appendix e hydraulic modelling of design floods · · 2015-05-11appendix e - hydraulic modelling...

APPENDIX E

HYDRAULIC MODELLING OF DESIGN FLOODS

Combined Catchments of Whartons, Collins and Farrahars Creeks, Bellambi Gully and Bellambi Lake Flood Study

Appendix E - Hydraulic Modelling of Design Floods

Combined FS Appendix E.doc Ei Lyall & Associates 30 May 2011 Rev. 4.0 Consulting Water Engineers

TABLE OF CONTENTS Page No

E1. SYNOPSIS ...................................................................................................................... E1

E2. HYDRAULIC MODEL SETUP ......................................................................................... E3

E2.1 Adjustments to TUFLOW Model Structure .......................................................... E3 E2.2 Model Inflows ...................................................................................................... E4 E2.3 Selection of Ocean Outlet Conditions ................................................................. E4 E2.4 Derivation of Design Flood Envelopes ................................................................ E5 E2.5 Application of WCC’s Conduit Blockage Policy ................................................... E6

E3. HYDRAULIC MODEL RESULTS .................................................................................. E11

E3.1 Presentation of Results ..................................................................................... E11 E3.2 Whartons Creek Catchment .............................................................................. E11 E3.3 Collins Creek Catchment .................................................................................. E13 E3.4 Bellambi Gully/Farrahars Creek Catchment ...................................................... E16 E3.5 Bellambi Lake Catchment ................................................................................. E19

E4. SENSITIVITY STUDIES ................................................................................................ E23

E4.1 General ............................................................................................................. E23 E4.2 Changes in Hydraulic Roughness ..................................................................... E23

E4.2.1 Global Increase in Hydraulic Roughness Values .................................. E23 E4.2.2 Decrease in Hydraulic Roughness Values in Fenced Properties .......... E23

E4.3 Blockage of Primary Spillway in Gordon Hutton Park Flood Retarding Basin ... E24 E4.4 Changes to Basin Discharge Characteristics as a Result of Lead Rainfall ........ E24 E4.5 Inflows from Existing Dam in Slacky Creek Catchment ..................................... E25 E4.6 Removal of Artificial Floodplain Storage ........................................................... E26 E4.7 Deposition of Sediment Along Study Reaches .................................................. E26

E5. CLIMATE CHANGE ...................................................................................................... E27

E5.1 General ............................................................................................................. E27 E5.2 Sea Level Rise .................................................................................................. E27

E5.2.1 Background .......................................................................................... E27 E5.2.2 Scope of Investigation .......................................................................... E27

E5.3 Increased Rainfall Intensities ............................................................................ E28 E5.3.1 Background .......................................................................................... E28 E5.3.2 Scope of Investigation .......................................................................... E29

E6. REFERENCES .............................................................................................................. E31



Combined FS Appendix E.doc Eii Lyall & Associates 30 May 2011 Rev. 4.0 Consulting Water Engineers

LIST OF FIGURES BOUND IN VOLUME 2

E2.1 Afflux (100 year ARI Flood Envelope minus 100 year ARI Design Run No. 1) – Whartons Creek and Collins Creek Catchments

E2.2 Afflux (100 year ARI Flood Envelope minus 100 year ARI Design Run No. 1) – Farrahars Creek and Bellambi Gully Catchments

E2.3 Afflux (100 year ARI Flood Envelope minus 100 year ARI Design Run No. 1) – Bellambi Lake Catchment

E3.1 TUFLOW Model Results - Whartons Creek and Collins Creek Catchments - 5 year ARI

E3.2 TUFLOW Model Results - Farrahars Creek and Bellambi Gully Catchments - 5 year ARI

E3.3 TUFLOW Model Results - Bellambi Lake Catchment - 5 year ARI



















E3.22 TUFLOW Model Results - Whartons Creek and Collins Creek Catchments - PMF

E3.23 TUFLOW Model Results - Farrahars Creek and Bellambi Gully Catchments - PMF

E3.24 TUFLOW Model Results - Bellambi Lake Catchment - PMF

E3.25 Design Water Surface Profiles - Main Arm of Whartons Creek

E3.26 Design Water Surface Profiles - Tributary 1 of Whartons Creek

E3.27 Design Water Surface Profiles - Tributary 2 of Whartons Creek

E3.28 Design Water Surface Profiles - Main Arm of Collins Creek (Upper)

E3.29 Design Water Surface Profiles - Main Arm of Collins Creek (Lower)

E3.30 Design Water Surface Profiles - Tributary 1 of Collins Creek


E3.32 Design Water Surface Profiles – Tributaries 3 and 4 of Collins Creek




Combined FS Appendix E.doc Eiii Lyall & Associates 30 May 2011 Rev. 4.0 Consulting Water Engineers

LIST OF FIGURES BOUND IN VOLUME 2 (CONT’D)

E3.34 Design Water Surface Profiles - Main Arm of Bellambi Gully

E3.35 Design Water Surface Profiles - Tributary 1 of Bellambi Gully


E3.37 Design Water Surface Profiles - Tributary 3 of Bellambi Gully (Upper)

E3.38 Design Water Surface Profiles - Tributary 3 of Bellambi Gully (Lower)


E3.40 Design Water Surface Profiles - Tributaries 5 and 6 of Bellambi Gully

E3.41 Design Water Surface Profiles - Farrahars Creek

E3.42 Design Water Surface Profiles - Main Arm of Bellambi Lake (Upper)

E3.43 Design Water Surface Profiles - Main Arm of Bellambi Lake (Lower)

E3.44 TUFLOW Model Design Hydrographs – Whartons Creek Ocean Outlet, 120 Minute Storm

E3.45 TUFLOW Model Design Hydrographs – Collins Creek Ocean Outlet, 120 Minute Storm

E3.46 TUFLOW Model Design Hydrographs – Bellambi Gully/Farrahars Creek Ocean Outlet, 120 Minute Storm

E3.47 TUFLOW Model Design Hydrographs – Bellambi Lake Ocean Outlet, 120 Minute Storm

E4.1 Sensitivity Analysis – Afflux (20% Increase in Hydraulic Roughness minus Best Estimate) – Whartons Creek and Collins Creek Catchments – Design Run No. 1 – 100 year ARI 120 Minute

E4.2 Sensitivity Analysis – Afflux (20% Increase in Hydraulic Roughness minus Best Estimate) – Farrahars Creek and Bellambi Gully Catchments – Design Run No. 1 – 100 year ARI 120 Minute

E4.3 Sensitivity Analysis – Afflux (20% Increase in Hydraulic Roughness minus Best Estimate) – Bellambi Lake Catchment – Design Run No. 1 – 100 year ARI 120 Minute

E4.4 Sensitivity Analysis – Afflux (Allotment Roughness n = 0.1 minus Allotment Roughness n = 1.0) – Whartons Creek and Collins Creek Catchments – Design Run No. 1 – 100 year ARI 120 Minute

E4.5 Sensitivity Analysis – Afflux (Allotment Roughness n = 0.1 minus Allotment Roughness n = 1.0) – Farrahars Creek and Bellambi Gully Catchments – Design Run No. 1 – 100 year ARI 120 Minute

E4.6 Sensitivity Analysis – Afflux (Allotment Roughness n = 0.1 minus Allotment Roughness n = 1.0) – Bellambi Lake Catchment – Design Run No. 1 – 100 year ARI 120 Minute

E4.7 Discharge Hydrographs at Ocean Outlet – Results of Whartons Creek Sensitivity Studies – Design Run No. 1 – 100 year ARI 120 Minute

E4.8 Sensitivity Analysis – Afflux (Gordon Hutton Park Flood Retarding Basin) – Whartons Creek Catchment – 100 year ARI 120 Minute

E4.9 Sensitivity Analysis – Afflux (Post-Stream Deposition minus Present Day Conditions) – Whartons Creek and Collins Creek Catchments – Design Run No. 1 – 100 year ARI 120 Minute



Combined FS Appendix E.doc Eiv Lyall & Associates 30 May 2011 Rev. 4.0 Consulting Water Engineers

LIST OF FIGURES BOUND IN VOLUME 2 (CONT’D)

E4.10 Sensitivity Analysis – Afflux (Post-Stream Deposition minus Present Day Conditions) – Farrahars Creek and Bellambi Gully Catchments – Design Run No. 1 – 100 year ARI 120 Minute

E4.11 Sensitivity Analysis – Afflux (Post-Stream Deposition minus Present Day Conditions) – Bellambi Lake Catchment – Design Run No. 1 – 100 year ARI 120 Minute

E5.1 Afflux (2050 Sea Level Rise Increase minus Present Day Conditions) – 100 year ARI

E5.2 Afflux (2100 Sea Level Rise Increase minus Present Day Conditions) – 100 year ARI

E5.3 Sensitivity of Design Flood Levels to Varying Entrance Berm and Inshore Ocean Conditions – Design Water Surface Profiles – 100 year ARI

E5.4 Extents of Increased Inundation Resulting from Increased Rainfall – Whartons Creek and Collins Creek Catchments – 100 year ARI Flood Envelope

E5.5 Extents of Increased Inundation Resulting from Increased Rainfall – Farrahars Creek and Bellambi Gully Catchments – 100 year ARI Flood Envelope

E5.6 Extents of Increased Inundation Resulting from Increased Rainfall – Bellambi Lake Catchment – 100 year ARI Flood Envelope

E5.7 Afflux (200 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Whartons Creek and Collins Creek Catchments

E5.8 Afflux (200 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Farrahars Creek and Bellambi Gully Catchments

E5.9 Afflux (200 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Bellambi Lake Catchment

E5.10 Afflux (500 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Whartons Creek and Collins Creek Catchments

E5.11 Afflux (500 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Farrahars Creek and Bellambi Gully Catchments

E5.12 Afflux (500 year ARI Flood Envelope minus 100 year ARI Flood Envelope) – Bellambi Lake Catchment



Combined FS Appendix E.doc Page E1 Lyall & Associates 30 May 2011 Rev. 4.0 Consulting Water Engineers

E1. SYNOPSIS This Appendix deals with the derivation of design flooding patterns on the study creeks. The hydraulic models used in the derivation of design flood envelopes were based on those described in Appendix C. Discharge hydrographs generated by the RAFTS catchment models described in Chapter 3 of the Main Report were applied to the hydraulic models which were tuned to the August 1998 storm. No historic data was available which provided information on the behaviour of the entrance berms during periods of high flow on the study creeks. Scoured and shoaled entrance conditions were therefore determined from the available literature and a review of the ALS data and aerial photography. Design flood envelopes were derived for each recurrence internal event after application of WCC’s Conduit Blockage Policy. This involved running seven separate TUFLOW models representing critical bridge and culvert blockage conditions on each creek system. The results of the modelling were then combined for each recurrence interval event to provide an upper limit of potential flooding in each catchment. A number of studies were carried out to test the sensitivity of the hydraulic model results. Details of the studies and findings are contained in Section E4 of this Appendix. The impact of climate change on flood behaviour in the study area was also assessed. Details of the study findings are contained in Section E5 of this Appendix.




[Page Intentionally Left Blank]




E2. HYDRAULIC MODEL SETUP E2.1 Adjustments to TUFLOW Model Structure Study Area Wide The following adjustments were made to the structure of the tuned TUFLOW models to reflect changes which have occurred broadly across the study area since the 17 August 1998 storm:

The inclusion of the Northern Distributor Extension road works. A three-dimensional model of the road works was provided by the RTA and used as the basis for updating the DTM in the tuned TUFLOW model. Details of the major cross drainage along the route of the Northern Distributor Extension were also incorporated into the tuned TUFLOW model. Table 2.2 in the Main Report contains details of the major cross drainage along the road corridor.

An increase in hydraulic roughness values to reflect the increase in the density of vegetation which has occurred along several watercourses in the past decade.

The works associated with the Edgewood Estate development. This included the flood retarding basins which have been constructed on Hollymount Creek and to the south of the development on Tributary 3 of Bellambi Gully system (Basin No. 2). The engineered sections of channel which lie upstream of Red Ash Drive on Hollymount Creek and the main arm of Collins Creek were also incorporated into the model. Following receipt of correspondence sent to Council during the public exhibition of the draft flood study report, the footprint of properties located along Forestview Drive were also raised by 300 mm to reflect general filling which has occurred in these properties since the ALS was taken.

Whartons Creek Catchment

As described in Appendix A, survey of the existing dam which has been constructed on the Illawarra Escarpment in the adjacent catchment of Slacky Creek shows that flow discharging from the dam will contribute to flood flows in the main arm of Whartons Creek. For the purpose of defining the design flood envelopes in the Whartons Creek catchment, 50 per cent of the flow discharging from the existing dam was assumed to contribute to flood flows in the main arm.

Details of the Bulli Spinners development located at the intersection of Franklin Avenue and Ursula Street, the construction of which commenced during the preparation of the flood study, was incorporated into the TUFLOW model. Details of the development were based on information contained in a letter addressed to WCC headed “Bulli Spinners Subdivision DA 2005-1839 Response to Council Request for Additional Drainage Information” (Cardno Forbes Rigby, 2006).

Collins Creek Catchment

Medium density residential type development has been constructed along the western (upstream) side of Balls Street on Tributary 2 in recent years. During the tuning process it was found that WCC’s ALS data reflected ground levels post the filling of this area, which at the time of the 17 August 1998 storm, was subject to flooding.

The area of fill which was removed as part of the tuning process was re-instated into the TUFLOW model in order that the model reflect present day ground conditions in this area.




Bellambi Gully Catchment

In recognition that flood producing rainfall which is critical for maximising flood flows on the study creeks is generally associated with more general rainfall (Rigby et al, 2003), the starting water level in the large tailings dam which is located to the west of the Princess Highway on the main arm of Bellambi Gully was set at the level of the spillway.

The adjustments which were made to several of the model cross sections to reflect the deposition of material which occurred during the 17 August 1998 storm between the Illawarra Railway Line and Gladstone Street on the main arm of Bellambi Gully were removed from the model (i.e. the raw surveyed cross section data was adopted for modelling the design flood events).

Bellambi Lake Catchment

No catchment specific changes were made to the TUFLOW model representing the Bellambi Lake system.

E2.2 Model Inflows

Discharge hydrographs derived from RAFTS (refer Chapter 3 of the Main Report) provided the boundary conditions at the upstream ends of the TUFLOW models.

Rainfall falling directly on the area covered by the two-dimensional domain was firstly converted to runoff using RAFTS before being applied to the TUFLOW model as a digitised rain boundary.

E2.3 Selection of Ocean Outlet Conditions

As the primary breakout mechanism for an ICOLL is heavy rainfall (Gordon, 1981), the adoption of appropriate entrance conditions for developing design flood envelopes is dependent on the conditional probability of local catchment runoff, entrance berm heights, standing water levels upstream of the entrance berm at the onset of rainfall and inshore ocean conditions.

Design Entrance Berm Heights

A study carried out on Bellambi Lagoon (DECC, 2008) found that the entrance berm reached a maximum of 2 m AHD over an eight month period commencing June 2007. A more recent report prepared for WCC entitled “A Climate Change Impact Assessment of Wollongong’s Estuaries – Implications for Ongoing Estuary Management Planning” (Cardno Lawson Trealor, 2009), includes additional information on observed entrance berm heights in the study area. The report notes that the average height of the entrance berms is typically between 1.8-2.0 m AHD, and adopted the higher value as a conservative estimate for setting baseline conditions from which climate change impacts were assessed.

Rock shelves at the ocean outlets were observed at about mean sea level, indicating that the entrance berms are likely to scour to about 0 m AHD after being breached. Following discussions with WCC and DECCW, the decision was made to adopt the following entrance berm conditions for deriving the envelope of maximum flood levels and flow velocities for each design flood event:

• Maximum elevation of shoaled entrance - 2.0 m AHD

• Minimum elevation of scoured entrance - 0.0 m AHD




Standing Water Level Upstream of Entrance Berm

During periods of little or no rainfall, water levels upstream of the entrance berms are governed primarily by the rate of groundwater outflows through the sand berm which separates the freshwater bodies from the ocean.

Water levels were observed to typically remain between 100-300 mm below the saddle point in the entrance berms. This observation is consistent with those found by LACE, 2002. Based on these observations, a starting water level of 1.8 m AHD was adopted upstream of the entrance berms when modelling shoaled entrance conditions.

Inshore Ocean Conditions

DECCW’s guideline entitled Flood Risk Management Guide: Incorporating Sea Level Rise Benchmarks in Flood Risk Assessments (DECCW, 2010) contains interim advice in relation to the coincident catchment and ocean flooding conditions which should be adopted when preparing flood studies in coastal areas. The interim advice is an update of DECC’s draft Floodplain Management Guideline No. 5 Ocean Boundary Conditions, 2004 and will be subject to review following the release of the update of Australian Rainfall and Runoff (IEAust, 1998).

The interim advice recommends that peak “Storm Tide” levels of 2.3 m AHD and 2.6 m AHD be adopted for deriving design flood envelopes for events of 20 and 100 year ARI, respectively. When modelling “Storm Tide” conditions, a dynamic boundary condition was adopted.

E2.4 Derivation of Design Flood Envelopes

The process undertaken for deriving the design flood envelopes for the study area was as follows:

� Step 1 – Run the hydraulic model for local catchment storms of various return period and duration in combination with a “Normal Tide”. [Note that a static water level of 0.63 m AHD was adopted as the downstream boundary of the hydraulic model for these runs].

� Step 2 - Combine the results of Step 1 to create an envelope of maximum local catchment flood levels for each return period (i.e. the results of running storms of the same return period but different duration were combined to create a single envelope).

� Step 3 – Run the dynamic 20 year ARI “Storm Tide” as presented in DECCW (2010) with the 5 year ARI local catchment storms of varying duration.

� Step 4 - Combine the results of Step 3 to create a single envelope representing 20 year ARI “Storm Tide” conditions.

� Step 5 – Run the dynamic 100 year ARI “Storm Tide” as presented in DECCW (2010) with the 20 year ARI local catchment storms of varying duration.

� Step 6 - Combine the results of Step 5 to create a single envelope representing 100 year ARI “Storm Tide” conditions.

� Step 7 – Prepare a final set of flood envelopes for each return period using a combination of the envelopes derived from the steps above. Tables E2.1 and E2.2 set out the combination of local catchment and storm tide conditions which were used to compile the design flood envelopes for the study area. Note that the results of running the hydraulic model for the critical 100 year ARI local catchment storm in combination with the dynamic 20 year ARI “Storm Tide” were also used to compile the 100 year ARI design flood envelope (refer Table E2.1 for details).




TABLE E.2.1 DERIVATION OF DESIGN 100 YEAR ARI FLOOD LEVEL ENVELOPE

Design Flood Envelope Local Catchment Flood Ocean Boundary Condition

100 year ARI 100 year ARI

• Normal Tide • 20 year ARI Storm Tide

20 year ARI 100 year ARI Storm Tide

TABLE E.2.2 DERIVATION OF DESIGN FLOOD LEVEL ENVELOPES FOR OTHER EVENTS

Design Flood Envelope Local Catchment Flood Ocean Boundary Condition

5 year ARI 5 year ARI Normal Tide










PMF PMF Normal Tide


E2.5 Application of WCC’s Conduit Blockage Policy

Following the major floods of 17 August 1998 and 24 October 1999, WCC developed a Conduit Blockage Policy (WCC, 2009), whereby catchment wide flood studies (as well as those carried out in support of development applications) must take into account the impact potential blockages will have on local flooding conditions.

Based on a detailed evaluation of flooding behaviour following the two historic floods, WCC developed the following set of blockage criteria which are to be applied to drainage structures as part of any flood study investigation:




“i) 100% blockage for structures with a major diagonal opening width of < 6 m.

ii) 25% bottom up blockage for structures with a major diagonal opening width > 6m.

For bridge structures involving piers or bracing, the major diagonal length is defined as the clear diagonal opening between piers/bracing, not the width of the channel at the cross section.

iii) 100% blockage for handrails over the structures covered in (i) and structures covered in (ii) when overtopping occurs.”

In developing the design flood envelopes for the study catchments, several runs of the TUFLOW model were carried out, whereby selected culverts and bridges were blocked in accordance with the Conduit Blockage Policy. Table E2.3 summarises the set of design runs which were used to compile the flood envelopes for design storms of 20, 50, 100 year ARI and the PMF. Tables E2.4 and E2.5 provide a record of the locations where the inlet of selected drainage structures were blocked as part of Design Run Nos. 6 and 7, respectively. Note that the results of Design Run No. 1 were adopted when developing flood envelopes for the 5 and 10 year ARI events, as WCC considers blockage of culverts during these more frequent events to be less of a risk.

TABLE E2.3 RUNS USED TO COMPILE DESIGN FLOOD ENVELOPES

20 to 500 YEAR ARI AND PMF EVENTS

Design Run No. Blockage Conditions

1(1) Ideal flow conditions (i.e. no blockage of drainage structures)

2 Blockage policy applied to all culverts/bridges in the study area

3 Blockage policy applied to the Princes Highway culverts

4 Blockage policy applied to the Illawarra Railway Line bridges/culverts

5 Blockage policy applied the Northern Distributor Extension bridges/culverts

6 Blockage policy applied to selected structures in each catchment (refer Table E2.4).

7 Blockage policy applied to selected structures in each catchment (refer Table E2.5).

1. A variant of Design Run No. 1 was also run for the Whartons Creek catchment, whereby the primary spillway of the flood retarding basin in Gordon Hutton Park was artificially roughened to reflect the partial blockage of the fences which cross it. Figures E2.1 to E2.3 show the impact the application of WCC’s Conduit Blockage Policy has on peak 100 year ARI flood levels in the study catchments. The figures show that peak 100 year ARI flood levels will increase by over 1 m upstream of several structures should they experience a major blockage during a flood of this magnitude. The extent of land inundated by a flood of this magnitude is also significantly increased by blockage of structures, especially along the main arms of Bellambi Gully and Bellambi Lake.




TABLE E2.4 LOCATION OF BLOCKED STRUCTURES

DESIGN RUN No. 6

Catchment Watercourse Location of Blocked Structure

Whartons Creek

Main Arm

• Inlet of flood retarding basin in Gordon Hutton Park;

• Dumbrell Road;

• Hospital Road;

• Organs Road; and

• Franklin Avenue.

Tributary 1 • None


Collins Creek

Main Arm • Princes Highway;

• Illawarra Railway Line; and

• Pedestrian footbridge at ocean outlet.

Hollymount Creek • None


Tributary 2 • Mountain Avenue;

• Woonona Parade; and

• Campbell Street.

Tributary 3 • Mountain Avenue; and

• Woonona Parade.

Tributary 4 • Robert Street;

• Campbell Street; and

• Illawarra Railway Line.

Tributary 5 • Illawarra Railway Line; and

• Carrington Street.

Bellambi Gully/ Farrahars Creek

Main Arm • Illawarra Railway Line; and


Tributary 1 • Channon Street;

• Illawarra Railway Line; and

• Pioneer Drive.

Tributary 2 • Hicks Street

Farrahars Creek • Illawarra Railway Line

Tributary 3 • Illawarra Railway Line.




TABLE E2.4 (Cont’d) LOCATION OF BLOCKED STRUCTURES

DESIGN RUN No. 6



Tributary 4 • Pipe draining low point south of Stanhope Street; and

• Local access driveway approximately 70 m downstream of Northern Distributor road corridor

Tributary 5 • 1200 mm diameter pipe controlling runoff from Edgewood Estate development.


Bellambi Lake Main Arm

• Midgley Street;

• Wilga Street;

• Rothery Street;

• Cross Street;

• Northern Distributor;

• Pioneer Road;

• Cawley Street;

• Kells Crescent;

• Sellers Crescent; and

TABLE E2.5 LOCATION OF BLOCKED STRUCTURES

DESIGN RUN No. 7


Whartons Creek

Main Arm

• Inlet of flood retarding basin in Gordon Hutton Park;

• Princes Highway;

• Illawarra Railway Line;

• Farrell Road; and




Collins Creek Main Arm

• Red Ash Drive


• Liddle Street;

• Northern Distributor Extension; and

• Kulgoa Road.




TABLE E2.5 (Cont’d) LOCATION OF BLOCKED STRUCTURES

DESIGN RUN No. 7


Collins Creek

Hollymount Creek • Outlet of flood retarding basin in Edgewood Estate.

Tributary 1 • High Street.

Tributary 2 • Inlet of piped system running beneath Woonona Shops;

and

• Nicholson Road


Tributary 4 • Northern Distributor Extension

Tributary 5 • Waterloo Street;

• Carrington Street; and

• Entrance road into Ocean Park.


Main Arm



• Pioneer Road; and

• Gladestone Street

Tributary 1 • Princes Highway

Tributary 2 • Princes Highway

Farrahars Creek • Princes Highway; and

• Inlet of structure north of Doris Avenue.


Tributary 3

• Outlet structure of flood retarding basin north of Duke Street;

• Princes Highway; and

• Northern Distributor Extension.

Tributary 4 • Inlet of structure west of Greta Street; and

• Northern Distributor Extension.

Tributary 5 • Inlet of structure west of Albert Street; and

• Hale Street.

Tributary 6 • Northern Distributor Extension.

Bellambi Lake Main Arm



• Park Road; and

• Outlet of flood retarding basin in Corrimal High School.




E3. HYDRAULIC MODEL RESULTS E3.1 Presentation of Results Figures E3.1 to E3.24 are a series of plans showing the characteristics of flooding on the floodplains of the various study creeks for the full range of recurrence interval events. Figures E3.25 to E3.43 show the design water surface profiles for the full range of recurrence interval events along both the main and tributary arms of the study creeks. Note that these figures show the upper envelope of flooding, which includes both blockage and storm tide conditions. Note also that for the Whartons Creek catchment, the envelope of flooding included two sets of Design Run No. 1 model runs, whereby the primary spillway of the flood retarding basin in Gordon Hutton Park was artificially roughened to reflect the possible partial blockage of the many fences which cross it. As is described in Section E4.3, the potential for these fences to experience a partial blockage during a major flood event has a significant impact on flood behaviour in the Whartons Creek catchment. Figures E3.44 to E3.47 are design discharge hydrographs at the outlet of each creek system for the critical 120 minute design storm event as modelled in TUFLOW. The discharge and water level characteristics of the five major flood retarding basins which have been constructed in the study area are summarised in Table 5.2 of the Main Report. E3.2 Whartons Creek Catchment Main Arm Flood flows on the main arm of Whartons Creek upstream of Gordon Hutton Park are confined to the inbank area of the watercourse for events up to 100 year ARI. It is only during larger events that floodwaters surcharge the right bank of the creek and flow in an easterly direction along Corrie Road and Athol Street where they discharge to the flood retarding basin in Gordon Hutton Park as overland flow. The primary spillway in the Gordon Hutton Park flood retarding basin commences to operate for floods of 20 year ARI and larger. For more frequent events, floodwaters enter the basin via the entrance spillway and drain slowly via the 450 mm diameter low flow outlet pipe following the passage of the flood wave. Several rows of fences were observed to cross the primary spillway of the basin where it runs through the back yards of several residential properties. Results of a sensitivity study carried out on the impact a partial blockage of these fences will have on flood behaviour in the study area is presented in Section E4.3. Floodwaters surcharge the culverts under Dumbrell Road and Hospital Roads for storms as frequent as 5 year ARI. Floodwaters which surcharge the road culverts follow the prevailing grade in the local roads (i.e. they flow in an easterly direction) where they discharge onto the Princes Highway. Floodwaters then turn and flow in a north to north-easterly direction where they converge on the existing four span bridge at the Illawarra Railway Line.




Floodwaters surcharging the secondary spillway of the flood retarding basin in Gordon Hutton Park combine with local runoff ponding in the depressed tennis court area of the Magnolia Green development. Floodwaters thence flow onto the Princes Highway immediately south of the sag which is located north of Hopetoun Street. Floodwaters discharging to the sag in the highway spilt and flow in either a northerly direction along the highway, or alternatively, flow in an easterly direction through a caryard, before entering a large unit block development which is located to the south of a sag in Farrell Road. Floodwaters which surcharge the sag in Farrell Road are conveyed beneath the Northern Distributor Extension via 3 off single span single bridges which have been constructed in series along the line of the watercourse. Floodwaters which discharge from beneath the northern bridge structure join flows on the main arm of Whartons Creek at the inlet of the four span bridge at the Illawarra Railway Line. Downstream of the railway corridor, floodwaters surcharge the bank of culverts which are located beneath Franklin Road for storms as frequent as 5 year ARI. Floodwaters which escape the channel at this location traverse the relatively flat left overbank area at relatively shallow depths, inundating the grounds of the Waniora Primary and Bulli High schools. Residential homes located in Benelong Street, as well as those located between Ursula Road and Colemans Lane, are also impacted by floods of 5 year ARI and greater. Floodwaters which traverse the relatively flat left overbank of the creek pond upstream of Trinity Row where it runs between Bulli Beach and Sandon Point Beach. Depths of ponding immediately to the west (upstream) of Trinity Row reach around 700 mm in Jardine Street and Godolphin Street, and up to 1 m near the eastern end of Ursula Road, during in a 20 year ARI event. Floodwaters ponding along the western (upstream) side of Trinity Row either flow in a southerly direction where they join flood flows discharging to the ocean outlet on the main arm of Whartons Creek, or alternatively, flow across Trinity Row near its intersection with Ursula Road and discharge onto Sandon Point Beach immediately to the south of the Slacky Creek ocean outlet. Tributary 1 Flooding along Tributary 1 is confined mainly to the inbank area of the watercourse over the full range of recurrence interval events, with the exception of property which is located on the right bank of the watercourse, immediately upstream of its confluence with the main arm. Property in this area is inundated by a PMF event. Floodwaters which surcharge Tributary 2 north of Organs Road for storms as frequent as 5 year ARI join flood flows in Tributary 1 upstream of its confluence with the main arm. Tributary 2 Floodwaters are confined mainly to the inbank area of the watercourse in the heavily wooded area which lies to the east (downstream) of Hospital Road. The results along this reach of Tributary 2 should be treated with caution as it is believed the ALS data is not necessarily accurate given the density of the vegetation in this area.




Floodwaters surcharge the right bank of the watercourse north (upstream) of the residential homes which are located along the northern side of Organs Road for storms as frequent as 5 year ARI. Floodwater which surcharges the watercourse at this location bypasses the inlet of the twin 1200 mm diameter pipes which convey flows through the residential properties and flows in a southerly direction across Organs Road, where it joins flow in Tributary 1 north of Bulli Hospital. E3.3 Collins Creek Catchment Main Arm Floodwaters on the main arm of Collins Creek are confined to the inbank area of the engineered section of channel where it runs through the Edgewood Estate development over the full range of recurrence interval storms. Floodwaters surcharge the Princess Highway culverts during storms larger than 5 year ARI as a result of tailwater influences. Depths of flow over the road will exceed 1 m at the location of the sag in the highway if a blockage of the culverts were to occur during in a 100 year ARI event. Nicholson Road operates as a floodway during storms as frequent as 5 year ARI. The relatively flat grassed right overbank area opposite Nicholson Road is also inundated for an event of this magnitude. Properties located at the northern end of Austin Street are affected by floodwaters which backwater behind a reinforced concrete wall which has been constructed along the southern side of the Woonona High School carpark. Properties in this area are also affected by floodwaters which overtop a low earth embankment which extends west of Austin Street toward Liddle Street. Immediately to the east (downstream) of the Northern Distributor Extension, floodwaters surcharge the left bank of the main arm and flow in a north-easterly direction towards Tributary 4 for events of 20 year ARI and greater. Several houses located immediately downstream of the Northern Distributor Extension on the right overbank of the watercourse are also affected by floodwaters for events as frequent as 5 year ARI. Floodwaters on the main arm are joined by flood flows which originate from Tributary 5 on the Bellambi Gully system as a result of a breakout which occurs along the western (upstream) side of the Illawarra Railway Line for events of 20 year ARI and greater. Further details of the breakout are given in Section E3.4. Several residential properties are shown to be affected by floodwaters on both the upstream and downstream sides of the railway corridor during a 100 year ARI flood. Further downstream, floodwaters are mainly confined to the inbank area of the main arm for events up to 100 year ARI, although a large parcel of undeveloped land is inundated near the confluence with Tributary 4. Several properties which lie between Carrington Street and the railway corridor are also affected by a flood of this magnitude.




Hollymount Creek The spillway of the basin was found to operate during a 100 year ARI event under ideal flow conditions. A peak flow of approximately 2.9 m3/s was found to surcharge the basin via its spillway during the critical 120 minute duration storm. The volume of temporary flood storage in the retarding basin at the level of the spillway is 11,900 m3, which represents only 11 per cent of the total volume of flow discharging to the basin for an event of this frequency and duration. The volume of temporary flood storage in the basin is equivalent to only the first 17 mm of rainfall excess. Floodwater which surcharge the spillway of the basin do not re-enter the inbank area of Hollymount Creek, but rather flow in a north-easterly direction across Hollymount View at relatively shallow depths where it discharges to the water quality pond which is located to the east of Cedar Terrace. Tributary 1 Floodwaters surcharge the left bank of Tributary 1 and flow across the relatively flat left overbank area north of Fretus Avenue during storms as frequent as 5 year ARI. Fretus Avenue commences to function as an overland flowpath for events larger than 10 year ARI, as floodwaters which surcharge the right bank of the watercourse flow in an easterly direction along the road. The twin cell 2150 mm wide by 1500 mm high RCBC’s crossing High Street are surcharged by a 5 year ARI event, with flow across the road reaching a depth of 0.5 m in a 100 year ARI event. Tributary 2 Residential property which is located along Mountain Avenue is subject to flooding when the banks of the watercourse are surcharged during a 5 year ARI event. A breakout from Tributary 2 occurs immediately downstream of Mountain Avenue for events larger than 10 year ARI. Floodwaters which surcharge the right bank of the watercourse at this location flow across Popes Road in a southerly direction where they combined with flow discharging to the inlet of a 600 mm diameter pipe which is located on the western side of the Princes Highway opposite the Woonona Bulli RSL Club. The inlet of the 600 mm diameter pipe is surcharged by storms as frequent as 5 year ARI event, with floodwaters found to inundate the adjacent sag in the highway. A number of properties which are located downstream of Mountain Avenue are affected by floodwaters, including the Woonona Shops. Floodwaters which flow onto Ball Street from the adjacent sealed carpark do not rejoin flow in the main channel until they reach a location south (downstream) of Campbell Street.




Tributary 3 The relatively steep section of pipe drainage system which controls runoff converging on the sag in Stephen Drive is surcharged for storms as frequent as 5 year ARI. Floodwaters which surcharge the inlet pit system in the street flow in an easterly direction through several residential properties before discharging onto Mountain Avenue. Floodwaters which discharge onto Mountain Avenue cross the road reserve and enter residential property located to the east. A 600 mm diameter pipe which controls flow in a short reach of channel is located in the rear of several properties which front onto Lang Street. The inlet of this pipe is surcharged by storms as frequent as 5 year ARI. Floodwaters pond to depths exceeding 1 m in a natural depression which is located in the rear of several residential properties that lie along the northern side of Lang Street for storms as frequent as 5 year ARI. Floodwaters which surcharge the natural depression flow onto Lang Street and follow the prevailing grade in the road in an easterly direction. Floodwaters traversing the roadway are joined by floodwaters which surcharge the left bank of Tributary 1 east of Woonona Parade. Tributary 4 Several residential unit developments located along the northern side of Campbell Street are affected by flooding during a 5 year ARI event. Floodwaters which surcharge the pipe drainage system to the west (upstream) of the unit developments discharge onto Campbell Street before entering several residential properties which are located on the southern (downstream) side of the road reserve. Depths of inundation exceeding 700 mm occur in several residential properties which are located between Campbell Street and the Northern Distributor Extension for storms greater than 5 year ARI. Downstream of the Northern Distributor Extension, floodwaters are confined mainly to the inbank area of the channel, although several residential properties which are located on the eastern side of Thompson Street will be directly impacted upon by floodwaters discharging from beneath the adjacent Northern Distributor Extension on-ramp. Tributary 5 Several properties are affected by stormwater which surcharges the pipe drainage system at the eastern end of Russell Street. Runoff from the residential properties which lie to the west of the Northern Distributor Extension ponds to depths exceeding 1 m at the inlet of the 2400 mm wide by 900 mm high RCBC which crosses the road corridor north of Russell Street for storms as frequent as 5 year ARI.

A large ponding area also forms upstream of the 1500 mm diameter pipe which crosses the Illawarra Railway Line west of Thompson Street during storms of 20 year ARI and larger.

Several properties which are located on both the left and right overbanks of the watercourse downstream of the railway corridor are inundated by floodwaters. Flooding in these properties occurs for events as frequent as 5 year ARI.




Both Lawrence Street and Carrington Street are inundated by floodwaters during relatively frequent events. For example, floodwaters pond to depths of approximately 700 mm along the western side of Carrington Drive during a 5 year ARI event. Flooding of the unit development which is located in the south-west corner of the road intersection occurs during storms larger than 10 year ARI.

Depths of ponding in both Lawrence Street and Carrington Street exceed 1 m during a 20 year ARI event and are a result of a backwater which forms upstream along the main arm of Collins Creek.

E3.4 Bellambi Gully/Farrahars Creek Catchment

Main Arm

Floodwaters pond along the western (upstream) side of the Princes Highway before discharging across the road corridor during storms as frequent as 5 year ARI. Floodwaters which cross the road corridor at the location of the sag in the highway discharge through a number of properties before rejoining flow in the main channel opposite Arthur Street.

The rear of several residential properties which are located on the right bank of the channel north of Albert Street are inundated during storms as frequent as 5 year ARI.

Floodwaters which approach the Northern Distributor Extension are joined by those which break out of the Bellambi Lake system along the western side of the road corridor for events as frequent as 20 year ARI. Significant depths of ponding occur on the upstream side of the Northern Distributor Extension as a result of a blockage of the cross drainage system.

Significant depths of ponding also occur to the west (upstream) of Brompton Street and the Illawarra Railway Line as a result of blockage to drainage structures in this area. Residential properties in this area are inundated by floodwaters during storms as frequent as 5 year ARI. A significant number of properties which are located between the Illawarra Railway Line and Gladstone Street are also affected by relatively frequent flooding.

Downstream of Gladstone Street, floodwaters traverse the grounds of the Holy Spirit College before combining with flood flows discharging from Farrahars Creek.

Farrahars Creek

Floodwaters on Farrahars Creek are generally confined to the inbank area of the watercourse for storms of up to 10 year ARI. However, floodwaters surcharge the inlet of the existing pipe drainage systems which cross Doris Avenue, Collaery Road and Williams Crescent for events as frequent as 5 year ARI.

Floodwaters which surcharge the inlet of the pipe drainage system north (upstream) of Doris Avenue during storms of 20 year ARI and larger flow in an easterly direction along the roadway before discharging through several residential properties which are located immediately to the west of Noosa Avenue.

Floodwaters are joined by flow in Tributary 2 upstream of the Northern Distributor Extension bridge crossing. The right bank of Farrahars Creek immediately downstream of the confluence of the two streams (i.e. immediately upstream of the Northern Distributor Extension) is surcharged for events larger than 10 year ARI.




Floodwaters which surcharge the inbank area of stream at this location flow in a south-easterly direction through several residential properties before combining with flow discharging to the Northern Distributor Extension road bridge on Tributary 1. As a result of this breakout of flow, the Northern Distributor will be inundated by floodwaters during a 100 year ARI event under ideal flow conditions, or during a 20 year ARI event should a partial blockage of the road bridge occur. Downstream of the Northern Distributor Extension, floodwaters on Farrahars Creek are joined by flow on Tributary 1. Floodwaters flow in a south-easterly direction across land which is largely undeveloped before discharging beneath the Illawarra Railway Line. Floodwaters are largely confined to the section of engineered channel which runs east from the Illawarra Railway Line over the full range of recurrence interval events, with the exception of several parcels of land which lie to the south of the watercourse which are inundated to relatively shallow depths during the PMF event. Tributary 1 Flooding of the Princes Highway on Tributary 1 occurs when the inlet of an existing 900 mm diameter pipe which is located to the west (upstream) of East Street is surcharged. Depths of ponding on the highway may not be as deep as predicted by the TUFLOW model, as no details were available on the diameter of the pipe which drains the sag in the road. Floodwaters which cross the highway as overland flow discharge through several properties which are located on the eastern (downstream) side of the road corridor. A row of unit developments which are located on the right bank of the watercourse are affected by floodwaters during events larger than 10 year ARI. Depths of inundation in the unit developments is relatively shallow and are generally not greater than 200 mm for events up to 100 year ARI. Shallow depths of flooding are also experienced in several properties which border Cawley Park on its southern side for events larger than 50 year ARI. As mentioned, floodwaters which surcharge the right bank of Farrahars Creek join flow in Tributary 1 immediately to the west (upstream) of the Northern Distributor Extension. Floodwaters which are conveyed across the road corridor join flow in Farrahars Creek on the eastern (downstream) side of the Northern Distributor Extension. Tributary 2 A number of residential properties located along West Street are shown to be affected by floodwaters which surcharge a retarding basin which is located immediately to their west for events as frequent as 5 year ARI. Flooding of the Princess Highway occurs for events as frequent as 5 year ARI, with depths of ponding exceeding 600 mm along the northbound lanes. Floodwaters which flow across the highway as overland flow inundate a commercial property which is located on the eastern (downstream) side of the road corridor. Depths of inundation in this property exceed 1 m in a 5 year ARI event.




Further downstream, residential properties located on both sides of the watercourse are inundated by floodwaters during events as frequent as 5 year ARI. John Park Reserve is also inundated to relatively shallow depths during a flood of this magnitude. Depths of inundation exceeding 600 mm will occur in the rear of several properties which are located on the left bank of the watercourse in Williams Crescent during a 100 year ARI event. Tributary 3 Several properties located to the north of Duke Street in the upper reaches of Tributary 3 are affected by floodwaters which surcharge the inlet of the 1500 mm diameter outlet pipe from Basin No. 1. Flow conveyed by the 1500 mm diameter pipe discharges to the flood retarding basin which has been recently constructed south of the Edgewood Estate development (i.e Basin No. 2). Floodwaters discharging from the flood retarding basin are controlled by the large reinforced concrete outlet structure which is located at its eastern end. The outlet structure controls the rate flow discharging from the basin for events up to the PMF. Floodwaters initially pond on the upstream side of the Princess Highway in Basin No. 3 before surcharging the inlet of the 1500 mm diameter outlet pipe during storms larger than 10 year ARI. Floodwaters which flow onto the highway at this location inundate several properties which are located on the eastern (downstream) side of the road corridor. Floodwaters flow across Hollymount Park at relatively shallow depths where it discharges into the rear of several residential properties which are located along the southern side of the park in Kathleen Crescent. Floodwaters also discharge to a depression which is located along the northern side of Hollymount Park on Tributary 4. Flows discharging from the outlet of the 1500 mm diameter pipe which drains Basin No. 3 inundate low lying property located on the right overbank of the watercourse in Kathleen Crescent. Depths of inundation in the sag in the road exceed 700 mm in a 5 year ARI event and 1 m in a 10 year ARI event. Flows which surcharge the right bank of Tributary 5 north of Alice Street join flow in the channel upstream of the Northern Distributor Extension. Floodwaters are confined mainly to the inbank area of the watercourse where it runs between the Northern Distributor Extension and the Illawarra Railway Line. Floodwaters are confined to the inbank area of the reach of engineered channel which runs along the eastern side of the Illawarra Railway Line for events up to 100 year ARI. During larger, less frequent events, floodwaters surcharge the left bank of the channel and join flow which traverses a floodway which runs in an easterly direction south of George Tate Close. [The floodway itself operates for events as frequent as 10 year ARI, when floodwaters which surcharge the left bank of Tributary 5 west of the Illawarra Railway Line, flow south along Pioneer Drive].




Tributary 4 The headwaters of Tributary 4 are located in the south-east corner of the Edgewood Estate development. The twin cell 600 by 350 box culvert system which controls flows which approach the Princes Highway corridor are surcharged for events larger than 10 year ARI. Floodwaters which surcharge the inlet to the cross drainage structure at this location combine with those which originate from Tributary 3 where they flow into Hollymount Park. Stormwater discharging from the cross drainage system flows in an easterly direction along a section of channel before ponding in the depression which is located along the northern side of Hollymount Park. Floodwaters pond to depths exceeding 1 m in several residential properties which are located along Stanhope Street as a result of a lack of capacity in the pipe drainage system in this area. Tributary 5 Flooding of the Princes Highway east of the Edgewood Estate occurs for events as frequent as 5 year ARI. Floodwaters flow through several residential properties which are located on the eastern (downstream) side of the highway, before combining with flows discharging from the 900 mm diameter pipe which drains the adjacent sag in the highway. The inlet of the pipe which controls flow in the section of channel which runs in a southerly direction west of Albert Street is surcharged by a 5 year ARI event. Floodwaters which surcharge the inlet of the pipe flow cross Albert Street in an easterly direction and thence into several residential properties which are located on the eastern (downstream) side of the road corridor. Floodwaters break out of the Tributary 5 drainage system east of Albert Street and flow into the adjacent catchment of Tributary 6 for events as frequent as 5 year ARI. Floodwaters which flow onto Albert Street also follow the prevailing grade in the road towards the south where they discharge onto Hale Street. Several residential properties which are located to the south of the intersection of Albert Street and Hale Street are inundated by shallow sheet type flow in a 5 year ARI event. Tributary 6 As described in Appendix A, Tributary 6 was only incorporated into the hydraulic model after initial runs of the TUFLOW model showed floodwaters breaking out of the Tributary 5 drainage system and flowing east into Chenhalls Street and Monie Street. E3.5 Bellambi Lake Catchment West (Upstream) of the Northern Distributor Floodwaters are confined to the channel and its immediate overbank west (upstream) of Bloomfield Avenue over the full range of recurrence interval events. The 1500 mm diameter pipe crossing Bloomfield Avenue is surcharged by a 5 year ARI event. Floodwaters which cross the road discharge through several residential allotments which are located on the eastern (downstream) side of the road reserve.




Overland flows rejoin flow in the main channel immediately to the west (upstream) of a large unit complex which is located on Wilford Street. Floodwaters are confined to the channel where they flow to the south of the large unit complex. A major breakout of flow occurs when floodwaters surcharge the inlet of the 1500 mm diameter pipe which crosses Wilford Street. Floodwaters which surcharge the inlet of the pipe are forced to pond in the sag in the road to depths approaching 1 m in a 5 year ARI event as a result of high ground which is present on the eastern (downstream) side of the road reserve. Floodwaters escape the sag in the road by flowing in a southerly direction along Wilford Street and thence into Robson Street. After entering onto Robson Street, floodwaters follow the prevailing grade in the road in an easterly direction where they then turn north and flow into Midgley Street. A number of residential properties which lie between Wilford Street and Midgley Street are inundated by floodwaters, although depths of flooding in these properties is relatively shallow. East (downstream) of Midgley Street, floodwaters inundate the relatively flat left overbank of the watercourse and inundate property as far east as the Northern Distributor. The width of overland flow east of Midgley Street exceeds 130 m. Depths of inundation exceed 700 mm in several properties in a 5 year ARI event. Floodwaters which traverse the left floodplain of the Bellambi Lake system north of Rothery Street converge at the inlet of the three cell 1200 mm wide by 900 mm high box culvert system which crosses the Northern Distributor east of Eager Street. Floods of up to 100 year ARI are conveyed across the Northern Distributor corridor at this location under ideal flow conditions. In the event that these culverts were to experience a blockage during a major flood, floodwaters will flow in a northerly direction along the western side of the road corridor where they will join flood flows ponding along the upstream side of the Northern Distributor on the main arm of the Bellambi Gully system. Floodwaters flow across the Northern Distributor at its intersection with Rothery Street during events larger than 10 year ARI. Northern Distributor to Illawarra Railway Line The main arm of the Bellambi Lake system runs along the southern fringe of the floodplain between the Northern Distributor and the Illawarra Railway Line. Floodwaters extend over a width of approximately 150 m on the left overbank in this area in a 5 year ARI event. Depths of inundation exceeding 600 mm occur in several residential properties which are located to the south of Rothery Street during an event of this magnitude. Floodwaters pond along the western (upstream) side of the Illawarra Railway Line to depths of over 1 m in a 20 year ARI event. Floodwaters surcharge the railway embankment during an event of this magnitude. A portion of the flow which surcharges the railway bridge at this location flows south along the railway corridor before turning and flowing east near Railway Street. A small percentage of this flow crosses into the Towradgi Creek catchment south of the Railway Street crossing. Illawarra Railway Line to Cawley Street




Floodwaters which surcharge the railway embankment inundate a number of residential allotments which are located along the northern and southern sides of Collins Street. Depths of inundation in these properties reach up to 0.5 m in several locations in a 20 year ARI event and 700 mm in a 100 year ARI event. Flood levels in the flood retarding basin located in the Corrimal High School grounds reach an elevation of 9.23 m AHD in a 100 year ARI event under ideal flow conditions. This elevation equates to a depth of 3.5 m on the upstream side of the twin 1800 mm diameter outlet pipes. The spillway operates for events larger than 50 year ARI, when flood levels in the basin exceed an elevation of 9.13 m AHD. The peak outflow from the basin in a 100 year ARI event is 27.6 m3/s, comprising a peak flow of 27.2 m3/s in the outlet pipes and 0.4 m3/s via the spillway. In the event that the twin 1800 mm diameter pipes were to experience a blockage, peak water levels in the basin could rise as high as 10.1 m AHD during a 100 year ARI flood (i.e. an increase of up to 600 mm when compared to ideal flow conditions). Floodwaters inundate a number of residential properties located on the right overbank of the channel which runs from the basin outlet to Cawley Street for events larger than 10 year ARI. Cawley Road to Ocean Outlet The main arm of the Bellambi Lake system is piped between Cawley Street and Bellambi Lagoon. Floodwaters which surcharge the inlet of this system generally flow in an easterly direction after crossing Cawley Street, inundating a number of residential properties which are located in Gleeson Crescent, Kells Crescent, Sellers Crescent, Turner Esplanade, Birch Crescent and Connaghan Avenue. Depths of inundation exceed 1 m at the location of sags in Kells Crescent and Birch Crescent in a 100 year ARI event.




E4. SENSITIVITY STUDIES

E4.1 General

This chapter deals with the findings of a range of studies which were carried out to test the sensitivity of the hydraulic model results to the adopted set of parameters. Also dealt with in this chapter are the results of studies which were carried out to test several assumptions which have been made in the development of the TUFLOW model, as well as the sensitivity of hydraulic model results to various start-up conditions. E4.2 Changes in Hydraulic Roughness

E4.2.1 Global Increase in Hydraulic Roughness Values Prior to the adoption of the peak water levels for the design floods, model runs were undertaken whereby the “best estimate” values of hydraulic roughness in the TUFLOW hydraulic model were globally increased by 20 per cent to test the sensitivity of hydraulic model results. Figures E4.1 to E4.3 show the impact increasing hydraulic roughness values by 20 per cent will have on peak flood levels for the 100 year ARI event of 120 minutes duration. Note Design Run No. 1 was used as the basis for assessing the sensitivity of model results to this change. A global 20 per cent increase in the “best estimate” of hydraulic roughness values results in increases in peak flood levels generally in the range 0 to 50 mm. Increases in peak flood levels of between 100-200 mm were found to occur along sections of channel which are more densely vegetated. For example, those reaches of channel which are located to the east (downstream) of the Northern Distributor and Illawarra Railway Line corridors. The increase in peak 100 year ARI flood levels does not generally translate to a significant increase in the extent of inundation in the study area, as demonstrated in Figures E4.1 to E4.3. The major exception is in the Whartons Creek catchment, where a breakout of flow occurs on the right bank of Whartons Creek in the vicinity of Corrie Road and Athol Street. Two other minor breakouts of flow are shown to occur in the Collins Creek catchment. The first is located on Tributary 2 to the west of the Woonona Shops, whilst the second is located to the east (downstream) of the Illawarra Railway Line between Park Road and Kiandra Road. Several minor breakouts of flow which were found to occur using the “best estimate” values of hydraulic roughness were removed after hydraulic roughness values were increased by 20% (e.g. along Tributary 2 of Collins Creek in the vicinity of Popes Road). The reason for this is attributed to the change in local flooding patterns which occurs as a result of the increased roughness.

E4.2.2 Decrease in Hydraulic Roughness Values in Fenced Properties The “best estimate” value of hydraulic roughness for fenced properties was reduced in isolation to assess the impact a more hydraulically efficient floodplain could have on predicted flood behaviour. Figures E4.4 to 4.6 show the impact reducing the “best estimate” of hydraulic roughness value in fenced properties from a value of 1.0 to 0.1 will have on 100 year ARI flood levels.




The impact on flood behaviour resulting from a reduction in the hydraulic roughness value is mixed, with both reductions and increases in peak flood levels shown to occur throughout the study catchments. The reduction in peak flood levels is a result of the more efficient nature of the floodplain, whilst the increase is a function of the faster moving flood wave which results in increases in peak flows in the lower reaches of the drainage system. Increases in 100 year ARI flood levels are less than 500 mm. E4.3 Blockage of Primary Spillway in Gordon Hutton Park Flood Retarding Basin Several fences were observed to cross the primary spillway of the Gordon Hutton Park flood retarding basin where it runs through the back yards of several residential homes. It was found that the partial blockage of these fences will have a significant impact on flood behaviour in the Whartons Creek catchment. Under ideal flow conditions (i.e. when flow surcharging the basin is unobstructed by the fences), approximately 16.9 m3/s surcharges the basin via the primary spillway, 2.6 m3/s via the secondary spillway, with no flow surcharging the basin via the tertiary spillway in a 100 year ARI event of 120 minutes duration. When the hydraulic roughness value representing the primary spillway where it runs through the rear of the residential properties is increased to 0.25, the peak flow surcharging the basin via the primary spillway is reduced by approximately half to 8.5 m3/s and the peak flow surcharging the basin via the secondary spillway more than doubles to 6.9 m3/s. Flow also commences to surcharge the basin via the tertiary spillway, with a peak flow of 0.8 m3/s shown to flow out of the basin via the higher level spillway. The partial blockage of the primary spillway translates to a 6 per cent reduction in the peak 100 year ARI flow discharging to the ocean (refer Figure E4.7), as more water surcharges the secondary spillway and flows overland, inundating property which lies to the east of the Princes Highway near Hopetoun Street and Farrell Road. Figure E4.8 shows the impact a partial blockage of the primary spillway will have on peak 100 year ARI flood levels downstream of the retarding basin. As expected, peak flood levels will be increased within the basin and along the overland flow path which extends downstream of the secondary spillway. Increases in peak 100 year ARI flood levels in this area are generally in the range 50-200 mm. Reductions in peak 100 year ARI flood levels of up to 100 mm will occur downstream of the primary spillway along the main arm of Whartons Creek. Reductions in peak flood levels will extend as far east as Trinity Row near Wamiora Point. E4.4 Changes to Basin Discharge Characteristics as a Result of Lead Rainfall Lead rainfall can under certain circumstances result in the filling of flood mitigation storages prior to the onset of flood producing rain. This characteristic was observed during the August 1998 storm (Rigby et al, 2003).




The storage associated with the flood retarding basin in Gordon Hutton Park is filled when floodwaters surcharge the inlet of the twin 1500 mm diameter pipes which run along its northern side and enter the basin via the entry spillway. Once having entered the basin, floodwaters can only drain from the basin via a 450 mm diameter low flow pipe (i.e. unless they pond to a depth of 2 m and discharge from the basin via its primary spillway). To test the sensitivity of downstream peak flows to a prior filling of the basin, the available storage below the level of the primary spillway (i.e. the 14,850 m3 of temporary flood storage which is present below an elevation of 28.0 m AHD), was assumed to be filled prior at the onset of flood producing rain. It was found that the peak 100 year ARI flow at the ocean outlet on Whartons Creek could increase by up to 18 per cent should runoff produced by lead rainfall result in the partial filling of the flood retarding basin prior to the onset of flood producing rain (refer Figure E4.7). Peak flood levels downstream of the flood retarding basin will be increased downstream of the flood retarding basin as a result of the loss of storage, as shown on Figure E4.8. Increases in the range 200-300 mm are shown to occur along the western (upstream) side of the Illawarra Railway Line and along the main arm of Whartons Creek downstream of Franklin Avenue. Land affected by a 100 year ARI flood event would increase as a result of a loss in flood storage. For example, the extent of land affected by a flood of this magnitude would increase in the vicinity of Dumbrell Road, Farrel Road and the Bulli Spinners site (refer Figure E4.8). E4.5 Inflows from Existing Dam in Slacky Creek Catchment Survey carried out as part of this present investigation showed that during storms which result in the surcharge of the piped outlet from the dam, flow from the storage will cross into the Whartons Creek catchment. Hydrologic modelling of the catchment which contributes flow to the dam in the Slacky Creek catchment (refer Sub-Catchment No. 40.00 on Figure C3.1) generates peak flows of between 4.7 m3/s and 10.8 m3/s for floods of between 5 and 100 year ARI, respectively. For the purpose of deriving design flood envelopes for this present study, 50 per cent of the flow discharging from the dam in the Slacky Creek catchment was assumed to contribute to flow in the Whartons Creek catchment. To assess the impact this diversion of flood flows from the Slacky Creek catchment has on peak flood levels in the study area, the TUFLOW model for Whartons Creek was run with zero flow contributing from the adjacent catchment. It was found that the transfer of up to 50 per cent of the flow which discharges from the dam results in a 10 mm increase in peak 100 year ARI flood levels downstream of the flood retarding basin in Gordon Hutton Park.




E4.6 Removal of Artificial Floodplain Storage The existing pipe drainage system which controls local runoff from the network of streets in the study area was not modelled as part of this present study. There are therefore several locations where water gets “locked up” in the two-dimensional model domain and do not ultimately drain to the model outlet. A run of the TUFLOW model was carried out to test the sensitivity of peak flows to the filling of these localised depressions prior to the introduction of the discharge hydrographs. It was found that the removal of depression storage from the two-dimensional hydraulic model domain resulted in a 6 per cent increase in peak 100 year ARI flows at the ocean outlet (refer Figure E4.7). E4.7 Deposition of Sediment Along Study Reaches In addition to the changes that have occurred in stream length as a result of development on the floodplain, significant change can occur to the inbank waterway area of the streams which drain the study catchments during major floods. Evidence suggests that a process of erosion and deposition of stream bed material occurs during periods of elevated flows in the study creeks, especially those that drain the Illawarra Escarpment. The deposition of material foreign to the streams which drain the study catchment (for example, coal wash from mine dams) has also been observed to impact flood behaviour in the study area (e.g. on the main arm of Bellambi Gully during the August 1998 storm). To reflect the loss of conveyance which can occur when material is deposited in the bed of a stream, the cross sectional area of those mildly steep reaches of creek which both lie to the west of the Princes Highway and drain the heavily wooded Illawarra Escarpment, were reduced by raising bed levels by a nominal 0.5 m. It was found that a rise in bed levels along the main arm of Whartons Creek and its tributaries west (upstream) of the Princes Highway will have a significant impact on flood behaviour in the upper reaches of the drainage system. For example, a breakout of flow will occur on the right bank of Whartons Creek in the vicinity of Corrie Road and Athol Street. Increased flood levels will also be experienced at the confluence of the Main Arm and Tributary 1. Impacts are less pronounced on the remainder of the study creeks (refer Figures E4.9 to E4.11).




E5. CLIMATE CHANGE

E5.1 General The following sections of the report provide an overview of several guidelines which have been issued by the NSW Government dealing with the flood related impacts of climate change. They also contain a discussion on the potential impacts of climate change on flood behaviour in the study area. E5.2 Sea Level Rise

E5.2.1 Background The NSW Government has adopted a Sea Level Rise Policy Statement (DECCW, 2009) to support adaptation to projected sea level rise impacts. The policy statement includes sea level rise planning benchmarks for use in assessing potential impacts of projected sea level rise in coastal areas, including flood risk and coastal hazard assessment. These benchmarks are a projected rise in sea level (relative to 1990 mean sea level) of 0.4 m by 2050 and 0.9 m by 2100 (DECCW, 2010) and are based on work carried out by the Intergovernmental Panel on Climate Change (IPCC) and the CSIRO. The NSW Government has released a guideline Flood Risk Management Guide: Incorporating Sea Level Rise Benchmarks in Flood Risk Assessments (DECCW, 2010), which is to be used as the basis for examining sea level rise in projects undertaken under the State Floodplain Management program and the Floodplain Development Manual, 2005. The information in this guide updates the sea level rise information contained in the guideline Practical Considerations of Climate Change (DECC, 2007) and the Floodplain Development Manual, 2005. In addition to the aforementioned guideline, the NSW Government has also released the NSW Coastal Planning Guideline: Adapting to Sea Level Rise (DoP, 2010). The guideline outlines an approach to assist councils, State agencies, planners and development proponents when addressing sea level rise in land-use planning and development assessment Appendix A of DECCW, 2010 provides interim advice in relation to the coincident catchment and ocean flooding conditions which should be adopted when preparing flood studies in coastal areas and is an update of the draft Floodplain Management Guideline No. 5 Ocean Boundary Conditions (DIPNR, 2004a). The NSW guidelines will be reviewed following the release of the update of Australian Rainfall and Runoff (IEAust, 1998), which is currently underway at a National level. Section E2.3 of this report contains further details on the methodology set out in Appendix A of the guide.

E5.2.2 Scope of Investigation In accordance with the requirements of Flood Risk Management Guide: Incorporating Sea Level Rise Benchmarks in Flood Risk Assessments (DECCW, 2010), the sea level rise benchmarks of 0.4 m and 0.9 m were added to the 100 year ARI ocean boundary condition level to represent coastal flooding conditions in 2050 and 2100, respectively.




Application of the sea level rise benchmarks to the 100 year ARI ocean boundary condition results in maximum inshore ocean water levels of 3.0 m AHD and 3.5 m AHD for 2050 and 2100 conditions, respectively. A recent report prepared for WCC entitled “A Climate Change Impact Assessment of Wollongong’s Estuaries – Implications for Ongoing Estuary Management Planning” (Cardno Lawson Treloar, 2009), found that provided there is sufficient supply of sediment within the beach compartment, the height of the entrance berms will increase by an amount equal to sea level rise. Consistent with these findings, the heights of the entrance berms were increased to 2.4 m AHD and 2.9 m AHD to reflect conditions in 2050 and 2100, respectively. Note that the current position of the entrance berms was maintained, as consideration of shoreline recession resulting from climate change is outside the scope of this present study. The same approach as set out in Section E2.4 for developing the present day 100 year ARI design flood envelope was adopted for developing flood envelopes representative of 2050 and 2100 flooding conditions. Note that present day design rainfall estimates were used in the development of the flood envelopes. Figures E5.1 and E5.2 show the afflux resulting from an increase in sea level and entrance berm heights of 0.4 m and 0.9 m, respectively for 100 year ARI catchment and ocean flooding conditions. Figure E5.3 contains a series of design water surface profiles showing the sensitivity of peak 100 year ARI flood levels to varying berm height and inshore ocean conditions near the catchment outlets. Note that local catchment flooding is based on Design Run No. 1 conditions. The impact of sea level rise on flood behaviour in the study area is summarised in Table 5.3 of the Main Report. E5.3 Increased Rainfall Intensities

E5.3.1 Background CSIRO prepared reports for the NSW Government on the impacts of climate change on rainfall intensities in the major river basins in the state (CSIRO, 2007). In the Sydney Metropolitan catchments, the 40 year ARI, 1 day rainfall was predicted to change by between -3 per cent to +12 per cent by 2030 and by between +1 per cent to +10 per cent by 2070. DECCW recommends that its guideline Practical Considerations of Climate Change (DECC, 2007) be used as the basis for examining climate change in projects undertaken under the State Floodplain Management program and the Floodplain Development Manual, 2005. The guideline recommends that until more work is completed in relation to the climate change impacts on rainfall intensities, sensitivity analyses should be undertaken based on increases in rainfall intensities ranging between 10 and 30 per cent. On current projections the increase in rainfalls within the service life of developments or flood management measures is likely to be around 10 per cent, with the higher value of 30 per cent representing an upper limit. Under present day climatic conditions, increasing the 100 year ARI design rainfall intensities by 10 per cent would produce about a 200 year ARI flood; and increasing those rainfalls by 30 per cent would produce about a 500 year ARI event.




E5.3.2 Scope of Investigation As previously mentioned, the investigations undertaken at the flood study stage are mainly seen as sensitivity studies pending more detailed consideration in the Floodplain Risk Management Study. For the purposes of the present investigation, the results of modelling the 200 and 500 year ARI events were adopted as being representative of increases in 100 year ARI rainfall intensities of about 10 and 30 per cent, respectively. Figures 3.4 to 3.7 in the Main Report show the discharge hydrographs at the catchment outlets for present day conditions, whilst Table E5.1 gives a comparison of peak flows at the catchment outlets.

TABLE E5.1 COMPARISON OF PEAK FLOWS AT CATCHMENT OUTLETS(1)

Study Catchment

Peak Flow at Catchment Outlet (m3/s) Increase in Peak Flow at Catchment Outlet (m3/s)(2)

100 year ARI 200 year ARI 500 year ARI 200 – 100 year ARI

500 – 100 year ARI

(1) (2) (3) (4)=(2)–(1) (5)=(3)–(1)

Whartons Creek 72 89 101 17 (23) 29 (41)

Collins Creek 132 151 176 19 (14) 44 (33)


189 230 261 41 (22) 72 (38)

Bellambi Lake 79 89 102 10 (13) 23 (29)

(1) Peak flows obtained from RAFTS model and relate to a design storm of 120 minutes duration

(2) Numbers in ( ) are percentage increase in peak flow when compared to 100 year ARI event. Figures E5.4 to E5.6 show the increased extents of inundation for the 200 and 500 year ARI flood envelopes, compared with 100 year ARI flood envelope. Figures E5.7 to E5.9 show the difference in peak flood levels between a flood of 100 and 200 year ARI, whilst Figures E5.10 to E5.12 show the difference in peak flood levels between a flood of 100 and 500 year ARI. An assessment was also carried out to demonstrate the increased frequency at which major breakouts of flow in the study area will occur as a result of an increase in rainfall intensity. The impact of increased rainfall intensity on flood behaviour in the study area is summarised in Table 5.4 of the Main Report. The findings of the investigations into how increased rainfall intensities will affect the frequency of occurrence of major breakouts in the study area are presented in Table 5.5 of the Main Report.




E6. REFERENCES

Cardno Lawson Treloar, 2009. “A Climate Change Impact Assessment of Wollongong’s Estuaries – Implications for Ongoing Estuary Management Planning” Report No. LJ2816/Rep2574/V3.

Cardno Forbes Rigby, 2006. “Bulli Spinners Subdivision DA 2005-1839 Response to Council Request for Additional Drainage Information” Letter addressed to Wollongong City Council dated 29 August 2006.

Department of Infrastructure, Planning and Natural Resources, 2004a. “Floodplain Management Guideline No. 5 Ocean Boundary Conditions”. Status: Draft.

Department of Infrastructure, Planning and Natural Resources, 2004b. “Riparian Corridor Management Study”

Department of Environment & Climate Change, 2007. “Floodplain Risk Management Guideline – Practical Consideration of Climate Change”

Department of Environment, & Climate Change, 2008. “Bellambi Lagoon Entrance Channel Northern Dune Erosion – Preliminary Technical Study”

Department of Environment, & Climate Change, 2009. “Bellambi Lagoon Entrance Channel Northern Dune Erosion – Preliminary Technical Study”

Department of Environment, Climate Change and Water, 2009. “Technical Note: Derivation of the NSW Government’s Sea level Rise Planning Benchmarks”

Department of Environment, Climate Change and Water, 2009. “NSW Sea Level Rise Policy Statement”

Department of Environment, Climate Change and Water, 2010. “Flood Risk Management Guideline – Incorporating Sea Level Rise in Flood Risk Assessments”

Department of Planning, 2010. “NSW Coastal Planning Guideline: Adapting to Sea Level Rise”

The Institution of Engineers Australia (Reprinted Edition 1998). “Australian Rainfall and Runoff: a guide to flood estimation”

Gordon, A.D. (1981). “The Behaviour of Lagoon Inlets”. Proc. 5th Aust. Conf. On Coastal and Ocean Eng, Perth.

Hanslow. D.J, Davis. G.A, You. B.Z, Zastawny.J, 2000. “Berm Height at Coastal Lagoon Entrances in NSW” 10th NSW Coastal Conference.

Lyall & Associates Consulting Water Engineers, 2002. “Dee Why Lagoon and Curl Curl lagoon Flood Studies”

New South Wales Government, 2005. “Floodplain Development Manual the management of flood liable land”

Rigby et al (2003). “Storms, Storm Bursts and Flood Estimation A Need for Review of the AR&R Procedures”. Paper presented at the 28th International Hydrology and Water Resources Symposium, Wollongong, NSW.

Wollongong City Council (2009) “Wollongong Development Control Plan 2009” Chapter E14: Stormwater Management.

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