no. 39 silverleaves ave. silverleaves phillip island

No. 39 SILVERLEAVES AVE.

SILVERLEAVES

PHILLIP ISLAND

COASTAL HAZARD

VULNERABILITY ASSESSMENT

prepared for

Zacacorp Builders Pty Ltd

March 2013

Coastal Hazard Vulnerability Assessment – Rev C

Document Title Coastal Hazard Vulnerability Assessment

Property at No. 39 Silverleaves Ave., Silverleaves

Client Zacacorp Pty Ltd

Document Code 13-756vic-pobrp

First Issue Date 05th March 2013

Document Status Record

Revision Date

Chapter/section/pages revised,

plus any remarks.

Authority

code Revised Author Reviewed

A 05Mar13 Initial Release POB HPR

B 07Mar13 Arithmetic correction made POB HPR

C 04Jun13 Include Melbourne Water levels HPR POB

Coastal Engineering Solutions Pty Ltd

25 Wirilda Way

Fish Creek

VIC 3959

Australia

tel : + 61 3 5683 2495

email : [email protected]

Coastal Engineering Solutions Pty Ltd

P.O. Box 677

59 Hulcombe Road

Samford QLD 4520

Australia

tel : + 61 7 3289 7011

fax : + 61 7 3289 7022

email : [email protected]

Coastal Hazard Vulnerability Assessment – Rev C

Table of Contents

SUMMARY........................................................................................................................................... 1

1 INTRODUCTION ........................................................................................................................ 2

1.1 General .................................................................................................................................. 2

1.2 Location and Property Characteristics .................................................................................. 2

2 ASSESSMENT PARAMETERS .............................................................................................. 4

2.1 Overview ................................................................................................................................ 4

2.2 Storm Tide and Surge ............................................................................................................ 5

2.3 Coastal Processes .................................................................................................................. 7

3 PRESENT-DAY VULNERABILITY ......................................................................................... 9

3.1 Vulnerability to the 100 year Return Period Storm Event..................................................... 9

3.1.1 Existing Inundation Risk ............................................................................................ 9

3.1.2 Existing Coastal Erosion Risk ................................................................................... 9

3.2 Mitigation .............................................................................................................................. 9

4 VULNERABILITY OVER A PLANNING PERIOD TO 2040 ......................................... 10

4.1 Storm Parameters................................................................................................................ 10

4.2 Vulnerability to the 100 year Return Period Storm Event................................................... 10

4.2.1 Inundation Risk in 2040 ........................................................................................... 10

4.2.2 Coastal Erosion Risk in 2040 .................................................................................. 10

4.3 Mitigation ............................................................................................................................ 10

5 VULNERABILITY OVER A PLANNING PERIOD TO 2100 ......................................... 11

5.1 Storm Parameters................................................................................................................ 11

5.2 Vulnerability to the 100 year Return Period Storm Event................................................... 11

5.2.1 Inundation Risk in 2100 ........................................................................................... 11

5.2.2 Coastal Erosion Risk in 2100 .................................................................................. 11

5.3 Mitigation ............................................................................................................................ 11

6 REFERENCES .......................................................................................................................... 13

Coastal Hazard Vulnerability Assessment – Rev C Page | 1

SUMMARY

The property at No. 39 Silverleaves Avenue in Silverleaves on Phillip Island is the subject

of a proposed planning application for a double storey house. In accordance with the

Victorian Coastal Strategy 2008, this Coastal Hazard Vulnerability Assessment (CHVA)

has been completed for the site.

At the time that the original CHVA was prepared in March 2012 the Client was not aware

that Melbourne Water had prepared over-riding flood levels for Westernport and Port Phillip

Bays. This revised CHVA has been prepared to include the Melbourne Water criteria.

No. 39 Silverleaves Avenue is set back some 78 metres from the natural shoreline of

Westernport Bay. This considerable physical buffer consists of naturally vegetated sand

ridges - with ridge levels exceeding +5 metres AHD in height. Consideration of existing

and likely future coastal processes indicate that the proposed development is well beyond

the effects of any shoreline erosion in the period up to and including 2100.

Existing land levels vary over the property - being approximately +3.04 metres AHD at the

northern (rear) property boundary, sloping down to approximately +1.94 metres AHD at the

south-west corner of the block (fronting Silverleaves Avenue). Apart from a small area at

the south-west corner of the property, these levels are above the influences of 100 year

return period storm tide events as defined by CSIRO. However, Melbourne Water

nominates a 100 year flood level for Westernport Bay of +2.7 metres AHD; consequently

whilst the property is currently not at risk of storm tide inundation it is at risk from flooding.

By 2040, the combined effects of future climate change (including predicted sea level rise

and increased “storminess”) mean that the occurrence of a 100 year return period storm at

that time will be such that there will be minor inundation from elevated sea levels due to the

storm tide across the street-side property boundary - up to a depth of around 51cm only.

Habitable areas of the proposed redevelopment will not be at risk of inundation by this

ocean storm tide during this possible future scenario. However, Melbourne Water

nominates a 100 year flood level for Westernport Bay of +2.9 metres AHD for the year

2040 and also requires a 600mm freeboard above this level. Consequently to

accommodate this criterion, the minimum floor level for the redevelopment will need to be

set at +3.5 metres AHD.

For the predicted climate change scenario in the year 2100, existing ground levels are such

that there would be some inundation across the property during a 100 year ARI event due

to the storm tide at that time. Based on the existing ground levels, the maximum depth of

such inundation could be around 111cm at the front of the property - reducing to around nil

at the rear boundary. However, the Melbourne Water requirement for flooding in the year

2100, nominates a flood level of +3.5 metres AHD - requiring a minimum habitable floor

level of +4.1 metres AHD so as to include a 600mm freeboard.

Whilst much of the land may be inundated during a 100 year ARI flood event in future

years of 2040 and 2100, safe egress from the property would still be available via the

higher coastal dune system.


1 INTRODUCTION

1.1 General

A double storey house is proposed on the property at No. 39 Silverleaves Avenue in

Silverleaves on Phillip Island. Drawings for the works have been prepared by Chaule Architect

Pty Ltd as part of the planning approvals process. The applicant has requested that a Coastal

Hazard Vulnerability Assessment (CHVA) for the subject site be prepared in accordance with

the requirements of the Victorian Coastal Strategy 2008 and the Victorian Coastal Hazard

Guide 2012. Flooding criteria introduced by Melbourne Water in 2012 for Westernport and

Port Phillip bays will be the dominant criteria in terms of inundation.

This Coastal Hazard Vulnerability Assessment includes:

A description of the relevant sea level, storm and foreshore characteristics.

Shoreline and site vulnerability to a storm having a 100 year return period for the present-

day climate scenario.

Corresponding shoreline and site vulnerability (including future climate change influences)

over a planning period that extends to the year 2040.

Shoreline and site vulnerability (including future climate change influences) over a planning

period to 2100.

The vulnerability of the site to coastal hazards over these various timeframes include the

effects of future sea level rise (and other associated increases in storm parameters) as

nominated in the Victorian Coastal Strategy 2008 and flood levels as nominated in

Melbourne Water, 2012.

1.2 Location and Property Characteristics

The location of 39 Silverleaves Avenue in relation to Westernport Bay is shown conceptually

on Figure 1.

The property is set back some 78 metres from the natural shoreline of the Bay. This physical

buffer consists of naturally vegetated sand ridges - with ridge levels exceeding +5 metres AHD

in height.

A survey1 of the site indicates that existing land levels vary over the property - being

approximately +3.04 metres AHD at the northern (rear) property boundary, sloping down to

approximately +1.94 metres AHD at the south-west corner of the block (fronting Silverleaves

Avenue).

Concept plans show that the proposed Finished Floor Levels of habitable areas throughout the

lower levels of the house are proposed at +2.94 metres AHD.

1 Drawing Reference No.A1, dated May 2012 prepared by Chaule Architect Pty Ltd. (under Job No. 1111).


Figure 1 : Location of No. 39 Silverleaves Ave in relation to Westernport Bay

Figure 2 : Local foreshore frontage to Westernport Bay

No. 39 Silverleaves Ave

Westernport

Bay

Phillip

Island

French

Island


2 ASSESSMENT PARAMETERS

2.1 Overview

The Victorian Coastal Strategy 2008 includes a policy to plan for a rise above present-day sea

levels of not less than 0.8m by the year 2100. In June 2009 the Victorian State Government

appointed a Coastal Climate Change Advisory Committee (CCCAC) to consider and

recommend appropriate planning responses to the implications of future climate change to the

state's coastal regions.

The CCCAC’s Final Report was released by the Minister for Planning on the 5th June 2012. A

number of recommendations have been accepted by the Government - including the following

amendments to the State Planning Policy Framework which have particular relevance to the

proposed development:

In planning for possible sea level rise, an increase of 0.2 metres over current 1 in 100 year

flood levels by 2040 may be used for new development in close proximity to existing

development (urban infill).

Plan for possible sea level rise of 0.8 metres by 2100, and allow for the combined effects of

tides, storm surges, coastal processes and local conditions such as topography and

geology when assessing risks and coastal impacts associated with climate change.

In conjunction with predicted sea level rise, there are other climate change influences that

need to be addressed - primarily as a consequence of increased “storminess” and the

associated increase in storm tide levels along Victoria’s shoreline.

These factors have been addressed in a CSIRO report (McInnes, et. al. 2005): “Climate

change in Eastern Victoria: Stage 2 report: the effect of climate change on storm surges: a

project undertaken for the Gippsland Coastal Board”. That 2005 study included the

Westernport Bay region.

At about the same time that the CCCAC’s Final Report was released, two further documents

were issued:

Victorian Coastal Hazard Guide (DSE, 2012)

Melbourne Water produced a document Planning for sea level rise, 2012, which

defined flood levels from present-day through to 2100; and also nominated a

mandatory freeboard of 600mm above these levels for habitable floor levels. This

freeboard is nominated to allow for wave effects on top of the flood level.

The information from these documents has been considered in the preparation of the CHVA.


2.2 Storm Tide and Surge

Figure 3 illustrates the primary water level components of a storm tide event. Any increase in

ocean water levels as a consequence of future climate change would be in addition to these

various natural phenomena. A brief discussion of these components is offered below.

Figure 3 : Components of a Storm Tide Event

Astronomical Tide: The astronomical tide is the normal day-to-day rising and falling of

ocean waters in response to the gravitational influences of the sun and the moon. The

astronomical tide can be predicted with considerable accuracy. Astronomical tide is an

important component of the overall storm tide because if the peak of a severe storm were

to coincide with a high spring tide for instance, severe flooding of low lying coastal areas

can occur and the upper sections of coastal structures can be subjected to severe wave

action.

Storm Surge : This increase in ocean water levels is caused by meteorological effects

during severe storms. Strong winds blowing over the surface of the ocean forces water

against the coast at a greater rate that it can flow back to sea. Furthermore sea levels can

rise locally when a low pressure system occurs over the sea - resulting in what is termed

an “inverted barometer” effect. A 10mb drop in atmospheric pressure results in an

approximate 10 cm rise in sea level. In order to predict the height of storm surges, these

various influences and their complex interaction are typically replicated by numerical

modelling techniques using computers - such as has been done for the CSIRO study for

Victoria (McInnes et al, 2005).

Breaking Wave Setup: As storm waves propagate into shallower coastal waters, they

begin to shoal and will break as they encounter the nearshore region. The dissipation of

wave energy during the wave breaking process induces a localised increase in the ocean

water level shoreward of the breaking point which is called breaking wave setup. Through

the continued action of many breaking waves, the setup experienced on a foreshore during

a severe wave event can be sustained for a significant timeframe and needs to be

considered as an important component of the overall storm tide on a foreshore.

ASTRONOMICAL TIDE

SURGE

WAVE SETUP

WAVE RUNUP

STORM TIDE

INCOMING WAVES BROKEN WAVES WAVES ARE BREAKING

LOW WATER DATUM

COASTLINE

Storm Tide = Astronomical Tide + Storm Surge + Breaking Wave Setup


Wave Runup: Wave runup is the vertical height above the local water level up to which

incoming waves will rush when they encounter the land/sea interface. The level to which

waves will run up a structure or natural foreshore depends significantly on the nature, slope

and extent of the land boundary, as well as the characteristics of the incident waves.

Future climate change scenarios indicate there will be increases in the magnitude of storm

surges due to changed meteorological conditions. The CSIRO report (McInnes, et. al. 2005)

determines the combined effects of future sea level rise and storm tide for a 100 year return

period in Westernport Bay at the nearby coastal location of Stony Point, some 12km north-

north-east of Silverleaves. Given the close proximity of Stony Point to Silverleaves, it is

appropriate for the purposes of this CHVA to adopt the reported storm tide levels at Stony

Point as being indicative of those on foreshores fronting Silverleaves.

The CSIRO report provides predictions for the years 2005, 2030 and 2070 under a range of

possible climate change scenarios - termed low, mid and high-range scenarios. The high-

range storm tide levels are shown plotted below on Figure 4, with storm tide levels

extrapolated to the year 2100.

Figure 4 : Plots of predictions for sea level rise and associated storm tide levels

Based on predictions by CSIRO (McInnes et al, 2005)

The storm tide levels in Figure 4 are consistent with those cited in DSE, 2012. Reference to

these results indicate that the ocean water levels summarised below in Table 1 can be used as

predictions for 100 year return period storm tide levels at Silverleaves under future climate

change scenarios.


Location Current Climate 2040 2100

Silverleaves +2.12 +2.45 +3.05

Table 1 : Predicted 100 year Return Period Storm Tide Levels at Silverleaves

(metres above AHD)

It is pertinent to note that the predicted storm tide level of +2.45m AHD in the year 2040 is 0.33

metres above the predicted present-day level; and differs from the 0.2 metre stated in the

State Planning Policy Framework. This is because by necessity it also includes the effects that

climate change has on meteorological conditions and therefore on future storm surges - not

just a 0.2 metre sea level rise.

However, the CSIRO determinations do not allow for flood levels defined by Melbourne Water.

These flood levels are presented in Table 2.

Location Current Climate 2040 2100

Westernport Bay

(Silverleaves) +2.7 +2.9 +3. 5

Table 2 : Predicted 100 year Return Period Flood Levels at Silverleaves (Melbourne Water) (metres above AHD)

2.3 Coastal Processes

The term “coastal processes” is used to classify the naturally prevailing conditions of waves

and currents that shape and control the stability of foreshores. In this instance, the property at

No. 39 Silverleaves Avenue is located some 78 metres inland from the natural shoreline that

exists along Westernport Bay in this area of Silverleaves. This high naturally vegetated dune

area provides a substantial physical buffer between the property and the waters of the Bay.

The preceding Figure 2 provides a general appreciation of the local setting and nature of the

foreshore opposite No. 39 Silverleaves Avenue.

The coastal processes at the shoreline are driven by the prevailing north-west (winter) to

south-west (summer) winds which generate waves across the Bay from these respective

directions. There will also be some significantly attenuated residual swell entering nearshore

waters from Bass Strait.

The result is a coastal regime where the predominant movement of sand on the local foreshore

is from west towards east. Whilst the beach is mobile, the position of the shoreline has

remained fairly constant in recent decades, implying that there is a sufficient natural supply of

sand to match the volume of sand moved eastward by the waves.

Nevertheless there will be storm events that cause cross-shore erosion of the beach. However

ambient wave conditions following an erosion event will act to restore the beach over time.


The foreshore dunes have a crest level of approximately +5 metres AHD. Natural land levels

behind the dune system are lower - tending to be in the range of +2 metres to +3 metres AHD.

The very substantial 78 metre wide physical buffer between the intertidal beach and the

seaward-most property boundary of 39 Silverleaves Avenue is adequate to accommodate

episodic erosion associated with 100 year ARI storms - even accounting for future climate

change influences to the year 2100. This conclusion is based on experience of similar wave

climate and coastal environments worldwide rather than site specific modelling or calculations

for Solverleaves.

The metocean and nearshore bathymetric survey data required to undertake meaningful site

specific modelling at Silverleaves does not exist. Given the very substantial costs associated

with a detailed Coastal Process Study for the local foreshore it is not practical or equitable to

expect the project proponent to provide it. It is understood that a regional CHVA is presently

being produced for Westernport Bay which should include the necessary modelling or

computations.


3 PRESENT-DAY VULNERABILITY

3.1 Vulnerability to the 100 year Return Period Storm Event

3.1.1 Existing Inundation Risk

Land levels across the property at No. 39 Silverleaves Avenue vary between approximately

+1.94 metres AHD at the front street boundary and +3.04 metres AHD at the rear property

boundary nearest Westernport Bay.

As noted previously in Table 1, the predicted ocean water level currently associated with a 100

year return period event is +2.12 metres AHD along local foreshores. This ocean level is some

0.18 metres above the lowest land level in the south-west corner of the property at No. 39

Silverleaves Avenue. This suggests that whilst the high dune system between the property

and the Bay will prevent the direct access of storm tide reaching the property. However,

Melbourne Water nominate that the 100 year ARI flood event has a level +2.7 metres AHD,

which would inundate up to 50% of the land.

Whilst the 100 year ARI storm tide level is below the proposed Finished Floor Level of +2.94

metres AHD, the Melbourne Water requirement is for a 600 mm freeboard. The proposed

finished floor level would need to be raised to +3.3 metres AHD for present day sea levels and

to allow for a 100 year return period flood event.

3.1.2 Existing Coastal Erosion Risk

The property at No. 39 Silverleaves Avenue is located some 78 metres inland and will

therefore be well beyond the effects of any storm erosion.

3.2 Mitigation

The finished floor level of the proposed development of the property at No. 39 Silverleaves

Avenue would need to be raised to +3.3 metres AHD to accommodate the effects of a 100

year return period flood event occurring under the present-day climate scenario.


4 VULNERABILITY OVER A PLANNING PERIOD TO 2040

4.1 Storm Parameters

The CSIRO study of future climate change effects in Westernport Bay (McInnes et al, 2005)

predicts an increase in the strength of winds during storms by the year 2040. This increased

“storminess” in conjunction with predicted sea level rise at 2040 results in an estimated 100

year return period storm tide level of +2.45 metres AHD - refer to Table 1.

However, Melbourne Water nominate that the 100 year ARI flood event has a level +2.9

metres AHD, which would inundate up to 70% of the land.


4.2.1 Inundation Risk in 2040

As stated previously, land levels on the property at No. 39 Silverleaves Avenue vary between

approximately +1.94 metres AHD at the front street boundary and +3.04 metres AHD at the

rear property boundary nearest the Bay.

Since the predicted ocean water level in 2040 as a consequence of a 100 year return period

event occurring at that time is +2.45 metres AHD, inundation up to 51cm is expected at the

front of the property due to storm tide. The predicted 100 year ARI flood level by Melbourne

Water is 2.9 metres AHD.

Whilst the 100 year ARI flood level is below the proposed Finished Floor Level of +2.94 metres

AHD, the Melbourne Water requirement is for a 600 mm freeboard. The proposed finished

floor level would need to be raised to +3.5 metres AHD for the 2040 sea level and to allow for a

100 year return period flood event.

4.2.2 Coastal Erosion Risk in 2040

As stated in Section 2.3, the 78 metre setback from the Bay at this location provides a wide

physical buffer that can adequately accommodate these predicted shoreline responses.

Consequently coastal processes will not adversely affect the property at No. 39 Silverleaves

Avenue in future years to 2040.

4.3 Mitigation



year return period flood event occurring under the 2040 climate scenario.


5 VULNERABILITY OVER A PLANNING PERIOD TO 2100

5.1 Storm Parameters

The Victorian Coastal Strategy requires consideration of a 0.8 metre rise in sea levels by the

year 2100. The CSIRO study of future climate change effects (McInnes et al, 2005) predicts

an increase in the strength of winds during storms by the year 2100. This increased

“storminess” in conjunction with 0.8 metre sea level rise results in a 100 year return period

storm tide level of +3.05 metres AHD by the year 2100 - refer to Table 1.

However, Melbourne Water nominate that the 100 year ARI flood event has a level +3.5

metres AHD, which would inundate all of the land.


5.2.1 Inundation Risk in 2100

As stated previously, land levels on the property at No. 39 Silverleaves Avenue vary between

approximately +1.94 metres AHD at the front street boundary and +3.04 metres AHD at the

rear property boundary nearest the Bay.

Since the predicted ocean water level in 2100 as a consequence of a 100 year return period

event occurring at that time is +3.05 metres AHD, inundation is expected across most of the

property - to a maximum depth of around 111cm at the front of the property. The predicted

100 year ARI flood level by Melbourne Water is 3.5 metres AHD which would inundate the

whole site.

The 100 year ARI flood level is above the proposed Finished Floor Level of +2.94 metres AHD.

Allowing for the Melbourne Water requirement for a 600 mm freeboard the proposed finished

floor level would need to be raised to +4.1 metres AHD for the 2100 sea level and to allow for a

100 year return period flood event.

5.2.2 Coastal Erosion Risk in 2100

As stated in Section 2.3, the approximately 78 metre setback from the Bay at this location

provides a wide, high physical buffer that can adequately accommodate these predicted

shoreline responses. Consequently coastal processes will not adversely affect the property at

No. 39 Silverleaves Avenue in future years to 2100.

5.3 Mitigation



year return period flood event occurring under the 2100 climate scenario.


It is noted that the dune seaward of 39 Silverleaves is significantly higher than the flood level

and during such a flood event with 2100 water levels, there would be a safe exit from the

property via the coastal dunes.


6 REFERENCES

Department of Sustainability and Environment (2012). “Victorian Coastal Hazard Guide”.

Published by Victorian Government Department of Sustainability and Environment. June 2012.

ISBN 978-1-74287-401-2 (print). ISBN 978-1-74287-402-9 (online).

http://www.climatechange.vic.gov.au/adapting-to-climate-change/future-coasts/victorian-coastal-

hazard-guide.

McInnes, K. L.; Macadam, I.; Hubbert, G. D.; Abbs, D. J.; Bathols, J. M. (2005). “Climate

change in Eastern Victoria: Stage 2 report: the effect of climate change on storm surges: a

project undertaken for the Gippsland Coastal Board.”. Prepared for the Gippsland Coastal

Board. CSIRO Marine and Atmospheric Research. June 2005. procite:070d5e68-da36-

4425-8737-76191b418452

Melbourne Water (2012). “Planning for sea level rise - Assessing development in areas prone

to tidal inundation from sea level rise in the Port Phillip and Westernport Region”. Published

by Melbourne Water. June 2010. ISBN 978-1-921603-62-4 (Print) 978-1-921603-63-1 (Web). http://www.dpcd.vic.gov.au/__data/assets/pdf_file/0017/111950/Melbourne-Water-Planning-for-sea-

level-rise-guidelines.pdf.

Victorian Government (2008). “Victorian Coastal Strategy 2008”. Published by the Victorian

Coastal Council. ISBN 978-1-74208-697-2 (print) ISBN 978-1-74208-698-9 (pdf).

ISBN 978-1-74208-698-9 (PDF)

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