattern for the 9 hour event. Spatial Distribution ≤100 AWE 1998 a Estimate design rainfall at the centroid of each model sub-catchment and apply ARF based on whole Recommended. catchment, as recommended in ARR 1987. >100 – 2000 CRCFORGE Estimate CRCFORGE rainfall at the centroid of each model sub-catchment. Recommended. >2000 - PMP GSDM (≤ 6 hours) Adopt PMP spatial distribution for events greater than 2000 year ARI as recommended by ARR 1999. Recommended. GTSMR (> 6 hours) Very little Queensland data used in recommended loss values for Queensland. Suggests Initial losses Rainfall Losses ≤100 ARR 1987 in the range 15-35mm and a continuing loss rate of 2.5mm/hr. Recommends adoption of median values from catchment-specific model calibration. Comprehensive study based on data for 48 Queensland catchments. Estimated the median initial and Recommended. Adopt 38mm for initial loss and median continuing loss Ilahee 2005 continuing loss rates for eastern Queensland catchment to be 38mm and 1.52mm/hr respectively. values from catchment specific model calibration. Interpolate losses between 100 year ARI and PMP Design Flood using approach recommended by >100 - PMP ARR 1999 Adopt minimum values from catchment-specific model calibration, as recommended by ARR 1999. Recommended . Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 37 of 65 8.2 Frequent to Large Design Events (up to and including 100 Year ARI) 8.2.1 Rainfall Depth Estimation Design rainfall intensities at the centroid of each sub-area are determined using the Council’s Intensity Frequency Distribution utility program (WRM 2008b 12.11 ) for storm durations up to 72 hours for all ARIs up to and including 100 years. The average rainfall intensities for each duration and ARI are then converted to rainfall depths. The IFD calculation parameters are originally based on ‘Review of Gold Coast Rainfall Data’ (AWE, 1998 12.6 ). The AWE study produced a revised set of design rainfall intensity maps and F2 and F50 factor maps for the City of Gold Coast area. These maps replaced Maps 1.5, 6.5 and Map 8 and 9, Volume 2 of IEAust (1998) for the Gold Coast area. In the AWE 1998 study the skew coefficient for Gold Coast was adopted as zero. WRM updated Council’s IFD utility by introducing a skew coefficient as per section 1, book II volume 1 of IEAust (1998) in 2008. 8.2.2 Areal Reduction Factors The point design rainfall estimates at the centroid of each sub-area are converted to average values for each sub-area using areal reduction factors (ARFs). The Queensland Extreme Rainfall Estimation Project (EREP, Hargraves c.2004)) developed the following relationship between ARF, catchment area and storm duration for Queensland catchments: ARF = 1 – 0.226 X (Area0.1685 – 0.8306 X log (Duration)) X Duration-0.3994 Where ARF = Aerial Reduction Factor Area = Catchment Area in Km2 and Duration = Storm Duration in Hours The derivation of the EREP ARF equation is based on daily rainfall data. Hence the applicability of ARFs derived from the above equation for durations shorter than 24 hours is uncertain. For consistency with the approach adopted in two recent major dam design studies in the study area, namely Hinze Dam upgrade study in the Nerang River catchment (HDA, 2007) and Wyaralong Dam design study in the Logan River catchment (Sunwater, 2007), the 24 hour ARFs are adopted in this study for durations shorter than 24 hours. Table 16 shows the adopted ARFs for design event simulation. For design event simulation the ARF calculation is based on total catchment area and for comparison with FFA results the ARF calculation is based on the catchment area upstream of Nicolls Bridge gauging station. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 38 of 65 Table 16: Adopted Aerial Reduction Factors for Currumbin Creek Catchment Aerial Reduction Factor Catchment U/S of Nicolls Storm Duration (Hour) Total Catchment (51.7 Bridge ALERT Station km2) (30.6 km2) 72 0.984 0.990 48 0.974 0.982 36 0.965 0.974 24 0.949 0.960 18 0.949 0.960 12 0.949 0.960 9 0.949 0.960 6 0.949 0.960 4.5 0.949 0.960 3 0.949 0.960 1.5 0.949 0.960 1 0.949 0.960 0.5 0.949 0.960 8.2.3 Temporal Patterns A sub-duration burst inconsistency within the Council’s temporal patterns (AWE, 2000) has been identified recently. These temporal patterns developed by AWE (2000), adopted by Council in 2000, produces design rainfalls that contain sub-burst of greater ARI than storm burst duration itself. For instance at a number of locations the 72 hour storm temporal pattern has within it 6 hour, 12 hour, 24 hour and 48 hour design burst rainfalls that have a greater ARI than the equivalent design rainfalls given by the 6 hour, 12 hour, 24 hour and 48 hour storm temporal patterns. Further this problem was evident for the full range of ARI’s from 2 to 100 years. The inconsistencies within the AWE (2000) temporal patterns produce unrealistically long critical storm durations and large design discharges for Gold Coast catchments. As part of WRM’s study, the sub-duration burst inconsistencies within the AWE (2000) patterns were filtered and smoothened by adjusting the longer duration storm temporal patterns according to the methodology outlined in Australian Rainfall and Runoff (1998) and BoM (1991). The filtering and smoothing were undertaken until the inconsistencies were removed for all ARI’s up to 100 years, whilst maintaining the basic shape and integrity of the patterns. Table 17 shows the adopted temporal patterns for ARI’s up to and including 100 years. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 39 of 65 Table 17: Adopted Temporal Patterns for All Storm Durations and ARI’s up to 100 Years PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10 MINUTE DURATION in 2 PERIODS OF 5 MINUTES ARI<=30 0.399 0.601 ARI>30 0.459 0.541 15 MINUTE DURATION in 3 PERIODS OF 5 MINUTES ARI<=30 0.322 0.452 0.226 ARI>30 0.339 0.408 0.253 20 MINUTE DURATION in 4 PERIODS OF 5 MINUTES ARI<=30 0.195 0.36 0.261 0.184 ARI>30 0.204 0.338 0.269 0.189 25 MINUTE DURATION in 5 PERIODS OF 5 MINUTES ARI<=30 0.159 0.31 0.225 0.177 0.129 ARI>30 0.166 0.296 0.218 0.187 0.133 30 MINUTE DURATION in 6 PERIODS OF 5 MINUTES ARI<=30 0.139 0.266 0.202 0.156 0.124 0.113 ARI>30 0.139 0.256 0.195 0.17 0.12 0.12 45 MINUTE DURATION in 9 PERIODS OF 5 MINUTES ARI<=30 0.084 0.102 0.112 0.199 0.153 0.12 0.094 0.072 0.064 ARI>30 0.086 0.097 0.104 0.201 0.153 0.122 0.094 0.077 0.066 1 HOUR DURATION in 12 PERIODS OF 5 MINUTES ARI<=30 0.055 0.082 0.095 0.11 0.104 0.178 0.08 0.073 0.062 0.057 0.055 0.049 ARI>30 0.059 0.075 0.084 0.113 0.11 0.177 0.079 0.066 0.062 0.06 0.058 0.057 1.5 HOUR DURATION in 18 PERIODS OF 5 MINUTES ARI<=30 0.034 0.048 0.067 0.073 0.121 0.082 0.061 0.06 0.057 0.053 0.043 0.052 0.044 0.044 0.042 0.041 0.04 0.038 ARI>30 0.044 0.047 0.064 0.068 0.11 0.088 0.056 0.061 0.06 0.053 0.048 0.053 0.044 0.043 0.042 0.041 0.04 0.038 2 HOUR DURATION in 24 PERIODS OF 5 MINUTES ARI<=30 0.028 0.034 0.046 0.095 0.08 0.052 0.068 0.056 0.044 0.041 0.037 0.035 0.032 0.03 0.033 0.041 0.033 0.036 0.033 0.032 0.029 0.031 0.028 0.026 ARI>30 0.034 0.034 0.052 0.087 0.068 0.048 0.062 0.054 0.048 0.044 0.036 0.038 0.034 0.034 0.035 0.039 0.033 0.037 0.035 0.033 0.032 0.032 0.027 0.024 3 HOUR DURATION in 12 PERIODS OF 15 MINUTES ARI<=30 0.066 0.121 0.158 0.097 0.073 0.068 0.059 0.078 0.111 0.062 0.055 0.052 ARI>30 0.072 0.117 0.158 0.091 0.075 0.071 0.064 0.074 0.105 0.061 0.059 0.053 4.5 HOUR DURATION in 18 PERIODS OF 15 MINUTES ARI<=30 0.088 0.061 0.045 0.071 0.048 0.055 0.035 0.075 0.117 0.086 0.046 0.032 0.04 0.037 0.03 0.046 0.052 0.036 ARI>30 0.077 0.066 0.047 0.07 0.049 0.056 0.04 0.065 0.109 0.079 0.048 0.04 0.039 0.037 0.034 0.049 0.056 0.039 6 HOUR DURATION in 12 PERIODS OF 30 MINUTES ARI<=30 0.058 0.061 0.073 0.096 0.172 0.066 0.135 0.077 0.064 0.045 0.099 0.054 ARI>30 0.061 0.069 0.074 0.102 0.153 0.072 0.122 0.082 0.068 0.048 0.098 0.051 PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 40 of 65 Table 18: Adopted Temporal Patterns for All Storm Durations and ARI’s up to 100 Years (Continued) PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 9 HOUR DURATION in 18 PERIODS OF 30 MINUTES ARI<=30 0.038 0.043 0.026 0.034 0.049 0.045 0.061 0.071 0.085 0.074 0.117 0.067 0.058 0.052 0.037 0.029 0.069 0.045 ARI>30 0.037 0.044 0.03 0.038 0.056 0.045 0.058 0.069 0.082 0.072 0.112 0.067 0.057 0.054 0.041 0.029 0.069 0.04 12 HOUR DURATION in 24 PERIODS OF 30 MINUTES ARI<=30 0.032 0.025 0.026 0.028 0.031 0.032 0.035 0.044 0.051 0.04 0.046 0.074 0.047 0.097 0.058 0.053 0.044 0.033 0.037 0.025 0.03 0.045 0.036 0.031 ARI>30 0.033 0.025 0.028 0.033 0.039 0.035 0.038 0.042 0.047 0.038 0.047 0.072 0.044 0.095 0.055 0.049 0.044 0.038 0.038 0.025 0.031 0.042 0.033 0.029 18 HOUR DURATION in 18 PERIODS OF 1 HOUR ARI<=30 0.035 0.027 0.023 0.047 0.049 0.071 0.09 0.144 0.05 0.078 0.07 0.057 0.063 0.052 0.042 0.034 0.039 0.029 ARI>30 0.036 0.027 0.022 0.048 0.052 0.068 0.095 0.14 0.05 0.076 0.067 0.048 0.065 0.052 0.044 0.037 0.042 0.031 24 HOUR DURATION in 24 PERIODS OF 1 HOUR ARI<=30 0.033 0.02 0.025 0.02 0.024 0.038 0.049 0.054 0.038 0.051 0.061 0.068 0.115 0.031 0.065 0.079 0.042 0.036 0.026 0.03 0.024 0.034 0.02 0.017 ARI>30 0.034 0.017 0.025 0.019 0.025 0.036 0.051 0.055 0.034 0.053 0.062 0.069 0.122 0.03 0.064 0.078 0.037 0.034 0.024 0.029 0.022 0.033 0.025 0.022 30 HOUR DURATION in 15 PERIODS OF 2 HOURS ARI<=30 0.039 0.036 0.031 0.039 0.042 0.057 0.063 0.099 0.082 0.171 0.13 0.068 0.046 0.039 0.058 ARI>30 0.051 0.047 0.039 0.036 0.038 0.057 0.06 0.098 0.079 0.179 0.119 0.068 0.041 0.035 0.053 36 HOUR DURATION in 18 PERIODS OF 2 HOURS ARI<=30 0.035 0.025 0.028 0.034 0.036 0.069 0.04 0.047 0.058 0.034 0.152 0.089 0.077 0.118 0.059 0.037 0.034 0.028 ARI>30 0.039 0.031 0.028 0.036 0.032 0.065 0.036 0.043 0.059 0.036 0.143 0.099 0.083 0.109 0.065 0.037 0.032 0.027 48 HOUR DURATION in 24 PERIODS OF 2 HOURS ARI<=30 0.024 0.022 0.023 0.023 0.032 0.034 0.025 0.036 0.027 0.037 0.035 0.055 0.135 0.089 0.062 0.034 0.053 0.087 0.035 0.041 0.028 0.023 0.02 0.02 ARI>30 0.028 0.025 0.024 0.021 0.03 0.032 0.021 0.036 0.027 0.035 0.035 0.058 0.118 0.093 0.067 0.041 0.059 0.086 0.033 0.041 0.025 0.023 0.021 0.021 72 HOUR DURATION in 18 PERIODS OF 4 HOURS ARI<=30 0.063 0.029 0.042 0.073 0.047 0.034 0.092 0.117 0.168 0.061 0.042 0.039 0.039 0.034 0.032 0.031 0.029 0.028 ARI>30 0.065 0.034 0.042 0.067 0.045 0.033 0.08 0.111 0.169 0.068 0.048 0.039 0.038 0.035 0.034 0.033 0.03 0.029 PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 41 of 65 8.2.4 Spatial Distribution The design rainfalls for durations from 30 minutes to 72 hours for all ARI’s up to and including 100 years are estimated at the centroid of each model sub-catchment as described in Section 8.2.1 . This is in accordance with the methodology described in Table 15. 8.2.5 Rainfall Losses The Initial Loss (IL) and Continuing Loss (CL) for design event simulations up to 100 year ARI were adopted as per Table 19 below: Table 19: Adopted Initial and Continuing Loss Rates ARI Adopted Loss (Years) IL (mm) CL (mm/hr) 2 50 3.0 5 50 3.0 10 40 2.5 20 30 2.0 50 20 1.0 100 10 0.0 8.2.6 Design Peak Discharges Table 20 and Table 21 show the design discharges and critical storm durations respectively at key locations throughout the Currumbin Creek catchment for ARI’s up to 100 years. It is of note that for all design event simulations the applied ARF is calculated based on total catchment area. Table 20: Peak Design Discharge at Key Locations, 2 to 100 Year ARI Peak Design Discharge (m3/s) 2 5 10 20 50 100 Location Stream Name Year Year Year Year Year Year ARI ARI ARI ARI ARI ARI Camberra Currumbin Creek 84 136 176 226 284 331 Nicolls Bridge AL Currumbin Creek 105 167 217 278 348 409 Catchment Outlet Currumbin Creek 152 239 306 386 498 593 Table 21: Critical Storm Duration at Key Locations, 2 to 100 Year ARI Critical Storm Duration (Hrs) 2 5 10 20 50 100 Location Stream Name Year Year Year Year Year Year ARI ARI ARI ARI ARI ARI Camberra Currumbin Creek 12.0 12.0 9.0 9.0 9.0 9.0 Nicolls Bridge AL Currumbin Creek 12.0 12.0 9.0 9.0 9.0 9.0 Catchment Outlet Currumbin Creek 9.0 9.0 9.0 9.0 6.0 6.0 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 42 of 65 8.3 Rare Design Events (200 to 2000 Year ARI) 8.3.1 Rainfall Depth Estimation The design rainfall depths for the rare events are estimated using the guidelines given in theTable 22 below: Table 22: Design Rainfall Depth Estimation Event Storm Duration Adopted Methodology 200 to 500 Year ARI Up to 12 hours Updated AWE 1998 Interpolated between updated AWE 18 hours 1998 and CRC Forge 24 to 72 hours CRC Forge (Hargraves, 2004) 1000 to 2000 Year ARI Up to 72 hours CRC Forge 8.3.2 Areal Reduction Factors The Areal Reduction Factors (ARFs) are applied to 200, 500, 1000 and 2000 Year ARI design rainfalls as described in the Section 8.2.2 . 8.3.3 Temporal Patterns The design rainfall temporal patterns for the rare design events are sourced from ‘The Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method’ (GSDM, BoM, 2003a) and the ‘Generalised Tropical Storm method (GTSMR, BoM, 2003b)’ as outlined in Table 23. Table 24 shows the adopted rainfall temporal patterns for rare to extreme design events. Table 23: Design Rainfall Temporal Patterns Event Storm Duration Adopted 200 to 2000 Year ARI Up to 6 hours GSDM 9 Hours Extrapolated from GSDM 12 and 18 Hours Extrapolated from GTSMR 24 to 72 hours GTSMR Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 43 of 65 Table 24: Adopted Design Rainfall Temporal Patterns for 200 to 2000 Year ARI PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 30 MINUTE DURATION in 6 PERIODS OF 5 MINUTES Area 100Km2 0.204 0.237 0.205 0.172 0.124 0.059 Area 500Km2 0.204 0.237 0.205 0.172 0.124 0.059 1 HOUR DURATION in 12 PERIODS OF 5 MINUTES Area 100Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 Area 500Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 1.5 HOUR DURATION in 18 PERIODS OF 5 MINUTES Area 100Km2 0.051 0.068 0.085 0.083 0.077 0.076 0.072 0.068 0.065 0.061 0.056 0.055 0.05 0.042 0.032 0.027 0.019 0.013 Area 500Km2 0.051 0.068 0.085 0.083 0.077 0.076 0.072 0.068 0.065 0.061 0.056 0.055 0.05 0.042 0.032 0.027 0.019 0.013 3 HOUR DURATION in 12 PERIODS OF 15 MINUTES Area 100Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 Area 500Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 4.5 HOUR DURATION in 18 PERIODS OF 15 MINUTES Area 100Km2 0.051 0.068 0.085 0.083 0.077 0.076 0.072 0.068 0.065 0.061 0.056 0.055 0.05 0.042 0.032 0.027 0.019 0.013 Area 500Km2 0.051 0.068 0.085 0.083 0.077 0.076 0.072 0.068 0.065 0.061 0.056 0.055 0.05 0.042 0.032 0.027 0.019 0.013 6 HOUR DURATION in 12 PERIODS OF 30 MINUTES Area 100Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 Area 500Km2 0.081 0.123 0.121 0.116 0.108 0.097 0.088 0.084 0.073 0.051 0.038 0.021 9 HOUR DURATION in 9 PERIODS OF 1 HOUR Area 100Km2 0.117 0.160 0.152 0.146 0.130 0.110 0.089 0.063 0.036 Area 500Km2 0.121 0.159 0.146 0.143 0.136 0.113 0.089 0.061 0.035 12 HOUR DURATION in 12 PERIODS OF 1 HOUR Area 100Km2 0.080 0.112 0.113 0.112 0.114 0.116 0.086 0.077 0.067 0.053 0.043 0.027 Area 500Km2 0.087 0.111 0.111 0.101 0.108 0.118 0.098 0.082 0.067 0.050 0.042 0.026 18 HOUR DURATION in 12PERIODS OF 1.5 HOUR Area 100Km2 0.074 0.104 0.105 0.113 0.121 0.133 0.084 0.072 0.06 0.057 0.049 0.031 Area 500Km2 0.091 0.100 0.099 0.092 0.107 0.139 0.108 0.078 0.061 0.053 0.044 0.029 24 HOUR DURATION in 8 PERIODS OF 3 HOUR Area 100Km2 0.119 0.141 0.168 0.223 0.125 0.074 0.097 0.053 Area 500Km2 0.143 0.129 0.109 0.238 0.178 0.069 0.085 0.050 36 HOUR DURATION in 12 PERIODS OF 3 HOURS Area 100Km2 0.053 0.034 0.070 0.089 0.055 0.062 0.107 0.094 0.185 0.132 0.076 0.043 Area 500Km2 0.033 0.046 0.052 0.098 0.074 0.053 0.115 0.177 0.131 0.091 0.060 0.069 PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 44 of 65 Table 8.10 – Adopted Design Rainfall temporal patterns for 200 to 2000 Year ARI Events (Continued) PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 48 HOUR DURATION in 16 PERIODS OF 3 HOURS Area 100Km2 0.062 0.039 0.019 0.040 0.058 0.054 0.065 0.114 0.148 0.076 0.050 0.089 0.081 0.032 0.028 0.043 Area 500Km2 0.062 0.048 0.022 0.042 0.059 0.052 0.085 0.093 0.140 0.079 0.037 0.110 0.071 0.025 0.040 0.033 72 HOUR DURATION in 24 PERIODS OF 3 HOURS Area 100Km2 0.044 0.053 0.077 0.125 0.064 0.010 0.034 0.071 0.083 0.039 0.025 0.008 0.006 0.017 0.022 0.028 0.015 0.059 0.094 0.048 0.033 0.020 0.011 0.013 Area 500Km2 0.037 0.056 0.074 0.068 0.046 0.013 0.022 0.035 0.115 0.042 0.018 0.015 0.009 0.040 0.021 0.061 0.027 0.051 0.096 0.081 0.025 0.030 0.007 0.012 PERIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 45 of 65 8.3.4 Spatial Distribution The design rainfalls for durations from 30 minutes to 72 hours for all ARIs between 200 and 2000 Year are estimated at the centroid of each model sub-catchment as described in Section 8.3.1 . This is in accordance with the methodology described in Table 15. 8.3.5 Losses An initial loss (IL) of 10.0 mm is adopted for the 100 year ARI event (refer Section 8.2.5 ). It is considered appropriate to adopt the same IL for all events greater than 100 year ARI as well. For the same reason, a CL of 0.0 mm/hr (refer Section 8.2.5 ) is adopted for all ARI’s greater than 100 years. 8.3.6 Design Discharges Table 25 and Table 26 show the design discharges and critical storm durations respectively at key locations throughout the Currumbin Creek catchment for 200 to 2000 year ARI events. Table 25 – Design Discharge at Key Locations, 200 to 2000 Year ARI Peak Design Discharge (m3/s) 200 500 1000 2000 Location Stream Name Year Year Year Year ARI ARI ARI ARI Camberra Currumbin Creek 372 427 537 592 Nicolls Bridge AL Currumbin Creek 458 526 656 721 Catchment Outlet Currumbin Creek 656 752 951 1042 Table 26 – Critical Storm Duration at Key Locations, 200 to 2000 Year ARI Critical Storm Duration (Hour) 200 500 1000 2000 Location Stream Name Year Year Year Year ARI ARI ARI ARI Camberra Currumbin Creek 4.5 4.5 3.0 3.0 Nicolls Bridge AL Currumbin Creek 4.5 4.5 4.5 4.5 Catchment Outlet Currumbin Creek 6.0 6.0 6.0 6.0 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 46 of 65 8.4 Extreme Design Events - Probable Maximum Precipitation Design Flood (PMPDF) 8.4.1 Rainfall Depth Estimation PMP rainfall depths for durations up to 6 hours are estimated using the methodology given in the Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method (GSDM, BoM 2003a). The PMP rainfall depths for 24 to 72 hour durations are estimated using the standard methodology given in the Generalised Tropical Storm Method (GTSMR, BoM 2003b). The PMP depths for 12 and 18 hour durations are obtained by extrapolating the GTSMR estimates and the PMP depths for 9 hour duration are estimated by extrapolating the GSDM estimates. A smooth transition between GSDM and GTSMR estimates are also adopted. 8.4.2 Areal Reduction Factors Areal Reduction Factors are already included in the GSDM and GTSMR methodology (BoM 2003a and BoM 2003b) so the additional ARFs are not required to apply on individual sub-area rainfall. 8.4.3 Temporal Patterns The temporal patterns for durations up to 6 hours are obtained from the GSDM and the durations from 24 to 72 hours are obtained from the GTSMR methodology. The temporal patterns for the 12 and 18 hour durations are extrapolated from the GTSMR and the temporal patterns for the 9 hour duration are extrapolated from the GSDM. A smooth transition between GSDM and GTSMR estimates are also adopted. 8.4.4 Spatial Distribution Rainfall spatial distributions are already included in the GSDM and GTSMR rainfall depth estimation methodology. 8.4.5 Losses An initial loss (IL) of 0.0 mm and continuing loss (CL) of 0.25 mm/hr are adopted for all the durations of PMPDF. 8.4.6 Design Discharges Table 27 shows the PMPDF discharge and critical storm duration at key locations throughout the Currumbin Creek catchment. Table 27 – Design Discharge and Critical Storm Duration at Key Locations for PMPDF PMPDF Location Stream Name Peak Design Critical Discharge Storm Duration (m3/s) (hours) Camberra Currumbin Creek 1268 3.0 Nicolls Bridge AL Currumbin Creek 1548 3.0 Catchment Outlet Currumbin Creek 2050 6.0 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 47 of 65 8.5 Joint Probability Approach The Joint Probability Approach (JPA) is also known as Monte Carlo simulation technique has been under development for last few years. There are two Monte Carlo techniques available: the Total Probability Theorem (TPT) developed by Nathan, Weinmann and Kuczera (Laurenson et al. 2005) and Cooperative Research Centre – Catchment Hydrology (CRC-CH) developed by Rahman (et al. 2001 and 2002a). The TPT methodology is based on current critical storm duration approach however the CRC-CH methodology is based on design storms of variable storm durations. In TPT methodology the BOM burst IFD tables are used. On the other hand the event based IFD tables derived from the raw pluviographs are used in CRC-CH. Don Carroll Project Management developed a relationship between the complete storm IFD table and the burst IFD table for Gold Coast region as part of Council’s Hydrological Study Review in April 2013 (12.18 ). The following relationships are established based on raw pluvio data from the BOM. q r Ie = p D T Ib Where I is intensity (mm/hour) D is duration (hours) T is ARI (years) b is burst e is event and p, q and r are constant. For Gold Coast region Don Carroll has recommended the following values: p = 0.1 x 12D24 – 0.25 q = 0.6 x (1 – p) r = - 0.025 The mean duration is 0.9 x 12D241.56 where 12D24 is the 2 year 24 hour burst intensity. It is of note that both approaches have some limitations. The TPT is developed for large to extreme floods and its applications for more frequent events are questionable. The CRC-CH often applied in the derivation of design flow estimates up to large floods so this approach is not robust in the estimation of rare and extreme floods. In this study the TPT and CRC-CH Monte Carlo simulations are undertaken only to verify the results of Design Event Approach. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 48 of 65 8.5.1 TPT The adopted steps in TPT can be summarized as below: Choose a range of storm durations around the critical storm duration from the Design Event Approach (DEA). Generate thousands of rainfall bursts through stratified sampling of the rainfall frequency curve. Randomly select a temporal pattern from the database obtained from the pluvio data. Apply random initial loss and fixed continuing loss. Table 28 shows the adopted parameters and losses for TPT. Table 28: Adopted Parameters and Losses for TPT Adopted Parameters and Losses α 0.14 β 1.8 m 0.65 n 1 F F*0.5 CL (mm/hr) 2.5 Min IL (mm) 0 Stdv IL (mm) 30 Mean IL (mm) 30 Max IL (mm) 100 Table 29 shows the peak design discharge at key locations within the catchment estimated by TPT Monte Carlo simulations for 2 year to 2000 year ARI. Table 29: Design Discharge at Key Locations, 2 to 2000 Year ARI estimated by TPT 3 Peak Design Discharge (m /s) 2 5 10 20 50 100 200 500 1000 2000 Location Year Year Year Year Year Year Year Year Year Year ARI ARI ARI ARI ARI ARI ARI ARI ARI ARI Camberra 109 156 192 232 286 335 377 446 516 586 Nicolls Br 135 190 232 278 340 401 458 543 622 725 Outlet 188 265 330 391 479 560 643 779 886 1039 Table 30 shows the critical storm duration at key locations within the catchment estimated by TPT Monte Carlo simulations for 2 year to 2000 year ARI. Table 30: Critical Storm Duration at Key Locations, 2 to 2000 Year ARI estimated by TPT Critical Storm Duration (hours) 2 5 10 20 50 100 200 500 1000 2000 Location Year Year Year Year Year Year Year Year Year Year ARI ARI ARI ARI ARI ARI ARI ARI ARI ARI Camberra 4.5 9.0 9.0 9.0 12.0 12.0 6.0 4.5 6.0 4.5 Nicolls Br 4.5 9.0 9.0 9.0 12.0 6.0 4.5 4.5 6.0 12.0 Outlet 4.5 9.0 9.0 6.0 12.0 6.0 9.0 6.0 6.0 6.0 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 49 of 65 8.5.2 CRC-CH The adopted steps in CRC-CH can be summarized as below: Storm core duration is selected from an exponential or gamma probability distribution fitted to the available pluvio data of the area. A conditional distribution in the form of IFD is established. Temporal patterns are randomly selected based on multiplicative cascade model. Thousands of combinations of the design inputs are generated and routed through the runoff routing model. Table 31 shows the adopted parameters and losses for CRC-CH Table 31: Adopted Parameters and Losses for CRC-CH Adopted Parameters and Losses α 0.14 β 1.8 m 0.65 n 1 F F*0.5 CL (mm/hr) 2.5 Min IL (mm) 0 Stdv IL (mm) 30 Mean IL (mm) 30 Max IL (mm) 100 Table 32 shows the peak design discharge at key locations within the catchment estimated by CRC-CH Monte Carlo simulations for 2 year to 2000 year ARI. Table 32: Design Discharge at Key Locations, 2 to 2000 Year ARI estimated by CRC-CH 3 Peak Design Discharge (m /s) 2 5 10 20 50 100 200 500 1000 2000 Location Year Year Year Year Year Year Year Year Year Year ARI ARI ARI ARI ARI ARI ARI ARI ARI ARI Camberra 110 158 201 242 303 348 400 575 613 656 Nicolls Br 121 172 218 269 335 381 442 583 668 676 Outlet 164 231 286 342 424 498 627 804 896 1044 It is of note that all the design discharges estimated in section 8, the ARF calculation is based on total catchment area and for comparison with the other approaches in section 9 the ARF calculation is based on the catchment area upstream of Nicolls Bridge gauging station only. JPA estimated design discharges in this study are compared with other methodologies and other studies in Section 9 of this report. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 50 of 65 9. Comparison In the current study peak design discharges at Nicolls Bridge gauging station were estimated by using the Design Event Approach (DEA), Flood Frequency Analysis (FFA), Total Probability Theorem Monte Carlo (TPT MC) and Cooperative Research Centre – Catchment Hydrology Monte Carlo (CRC-CH MC) methodologies. Figure 9 shows the peak design discharge for different ARIs at Nicolls Bridge Gauging Station as estimated by DEA, FFA, TPT MC and CRC- CH MC approach. As the comparison was made at Nicolls Bridge so the adopted ARF for all estimations are based on catchment area only upstream of Nicolls Bridge. Figure 9: Peak Design Discharge comparison plot for different modelling approaches Table 33 shows the peak design discharges at Nicolls Bridge GS estimated by different modelling approaches for different ARIs from current and previous studies. Table 33: Peak Design Discharge estimated by different modelling approach from current and previous studies at Nicolls Bridge Peak Design Discharge (m3/s) ARI Current Study GCCC 2001 WBM 1995 (Year) DEA FFA TPT MC CRC-CH MC Modelled FFA Modelled FFA 2 107 101 131 123 - - - - 5 170 167 185 175 255 259 207 232 10 220 214 235 220 324 193 257 304 20 281 266 280 272 388 233 324 361 50 353 343 343 340 441 296 380 419 100 413 410 398 385 498 354 445 452 200 464 486 458 446 - - - - Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 51 of 65 The following is of note with regards to Figure 9 and Table 33: The agreement between DEA and FFA estimates of the current study is excellent. The agreement between DEA, TPT MC and CRC-CH MC estimates of the current study is excellent. The agreement improves as ARI increases. The agreement between the DEA and the FFA estimated design discharges is better in the current study when compared to the GCCC 2001 and WBM 1995 studies. The FFA in the current study is based on a longer (1970 – 2013) period of recorded data. The peak design discharges estimated in this study are generally lower than the discharges estimated by GCCC 2001 and WBM 1995 studies. The differences in design peak discharges are likely due to the following reasons: The Areal Reduction Factors were not applied to the design rainfalls in the GCCC 2001 study. WBM 1995 used ARR 87 temporal patterns and GCCC 2001 used AWE 2000 temporal patterns. WBM filtered the ARR 87 temporal patterns to remove the sub-duration burst inconsistencies but the inconsistencies were not removed completely. On the other hand AWE 2000 temporal patterns had significant sub-duration burst inconsistencies and these were not filtered in GCCC 2001 study. In the current study a sensitivity analysis were undertaken and it is found that temporal patterns with Sub-duration burst inconsistencies were generating unrealistically long critical storm durations and higher design peak discharges in the Gold Coast region. In the current study the AWE 2000 temporal patterns were filtered and smoothened by adjusting the longer duration storm temporal patterns according to the methodology outlined in Australian Rainfall and Runoff (1998) and BoM (1991). The filtering and smoothing were undertaken until the sub-duration burst inconsistencies were removed for all ARI’s up to 100 years, whilst maintaining the basic shape and integrity of the patterns (WRM 2008c 12.12 ). The filtered temporal patterns are giving realistic critical storm durations and lower design peak discharges than the ARR 87 and AWE 2000 Temporal patterns. The previous studies did not attempted a reconciliation between model estimates and the FFA results Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 52 of 65 10. Conclusion An URBS hydrological model for the Currumbin Creek catchment has been developed, calibrated and verified successfully using the best available data and current industry standard modelling practices. The model is calibrated for January 2013, January 2008, June 2005 and February 2004 historical events and verified against November 2004, February 1990, April 1972 and February 1972 historical events. The calibrated model was then used to estimate the design flood discharges at different locations within the Currumbin Creek catchment for 2, 5, 10, 20, 50, 100, 200, 500, 1000 and 2000 year ARI and PMP design flood events. The design discharges estimated by the model have been compared and reconciled with FFA estimated design discharges at Nicolls Bridge gauging station. The FFA was undertaken for 43 (1970 to 2013) years of recorded data. A Joint Probability Analysis for Total Probability Theorem Monte Carlo and Cooperative Research Centre – Catchment Hydrology Monte Carlo simulations are also undertaken to verify the results of current Design Event Approach. In conclusion, the current Design Event hydrological study for Currumbin Creek catchment is robust – it is supported by Flood Frequency Analysis and Joint Probability Analysis. The output of this study can be used as input to the hydraulic modelling for further studies of Currumbin creek catchment. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 53 of 65 11. Recommendation The following recommendations are made: The model calibration should be revisited and reviewed when a significant storm event occurs and the data are available. Currumbin Creek Catchment URBS model design rainfall and temporal pattern to be reviewed after the next release of Australian Rainfall and Runoff. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 54 of 65 12. Reference 12.1 GCCC (2001) Tallebudgera and Currumbin Creeks Flood Risk Management Study - Interim Hydrology Report prepared by Don Carroll for Gold Coast City Council, February 2001 12.2 WBM (1995) Currumbin Creek and Tallebudgera Creek Flood Study Report prepared by WBM for Gold Coast City Council, August 1995 12.3 GHD (2007) Tallebudgera Creek Dam Upgrade – Detailed Design Report prepared by GHD for Gold Coast City Council, July 2007 12.4 GHD (2001) Tallebudgera Dam – Report on Risk Assessment Prepared by GHD for Gold Coast City Council, May 2001 12.5 GHD (2003) Tallebudgera Dam – Risk Assessment Addendum Report prepared by GHD for Gold Coast Water, July 2003 12.6 AWE (1998) Review of Gold Coast Rainfall Data Volume 1 Report prepared by Australian Water Engineering Pty Ltd for Gold Coast City Council, May 1998 12.7 AWE (2000) Review of Gold Coast Rainfall Data Volume 3 Temporal Patterns Report prepared by Australian Water Engineering Pty Ltd for Gold Coast City Council, October 2000 12.8 Pilgrim (2000) Peer Review Gold Coast Rainfall Temporal Patterns Report prepared by D H Pilgrim & Associates Pty Ltd for Gold Coast City Council, October 2000 12.9 UWS (2006) Design Temporal Patterns in the Gold Coast Region Report prepared by the University of Western Sydney for Gold Coast City Council, August 2006 12.10 WRM (2008a) Summary Findings of the Review of Hydrological Models for the Gold Coast City Catchments Report prepared by WRM Water & Environment Pty Ltd for Gold Coast City Council, March 2008 12.11 WRM (2008b) GCCC IFD Utility Modifications Report prepared by WRM Water & Environment Pty Ltd for Gold Coast City Council, July 2008 12.12 WRM (2008c) Revision of Design Rainfall Temporal Patterns for Gold Coast Catchments Report prepared by WRM Water & Environment Pty Ltd for Gold Coast City Council, October 2008 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 55 of 65 12.13 WRM (2008d) Design Event Hydrology Report prepared by WRM Water & Environment Pty Ltd for Gold Coast City Council, August 2008 12.14 BoM (2003) Guide to the Estimation of Probable Maximum Precipitation: Generalised Tropical Storm Method Report prepared by the Hydro Meteorological Advisory Service, Australian Government Bureau of Meteorology, November 2003 12.15 BoM (2003) The Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method Report prepared by the Hydro Meteorological Advisory Service, Australian Government Bureau of Meteorology, June 2003 12.16 Carroll (2012) URBS – A Rainfall Runoff Routing Model for Flood Forecasting and Design, Software v5.00 Manual developed by D G Carroll, December 2012 12.17 Hargraves (2004) Extreme Rainfall Estimation Project, CRC FORGE and ARF Techniques, Queensland and Border Locations Report prepared by Gary Hargraves, Water Assessment & Planning Resource Science Centre, Circa 2004 12.18 Carroll (2013) Review and Update of The City of Gold Coast’s Hydrological Models Report prepared by D G Carroll for The City of Gold Coast, April 2013 12.19 Kuczera (2006) Joint Probability and Design Storms at the Cross Roads Report prepared by George Kuczera for Australian Journal of Water Resources, Vol 10 No 1, 63-80 12.20 Rahman (2001) Monte Carlo Simulation of Flood Frequency Curves from Rainfall Technical report prepared for CRC-CH 12.21 Mirfenderesk (2013) Comparison between Design Event and Joint Probability Hydrological Modelling Report prepared by Hamid Mirfenderesk for FMA National Conference, May 2013 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 56 of 65 13. Appendix A: URBS Model Sub-catchment Areas and Land Uses Table 34: Currumbin Sub-catchment Areas and Land Uses Sub Catchment Area UL UM UH UR UF ID (km2) (km2) (km2) (km2) (km2) (km2) 1 2.22 0.000 0.000 0.024 0.000 0.976 2 1.75 0.000 0.000 0.016 0.000 0.984 3 3.62 0.000 0.000 0.047 0.304 0.649 4 1.95 0.000 0.000 0.047 0.688 0.265 5 4.39 0.000 0.000 0.054 0.024 0.922 6 4.61 0.000 0.000 0.053 0.690 0.257 7 2.30 0.000 0.000 0.066 0.544 0.390 8 2.32 0.000 0.000 0.098 0.778 0.124 9 3.99 0.001 0.000 0.082 0.882 0.035 10 0.96 0.000 0.000 0.077 0.591 0.331 11 2.53 0.005 0.000 0.055 0.746 0.194 12 3.64 0.001 0.000 0.127 0.603 0.269 13 1.31 0.000 0.029 0.164 0.726 0.081 14 1.33 0.000 0.000 0.295 0.590 0.114 15 1.02 0.002 0.000 0.048 0.517 0.434 16 1.00 0.006 0.000 0.037 0.023 0.935 17 1.93 0.000 0.000 0.050 0.950 0.000 18 3.36 0.000 0.303 0.390 0.211 0.096 19 2.23 0.000 0.000 0.917 0.078 0.005 20 2.65 0.000 0.000 0.899 0.052 0.049 21 2.58 0.000 0.000 0.810 0.178 0.013 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 57 of 65 14. Appendix B: URBS Catchment Definition File CURRUMBIN CREEK {Model built by , reviewed by (2009) and (2013)} MODEL: SPLIT USES: L,U,F*0.5 DEFAULT PARAMETERS: alpha=0.14 m=0.65 beta=1.8 n=1.0 CATCHMENT DATA FILE = CURRU_00.csv RAIN #1 L = 0.82 ROUTE THRU #2 L = 0.41 ADD RAIN #2 L = 0.34 ROUTE THRU #3 L = 0.83 ADD RAIN #3 L = 0.73 ROUTE THRU #4 L = 0.65 ADD RAIN #4 L = 0.49 ROUTE THRU #5 L = 0.86 ADD RAIN #5 L = 1.2 ROUTE THRU #6 L = 1.87 ADD RAIN #6 L = 0.97 ROUTE THRU #7 L = 0.34 ADD RAIN #7 L = 0.52 ROUTE THRU #8 L = 0.71 ADD RAIN #8 L = 0.83 ROUTE THRU #9 L = 0.96 PRINT. CAMBERRA {X=540502.75, Y=6881175.11 SC1-8} ADD RAIN #9 L = 2.13 PRINT. TH_SC1-9 STORE. RAIN #10 L = 0.57 PRINT. LH_SC10 {X=541389.08, Y=6882029.08} GET. ROUTE THRU #11 L = 1.07 STORE. RAIN #11 L = 0 PRINT. LH_SC11 {X=541221.55, Y=6882909.71} GET. ROUTE THRU #11 L = 0.92 PRINT. NICOLLS {X=541874.86, Y=6882831.20 SC1-11} ROUTE THRU #12 L = 1.41 STORE. RAIN #12 L = 0 PRINT. LH_SC12 {X=542956.16, Y=6883496.78} GET. ROUTE THRU #12 L = 1.12 ROUTE THRU #13 L = 0.58 STORE. RAIN #13 L = 0 PRINT. LH_SC13 {X=544139.53, Y=6.884349.63} GET. ROUTE THRU #13 L = 0.57 ROUTE THRU #14 L = 0.54 STORE. RAIN #14 L = 0 PRINT. LH_SC14 {X=545157.00, Y=6884663.96} GET. ROUTE THRU #14 L = 0.46 STORE. RAIN #15 L = 0.72 ROUTE THRU #16 L = 0.69 ADD RAIN #16 L = 0.58 PRINT. TH_SC15-16 ROUTE THRU #17 L = 1.59 STORE. RAIN #17 L = 0 PRINT. LH_SC17 GET. ROUTE THRU #17 L = 0.99 Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 58 of 65 {ADD RAIN #17 L = 0.99} {PRINT. TH_SC15-17} {X=545547.78, Y=6884887.97} GET. ROUTE THRU #18 L = 1.79 STORE. RAIN #18 L = 0 PRINT. LH_SC18 {X=545508.95, Y=6886189.40} GET. ROUTE THRU #18 L = 0.49 ROUTE THRU #19 L = 0.81 STORE. RAIN #19 L = 0 PRINT. LH_SC19 {X=546064.35, Y=6887311.52} GET. ROUTE THRU #19 L = 0.65 STORE. RAIN #20 L = 0 PRINT. LH_SC20 {X=546789.77, Y=6886210.48} ROUTE THRU #20 L = 1.54 GET. ROUTE THRU #21 L = 0.52 STORE. RAIN #21 L = 0 PRINT. LH_SC21 {X=546950.02, Y=6887899.73} GET. ROUTE THRU #21 L = 0.82 {PRINT. TH_CATOUT X=547177.06, Y=6888522.96} END OF CATCHMENT DATA. Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 59 of 65 15. Appendix C: Calibration and Verification Plot Figure 10: January 2013 Calibration @ Nicolls Bridge Gauging Station Figure 11: January 2008 Calibration @ Nicolls Bridge Gauging Station Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 60 of 65 Figure 12: June 2005 Calibration @ Nicolls Bridge Gauging Station Figure 13: February 2004 Calibration @ Nicolls Bridge Gauging Station Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 61 of 65 Figure 14: November 2004 Verification @ Nicolls Bridge Gauging Station Figure 15: February 1990 Verification @ Nicolls Bridge Gauging Station Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 62 of 65 Figure 16: April 1972 Verification @ Camberra Gauging Station Figure 17: April 1972 Verification @ Nicolls Bridge Gauging Station Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 63 of 65 Figure 18: February 1972 Verification @ Camberra Gauging Station Figure 19: February 1972 Verification @ Nicolls Bridge Gauging Station Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 64 of 65 Council of the City of Gold Coast PO Box 5042 GCMC Qld 9729 P 1300 GOLDCOAST E [email protected] W cityofgoldcoast.com.au Currumbin Creek Catchment – Hydrological Study, April 2014 TRACKS-#43749416-v1 Page 65 of 65