LOCATION ON INVESTIGATED...John Shore Park A Maroubra Bay Primary Broadarrow Reserve MAROUBRA Maroubra Bay D U N C A N O S T M O NS A V FITZ G E R AL D AV B O N D S T M A R O B R A

Maroubra Bay Floodplain Risk Management Study and Plan

WMAwater: 113044:Maroubra_FRMSP: 7 December 2016 D1

APPENDIX D MITIGATION OPTIONS MAPPING

FIGURES

Figure D1: Location of Investigated Mitigation Options

Figure D2: Peak Flood Level Impact – Option B - 1% AEP Event

Figure D3: Peak Flood Level Impact – Option C - 1% AEP Event

Figure D4: Peak Flood Level Impact – Option F - 1% AEP Event

Figure D5: Peak Flood Level Impact – Options G & J – 1% AEP Event

Figure D6: Peak Flood Level Impact – Option I – 1% AEP Event

Figure D7: Peak Flood Level Impact – Options L, M & K – 1% AEP Event

Figure D8: Peak Flood Level Impact – Option N – 1% AEP Event

Figure D9: Peak Flood Level Impact – Option P – 1% AEP Event

Figure D10: Peak Flood Level Impact – Option D – 1% AEP Event

ArthurByrne

Reserve

JohnShorePark

Maroubra BayPrimary

Broadarrow Reserve

MAROUBRA

Maroubra Bay

RifleRange

F

G

I

J

K

L M

N

P

B

C D

H

A

MAROUBRA RD

ANZACPDEANZACPDE

BERN

IE KE

LLY D

R

THE CAUSEWAYNICOL AV

BYNG ST

ANZAC PDE

SCOTT ST

KYOG

LE ST

MARINEPDE

DUNC

ANST MA

LABA

RRD

TYRWHITT ST

SACKVILLE ST

WISEST

ANZA

CPD

E

BEATTY ST

MONS AVMALABAR

RD

FRENCH ST

FORD RD

WADE ST

MURRAY ST

MARINE PDE

FITZGERALD AV

ASTORIACCT

DUNCAN ST

HAIG ST

CHAPMAN AV

NEW ORLEANS CR

DUNCAN L

BROOME ST

THOMPSON AV

BENT L

MALA

BAR

RD

MONS AV

BROWNRD

MCKEON ST

RODMAN AV

LITTLE LFLOW

ER ST

FITZGERALD AV

RUSSELLCT

MORRIS PL

MARINE PDE

CHICAGO AV

BENNETT PL

FENTON AV

MAROUBRA RD

YORKTOWN PDE

CHES

TER A

V

MALABAR RD

HEREWARD ST

BEAUCHAMP RD

DAN AV

ROSSITER L

COLLIER PL

OSPREY CT

SCOTT L

COOPER L

BOND ST

BYNG L

BOUGAINVILLE CT

WHITE A

V

SEVERN ST

NEOSHO WY

DUNCAN ST

PERKINS WY

FITZG

ERAL

D L

CURT

IN C

R

ANZAC PDEBE

LLEVU

E ST

HARGRAVES PL

MARINEPDE

DEWEY CT

PETERS PL

MCIVER PL

FLOWER L

BONA

VIST

A AV

MINNEAPOLIS CR

BOYCE L

LITTL

E ST

LEXINGTON PL

GLENUGIE ST

BYRNE CR

MEAGHER AV

BEAU

MOND AV

SIMS L

SIMS GR

PORTLAND CR

ASTORIA CCT

BEATTY L

WRIDE ST

KITCHENER ST

MALA

BAR R

D

MIDWAY DR

MARINE PDE

FIGURE D1LOCATION ON INVESTIGATED

MITIGATION OPTIONS

Existing PitsExisting PipesHotspotCatchment Area

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E

O

0 200 400 600100Meters

ArthurByrne

Reserve

JohnShorePark

Maroubra BayPrimary

Broadarrow Reserve

MAROUBRA

Maroubra Bay

DUNC

AN ST

MONS AV

FITZGERALD AV

BOND ST

MAROUBRA RD

FRENCH ST

MARINE

PDE

WRIDE ST

MALA

BAR

RD

YORKTOWN PDE

SACKVILLE ST

THE CORSO

KYOGLE ST

HAIG ST

FLOW

ER ST

BOYCE RD

NEW

ORLE

ANS C

R

FIRST AV

SIMS L

BROO

ME ST

MCKEON ST

LITTL

E ST

NICOL

L

SCOTT ST

SECOND AV

SIMS GR

PORT

LAND

CR

BONA V

ISTA A

V

THE C

AUSE

WAY

FENTON AV

BELL

ST

FITZG

ERAL

D L

CAMI

RA ST

ROSSITER L

BOYCE L

HEREWARD ST

BRIDGES ST

MINN

EAPO

LIS CR

BERN

IE KE

LLY D

R

CHAPMAN AV

BELLE

VUE S

T

FLOWER L

BEAU

MOND AV

GLENUGIE ST

GALVIN ST

LEXINGTON PL

SEVERN ST

NEOSHO WY

NICOL AV

LITTLE L

BENT L

COLLIER PL

MORRIS PL

SCOTT L

MAXW

ELL A

V

RUSSELL CT

MULC

RAY A

V

DUNCAN L

MAROUBRA RD

MONS AV

DUNCAN ST

0 0.250.125Km

FIGURE D2PEAK FLOOD LEVEL IMPACT

OPTION B1 % AEP EVENT

Pits/ NodesPipesHotspot

Peak Flood Level Impact (m)No Longer Flooded< -0.5-0.5 - -0.25-0.25 - -0.1-0.1 - -0.01-0.01 - 0.010.01 - 0.10.1 - 0.25> 0.25Newly Flooded

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Hotspot B:Flood retarding basin in Muraborah Reserve (Wride Street)

B

ArthurByrne

Reserve

JohnShorePark

Maroubra BayPrimary

Broadarrow Reserve

MAROUBRA

Maroubra Bay

DUNC

AN ST

MONS AV

FITZGERALD AV

BOND ST

MAROUBRA RD

FRENCH ST

MARINE

PDE

WRIDE ST

MALA

BAR

RD

YORKTOWN PDE

SACKVILLE ST

THE CORSO

KYOGLE ST

HAIG ST

FLOW

ER ST

BOYCE RD

NEW

ORLE

ANS C

R

FIRST AV

SIMS L

BROO

ME ST

MCKEON ST

LITTL

E ST

NICOL

L

SCOTT ST

SECOND AV

SIMS GR

PORT

LAND

CR

BONA V

ISTA A

V

THE C

AUSE

WAY

FENTON AV

BELL

ST

FITZG

ERAL

D L

CAMI

RA ST

ROSSITER L

BOYCE L

HEREWARD ST

BRIDGES ST

MINN

EAPO

LIS CR

BERN

IE KE

LLY D

R

CHAPMAN AV

BELLE

VUE S

T

FLOWER L

BEAU

MOND AV

GLENUGIE ST

GALVIN ST

LEXINGTON PL

SEVERN ST

NEOSHO WY

NICOL AV

LITTLE L

BENT L

COLLIER PL

MORRIS PL

SCOTT L

MAXW

ELL A

V

RUSSELL CT

MULC

RAY A

V

DUNCAN L

MAROUBRA RD

MONS AV

DUNCAN ST

0 0.250.125Km


OPTION C1 % AEP EVENT

Pits/ NodesPipesHotspot


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Hotspot C:Flood retarding basin in John Shore Park

C

Coral SeaPark

AB

Re

JohnShorePark

Broadarrow Reserve

MAROUBRA

FITZGERALD AV

PORTLAND CR

MALABAR RD

YORKTOWN PDE

BROO

ME ST

MONS AV

NEW

ORLE

ANS C

R

MIDWAY DRMI

NNEA

POLIS

CR

KYOGLE ST

SIMS L

SCOTT ST

SIMS GR

TYRWHITT ST

CURTIN CR

CHESTER AV

CHICAGO AV

FITZG

ERAL

D L

CAMI

RA ST

ROSSITER L

BRIDGES ST

BERN

IE KE

LLY D

R

SCOTT L

DEWEY CT

GLENUGIE ST

LEXINGTON PL

MEAGHER AV

BYRNE CR

PERKINS WY

NEOSHO WY

COLLIER PL

OSPREY CT

MORRIS PL RUSSELL CT

MONS AV

CHESTER AV

0 0.250.125Km


OPTION F1 % AEP EVENT

Pits/ NodesPipesOption F PitsOption F PipesHotspot


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Hotspot F:Added 3 new 1.2m pipes

F

JohnShorePark

Maroubra BayPrimary

Broadarrow Reserve

MAROUBRA1.2

1.2

1.2

1.2 1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.21.2

1.2

1.2

1.2

1.2

1.2

1.2

MONS AV

DUNCAN ST

FITZGERALD AV

FRENCH ST

MARINE PDE

MAROUBRA RD

WRIDE ST

YORKTOWN PDE

KYOGLE STMA

LABA

R RD

BOND ST

HAIG ST

MCKEON STLIT

TLE S

T

FLOW

ER ST

FENTON AV

CAMI

RA ST

ROSSITER L

SIMS GR

NICOL

L

BOYCE L

HEREWARD ST

THE C

AUSE

WAY

CHAPMAN AV

BELLE

VUE S

T

FLOWER L

BONA V

ISTA A

V

NEW

ORLE

ANS C

R

FIRST AV

GLENUGIE ST

GALVIN ST

SECOND AV

SIMS L

SEVERN ST

MINN

EAPO

LIS CR

BRIDGES ST

FITZG

ERAL

D L

NICOL AV

BROO

ME ST

LITTLE L

BENT L

BELL

ST

MAXW

ELL A

V

ROSS

ITER A

V

COLLIER PL

MULC

RAY A

V

DUNCAN L

BERNIE KE

LLY DR

PORT

LAND

CRDUNCAN ST

MAROU

BRA R

D

MONS AV

0 0.250.125Km


OPTION G & J1% AEP EVENT

PitsPipesOption GJ PipesHotspot


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J

G

Hotspot GJ:Increased pipe sizes to 1.2m fromGalvin Street to Fitzgerald Avenue

Hotspot GJ:Increased number of pipes to 2 alongFitzgerald Avenue and Marine Parade

1.2

BOND ST

MARINE

PDE

MAROUBRA RD

BONA V

ISTA A

V

BEAU

MOND AV

SACKVILLE ST

SEVERN ST

HEREWARD ST

DUNCAN ST

MCKEON ST

BELLE

VUE S

T

0 0.250.125Km


OPTION I1% AEP EVENT

PitsPipesOption I PipesHotspot


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d IHotspot I:Pipe size was changed from 0.6m to 1.2m

Coral SeaPark

MAROUBRA

0.9

1.05

0.6

0.9

0.9

1.05

FITZGERALD AV

MONS AV

ANZAC PDE

YORKTOWN PDE

CHESTER AV

MINNEAPOLIS CR

MIDWAY DR

KYOGLE ST

NEW ORLEANS CR

BEATTY L

SIMS L

PORT

LAND

CR

SIMS GR

KITCHENER ST

ASTORIA CCT

BEATTY ST

WADE ST

LEXINGTON PL

PERKINS WY

NEOSHO WY

COLLIER PL

MORRIS PL

MALABAR RD

GLENUGIE ST

FRENCH ST

RUSSELL CT

CHIC

AGO

AV

DUNCAN ST

ANZAC PDE

MALABAR RD

MONS AV

0 0.250.125Km


OPTION L, M & K1% AEP EVENT

PitsPipesOption LMK PitsOption LMK PipesHotspot


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.mxd L

M

K

Hotspot LMK:Added 6 new circular pipes (sizes as labelled)

WHITE

AV

BROO

ME ST

BEAUCHAMP RD

RODMAN AV

BENNETT PL

THOMPSON AV

HARGRAVES PL

0 0.250.125Km


OPTION N1% AEP EVENT

PitsPipesOption N PipeHotspotOutside The Study Area


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N

Hotspot N:Increased pipe size from 0.6m to 0.9m

ArthurByrne

Reserve

RifleRange

2

BROO

ME ST

MALABAR RD

SCOTT ST

CURTIN CR

TYRWHITT ST

MEAGHER AV

BERNIE KELLY DR

BYRN

E CR

FORD RD

DAN AV

SCOTT L

PORT

LAND

CR

RODMAN AV

RUSSELL CT

BROWN RD

PETERS PL

FITZG

ERAL

D L

MALABAR RD

0 0.250.125Km


OPTION P1% AEP EVENT

PitsPipesOption P PipesHotspot


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Hotspot P:Increased number of existing pipes to 2along Curtin Crescent Hotspot P:

Added a new pipe from ByrneCrescent to Arthur ByrneReserve

P

JohnShorePark

Broadarrow Reserve

MarB

MONS AV

MARINE PDE

MALA

BAR

RD

DUNCAN ST

FITZGERALD AV

MCKEON ST

FENTON AV

CAMI

RA ST

ROSSITER L

HEREWARD ST

BRIDGES ST

CHAPMAN AV

BROO

ME ST

FITZG

ERAL

D L

MAROUBRA RD

SEVERN ST

BERN

IE KE

LLY DR

MAXW

ELL A

V

LITTL

E ST

ROSS

ITER A

V

YORKTOWN PDE

BONA V

ISTA A

V

GLENUGIE ST

DUNCAN ST

MONS AV

0 0.20.1Km


OPTION D1 % AEP EVENT

Option D PipesHotspotPits/ NodesPipes


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Hotspot D:Replaced existing trunk system with 1.9 m x 1.05 mpipes from Fenton Avenue to Northease of Marine ParadeReplaced trunk system with 5.6 m x 1.68 m outlet pipealong Maroubra Beach

D

WMAwater: 113044:Maroubra_FRMSP: 7 December 2016 E1







SUBMISSIONS AND RESPONSES FROM PUBLIC EXHIBITION OF DRAFT REPORTS

Comments Response

The Maroubra study and plan only mentions acid sulfate soils in the glossary. Council's own fact sheet on this issue states that acid can form and cause damage after such soils are drained, so I suggest that this risk should be considered when Council proposes additional drainage works

The acid sulfate soils issue will be added into the report and will be taken into account when conducting drainage works that require excavation.

The drains in Chapman Av Maroubra are regularly blocked by leaves from the street trees. Thought needs to be given to catching water higher up around Maroubra Bay Public School and above. The ground in John Shore reserve does not absorb water readily. The soil has been compacted by it's previous use as a carpark for The Seals Club. Remedial work should be done to aerate the soil and provide adequate drainage, including unblocking the existing drain in this reserve. Thanks

Cleaning of pits and pipes is regularly undertaken by Council in this area as it is a known hot spot for flooding. There was a recurrent issue with the Gross Pollutant Trap located at the corner of Marine Parade and McKeon Street. It was creating backwatering in the drainage network and increasing flooding issues. Work is currenltly undertaken by Council to solve this issue. Regarding John Shore Park, the comment is noted and it will be taken into account when further investigating this option.

Thank you for the opportunity to meet representatives from council on Tuesday, 18th October. Option A Lowering Promenade This could be done easily enough and if landscaped well could pass community scrutiny. To the laymen it appears you would have to remove a fair amount of soil to lower the promenade increasing the risk of ingress of sea water higher into McKeon Street. Is there opportunity to capture water at the junction of McKeon and Marine Parade and pipe it underneath the existing promenade? Option B Muraborah Reserve Basin This is acceptable option. In my opinion we could make this basin more effective in capturing and retaining water by having it deeper, fencing it and incorporating a community garden into the basin reserve. An idea like this provides amenity to the area as well assisting in flood mitigation. Option C John Shore Park Basin Again this is an acceptable part solution, although it depends upon landscaping channels and higher islands to create a park with high amenity value. The park incorporates a number of maturing and established trees that should be kept as part of the basin design. Option D Fenton Avenue Drainage Upgrade Option G&J Upgrade Drainage from Galvin Street to Beach outlet These are the most promising options for controlling storm and floodwaters. However it appears little chance of happening unless the Maroubra Beach council masterplan is adopted. My conversations with neighbours and residents discussing the extra beach outfall produces mixed results. Many agree that it would prevent property damage but are unsure of the siting and appearance of the outfall on the beach.

Comments are noted.

Parking on weekends is a problem. Could the Park between Mons Avenue and Fitzgerald Avenue be used as a carpark on Sundays? Also why not use the park at South Maroubra for the Kingsford Markets now that they can't be held at Kingsford anymore?

Both of these suggestions will be forwarded to the Open Space Assets team for consideration and response.

If John Shore Park is used as a retention basin it is important that residents can still access it. Basin will have to be landscaped. If the lowering of the promenade is investigated in more details, the impact of king tides needs to be taken into account.

During detail investigations both these points will be taken into account. Residents will be consulted regarding these projects.

There is always rubbish and gravel in the driveway at 1 Fenton Avenue (access to the Maroubra SEALS). These debris are increasing the risk of blockage in the surrounding pits and pipes.

Council has opened discussions with the Maroubra SEALS club regarding the condition of the driveway.


WMAwater: 113044:Maroubra_FRMSP: 7 December 2016 F1

SITE SPECIFIC INFORMATION FOR RESIDENTIAL DAMAGE CURVE DEVELOPMENT

Version 3.01 June 2011 plus 2014 LW Edits Queries to [email protected]

PROJECT DETAILS DATE JOB No.

Maroubra Residential Only 12/05/2014 113048

BUILDINGS

Regional Cost Variation Factor 1.00 From Rawlinsons

Post late 2001 adjustments 1.50 Changes in AWE see AWE Stats Worksheet

Post Flood Inflation Factor 1.00 1.0 to 1.5

Multiply overall structural costs by this factor

Judgement to be used. Some suggestions

below

Regional City Regional Town

Houses Affected Factor Houses Affected Factor

Small scale impact < 50 1.00 < 10 1.00

Medium scale impacts in Regional City 100 1.20 30 1.30

Large scale impacts in Regional City > 150 1.40 > 50 1.50

Typical Duration of Immersion 1.00 hours

Building Damage Repair Limitation Factor 0.85 due to no insurance short duration

long

duration

Suggested range 0.85 to 1.00

Typical House Size 240 m^2 240 m^2 is Base

Building Size Adjustment 1.0

Total Building Adjustment Factor 1.28

CONTENTS

Average Contents Relevant to Site $

60,000

Base for 240 m^2 house $

60,000

Post late 2001 adjustments 1.50 From above

Contents Damage Repair Limitation Factor 0.75 due to no insurance short duration

long

duration

Sub-Total Adjustment Factor 1.13 Suggested range 0.75 to 0.90

Level of Flood Awareness LOW low or high only. Low default unless otherwise justifiable.

Effective Warning Time 0 hour

Interpolated DRF adjustment (Awareness/Time) 1.00 IDRF = Interpolated Damage Reduction Factor

Typical Table/Bench Height (TTBH) 0.90 0.9m is typical height. If typical is 2 storey house use 2.6m.

Total Contents Adjustment Factor AFD <= TTBH 1.13 AFD = Above Floor Depth

Total Contents Adjustment Factor AFD > TTBH 1.13

Most recent advice from Victorian Rapid Assessment Method

Low level of awareness is expected norm (long term average) any deviation needs to be justified.

Basic contents damages are based upon a DRF of 0.9

Effective Warning time (hours) 0 3 6 12 24

RAM Average IDRF Inexperienced (Low awareness) 0.90 0.80 0.80 0.80 0.70

DRF (ARF/0.9) 1.00 0.89 0.89 0.89 0.78

RAM AIDF Experienced (High awareness) 0.80 0.80 0.60 0.40 0.40

DRF (ARF/0.9) 0.89 0.89 0.67 0.44 0.44

Site Specific DRF (DRF/0.9) for Awareness level for iteration 1.00 0.89 0.89 0.89 0.78

Effective Warning time (hours) 0 3 0

Site Specific iterations 1.00 0.89 1.00

ADDITIONAL FACTORS




10050

External Damage $ 6,700 $6,700 recommended without justification

Clean Up Costs $ 4,000 $4,000 recommended without justification

Likely Time in Alternate Accommodation 3 weeks

Additional accommodation costs /Loss of Rent $ 220

$220 per week recommended without

justification

TWO STOREY HOUSE BUILDING & CONTENTS FACTORS

Up to Second Floor Level, less than 2.6 m 70% Single Storey Slab on Ground

From Second Storey up, greater than 2.6 m 115% Single Storey Slab on Ground

Base Curves AFD = Above Floor Depth

Single Storey Slab/Low Set 13164 + 4871 x AFD in metres

Structure with GST AFD greater

than 0.0 m

Validity Limits AFD less than or equal to 6 m

Single Storey High Set 16586 + 7454 x AFD


than -0.100 m


Contents 20000 + 20000 x AFD

Contents with GST AFD greater than

0

Validity Limits AFD less than or equal to 2

SITE SPECIFIC INFORMATION FOR RESIDENTIAL DAMAGE CURVE DEVELOPMENT

Version 3.01 June 2011 plus 2014 LW Edits Queries to [email protected]

PROJECT DETAILS DATE JOB No.

Maroubra Commercial\Industrial

Only (uses same curve as Residential)

12/5/2014 113048

BUILDINGS

Regional Cost Variation Factor 1.00 From Rawlinsons

Post late 2001 adjustments 1.50 Changes in AWE see AWE Stats Worksheet

Post Flood Inflation Factor 1.00 1.0 to 1.5

Multiply overall structural costs by this factor

Judgement to be used. Some suggestions below

Regional

City Regional Town

Houses Affected Factor Houses Affected Factor

Small scale impact < 50 1.00 < 10 1.00

Medium scale impacts in Regional City 100 1.20 30 1.30

Large scale impacts in Regional City > 150 1.40 > 50 1.50

Typical Duration of Immersion

hours

Building Damage Repair Limitation Factor 0.85 due to no insurance short

duration

long duration

Suggested range 0.85 to 1.00

Typical House Size 500 m^2 240 m^2 is Base

Building Size Adjustment 2.1

Total Building Adjustment Factor 2.66

CONTENTS

Average Contents Relevant to Site $

125,000

Base for 240 m^2

house

$

60,000 $ 187,500.00




Contents Damage Repair Limitation Factor 0.75 due to no insurance short

duration

long duration

Sub-Total Adjustment Factor 1.13 Suggested range 0.75 to 0.90

Level of Flood Awareness LOW low or high only. Low default unless otherwise justifiable.

Effective Warning Time 0 hour

Interpolated DRF adjustment (Awareness/Time) 1.00 IDRF = Interpolated Damage Reduction Factor

Typical Table/Bench Height (TTBH) 0.90 0.9m is typical height. If typical is 2 storey house use 2.6m.

Total Contents Adjustment Factor AFD <= TTBH 1.13 AFD = Above Floor Depth

Total Contents Adjustment Factor AFD > TTBH 1.13

Most recent advice from Victorian Rapid Assessment Method

Low level of awareness is expected norm (long term average) any deviation needs to be justified.

Basic contents damages are based upon a DRF of 0.9

Effective Warning time (hours) 0 3 6 12 24

RAM Average IDRF Inexperienced (Low awareness) 0.90 0.80 0.80 0.80 0.70

DRF (ARF/0.9) 1.00 0.89 0.89 0.89 0.78

RAM AIDF Experienced (High awareness) 0.80 0.80 0.60 0.40 0.40

DRF (ARF/0.9) 0.89 0.89 0.67 0.44 0.44

Site Specific DRF (DRF/0.9) for Awareness level for iteration 1.00 0.89 0.89 0.89 0.78

Effective Warning time (hours) 0 3 0

Site Specific iterations 1.00 0.89 1.00

ADDITIONAL FACTORS


External Damage $ 6,700 $6,700 recommended without justification

Clean Up Costs $ 9,000 $4,000 recommended without justification

Likely Time in Alternate Accommodation 3 weeks

Additional accommodation costs /Loss of Rent $ 220

$220 per week recommended without justification (for residential)

TWO STOREY HOUSE BUILDING & CONTENTS FACTORS

Up to Second Floor Level, less than 2.6 m 70% Single Storey Slab on Ground

From Second Storey up, greater than 2.6 m 115% Single Storey Slab on Ground

Base Curves AFD = Above Floor Depth

Single Storey Slab/Low Set 13164 + 4871 x AFD in metres


than 0.0 m


Single Storey High Set 16586 + 7454 x AFD


than -0.100 m


Contents 20000 + 20000 x AFD

Contents with GST AFD greater than

0

Validity Limits AFD less than or equal to 2



Diagram F1: Residential Flood Damages Curves for Maroubra Bay Catchment

Diagram F2: Commercial / Industrial Residential Flood Damages Curves for Maroubra Bay

Catchment

WMAwater Pty Ltd (Formerly Webb McKeown and Associates) ABN 14 600 315 053

DIRECTORS ASSOCIATES Level 2, 160 Clarence St, SYDNEY NSW 2000

M K Babister BE(Hons), MEngSc GradDipMgt, FIEAust R Hardwick Jones BE(Hons), MEngSc, MIEAust Phone: 02 9299 2855 Fax: 02 9262 6208 R W Dewar BSc(Hons), MEngSc, MAIG, MIEAust M E Retallick BE(Hons), BSc, MIEAust Email: [email protected] E J Askew BE(Hons), MIEAust Website: wmawater.com.au S D Gray BE, MEng

Randwick City Council

j:\jobs\113044\admin\byrnecrescent\141218_byrnecres_floodasessment_issued.docx

30 Frances Street Randwick NSW 2031

18 December 2014

Attention: Parissa Ghanem Drainage Engineer

Dear Parissa,

Re: Byrne Crescent Investigation for Maroubra Bay FRMS&P

1 INTRODUCTION

This flood impact assessment has been prepared in relation to the localised flooding of Byrne

Crescent, Maroubra. WMAwater have developed a TUFLOW hydraulic model of the catchment

for the Flood Study and FRMS&P for the Maroubra Bay area. This model has been modified to

investigate four options in an attempt to reduce the flood impacts in Byrne Crescent. It should

be noted that the hydraulic modelling system was established to cater for large design events

and thus has limitations in its use for small design events.

2 BACKGROUND

No. 11 Byrne Crescent is subject to above floor inundation on a frequent basis (possibly as

much as three times a year from discussions with the resident). A site visit was undertaken in

September 2014 during which WMAwater spoke with local residents. The resident has sand

bags ready to divert flow and opens doors to allow the free flow of water through the house.

This causes significant disruption, inconvenience and some internal house and yard damages.

The front door of the house is at ground level and runoff which overtops the kerb will largely flow

directly to the front door.

A CDS GPT unit was installed in 2005 in the cul-de-sac adjacent to No. 11 and residents report

that runoff emanates from the lid of the unit during heavy rain. There are reports of flooding in

the area from both before and after the installation of the CDS unit. There is a very steep

gradient from the intersection with Broome Street to Byrne Crescent and after the low point the

gradient flattens significantly. A 1050 mm diameter pipe runs from Broome Street to the CDS

unit in Byrne Crescent and then out towards open land at the rear of No. 15 Byrne Crescent on

a significantly flatter gradient. An approximately 2.5 m wide drainage easement is present

between Nos. 11 and 15. A smaller 375 mm diameter pipe also takes local flows from the other

side of Byrne Crescent into the CDS unit.

Flooding occurs as water flowing down the 1050 mm steep pipe, emanating from Broome

Street, suddenly meets a gentle gradient pipe and the change in gradient causes a slowing of

2

velocities and the CDS to surcharge. Since the CDS unit has been installed both the central

access shaft cover and diversion chamber cover have been known to pop off. The larger

central access shaft has been bolted down, and as a result this causes the diversion chamber

cover to pop. Reports from residents are of both covers “floating” or “hovering” during flooding.

Not only is there risk from flooding but there is also a significant hazard posed by the exposure

of a deep hole and the floating cover.

Council wish to resolve the flooding issue in the area and also the issue of the two CDS unit

covers surcharging. It is understood that there is a DA currently submitted at No. 15 and

Council would like to undertake any works, if required, at the time of reconstruction of No. 15.

3 METHODOLOGY

WMAwater has previously developed a TUFLOW hydraulic model for the Maroubra Bay area,

which is being used to determine design flood levels for the Maroubra Bay FRMS&P. Byrne

Crescent lies within the domain of this model and, as such, this model was used in the

assessment.

To assess the hydraulic effect of the proposed works, the existing model was firstly rerun to

establish base case results. The model was run for the 1 EY (approximately one year ARI), 0.5

EY (approximately two year ARI), 0.2 EY (approximately five year ARI), 10% AEP and 5% AEP

events.

Four options were considered in this study, namely:

1) Local kerbside modifications to prevent flood water flowing through the building at No. 11:

Raising area in grass verge in front of No. 11 and modifying local ground levels to

encourage water to flow through the easement area rather than through the building

at No. 11.

2) Increase capacity of downstream pipe:

Doubling the number of pipes downstream and a review of ALS ground levels to see

if it would be possible to increase the pipe gradient.

3) Addition of a second pipe which discharges to the open area of land immediately

downstream of Nos. 11 and 15.

4) Create an overland flow path through No. 15 (over existing easement):

Run as an addition to option one.

Should this option be feasible in terms of reducing flood risk, Council would need to

enter discussions with the owner of the land regarding a flow path easement through

the property.

4 ASSESSMENT

4.1 OPTION 1 – LOCAL PROPERTY AND KERBSIDE MODIFICATIONS

Option 1 assessed the construction of a bund that extends from the southern corner of No. 11

Byrne Crescent, into the footpath reserve before turning northwest and running along the street

reserve. The crest of the bund was modelled as 17.3m AHD (approximately the 1% AEP level).

The bund would continue at the same elevation and terminate in the vicinity of No. 9 Byrne

Crescent at the point where the natural ground surface elevation of 17.3m AHD was met. It is

estimated from the LiDAR data that the wall length would be in the order of 25 m. For reference

the surface levels in both the cul-de-sac and the front of No. 11 Byrne Crescent are in the order

of 17±0.1m AHD and hence the bund would be around 300mm above the existing ground.

3

Option 1 reduced localised flooding to the extent that flooding is unlikely to occur through the

building at No. 11 Byrne Crescent for events up to the 10 % AEP, as shown in Figures 1A

through D. At the 5 % AEP level, shown in Figure 1E, depths in the front yard are reduced in

the order of 200 mm. Thus, this option would considerably reduce the frequency of flooding

through the house at No. 11. However, this benefit to No. 11 increases ponding depths within

the road reserve of Byrne Crescent by the order of 10-50 mm with the impacts largely contained

within the road reserve.

This option provides a relatively failure proof means of reducing the incidence of runoff entering

No. 11 Byrne Crescent at low cost (say $10,000)and with no significant environmental,

aesthetic or social adverse impacts.

4.2 OPTION 2 – INCREASED CAPACITY OF DOWNSTREAM DRAINAGE

Option 2 is designed to reduce the localised surface flooding by providing additional

underground pipe capacity from the CDS unit in Byrne Crescent downstream to the existing

ocean outfall. Each of the pipes downstream of the CDS was doubled from one to two.

Option 2 is effective at reducing the localised stormwater inundation within and between Nos.

11 and 15 Byrne Crescent. At the 0.2 EY level the localised flooding in the front gardens of

Nos. 11 and 15 was reduced in the order of 10 to 20 mm such that flooding is unlikely to occur,

as shown in Figure 2C. For the 10 % and 20 %AEP events flooding will still occur within these

lots, however the peak inundation depths are likely to be reduced by at least 20 mm, as shown

in Figures 2D and 2E, respectively. These small reductions in peak level are likely to make little

difference to the magnitude of flood damages at No. 11 Byrne Crescent, or the considerable

inconvenience experienced by the resident during floods.

This option has the added benefit of reducing the depth of inundation in the adjacent reserve

and surf club car park by at least 10 mm, for the 0.2 EY through 5% AEP events, as shown in

Figures 2C to 2E, respectively.

An inspection of the ALS surface levels shows that the ground elevation at the rear of No. 11

Byrne Crescent is approximately 14.4 mAHD, whereas the ocean outfall, some 480 m

downstream, is at an elevation of 3.3 mAHD. Theoretically, this results in a potential 2.3 %

average grade, however due to variations in the above ground topography it is unlikely that this

grade would be achieved. Therefore, it is highly unlikely that works could be undertaken to

effectively improve the existing pipe gradients to increase their capacity.

This option is likely to be prohibitively expensive in terms of the associated earth and pipe

works as well as require significant negotiation with neighbouring landholders. Further, it is

unlikely that a cost benefit analysis would support this expenditure given the projected

reductions in localised stormwater inundation.

4.3 OPTION 3 – ADDITION OF A SECOND PIPE THAT DISCHARGES DOWNSTREAM

Option 3 assessed the addition of a 60 m length of 1050 mm diameter pipe that would run from

the existing kerbside pit in front of No. 15 Byrne Crescent and discharge into the land

immediately downstream of No. 11. Pipe duplication was explicitly not simulated between the

kerb pit and the CDS unit to minimise the volume of work to be executed within the road

reserve. This configuration will likely still result in surcharging of the CDS and adjacent pit,

however increased capacity of the kerbside pit and pipe network will not only capture the

surcharged flows but also the overland flows that already arrive at the cul-de-sac. While the

length of the pipe run along the boundary between Nos. 11 and 15 is in the order of 45 m, the

additional 15 m of run is required to reach a point where the natural surface elevation is

sufficiently low to maintain pipe grade and still discharge onto the natural surface.

4

It should be noted that there is likely to be little space within the existing easement to install a

second 1050mm diameter pipe and Council would need to investigate this further if they were to

pursue this option. This option assumes that all runoff in the cul-de-sac enters the pipe system

via new kerb inlet pits. In practice this is unlikely to occur due to blockage (leaves, parked

vehicle etc.) or the runoff bypassing the pits.

Option 3 will reduce the localised stormwater inundation within Byrne Crescent by at least

20 mm. At the 0.2 EY event localised ponding is largely eliminated, as shown in Figure 3C.

With the 10 % and 20 % AEP events, flooding is reduced within the front yards of Nos. 11 and

15 as well as the cul-de-sac of Byrne Crescent in the order of 10-20 mm, as shown in Figures

3D and E. However, it is likely that for these events, as well as less frequent events, that there

will still be some localised flooding that may still enter the building at No. 11 Byrne Crescent.

Unfortunately the results indicated that if this option was implemented the downstream lands in

the adjacent reserve and surf club car park will be subject to increased flood depths.

Specifically, for all events areas of flooding in the surf club car park increase by over 100 mm

depth.

4.4 OPTION 4 – PROVISION OF FORMAL OVERLAND FLOW PATH AND KERB

MODIFICATIONS

Option 4 is a superset of Option 1 in which the kerb modifications are combined with a formal

overland flow path defined along the boundary between Nos. 11 and 15 Byrne Crescent. There

is currently an informal vegetated overland flow path and it is anticipated that the provision of a

formalised overland flow path would increase the conveyance as well as define a known zone

of inundation and correspondingly reduce the associated risks and possibly external damages.

Option 4 reduces localised flooding within the front of No. 11 Byrne Crescent to such an extent

that localised flooding is unlikely to occur for events up to the 5 % AEP, as shown in Figures 4A

though E. Similarly there is a reduction in flow depth along the formalised flow path in the order

of 20 mm for all events simulated, although this reduction will depend on the assumed flow path

dimensions. As with Option 1, the ponding depth within Byrne Crescent increases in the order

of 10-50 mm. There was no measureable impact on downstream lands from this option.

4.5 IMPACT OF OPTIONS ON CDS UNIT FLOWS

With respect to the performance of the CDS unit there is essentially no change across all the

scenarios, as shown in Table 1 and Table 2.

Table 1 Peak Upstream Inflows to CDS (m3/s)

Option 1 EY 0.5 EY 0.2 EY 10% AEP 5% AEP

Base 0.44 1.34 1.44 1.44 1.43 1 0.44 1.34 1.44 1.44 1.43 2 0.44 1.35 1.60 1.62 1.64 3 0.40 1.23 1.54 1.61 1.64 4 0.44 1.34 1.44 1.43 1.43

Table 2 Peak Pipe Outflows Downstream of CDS (m3/s)

Option 1 EY 0.5 EY 0.2 EY 10% AEP 5% AEP

Base 0.47 1.42 1.51 1.51 1.51 1 0.47 1.42 1.51 1.51 1.51 2 0.47 1.46 1.75 1.77 1.81 3 0.42 1.33 1.69 1.76 1.79 4 0.47 1.42 1.51 1.51 1.51

5

5 CONCLUSIONS

An inspection of the ALS survey data indicated that it is unlikely that the pipe gradients

downstream of Byrne Crescent could be significantly increased.

From an examination of the impacts of the four options, Options 2 and 3 are both effective at

reducing the localised stormwater inundation in the locality of the Byrne Crescent cul-de-sac

and Nos. 11 and 15 Byrne Crescent. However, as both these options involve significant

construction works and associated costs they are unlikely to be feasible to implement.

Option 1 provides a relatively straightforward and cost effective measure to eliminate inundation

of No. 11 Byrne Crescent, to the crest of the embankment. This measure could be further

enhanced with formalisation of the overland flow easement (Option 4) but this would require

negotiation with the property owners.

The performance of the CDS unit was examined and it was found that there was little or no

change to the flows through the CDS unit for the options tested. The key factors that contribute

to the flooding issue at the Byrne Crescent cul-de-sac are likely to be a combination of the

relatively steep topography leading into the cul-de-sac, and where stormwater is partially

trapped in the cul-de-sac at an enclosed low point. Removal of the CDS unit would not

eliminate the problem. If the CDS unit lids were 100% sealed then water would still surcharge

at the kerb inlet pits.

However, in very small events the CDS unit is likely to restrict the capacity of the pipe system

and thus slightly increase the frequency of surcharging. This issue cannot be investigated in

the TUFLOW model as it would require a greater understanding of the hydraulics of the specific

CDS unit and how it performs under pressure.

Yours Sincerely,

WMAwater

Richard Dewar

Director

Enclosed Figures:

Figure 1A-E: Peak Flood Impact, Option 1 Property/Kerbside Modification, 1 EY Event to

5 % AEP event, respectively

Figure 2A-E: Peak Flood Impact, Option 2, Pipe Duplication Downstream, 1 EY Event to

5 % AEP event, respectively

Figure 3A-E: Peak Flood Impact, Option 3, Addition of Second Pipe, 1 EY Event to 5 %

AEP event, respectively

Figure 4A-E: Peak Flood Impact, Option 4, Overland Flow Path and Kerbside

Modification, 1 EY Event to 5 % AEP event, respectively

Existing Pipes

Kerbside Modification

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 1APEAK FLOOD IMPACT

OPTION ONE: PROPERTY/KERBSIDE MODIFICATION1EY EVENT

J:\Jobs\113044\ArcGIS\ArcMaps\BryneCresent\Figure01a_Impact_001yr_Opt1.mxd

Bund raised to 17.3 mAHD

MAROUBRA

MALABAR

Existing Pipes


Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 1B

PEAK FLOOD IMPACTOPTION ONE: PROPERTY/KERBSIDE MODIFICATION

0.5EY EVENT

J:\Jobs\113044\ArcGIS\ArcMaps\BryneCresent\Figure01b_Impact_002yr_Opt1.mxd


MAROUBRA

MALABAR

Existing Pipes


Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 1C


0.2EY EVENT

J:\Jobs\113044\ArcGIS\ArcMaps\BryneCresent\Figure01c_Impact_005yr_Opt1.mxd


MAROUBRA

MALABAR

Existing Pipes


Impact (m)

< - 0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 1D


10% AEP EVENT

J:\Jobs\113044\ArcGIS\ArcMaps\BryneCresent\Figure01d_Impact_010yr_Opt1.mxd


MAROUBRA

MALABAR

Existing Pipes


Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 1EPEAK FLOOD IMPACT

OPTION ONE: PROPERTY/KERBSIDE MODIFICATION5% AEP EVENT

J:\Jobs\113044\ArcGIS\ArcMaps\BryneCresent\Figure01e_Impact_020yr_Opt1.mxd


Existing Pipes

Pipe Upgraded

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION TWO: DOUBLING PIPES DOWNSTREAM1EY EVENT


Existing - 1 x 1050mm pipe

Option - 2 x 1050mm pipes









MAROUBRA

MALABAR

Existing Pipes

Pipe Upgraded

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 2B

PEAK FLOOD IMPACTOPTION TWO: DOUBLING PIPES DOWNSTREAM

0.5EY EVENT














MAROUBRA

MALABAR

Existing Pipes

Pipe Upgraded

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 2C


0.2EY EVENT












MAROUBRA

MALABAR

Existing Pipes

Pipe Upgraded

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 2D


10% AEP EVENT












MAROUBRA

MALABAR

Existing Pipes

Pipe Upgraded

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION TWO: DOUBLING PIPES DOWNSTREAM5% AEP EVENT












Additional Pipe

Existing Pipes

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION THREE: ADDITION OF SECOND PIPE1EY EVENT


Outlet for additional pipe

into open land. Invert level 12.9m

Second 1.05m pipe

added to existing pit

Additional Pipe

Existing Pipes

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 3B

PEAK FLOOD IMPACTOPTION THREE: ADDITION OF SECOND PIPE

0.5EY EVENT




Second 1.05m pipe


0 50 100 150 20025m

FIGURE 3C


0.2EY EVENT




Second 1.05m pipe


Additional Pipe

Existing Pipes

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

Additional Pipe

Existing Pipes

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 3D


10% AEP EVENT




Second 1.05m pipe


Additional Pipe

Existing Pipes

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION THREE: ADDITION OF SECOND PIPE5% AEP EVENT




Second 1.05m pipe


Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION FOUR: OVERLAND FLOW PATH AND KERBSIDE MODIFICATION1EY EVENT




Overland Flow Path

Existing Pipes

Newly created swale through existing drainage easement.

Approximate width - 1 m

Approximate average depth - 250 mm

Length - 36 m

Note: Existing 1050 mm pipe to remain in situ.

MAROUBRA

MALABAR

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 4B

PEAK FLOOD IMPACTOPTION FOUR: OVERLAND FLOW PATH AND KERBSIDE MODIFICATION

0.5EY EVENT




Overland Flow Path

Existing Pipes




Length - 36 m


MAROUBRA

MALABAR

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 4C


0.2EY EVENT




Overland Flow Path

Existing Pipes




Length - 36 m


MAROUBRA

MALABAR

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m

FIGURE 4D


10% AEP EVENT




Overland Flow Path

Existing Pipes




Length - 36 m


MAROUBRA

MALABAR

Impact (m)

< -0.02

-0.02 - -0.01

No Impact

0.01 - 0.05

0.05 - 0.1

> 0.1

No Longer Flooded

Newly Flooded

0 50 100 150 20025m


OPTION FOUR: OVERLAND FLOW PATH AND KERBSIDE MODIFICATION5% AEP EVENT




Overland Flow Path

Existing Pipes




Length - 36 m


Documents

LOCATION ON INVESTIGATED...John Shore Park A Maroubra Bay Primary Broadarrow Reserve MAROUBRA Maroubra Bay D U N C A N O S T M O NS A V FITZ G E R AL D AV B O N D S T M A R O B R A