nerang river catchment hydraulic study · 2019. 6. 23. · nerang river flood mitigation project,...

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Nerang River Catchment

Hydraulic Study June 2016

Version 4 – June 16 TRACKS-#45474511-v5-REPORT_NERANG_RIVER_HYDRAULIC_MODELLING_REPORT_2015

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Title: Nerang River Catchment Hydraulic Study

Author:

Study for: City Planning Branch

Planning and Environment Directorate

The City of Gold Coast

File Reference: WF50/44/01(P1)

TRACKS #45474511

Version history

Version Comments/Change Changed by & date Reviewed by &

date

1.0 Draft for Consultation (Nerang Model v2.0)

2.0 Based on Nerang Model v2.2

3.0 Based on Nerang Model v5.3.1

4.0 Based on Nerang Model v5.4

5.0 Grammar Review

Distribution list

Name Title Directorate Branch

NH Team Planning Environment City Planning


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1. Executive Summary

The Nerang River catchment is one of the largest water catchment areas (491.2km2) on the Gold Coast. It consists of the Nerang River and a number of tributaries including the Mudgeeraba, Bonogin and Worongary Creeks that drain out into the Broadwater system.

The hydraulic study’s main output is an updated Nerang hydraulic model for use in the development of the City of Gold Coast’s (City) designated flood level (DFL) for the upcoming City Plan scheduled for release in 2016 and for use in flood impact assessment for proposed development.

MIKE Flood software was used to build and calibrate the hydraulic model. MIKE Flood is capable of simulating complex flows and floodplain storage in the 2D topographical terrain using MIKE 21 and is able to provide flow constriction in hydraulic structures using MIKE 11. The combination is appropriate as the Nerang floodplain consists of numerous structures (weirs, culverts and bridges), as well as a number of canal systems. The Mike 21 2D topographical terrain is represented using a 20m rectangular grid created from City’s December 2014 2m Digital Terrain Model (DTM). The DTM is a combination of Aerial Laser Survey (ALS) data conducted in 2009, High Resolution ALS captured in 2010, and bathymetric surveys from multiple years.

The hydraulic model is calibrated to the January 1974 historical flood event and verified against the June 2005 and January 2013 historical flood events. Tide was also simulated to test the model’s ability to simulate conveyance through the multiple creeks and canals. The historical flood events produced reasonable agreement between modelled and recorded water levels. It is worthwhile to point out that the tidal calibration does not replicate tidal dynamics throughout the catchment, as a more refined grid would need to be used to more accurately represent the narrower creeks and canal systems.

The calibrated model has been setup to simulate design flood events ranging from a 2 year Average Recurrence Interval (ARI) flood event to a Probable Maximum Flood (PMF). The design runs include storm durations of 0.5, 1.0, 1.5, 3.0, 4.5, 6.0, 9.0, 12.0, 18.0, 24.0, 36.0, 48.0, 72.0, 96 and 120 hours. The hydrology used in this study for both calibration and design events are sourced from the Nerang River Catchment Hydrological Study (Ref 29).

The Design model setup is based on the 2100 planning horizon. This includes a 10% increase in rainfall intensity and storm surge levels that include 0.8m sea level rise and 10% increase in storm surge intensity.


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Table of Contents

1. Executive Summary ...................................................................................................................... 3

2. Introduction ................................................................................................................................... 6 Overview .............................................................................................................................. 6 2.1 Limitations ............................................................................................................................ 6 2.2 Acknowledgement ............................................................................................................... 6 2.3

3. Background ................................................................................................................................... 7 Catchment Description ........................................................................................................ 7 3.1 Previous Studies .................................................................................................................. 9 3.2

4. Hydraulic Model Development ................................................................................................... 10 Datum ................................................................................................................................ 11 4.1

Horizontal Datum ............................................................................................................... 11 Vertical Datum ................................................................................................................... 11

Available Data .................................................................................................................... 11 4.2 Model Extent (Figure 3) ..................................................................................................... 14 4.3 MIKE 21 ............................................................................................................................. 15 4.4

Topography ........................................................................................................................ 15 Roughness ......................................................................................................................... 16 Eddy Viscosity ................................................................................................................... 17 Tailwater Boundary ............................................................................................................ 18

MIKE 11 ............................................................................................................................. 18 4.5Roughness ......................................................................................................................... 19 Hydraulic Structures .......................................................................................................... 19

Inflow and Source Points ................................................................................................... 19 4.6

5. Model Calibration and Verification ............................................................................................ 21 January 1974 Calibration ................................................................................................... 21 5.1 June 2005 Verification ....................................................................................................... 24 5.2 January 2013 Verification .................................................................................................. 28 5.3 Tidal Calibration ................................................................................................................. 32 5.4

6. Design Floods .............................................................................................................................. 36 Introduction ........................................................................................................................ 36 6.1 Design Run Setup .............................................................................................................. 37 6.2 Initial Water Level .............................................................................................................. 38 6.3 Probable Maximum Precipitation (PMP) ............................................................................ 38 6.4 PMPDF Temporal Patterns ................................................................................................ 38 6.4.1 PMF Temporal Patterns ..................................................................................................... 39 6.4.2

7. Conclusion ................................................................................................................................... 39

8. References ................................................................................................................................... 40

Appendix A ......................................................................................................................................... 42


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Semidiurnal Tidal Planes - 2014 ........................................................................................................ 42

List of Figures

Figure 1. Nerang River Catchment which includes the Mudgeeraba and Worongary Creek Catchments ............................................................................................................................................ 8 Figure 2. Locations of Alert and TM Stations ....................................................................................... 13 Figure 3. MIKE Flood Model Extent ..................................................................................................... 14 Figure 4. DTM Data Source ................................................................................................................. 15 Figure 5. MIKE 21 DTM ........................................................................................................................ 16 Figure 6. MIKE 21 Roughness ............................................................................................................. 17 Figure 7. MIKE 11 Network with Hydraulic Structures and MIKE 21 DTM Extent ................................ 18 Figure 8. Inflow Source Points ............................................................................................................. 20 Figure 9. Modelled and Recorded Water Level @ Clearview TM - Jan 1974 ...................................... 22 Figure 10. Survey (red) and Modelled (blue) Debris Marks - Jan 1974 ............................................... 23 Figure 11. Modelled and Recorded Water Level @ Clearview AL - June 2005 ................................... 24 Figure 12. Modelled and Recorded Water Level @ Carrara AL - June 2005 ....................................... 25 Figure 13. Modelled and Recorded Water Level @ Evandale AL - June 2005 .................................... 25 Figure 14. Modelled and Recorded Water Level @ Mudgeeraba TM - June 2005 .............................. 26 Figure 15. Surveyed (red) and Modelled (blue) Debris Marks - June 2005 ......................................... 27 Figure 16. Modelled and Recorded Water Level @ Clearview AL - Jan 2013 ..................................... 28 Figure 17. Modelled and Recorded Water Level @ Carrara AL - Jan 2013 ......................................... 29 Figure 18. Modelled and Recorded Water Level @ Boobegan AL - Jan 2013 .................................... 29 Figure 19. Modelled and Recorded Water Level @ Mudgeeraba AL – Jan 2013 ................................ 30 Figure 20. Modelled and Recorded Water Level @ Mudgeeraba TM - Jan 2013 ................................ 30 Figure 21. Modelled and Recorded Water Level @ Neranwood AL - Jan 2013 .................................. 31 Figure 22. Modelled and Recorded Water Level @ Worongary Ck AL - Jan 2013 .............................. 31 Figure 23. 2005 Tide Station Locations ................................................................................................ 33 Figure 24. Modelled and Recorded Tidal Level @ B11 Nerang Mouth ................................................ 34 Figure 25. Modelled and Recorded Tidal Level @ N1 Evandale ......................................................... 34 Figure 26. Modelled and Recorded Tidal Level @ N2 Sorrento ........................................................... 35 Figure 27. Modelled and Recorded Tidal Level @ N3 Royal Pines ..................................................... 35 Figure 28. Modelled and Recorded Tidal Level @ N4 ARC Resort ..................................................... 36

List of Tables

Table 1. ARI and AEP Design Floods .................................................................................................. 37 Table 2. PMF Temporal Patterns ......................................................................................................... 39

Version 4 – June 16 TRACKS-#45474511-v5-REPORT_NERANG_RIVER_HYDRAULIC_MODELLING_REPORT_2015 Page 6 of 43

2. Introduction Overview 2.1

The Natural Hazards Unit updated the Nerang hydraulic model for use in the development of the City of Gold Coast’s (City) designated flood level (DFL) for the upcoming City Plan, scheduled for release in 2016 and for use in flood impact assessment of proposed development.

This report documents the most recent (at time of writing) hydraulic modelling methodology, data source and calibration results for the Nerang Catchment.

Limitations 2.2

The following limitations apply in the preparation of this report. General

The hydraulic report was prepared based on available information at the time of writing. The analysis and overall approach adopted by this study is specifically prepared for internal

use. For this reason, any third parties are not authorised to use any contents from this report and their use is prohibited unless a written approval from City is obtained.

City believes that the assessment is accurate for its intended purpose and disclaims any responsibility for any loss or damage suffered as a result of placing reliance upon information provided in this report.

Model Grid The model grid is based on aerial laser survey (ALS) conducted between the 30th of April 2009

and the 9th of June 2009 (Rural Areas), and High Resolution ALS data captured in 2010. Any development or a topographical change to the model grid after this date is not documented in this report.

The 2009 ALS data acquisition and post-processing has been controlled to achieve a vertical accuracy of about 0.15m and horizontal accuracy of 0.45m. The 2010 High Resolution ALS data is captured at a slightly higher accuracy (about 0.04m vertical accuracy).

The topographical data is supplemented by bathymetric surveys conducted from various years (1999 - 2013) which may influence flood conveyance and storage.

Hydraulic Model The hydraulic model does not include stormwater drainage systems (e.g. pipe, inlet pits, etc),

which may influence local flooding paths and inundation extent. A separate model is required to simulate this type of pressurised flow through three-way coupling.

Acknowledgement 2.3

The City would like to acknowledge Griffith University, Bureau of Meteorology and Marine Safety Queensland for providing calibration data for this project.


3. Background The following section briefly provides some background information.

Catchment Description 3.1

The Nerang River catchment is one of the largest water catchment areas (491.2km2) on the Gold Coast (Figure 1). It consists of the Nerang River and a number of tributaries, including the Mudgeeraba, Bonogin and Worongary Creeks, that drain out into the Broadwater system.

The Nerang River originates from the McPherson Ranges, which drains water to two dams, i.e. Little Nerang Dam and Hinze Dam, then meanders through numerous canal estates of Benowa before discharging to the Broadwater. It is mainly rural in the upstream of the dams and heavily urbanised in the floodplain.

There are a number of flood mitigation features in the catchment: Hinze Dam, Boobegan Weir, and Benowa Flood Channel.


Figure 1. Nerang River Catchment which includes the Mudgeeraba and Worongary Creek Catchments


Previous Studies 3.2

Many studies have been undertaken for the Nerang River catchments in response to several major floods in the 1950s, 1960s and 1970s. Some of the relevant studies are listed below. Nerang Flood Study 1992 (Ref 4 and 5) Based on the recommendation of the Joint Technical Steering Committee, a detailed mathematical model was developed to test a wide range of proposals and conditions. It is based on the USF software which was developed by the Water Resource Commission. Kinhill Cameron McNamara consultants provided the hydrological inputs for the model using the RORB model. The model was calibrated to the January 1974 flood. Once calibrated, the model went through a number of iterations to document the development changes from 1974 up to 1992. City adopted the 1992 model as the base case. Nerang River Flood Mitigation Project, Addendum. MIKE 11 Model: Hinze Dam to Pacific Highway, Hydraulic Study. 2001 (Ref 34) The objective of this study was to estimate peak flood levels for various frequencies from Hinze Dam to Pacific Highway. Nerang River Hydraulic Modelling Study 2001 (Ref 6 and 7) In 1997, SMEC was commissioned to develop a new model of the Nerang River using MIKE 21 with the structures implemented as “Stage/Area. The locations of identified structures are listed in Table 8.1 and shown in Figure 8.1 of the SMEC report. Development Option Testing using the Nerang MIKE 21 2003 (Ref 8) Lawson and Treloar updated the SMEC model in 2003 to include approved developments and to investigate the impacts of the current and future hypothetical development. The base grid uses the ALS conducted in mid-2001. Some of the approved developments include the Nerang Broadbeach Road, Gold Coast Springbrook Road, Nielsen Road and Birmingham Road upgrade, GC Convention Centre, Robina Town Centre, Benowa Waters, Lakelands, Royal Pines, Mermaid Cove, Varsity Lakes, Emerald Lakes, Tricare and The Glades (Figure 2.1 of the L&T report). It is important to note that Boobegan Weir has been reconstructed and the new details are being incorporated into the model. Mudgeeraba and Worongary Hydrological and Hydraulic Study 2004/2007 (Ref 9, 10 and 11) WFM developed URBS hydrological and MIKE 11 hydraulic models in 2003/2004 for the Mudgeeraba and Worongary Creeks (upstream of Pacific Motorway). These flood surfaces supplement the existing Nerang River MIKE 21 model. A further update to the hydrological model was undertaken in 2007 to calibrate to historical events and to regenerate all design events (Ref 11). Nerang MIKE Flood 2005 Update (Ref 12) In 2006, the Nerang River MIKE 21 model was upgraded to a MIKE Flood (version 2005). The update included converting the MIKE 21 hydraulic structures to a MIKE 11 structure and using updated bathymetry. Nerang MIKE 11 Development 2006 (Ref 13) In 2006, WFM developed a MIKE 11 model for the Nerang River catchment for the purpose of flood emergency management. It included the Mudgeeraba and Worongary Creeks.


The model was calibrated to tide and the June 2005 flood event. Hinze Dam Stage 3 (Ref 14) Hinze Dam Alliance reviewed and updated the flood hydrology for the Hinze Dam Stage 3 upgrade. The existing dam’s spillway was raised from 82.2 m AHD to 94.5 m AHD. Nerang River Flood Study Hydrological Modelling (Ref 3) WRM was commissioned by City to undertake a comprehensive study to review and to update the hydrological models to a consistent methodology and to calibrate to recent flood events. Hydrology Addendum Report (Ref 15) City updated the WRM hydrological model in 2011 to include the final storage curve for the Hinze Dam Stage 3 upgrade. Nerang MIKE FLOOD Model Upgrade 2009 (Ref 1 & 2) DHI upgraded the 2005 MIKE Flood model in 2009 to incorporate Bridges (rather than culvert/weir) and other structures and to calibrate the model to various flood events. Nerang Hydraulic Addendum Report 2011 (Ref 30) Natural Hazards upgraded the DHI 2009 MIKE Flood model with changes made to the MIKE Flood couple files, MIKE 11 structures and slight changes to the DTM at the locations of model structures. The changes improved model stability and improved flood water spatial conveyance. The model was recalibrated to numerous historical flood events to ensure the changes made did not compromise reliability. Nerang River Hydrological Modelling Report 2013 (Ref 29) Natural Hazards updated WRM’s hydrological study (Ref 3) in 2013 for use within the URBS control Centre. The update also combined the Nerang, Worongary and Mudgeeraba catchments into one hydrological model. The model was then re-calibrate and verified against multiple historical flood events. Monte Carlo simulations were also undertaken for further verification of the design event discharges. The outputs (historical and design discharges) from this hydrologic model were used as inputs into the hydraulic model described in this report. 4. Hydraulic Model Development MIKE Flood software was used to build and calibrate the hydraulic model. MIKE Flood is capable of simulating complex flows and floodplain storage in the 2D topographical terrain using MIKE 21 and is


able to provide flow constriction in hydraulic structures using MIKE 11. The combination is appropriate as the Nerang floodplain consists of numerous structures (weirs, culverts and bridges), as well as a number of canal systems.

The Nerang Mike Flood model is solely based on the model developed by DHI in 2009 (Ref 1), which was then updated by our own Natural Hazards team in 2011 (Ref 30). The model has since undergone further updates to improve its replication of the physical catchment and to improve stability.

This report will outline the overall model setup and calibration results as is present for the current, most up to date version of the Nerang River Catchment hydraulic model. It will not specifically highlight differences in the model to that seen in older versions.

Datum 4.1

Horizontal Datum

All horizontal coordinates that are used in this report are in Map Grid of Australia (MGA) Zone 56.

Vertical Datum

The vertical coordinates are referenced to Australian Height Datum (AHD) 1992.

Available Data 4.2

The following data is available for this study: Creek cross-sectional survey data conducted from various years Bathymetric survey data from various years 2009 Aerial Laser Survey (ALS) (Ref 16) 2010 High Resolution ALS data (Ref 35) Historical ALERT and TM Station Water Level Data Maximum Height Gauge and Historical Debris Mark Data ALERT and TM Stations The Bureau of Meteorology (BOM) operates a number of flood telemetry systems, known as ALERT stations, on the Gold Coast and Logan region for flood prediction and climate data collection. These ALERT stations record water level, rainfall and other meteorological data. Other telemetry stations, known as TM stations, are operated by The Marine Safety Queensland (MSQ) and Department of Environment and Resource Management (DERM). These stations generally record stream flow, water quality and tide data. In the Nerang catchment area, both ALERT and TM station data are available for tide and flood calibration (Figure 2). They are: Air Sea Rescue AL (040881) Gold Coast Seaway TM (540001) Evandale AL (540318) Carrara AL (540319) Clearview AL (040846) Clearview TM (040416) Boobegan Creek (540253)


Mudgeeraba AL (540254) Mudgeeraba TM (540399) Burleigh Waters AL (040981) Bonogin AL (540352) Worongary Creek AL (540597)


Figure 2. Locations of Alert and TM Stations


Model Extent (Figure 3) 4.3

The MIKE 21 component of the model extends to the north to include the southern Broadwater, to the south covering Burleigh Waters and Varsity Lakes, and to the west where the Pacific Motorway crosses the Nerang River. The MIKE 11 component includes the upper reaches of the Nerang River up to Hinze Dam, Worongary, Mudgeeraba, Bonogin and Reedy Creek.

Figure 3. MIKE Flood Model Extent


MIKE 21 4.4

Topography

A 20m rectangular grid was created from City’s December 2014 2m Digital Terrain Model (DTM). The DTM includes Aerial Laser Survey (ALS) data from 2009, high resolution ALS data captured in 2010, and numerous bathymetric surveys over a period from 1999 to 2013 (Figure 4).

Figure 4. DTM Data Source


Using a 20m grid to represent many of the small channels and canals can create issues with conveyance as some of these waterways can be less than the 20m wide. Conveyance through these narrow channels was maintained by manually digging out cells where necessary to preserve the channel connectivity. The final DTM is shown in Figure 5.

Figure 5. MIKE 21 DTM

Roughness

The key calibration parameter for the hydraulic model is the floodplain roughness or Manning’s M roughness coefficient. The roughness map was based on the 2009 DHI study (Ref 1), with some alterations made in Council’s 2011 update (Ref 30) and further improvements made in the current version to the Worongary, Mudgeeraba and Bonogin areas. These adjustments improved calibration in those areas significantly. Figure 6 below shows Nerang’s Manning’s (M) roughness map.


Figure 6. MIKE 21 Roughness

Eddy Viscosity

Velocity based eddy viscosity is estimated using the following formula:

e = 0.1xDepth

An eddy viscosity of 0.5 is adopted across the catchment, with 10 being applied around the hydraulic structures to aid in model stability.


Tailwater Boundary

The open tailwater boundary is located where the Nerang River mouth exists into the southern Broadwater, approximately at the Marine Operations Base Southport telemetry station (100035), operated by Marine Safety Queensland (MSQ).

MIKE 11 4.5

The MIKE 11 network can be seen below in Figure 7, with the main river and creek systems labelled. These MIKE 11 branches represent the upper reaches of the catchment, which are relatively step with well-defined river and creek banks, which feed into the flatter floodplain area represented by the MIKE 21 component. Smaller MIKE 11 branches are contained within the MIKE 21 floodplain area to represent significant hydraulic structures, such as bridges, weirs and culverts.

Figure 7. MIKE 11 Network with Hydraulic Structures and MIKE 21 DTM Extent


Roughness

Roughness values for the main MIKE 11 branches (Nerang River, Worongary, Mudgeeraba, Bonogin and Reedy Creek) have been based on the roughness values used in a previous calibrated MIKE 11 model (Ref 34). Adopting these informed values substantially aided in the modelled water levels reflecting the recorded levels documented in the calibration and verification events.

Hydraulic Structures

There are a total of 130 structures modelled within the MIKE 11 network, including 29 weirs, 54 culverts and 53 bridges (Figure 7).

Inflow and Source Points 4.6

Figure 8 displays the location of the inflow hydrographs for both MIKE 11 (red triangles) and MIKE 21 (yellow circles). The hydrographs were produced from the Nerang River Hydrological study 2013 (Ref 29). In total there are 42 MIKE 21 and 39 MIKE 11 source points.

To aid in model stability, the local hydrograph produced in sub-catchment 28 was split equally and applied at 3 locations within the sub-catchment.

To account for the 2 branches seen in sub-catchment 23, the hydrograph was again divided evenly and placed on the individual branches.


Figure 8. Inflow Source Points


5. Model Calibration and Verification The hydraulic model is calibrated to the January 1974 historical flood event and verified against the June 2005 and January 2013 historical flood events.

The January 1974 flood event was the main calibration event in the Nerang hydrology study (Ref 29), as it is significantly larger than the other recorded flood events. The January 1974 flood event displayed a peak discharge of approximately 1400m3/s at Clearview TM, with the next highest being the January 2013 flood event at roughly 200m3/s.

Model robustness is also tested against 2005 recorded tide data.

January 1974 Calibration 5.1

One of the worst flooding events occurred at the end of January 1974. Most of Gold Coast experienced flooding especially in the lower reaches of the catchment. It is worth noting that Hinze Dam was only built in the 1980s as a result of this event (although Little Nerang Dam was already constructed).

This event is used as the main calibration event in the hydrology review project (Ref 29). Although some land use changes have occurred, there is good hydraulic calibration data (e.g. surveyed debris mark and Clearview TM recordings). Some significant changes, like the construction of the Benowa canals, opening of the Seaway, construction of the Pacific Motorway, construction of the railways, etc. can change the flooding characteristics of this modelled event.

Figure 9 displays the modelled and recorded water level results at Clearview TM. The calibration plot shows reasonable agreement between the modelled and recorded time-series. Peak and trough shapes within the time-series’ are represented in the modelled data but are not exactly replicated in terms of all the levels reached. These discrepancies can be attributed to the difficulty in accurately representing catchment conditions within the model to an event that occurred over 40 years ago. It is good to note that the absolute peak water level is represented in the modelled results.

Figure 10 displays the recorded and modelled debris marks for the January 1974 flood event.

The accuracy of the surveyed marks are difficult to gain 100% confidence in when comparing them to the surrounding flood waters. Localised situations could significantly affect water level and resultant debris marks in a particular area, which the model may not be able to exactly replicate. Debris moving within flood waters may create partial blockages when they are held back from continuous flow. This may result in higher flood waters where the debris builds up compared to the surrounding waters. These types of situations are not replicated in the hydraulic model.

The modelled maximum water level results display reasonable comparison to the surveyed marks across the entire catchment, with a mix of higher and lower modelled results compared to the debris marks.


Figure 9. Modelled and Recorded Water Level @ Clearview TM - Jan 1974

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Figure 10. Survey (red) and Modelled (blue) Debris Marks - Jan 1974


June 2005 Verification 5.2

A major flood occurred in South East Queensland on 30 June 2005 when heavy rainfall caused flash flooding on some parts of the Gold Coast area (Ref 20). This event is used as a calibration event in the hydrology review project (Ref 29). Although the rainfall was not evenly distributed, there is good hydraulic calibration data (e.g. surveyed debris marks and recorded waters level at the ALERT stations).

Figure 11 to Figure 14 represents the modelled and recorded water levels at the Clearview AL, Carrara AL, Evandale AL and Mudgeeraba TM Stations. Timing of the flooding peak is reasonably represented, however the modelled results is slightly overestimating the flooding peak water level at the Carrara AL, Evandale AL and Mudgeeraba TM Stations.

Figure 15 displays the recorded and modelled debris marks for the June 2005 flood event. For the steep hydraulic gradient along the upper reaches of Bonogin Creek the modelled results are notably lower than the recorded water level. For the remaining debris marks below 10m AHD, the modelled results reflect that seen in the alert station water level hydrographs, whereby the modelled results are slightly higher than the recorded results.

Figure 11. Modelled and Recorded Water Level @ Clearview AL - June 2005

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Figure 12. Modelled and Recorded Water Level @ Carrara AL - June 2005

Figure 13. Modelled and Recorded Water Level @ Evandale AL - June 2005

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Figure 14. Modelled and Recorded Water Level @ Mudgeeraba TM - June 2005

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Figure 15. Surveyed (red) and Modelled (blue) Debris Marks - June 2005


January 2013 Verification 5.3

Significant flooding events occurred in January 2013 down the east coast of Australia coinciding with the path of Ex-Tropical Cyclone Oswald. Upper Springbrook recorded 744mm in 24 hours, becoming the highest recorded 24 hour rainfall total in Australia since 772mm fell in Noosa in 2007 (Ref 31).

Figure 16 to Figure 22 displays the modelled and recorded water level at the Clearview AL, Carrara AL, Boobegan AL, Mudgeeraba AL, Mudgeeraba TM, Neranwood AL and Worongary AL Stations. All graphs show a reasonable agreement between the modelled and recorded water levels in terms of the timing and shape of the water level hydrographs, but they do not always reach to the peak recorded water level.

Figure 16. Modelled and Recorded Water Level @ Clearview AL - Jan 2013

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Figure 17. Modelled and Recorded Water Level @ Carrara AL - Jan 2013

Figure 18. Modelled and Recorded Water Level @ Boobegan AL - Jan 2013

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Figure 19. Modelled and Recorded Water Level @ Mudgeeraba AL – Jan 2013

Figure 20. Modelled and Recorded Water Level @ Mudgeeraba TM - Jan 2013

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Figure 21. Modelled and Recorded Water Level @ Neranwood AL - Jan 2013

Figure 22. Modelled and Recorded Water Level @ Worongary Ck AL - Jan 2013

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Recorded

Modelled


It is worthwhile to point out that the June 2005 event produced slightly higher modelled water levels, whereas the January 2013 event produced slightly lower modelled water levels. These discrepancies are attributed to rainfall and resultant discharges in the hydrological model (Ref 29) being calculated for each sub-catchment from the availability of ALERT and Telemetry Stations for those flood events. Small deviations in the calibration of the hydrological model (Ref 29) have the potential to be magnified within the calibration of the hydraulic model.

Tidal Calibration 5.4

Griffith University undertook a comprehensive tidal water and current measurement data collection exercise in late 2004 to early 2005 (Ref 25). Within the Nerang River study area 5 water level stations were used to assess the Nerang hydraulic model’s ability to replicate tidal water fluctuations. The 5 tidal recording stations are listed below and their locations are displayed in Figure 23.

B11 Nerang River Mouth

N1 Evandale

N2 Sorrento

N3 Royal Pines

N4 Arc Resort

Figure 24 to Figure 28 display the modelled and recorded tidal fluctuations at the five identified locations. The results show that the modelled results become less replicating of the recorded results as they move further upstream. B11 Nerang Mouth shows good replication of the recorded results in terms of peaks, timing and shape. N1 Evandale displays a slight decline in the modelled results duplication of the recorded data, with a continued decline as the tidal waters move upstream along Nerang River to N2 Sorrento then N3 Royal Pines. Reasonable timing and shape is replicated at N4 ARC Resort, but the model results fail to successfully replicate the high and low peak tide levels seen in the recorded results.

To achieve good tidal calibration throughout the entire Nerang catchment river network a more refined DTM is required, or it would need a 1D model with refined cross-sectional information to more accurately represent the narrower creeks and canal systems. This would allow greater conveyance of the required water volume to be distributed throughout the tidal area.


Figure 23. 2005 Tide Recording Station Locations


Figure 24. Modelled and Recorded Tidal Level @ B11 Nerang Mouth

Figure 25. Modelled and Recorded Tidal Level @ N1 Evandale


Figure 26. Modelled and Recorded Tidal Level @ N2 Sorrento

Figure 27. Modelled and Recorded Tidal Level @ N3 Royal Pines


Figure 28. Modelled and Recorded Tidal Level @ N4 ARC Resort

6. Design Floods Introduction 6.1

Design floods are hypothetical statistical floods used for flood planning and designing of structures. They are based on a probability of occurrence, either as:


ARI – Average Recurrence Interval; or

AEP – Annual Exceedance Probability

For example, a 100 year ARI or 0.1 AEP flood has an average recurrence interval of 100 years, or, the flood has a 1% probability of being equalled or exceeded in any one year.

With ARI expressed in years, the relationship is:

Resulting in the following conversions seen in Table 1.

Table 1. ARI and AEP Design Floods

ARI (in Years) AEP (as Percentage)

2 39.3%

5 18.1%

10 9.5%

20 4.9%

50 2.0%

100 1.0%

200 0.5%

500 0.2%

1000 0.1%

2000 0.05%

The calibrated Nerang MIKE Flood hydrodynamic model has been setup to simulate design flood events ranging from 2 year ARI to PMF. The design runs include storm durations of 0.5, 1.0, 1.5, 3.0, 4.5, 6.0, 9.0, 12.0, 18.0, 24.0, 36.0, 48.0, 72.0, 96 and 120 hours. The hydrology of this study for both calibration and design events are sourced from the Nerang River Catchment Hydrological Study (Ref 29).

Tailwater boundary conditions range from Lowest Astronomical Tide (LAT) (see Section 9) to 2000 year ARI storm surge. Storm surge levels are based on the City of Gold Coast’s - Storm Tide Study May 2012, undertaken by GHD (Ref 26).

Design Run Setup 6.2

The design runs are based on the following: Hydrological Input from the 2015 hydrological study (Ref 29) with 10% increase in rainfall

intensity Nerang Hinze Dam Full Supply Level


Selected Tailwater Conditions based on GHD study (Ref 26) and general tidal planes o 0.8m Sea Level Rise with 10% increase in storm surge intensity (2100 year storm surge) o Does not include wave setup (Seaway and Wave Break Island)

Predetermined Initial Water Levels based on lake designed levels Bond University Weir (Structure “C”) assumed blocked

Initial Water Level 6.3

For the design events, the following initial water level was applied to all model setups:

Rivers, creeks and canal system = 0.66 m AHD Clear Island Waters and Robina Lakes = 0.56 m AHD Varsity Lake level = 0.56 m AHD

It is important to note that the initial water level is set at 0.66 m AHD to ensure the storage in the system partly full. For Clear Island Waters, Boobegan weir maintains water level of the lake. For Robina Lakes, the water level is maintained by Structure “C” which is assumed to be either malfunctioning or blocked by debris.

Probable Maximum Precipitation (PMP) 6.4

The Nerang model has been setup to simulate PMP events based on the hydrological outputs derived from the Nerang hydrological study (Ref 29).

PMP rainfall depths for durations from 0.5 to 6 hours were determined using the Bureau of Meteorology’s Guide to the Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method (GSDM) (Ref 32).

PMP rainfall depths for durations from 24 to 120 hours were determined using the Bureau of Meteorology’s Guide to the Estimation of Probable Maximum Precipitation Generalised Tropical Storm Method (GTSMR) (Ref 33). For 9, 12 and 18 hours, the rainfall is linearly interpolated between 6 and 24 hours.

The hydrological study developed a set of discharge outputs from the PMP rainfall depths using two differing approaches based on the temporal pattern source. The two differing suites of discharge outputs were categorised as PMPDF (Probable Maximum Precipitation Design Flood) and PMF (Probable Maximum Flood).

PMPDF Temporal Patterns 6.4.1

Durations from 0.5 hours to 6 hours utilises the design temporal pattern from the Bureau of Meteorology’s Guide to the Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method (GSDM) (Ref 32).

Durations from 24 hours to 72 hours uses the 500km2 Coastal AVM design temporal patterns from the Bureau of Meteorology’s Guide to the Estimation of Probable Maximum Precipitation Generalised Tropical Storm Method (GTSMR) (Ref 33).


For the 9, 12 and 18 hour durations the temporal pattern is interpolated between the 6 and 24 hour durations.

PMF Temporal Patterns 6.4.2

The PMF setup uses the top 10 individual storm temporal patterns from the GTSMR (Ref 33). As the GTSMR is a guide for PMP durations of 24 to 120 hours, these are the only durations for the PMF setup. For each of the PMF durations the 10 temporal patterns applied can be seen in Table 2.

Table 2. PMF Temporal Patterns

24HOURS 36HOURS 48HOURS 72HOURS 96HOURS 120HOURS PMP01 1893FEB03-1 1893FEB03-2 1893FEB03-2 PMP02 1898APR03-2 1898APR03-2 PMP03 1989MAR14-1 PMP04 1918JAN24-3 1918JAN24-3 1918JAN24-3 1918JAN25-5 1918JAN25-5 PMP05 1954FEB21-1 1954FEB21-2 PMP06 1955FEB25-2 1955FEB25-2 PMP07 1956JAN22-2 PMP08 1963APR16-4 1963APR16-4 PMP09 1970JAN19-1 PMP10 1972JAN12-5 1972JAN12-5 1972JAN12-5 1972JAN12-5 PMP11 1974JAN09-3 PMP12 1974JAN23-6 1974JAN23-6 1974JAN23-6 PMP13 1974JAN27-2 1974JAN27-2 1974JAN27-2 1974JAN28-4 1974JAN28-4 1974JAN27-9 PMP14 1974MAR13-4 1974MAR13-4 1974MAR13-4 PMP15 1975FEB25-6 1975FEB25-6 PMP16 1975DEC10-2 PMP17 1979JAN06-4 1979JAN06-4 1979JAN06-4 1979JAN06-4 1979JAN06-5 PMP18 1981JAN13-7 1981JAN13-7 PMP19 1982JAN22-2 PMP20 1989MAR14-2 1991JAN01-7 1991JAN01-7 PMP21 1995FEB28-4 1995FEB28-4 PMP22 1997MAR06-7 1997MAR06-7 PMP23 1998JAN29-4 PMP24 1998MAR05-7 1998MAR05-7 1998MAR05-7 PMP25 1998DEC10-2 PMP26 1999FEB13-2

7. Conclusion A MIKE Flood hydrodynamic model for the Nerang River catchment has been developed and successfully calibrated against the January 1974 and verified against the June 2005 and January 2013 historical flood events. The model was tested for its ability to simulate tidal conditions, but it was found that a more refined DTM was needed to confidently replicate tidal conveyance throughout the numerous narrow creeks and canals.

This calibrated model has been setup for 2, 5, 10, 20, 50, 100, 200, 500, 1000 and 2000 year ARI design flood events. The Design model setup accounts for climate change by the year 2100 by


increasing rainfall intensity by 10% and using predicted storm surge levels that include 0.8m in sea level rise and a 10% increase in storm surge intensity. The model has also been setup to simulate PMPDF and PMF flood events.

City of Gold Coast’s Designated Flood Level for the City Plan 2015 for Nerang River catchment is determined using this model. This model is also useful to assess flood impacts for proposed development

The model is an accumulation of years of development and the latest information that is available. It requires constant review and updates to take into account the changes in the floodplain.

8. References

1. DHI. Nerang MIKE Flood Model Upgrade. Final Report. June 2011. 2. DHI. Nerang MIKE Flood Model Upgrade. Appendices. June 2011. 3. WRM Water & Environment. Nerang River Flood Study Hydrological Modelling. Final

Report. April 2010. 4. Joint Technical Steering Committee. Nerang River Flood Study. September 1992. 5. Kinhill Cameron McNamara. Nerang River Hydrological Studies. February 1992.


6. SMEC Australia Pty Ltd. Nerang River Hydraulic Modelling Study. May 2001. 7. SMEC Australia Pty Ltd. Nerang River Hydraulic Modelling Study. Appendix A. Structure

Data. October 2001. 8. Lawson and Treloar Pty Ltd. Development Option Testing using the Nerang MIKE 21.

June 2003. 9. City of Gold Coast. Mudgeeraba & Worongary Creeks Hydrology Study Report. June

2004. 10. City of Gold Coast. Nerang River Flood Surfaces Development Project . June 2004. 11. City of Gold Coast. Mudgeeraba Creek Hydrological Model Update 2007. October 2007. 12. City of Gold Coast. Nerang MIKE 21 Model Update –Conversion To Mike Flood V2005.

July 2006. 13. City of Gold Coast. Nerang Mike 11 Hydraulic Model Development and Analysis.

November 2006. 14. HDA Alliance. Hinze Dam Flood Hydrology for Stage 3 Design. Report v6. 16 February

2009. 15. City of Gold Coast. Nerang Hydrology Addendum Report. November 2011. 16. Department of Environment and Resource Management. LiDAR and Orthophoto

Acquisition 2009. 17. Bureau of Meteorology website. http://www.bom.gov.au/hydro/flood/qld/ 18. Marine Safety Queensland website. http://www.msq.qld.gov.au/Home/Tides/ 19. Department of Environment and Resource Management website.

http://www.derm.qld.gov.au/water/monitoring/current_data/index.php 20. Bureau of Meteorology. Heavy Rainfall Gold Coast. June 2005. 21. Bureau of Meteorology. South East Queensland Floods. January 2008. 22. Bureau of Meteorology. Brisbane Floods. January 1974. 23. Bureau of Meteorology. South East Queensland Floods. November 2004. 24. Bureau of Meteorology. South East Queensland Floods. May 2009. 25. Mirfenderesk H., Tomlinson R., Hughes. L., (2005a), “Field Data Collection and analysis

at the Broadwater, Gold Coast”, 17th Australasian Conference on Coastal and Ocean Engineering, Adelaide, Australia. P477-482.

26. GHD. Storm Tide Study. May 2012. 27. DERM. Queensland Coastal Plan. January 2012. 28. Cardno Lawson Treloar. Joint Probability Literature Review. 2010. 29. City of Gold Coast. Nerang River Catchment, Hydrological Study. August 2015. 30. City of Gold Coast. Nerang Hydraulic Addendum Report. 2011. 31. Bureau of Meteorology. Ex-TC Oswald Floods. January and February 2013. 32. Bureau of Meteorology. The Estimation of Probable Maximum Precipitation in Australia:

Generalised Short Duration Method. June 2003. 33. Bureau of Meteorology. Guide to the Estimation of Probable Maximum Precipitation:

Generalised Tropical Storm Method. November 2003. 34. City of Gold Coast. MIKE 11 Model: Hinze Dam to Pacific Highway, Hydraulic Study.

September 2001. 35. Terranean Mapping Technologies. Project Report, 3D Mapping of the Gold Coast Urban

Footprint 2012.


Appendix A

Semidiurnal Tidal Planes - 2014


Council of the City of Gold Coast PO Box 5042 GCMC Qld 9729 P 1300 GOLDCOAST E [email protected] W cityofgoldcoast.com.au

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