Coastal Hazard Assessment 2018

R1009 Rev 0

March 2018

Northern Agricultural Catchment Council

Coastal Hazard Assessment of Islands in the Northern Agricultural Region

www.coastsandports.com.au

m p rogers & associates pl Coastal Hazard Assessment of Islands in the Northern Agricultural Region K1516, Report R1009 Rev 0, Page (i)

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K1516, Report R1009 Rev 0 Record of Document Revisions

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0 Issued for Client Use 20/03/2018

A Clapin C Doak C Doak

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Limitations of this Document This document has been prepared for use by the Client in accordance with the agreement between the Client and M P Rogers & Associates Pty Ltd. This agreement includes constraints on the scope, budget and time available for the services. The consulting services and this document have been completed with the degree of skill, care and diligence normally exercised by members of the engineering profession performing services of a similar nature. No other warranty, expressed or implied, is made as to the accuracy of the data and professional advice included. This document has not been prepared for use by parties other than the Client and its consulting advisers. It may not contain sufficient information for the purposes of other parties or for other uses.

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Table of Contents 1. Introduction 1 1.1 Site 1 2. Coastal Inundation Assessment 5 2.1 Sea Level Rise 5 2.2 Previous Studies 6 2.3 500 Year ARI Inundation Levels 8 2.4 Available Elevation Data 9 2.5 Inundation Hazard Assessment 10 3. Coastal Erosion Assessment 11 3.1 Available Data 13 3.2 Severe Storm Erosion Sand & Rubble Coasts - S1 Allowance 14 3.3 Historic Shoreline Movement Sand & Rubble Coasts - S2 Allowance 16 3.4 Sea Level Rise Sand & Rubble Coasts - S3 Allowance 16 3.5 Rocky Shoreline Allowance 18 3.6 Erosion Hazard Assessment 18 4. Coastal Hazard Mapping 20 5. Conclusion 21 6. References 22 7. Appendices 23 Appendix A Coastal Hazard Maps 24

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Table of Figures Figure 1.1 The Northern Agricultural Region Area 1 Figure 2.1 Storm Surge Components 5 Figure 2.2 Recommended Sea Level Rise Scenario for Coastal Planning in Western Australia (DoT 2010) 6 Figure 2.3 Model Output Showing a Tropical Cyclone Wind Field While Tracking to the Southwest of Geraldton 7 Figure 2.4 LiDAR Survey Showing Jurien Bay Islands 10 Figure 3.1 Abrolhos Island Types 12 Figure 3.2 Photograph of Typical Rubble Shoreline within the Abrolhos Islands (Image from Long Island) 13 Figure 3.3 CoastAdapt Shoreline Classification Showing Jurien Bay Islands 14 Figure 3.4 Storm Wave Attack 15

Table of Tables Table 1.1 NAR Islands Location 3 Table 2.1 Sea Level Rise Allowances 6 Table 2.2 Total Water Level During 500 Year ARI Event at the Abrolhos Islands 8 Table 2.3 500 Year ARI Storm Surge Inundation Levels for the NAR Islands 9 Table 3.1 S1 Erosion of Various NAR Island Profiles 16 Table 3.2 S3 Erosion of Various NAR Island Sandy Coasts 17 Table 3.3 S3 Erosion of Various NAR Island Rubble Coasts 18 Table 3.4 Erosion Allowances for Sandy Coasts, North, South and West Aspects 18 Table 3.5 Erosion Allowances for Sandy Coasts, East Aspect 19 Table 3.6 Erosion Allowances for Rubble Coasts, North, South and West Aspects 19 Table 3.7 Erosion Allowances for Rocky Coasts 19

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1. Introduction 1.1 Site The Northern Agricultural Region (NAR) covers an approximately 7.5 million hectare area of Western Australia, spanning the coastline between Gingin to the south and Kalbarri to the north. The coastline and waters that form part of the region, as identified in Figure 1.1, feature over 150 islands. The majority of these islands are part of either the Houtman-Abrolhos Group (Abrolhos) or the Turquoise Coast Island Nature Reserves (Turquoise Coast NR) and are significant sites of natural resource management investment - including revegetation/invasive flora and fauna control and feral-free sanctuaries for threatened species such as Dibblers and Australian Sea Lions.

Figure 1.1 The Northern Agricultural Region Area m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 1

Given the close proximity and susceptibility to coastal processes, the risks posed to natural resources on the NAR islands from coastal hazards both now and into the future need to be considered. It is especially important to understand how changes to the climate system and projected sea level rise may impact the islands over the relevant planning timeframe.

Within Western Australia, State Planning Policy 2.6 State Coastal Planning Policy (SPP2.6, WAPC 2013) provides guidance on the assessment of coastal hazard risks for infrastructure as well as natural assets located in close proximity to the coast. This guidance is in the form of a methodology to assess the potential extent of coastal hazard impacts.

To provide guidance regarding the risks posed by coastal hazards, the Northern Agricultural Catchments Council (NACC) engaged specialist coastal and port engineers, M P Rogers & Associates (MRA), to prepare a coastal hazard assessment for the islands within the NAR. NACC is one of Australia’s 56 regional Natural Resource Management (NRM) organisations working to accomplish nation-wide management, restoration and protection of Australia’s natural environment by addressing national environmental priorities at the regional level.

The outcomes of this assessment, in conjunction with the ecological requirements of target conservation species, will aid natural resource managers in determining the timeframes for which specific islands may become unsuitable for maintaining these species. This will in turn, guide future NRM investment decisions within the NAR.

A total of 30 islands in the NAR (22 in the Abrolhos and 8 in the Turquoise Coast NR) were identified by manager stakeholders as having significant terrestrial NRM values at risk from coastal hazards. The coastal inundation and erosion assessments in Sections 2 and 3 as well as the coastal hazard mapping in Section 4 focusses on the 30 islands, as listed (south to north) in Table 1.1.

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Table 1.1 NAR Islands Location

Island Location

Lancelin Island Lancelin, Turquoise Coast NR

Wedge Island Wedge Island, Turquoise Coast NR

South Green Island Grey, Turquoise Coast NR North Green Island

Escape Island

Whitlock Island Jurien Bay, Turquoise Coast NR Boullanger Island

Favorite Island

Pelsaert Island

Middle Island

Gun Island

Stick Island

Leo Island

Alexander Island

Serventy Island

Campbell Island

Suomi Island

Keru Island

Wooded Island Abrolhos Islands Morley Island

Rat Island

Little Rat Island

Long Island

Beacon Island

West Wallabi Island

East Wallabi Island

Pigeon Island

Seagull Island

Tattler Island

North Island

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It is important to recognise the differences in assessing natural flora and fauna resources on offshore islands, compared to developed sections of mainland coastlines. The Turquoise Coast NR islands listed are completely uninhabited and do not contain any developed infrastructure. Human influence on these islands is limited to temporary visitors on suitable boating days.

A number of the Abrolhos islands listed, including Pigeon, Little Rat, Rat and East Wallabi islands, have minimal infrastructure - generally limited to fisherman shacks, private mooring jetties and a single airport runway. Fisherman might stay for periods of time seasonally, however there is no public accommodation on the islands for visitors. Visitors are allowed to moor overnight at several locations or day trip to the islands by boat or plane, either privately or via several local charters who operate out of Geraldton. The difficulty in accessing the islands, approximately 60 km offshore, means that they are generally only accessed by experienced boaters, fisherman and charter operators. This helps to reduce the risks of public safety issues on the islands.

The consequences, and therefore risks, are much lower for the natural resources assessed in following Sections of this report. The fact that significant and widespread vegetation can be identified throughout the islands today, shows that flora and fauna has historically and can potentially, over the 100 year planning timeframe, survive severe storm events. It is vital that this is considered in analysing the coastal hazard assessment and subsequent maps presented in this report. This is particularly significant as the SPP2.6 methodology should not be viewed as a prediction of future shoreline location, but rather should be viewed to provide a conservative assessment of land areas that could potentially be impacted by coastal processes over the relevant planning horizons.

This coastal hazard assessment will provide a first pass assessment of the potential areas that could be impacted by inundation and erosion over a range of timeframes up to and including the “long term” 100 year planning timeframe to the year 2118 as required by SPP2.6. The intermediate planning horizons that will be considered are below.

 Present Day (2018)

 “Short term” - 25 years to 2043.

 “Medium term” - 50 years to 2068.

This report presents the outcomes of the assessment.

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2. Coastal Inundation Assessment SPP2.6 requires that the allowance for inundation (termed the S4 Allowance) be taken as the maximum extent of inundation experienced during a water level event with a 500 year average recurrence interval (ARI) plus the appropriate allowances for sea level rise.

Assessment of the inundation levels requires consideration of peak storm surge, including wave setup. A storm surge occurs when a storm with high winds and low pressures approaches the coastline (refer to Figure 2.1). The strong, onshore winds and large waves push water against the coastline (wind and wave setup) and the barometric pressure difference creates a region of high water level. These factors acting in concert create the storm surge. The size of the storm surge is influenced by the following factors.

 Wind strength and direction.

 Pressure gradient.

 Seafloor bathymetry.

 Coastal topography.

Figure 2.1 Storm Surge Components

The important difference for the assessment of inundation of islands, compared to the mainland, is the amount of wind and wave set up. This is due to the way in which wind and wave set up occurs as the bathymetric profile becomes shallower as it nears the shoreline. The scale is important however, as the scale of each of the islands or group of islands within the NAR would be much smaller than the scale of any storm event. Therefore, significant set up is unlikely as the elevated water levels would quickly dissipate around the islands. This is fundamentally different to the mainland, where such dissipation is not possible as the length of the shoreline is greater than the scale of the storm event.

2.1 Sea Level Rise The DoT (2010) completed an assessment of the potential increase in sea level that could be experienced on the Western Australian coast in the coming 100 years. This assessment m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 5

extrapolated work by Hunter (2009) to provide sea level rise values based on the IPCC (2007) A1F1 climate change scenario projections to the year 2110. The derived sea level rise scenario was subsequently adopted by the Western Australian Planning Commission (and SPP2.6) for use in coastal planning along the Western Australian coast. The adopted sea level rise scenario is presented in Figure 2.2.

Figure 2.2 Recommended Sea Level Rise Scenario for Coastal Planning in Western Australia (DoT 2010)

Based on Figure 2.2, the required allowances for sea level rise from 2018 to each of the three key 25, 50 and 100 year horizons to 2043, 2068 and 2118 respectively are presented in Table 2.1.

Table 2.1 Sea Level Rise Allowances

Planning Timeframe SLR Allowance (m)

Present day (2018) 0.00

25yr - 2043 0.14

50yr - 2068 0.36

100yr - 2118 0.90

2.2 Previous Studies MRA (2015) previously completed storm surge modelling of the Greater Geraldton region, located approximately in the middle of the NAR mainland and directly in line with the offshore Abrolhos Islands. The study assessed the following in order to determine the appropriate allowances for coastal inundation.

 Cyclonic storm surge inundation.

 Non-cyclonic storm surge inundation. m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 6

 Tsunami induced inundation.

The potential extent of inundation was determined for 20, 100 and 500 year ARI events. For both 100 and 500 year ARI events, the critical and therefore recommended inundation allowances were based on the cyclonic storm surge levels. Given the location of the Abrolhos Islands, it is considered appropriate to assess the 500 year ARI cyclone event previously determined for the mainland at Geraldton. An output from the modelling, showing a spatial plot of the wind field and storm surge during the passage of a severe cyclone event southwest of Geraldton, is illustrated in Figure 2.3.

Abrolhos Islands

Geraldton Storm Surge (m)

Cyclone Eye

Travel Direction

Figure 2.3 Model Output Showing a Tropical Cyclone Wind Field While Tracking to the Southwest of Geraldton m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 7

A review of the cyclone modelling completed for Geraldton determined that up to approximately 0.9 m of cyclone setup could be expected during the passage of the 500 year ARI event at present day. Wind and wave setup is also expected to occur during such events and the values from previous modelling on the mainland (MRA 2015) have been used to produce the following table of total water level, including sea level rise for each of the planning horizons.

Table 2.2 Total Water Level During 500 Year ARI Event at the Abrolhos Islands

Timeframe Sea Level Rise 500 year ARI Additional Wind Total Water Level Cyclone Setup & Wave Setup (mAHD) (m) (mAHD) (m)

Present Day 0 0.9 1.0 1.9

25yr - 2043 0.14 0.9 1.0 2.1

50yr - 2068 0.36 0.9 1.0 2.3

100yr - 2118 0.90 0.9 1.0 2.8

For the remaining coastal islands, all located within 5 kms from the mainland, coastal inundation levels are expected to be similar to those on the mainland.

MRA have previously completed extensive inundation assessments for a number of locations relevant to or within the NAR including:

 Joondalup - Cyclone Modelling (MRA 2016a).

 Cervantes - Coastal Hazard Risk Management and Adaptation Plan (MRA 2016b)

 Greenough (25 kms south of Geraldton) - Inundation and Coastal Processes Study (MRA 2017).

2.3 500 Year ARI Inundation Levels Results and outcomes of these assessments were used to determine 500 year ARI storm surge inundation levels for mainland locations adjacent to the islands to be assessed. Modelling in SBEACH (explained in detail in Section 3) was then completed to estimate the actual water levels at the islands during the passage of the 500 year ARI events. This was done by ensuring that the water levels at the mainland matched those determined from previous storm surge modelling and then simulating the unchanged event over the adjusted profiles for the islands.

The 500 year ARI storm surge inundation levels determined for each of the NAR islands considered, including appropriate allowances for sea level rise, are presented in Table 2.2.

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Table 2.3 500 Year ARI Storm Surge Inundation Levels for the NAR Islands

Island/Timeframe Present Day (2018) 25yr - 2043 50yr - 2068 100yr - 2118

Sea Level Rise 0 0.14 0.36 0.90

Abrolhos Islands 1.9 2.1 2.3 2.8

Whitlock Island 2.6 2.8 3.0 3.5

Escape Island 2.6 2.8 3.0 3.5

Favourite Island 2.5 2.7 2.9 3.4

Boullanger Island 2.9 3.1 3.3 3.8

Green Islands 2.4 2.6 2.8 3.3

Wedge Island 2.8 3.0 3.2 3.7

Lancelin Island 2.7 2.9 3.1 3.6

2.4 Available Elevation Data In 2016, Fugro LADS Corporation were engaged by the Department of Transport to undertake a high-resolution (5 x 5 m) bathymetric and terrestrial coastal survey of more than 400 kms of coastline. The area surveyed spanned the coast between Hillarys and Horrocks, including around the Abrolhos Islands and 5 inland waterways.

The survey was completed using LiDAR (Light Detection and Ranging) between March and May of 2016. The data is available from the Department of Transport and covers each of the offshore Abrolhos Islands and nearshore islands listed in Table 1.1, being assessed in this report. Figure 2.4 presents a screenshot of the survey area available that includes 3 of the Jurien Bay islands. The data were used to generate contours across each of the islands to be assessed for inundation during the passage of the 500 year ARI event.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 9

Boullanger Island

Jurien Bay

Whitlock Island

Escape Island

Figure 2.4 LiDAR Survey Showing Jurien Bay Islands

2.5 Inundation Hazard Assessment The water levels in Table 2.3 were used in conjunction with the generated island contours to determine the extent of “bathtub” like inundation for each of the NAR islands each of the respective planning horizons. These results are shown by the hazard mapping described in Section 4.

It must be noted that the inundation mapping has been completed to be consistent with the requirements of SPP2.6 and therefore presents the potential inundation during a very severe event. The duration of this inundation would be expected to be limited to less than a handful of hours given the nature of the cyclonic events that would likely cause this level of inundation. The impact of this inundation on the flora and fauna would need to be considered, however, it is expected that over the lifetime of the islands a level of inundation equivalent or greater than that plotted for the present day scenario would have been experienced. The fact that there is flora and fauna on islands that would have been entirely inundated is therefore testament to the potential resilience of these ecosystems.

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3. Coastal Erosion Assessment Schedule One of SPP2.6 presents the recommended methodology for calculation of coastal erosion hazards. For sandy coasts, the assessment requires that consideration be given to the potential impacts of each of the following.

 Severe storm erosion associated with the 100 year ARI event (termed S1 allowance).

 Long term shoreline movement (termed S2 allowance).

 Sea level rise (termed S3 allowance).

 An allowance of 0.2 m/yr for uncertainty.

For rocky coasts, the methodology is less defined in SPP2.6 due to the different slopes, durability, consistency and thickness of various shorelines under this classification. It is to be noted that SPP2.6 recognises that coastal erosion rates on the majority of rocky coasts is relatively slow and should therefore be treated differently to sandy coasts. The erosion allowances should be based on an assessment of the potential erodibility of the rock over the planning timeframe.

The 8 islands within the Turquoise Coast NR can be classified as either sandy or rocky coasts using the methods as discussed in Section 3.1.

The geological composition of the Abrolhos Islands however, is complex and differs significantly between the various islands as described in Geology of the Houtman Abrolhos Islands (Collins et al. 1997). The islands are the emergent parts of three large shallow reef carbonate platforms and have been classified into five different types based on their morphological and stratigraphic features. The five island types are described as follows and shown in Figure 3.1.

 Eolianite islands: Consisting of a core of reef limestone that is overlain by eolianites (lithified rock) and unconsolidated dune sands.

 High rock islands: Consisting of flat topped rocky islands with coastal morphology generally dominated by a well-developed intertidal notch.

 Cemented coral shingle cay: Consisting of coral shingles, bound and cemented by coralline algae and marine cements.

 Low coral shingle/sand cay: Consisting of coral shingle ridges and associated carbonate sands.

 Composite islands: Consisting of a core of emergent coral reef and cemented, imbricated coral rubble, overlain by elongated coral shingle ridges.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 11

Figure 3.1 Abrolhos Island Types

A significant number of the island shorelines assessed by this study are largely composed of both cemented coral shingles and imbricated coral rubble ridges as characteristic of the latter three island types outlined in Figure 3.1. These shorelines are typically quite steep in nature and behave very differently to sandy coastlines. An example of one such shoreline is shown in Figure 3.2, consisting of a combination of both cemented coral shingles and imbricated coral rubble.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 12

Figure 3.2 Photograph of Typical Rubble Shoreline within the Abrolhos Islands (Image from Long Island)

The coral shingles bound by coralline algae can cement over time, leading to the formation of conglomerate shorelines which are more likely to exhibit rocky coastline characteristics. However, as the level of cementation is generally quite weak compared to the rock types discussed in SPP2.6, it isn’t considered appropriate to treat these beaches as rocky coasts.

Similarly, the imbricated coral rubble ridges have significantly larger sediment sizes than those that make up a typical sandy coastline or which can be modelled reliably in SBEACH that features a maximum input grain size of 1.0 mm.

To account for these shorelines, and to appropriately determine likely erosion values throughout the Abrolhos Islands, a separate classification of “Rubble Coasts” was therefore included in this study.

The methodology for the assessment of erosion hazard risks for sandy, rubble and rocky coasts is explained in following Sections.

3.1 Available Data The LiDAR survey data between Hillarys and Horrocks discussed in Section 2, encompassing the NAR islands to be assessed, were again interrogated to generate shoreline profiles for each island. The shoreline response to potential erosion impacts is vastly different depending on the steepness of each profile. Therefore, a number of profiles with different aspects were developed for simulation in SBEACH. For the NAR islands, the West facing profiles were typically steep and the East facing profiles were typically steady and gradually sloping.

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Similarly, the shoreline response to potential erosion impacts also differs greatly depending on the type of shoreline being considered. The NAR islands feature a number of shoreline types including rock, rubble and sand.

CoastAdapt is a coastal specific information tool that is available online (via: https://coastadapt.com.au) and was developed by the National Climate Change Adaptation Research Facility for the Department of the Environment and Energy. The site has been made available to assist and support coastal decision-makers and managers in Australia and includes resources such as climate change causes, projections and potential adaptation strategies. Importantly for this study, the CoastAdapt site has a shoreline explorer that gives a comprehensive map of the location and extent of the various types of shores found in Australia. A handful of the islands assessed in this project have shoreline classifications given by CoastAdapt as demonstrated by Figure 3.3.

Jurien Bay

Boullanger Island

Whitlock Island

Escape Island

Figure 3.3 CoastAdapt Shoreline Classification Showing Jurien Bay Islands

A number of islands including Lancelin Island, Green Island and the Abrolhos Islands however, are not classified by the CoastAdapt shoreline explorer and required classification for further assessment.

Available aerial imagery is limited for the NAR islands. Where available from Landgate, aerial images were purchased and examined to confirm or fill in the shoreline classifications for each of the islands.

Available imagery was also used to map the current vegetation line and approximate the horizontal shoreline datum (HSD) - active limit of the shoreline under storm activity. The HSD was used by the erosion hazard mapping described in Section 4.

3.2 Severe Storm Erosion Sand & Rubble Coasts - S1 Allowance Severe storm events have the potential to cause increased erosion to a shoreline, through the combination of higher steeper waves generated by sustained strong winds, and increased water m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 14

levels. These two factors acting in concert allow waves to erode the upper parts of the beach not normally vulnerable to wave attack.

If the initial width of the surf zone is insufficient to dissipate the increased wave energy, this energy is often spent eroding the beach face, beach berm and sometimes the dunes. The eroded sand is transported offshore with the return water flow to form offshore bars. As these bars grow, they can cause incoming waves to break further offshore, decreasing the wave energy available to attack the beach. This is shown diagrammatically in Figure 3.4 for a sandy coastline.

Figure 3.4 Storm Wave Attack

SPP2.6 recommends that potential cross shore erosion be determined by modelling the impact of an appropriate storm sequence using acceptable models such as SBEACH (WAPC 2013). The SBEACH computer model was developed by the Coastal Engineering Research Centre to simulate beach profile evolution in response to storm events. It is described in detail by Larson & m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 15

Kraus (1989). Since this time the model has been further developed, updated and verified based on field measurements (Wise et al 1996). It is also specified in SPP2.6 that the modelled storm for the calculation of the severe storm erosion allowance should have an ARI of 100 years.

It is widely accepted that simulating 3 repeats of a severe storm sequence that affected south west Western Australia in July 1996 provides a conservative representation of the 100 year beach erosion event. This storm sequence had elevated water levels for a period of approximately 111 hours and caused significant coastal erosion at a number of locations in Western Australia. Modelling three consecutive repeats of this storm therefore simulates the effects of over 330 hours of storm conditions at the shoreline.

SBEACH modelling of the July 1996 event was completed for each of the profiles, covering a full range of shoreline aspects. Consideration of aspect was important, as island shorelines with an easterly aspect would not be exposed to the same wave conditions as those facing offshore.

Where erosion output values differed slightly between island profiles, the critical and more conservative values were applied to each of the general profiles of the same aspect. The outcomes of the SBEACH modelling for the general groups of profile classification and aspect are summarised by the following table.

Table 3.1 S1 Erosion of Various NAR Island Profiles

Profile Erosion (m)

Sandy Coast, North, South and West Aspects 25

Sandy Coast, East Aspect 5

Rubble Coast, North, South and West Aspects 10

3.3 Historic Shoreline Movement Sand & Rubble Coasts - S2 Allowance As mentioned previously, there is very little aerial imagery available for the NAR islands, especially the Abrolhos Islands, to enable review of discern historical shoreline movement. That being said, the majority of rock and rubble shorelines identified for the NAR islands mean that shoreline movement has likely been relatively slow historically. For this reason, the uncertainty factor of 0.2 m/yr has been applied and this, in combination with the potentially conservative allowance for erosion caused by sea level rise, should provide an appropriate allowance for this level of assessment.

3.4 Sea Level Rise Sand & Rubble Coasts - S3 Allowance The potential sea level rise that is required to be used for coastal planning within Western Australia, to be considered for the S3 erosion allowance, was previously outlined in Table 2.1.

The effect of sea level rise on the coast is difficult to predict. Komar (1998) provides a reasonable treatment for sandy shores, including examination of the Bruun Rule (Bruun 1962). The Bruun Rule relates the recession of the shoreline to the sea level rise and slope of the nearshore sediment bed:

1  SR )tan( m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 16

where: R = recession of the shore;

θ = average slope of the nearshore sediment bed; and

S = sea level rise.

Komar suggests that the usual range of recession is R = 50S – 100S. However, the “Bruun Rule” does not take into account possible changes in the balance of sediment transported along the shore in response to sea level rise. SPP2.6 recommends that for sandy shores the potential recession be taken as 100 times the estimated sea level rise.

Therefore, for sandy shores of the NAR islands, the S3 erosion allowances are given by the following table.

Table 3.2 S3 Erosion of Various NAR Island Sandy Coasts

Timeframe Sea Level Rise (m) S3 Erosion Allowance (m)

Present Day 0 0

25yr - 2043 0.14 14

50yr - 2068 0.36 36

100yr - 2118 0.90 90

For the rubble coastlines that exist within the Abrolhos islands, the average slope of the nearshore sediment bed is typically much steeper than that for sandy shorelines. A number of profile gradients were measured using the available LiDAR data and it was found that the gradient of these rubble profiles was generally quite steep, as shown in Figure 3.2. As a result, the application of the 100 multiplier for the Bruun Rule would be overly conservative for thes e locations. A more reasonable, though still conservative allowance was therefore adopted for the rubble shorelines within this study, with a profile slope in the order of 1V:40H used as the basis for this assessment.

Applying the Bruun Rule, the recession of the shoreline can be therefore calculated as approximately 40S. For the sea level rise values given previously in Table 2.1, the following S3 erosion allowance values can be determined for rock and rubble coasts of the NAR islands.

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Table 3.3 S3 Erosion of Various NAR Island Rubble Coasts

Timeframe Sea Level Rise (m) S3 Erosion Allowance (m)

Present Day 0 0

25yr - 2043 0.14 6

50yr - 2068 0.36 15

100yr - 2118 0.90 36

3.5 Rocky Shoreline Allowance SPP2.6 outlines that for rocky shorelines the appropriate allowance for coastal erosion should consider the potential erodibility of the rock matrix itself. This assessment should ideally be completed based on a geological/geotechnical review of the competence of the rock and its potential erodibility. Such assessments were beyond the scope of this initial study, however available information from other areas has been used to provide an indication of the potential shoreline response for planning and review purposes.

On the south-western coastline of Western Australia a number of the coastal limestone formations have been assessed by engineering geologists to provide indications of the potential future change. Based on a review of the history of previous change through review of aerial imagery, combined with review of typical rock strengths, it is generally accepted that the rate of erosion of these limestone shorelines should be less than 10 to 20 m over a 100 year period. However, in the absence of detailed review and investigations on the island shorelines within the study area, it is considered appropriate to incorporate a more conservative estimate of 0.3 m/year to allow for potential erosion of the rocky shorelines. This has been applied to estimate the erosion hazard allowance for rocky coasts.

3.6 Erosion Hazard Assessment A summary of the erosion allowances and totals for each of the shoreline classifications and aspects found on the NAR islands, is given below in Tables 3.4 to 3.7 for the respective planning horizons. These results are shown spatially by the hazard mapping described in Section 4.

Table 3.4 Erosion Allowances for Sandy Coasts, North, South and West Aspects

Timeframe S1 (m) S3 (m) U (m) Total Erosion (m)

Present Day 25 0 0 25

25yr - 2043 25 14 5 44

50yr - 2068 25 36 10 71

100yr - 2118 25 90 20 135

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 18

Table 3.5 Erosion Allowances for Sandy Coasts, East Aspect

Timeframe S1 (m) S3 (m) U (m) Total Erosion (m)

Present Day 5 0 0 5

25yr - 2043 5 14 5 24

50yr - 2068 5 36 10 51

100yr - 2118 5 90 20 115

Table 3.6 Erosion Allowances for Rubble Coasts, North, South and West Aspects

Timeframe S1 (m) S3 (m) U (m) Total Erosion (m)

Present Day 10 0 0 10

25yr - 2043 10 6 5 21

50yr - 2068 10 15 10 35

100yr - 2118 10 36 20 66

Table 3.7 Erosion Allowances for Rocky Coasts

Timeframe Total Erosion (m)

Present Day 0

25yr - 2043 8

50yr - 2068 15

100yr - 2118 30

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 19

4. Coastal Hazard Mapping Coastal hazard maps are included in Appendix A and display both inundation and erosion hazards for the 25, 50 and 100 year planning horizons to 2043, 2068 and 2118 respectively.

As discussed in Section 2, “bathtub” like inundation has been mapped for each of the NAR islands based on the contours generated from the LiDAR survey data available. The maps show widespread inundation during the passage of the 500 year ARI event, for each of the planning horizons including present day. This isn’t unexpected given the typical low elevations of large sections of the NAR islands.

The total erosion values calculated for each aspect and shoreline type in Section 3 have been used to map the coastal erosion hazards for each planning horizon. Similarly to inundation, the maps show significant erosion hazards for the NAR islands, especially small islands including Tattler Island and Beacon Island shown to be vulnerable to complete erosion by 2118. This can be largely attributed to the principal erosion component, sea level rise, with projections of up to 0.9 m forecast towards the end of this century.

The hazard mapping completed for both inundation and erosion over the 25, 50 and 100 year planning horizons has also been provided in an ESRI shapefile format to be used and viewed on GIS mapping software. This will enable future analysis and comparison throughout the 100 year planning timeframe.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 20

5. Conclusion The NAR spans a large portion of the Western Australian coastline and features over 150 islands, many of which are significant sites of natural resource management investment. 30 islands within the NAR (22 in the Abrolhos and 8 in the Turquoise Coast NR) were identified by manager stakeholders as having significant terrestrial NRM values at risk from coastal hazards and therefore considered for this assessment.

Section 2 outlined the methodology and determination of the 500 year ARI inundation event water levels for each island over the 100 year planning timeframe. The available LiDAR data for the NAR islands was used to plot these inundation water levels as displayed in Appendix A. As shown by the coastal hazard mapping, the majority of NAR islands are expected to be significantly inundated by the 500 year ARI event over the 100 year planning timeframe.

It is again important that consideration is given to the fact that large portions of these islands have likely experienced inundation from severe events in the past, however remain mostly vegetated. This may in part be due to the resilient nature of the vegetation in the NAR, but also likely due to the short duration of inundation expected during such cyclone induced storm surge events as described in Section 2.

Section 3 outlined the methodology and modelling to calculate erosion hazard distances for the various shoreline types and aspects found within the NAR islands. Spatial plots of these hazards were also mapped as shown in Appendix A.

Erosion hazards are considered to be more permanent than inundation and in the case that all allowances are realised over the planning timeframes, smaller islands such as Tattler Island and Beacon Island could be completely vulnerable to erosion.

The coastal mapping described in Section 4 and shown in Appendix A has been produced to provide guidance regarding the risks posed by coastal hazards both at present and into the future, over the 100 year planning timeframe. The methodology implemented, which was consistent with the requirements of SPP2.6, enabled the determination of erosion and inundation hazards to aid natural resource managers in determining the timeframes for which specific islands may become unsuitable for maintaining certain species.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 21

6. References Bruun, P. 1962, Sea level rise as a cause of shore erosion, Journal Waterways and Harbours Division, American Society of Civil Engineers. WWI, 88, pp. 117-130.

Collins, L. B., Zhu, Z. R. and Wyroll, K-H. 1997. Geology of the Houtman Abrolhos Islands, Chapter 28: Geology and Hydrogeology of Carbonate Islands: Developments in Sedimentology, Elsevier Science 54: 811-833.

Department of Transport, 2010. Sea Level Change in Western Australia – Application to Coastal Planning. Government of Western Australia, Perth.

Hunter, J., 2009. Estimating sea-level extremes under conditions of uncertain sea-level rise. Climatic Change, DOI:10.1007/s10584-009-9671-6, published online at www.springerlink.com.

IPCC. 2007, Fourth Assessment Report - Climate Change 2007. Published by the IPCC.

Komar, P D 1998. Beach Processes and Sedimentation (2nd Edition). Prentice Hall Inc, New Jersey, USA.

Larson, M. & Kraus, N. C. 1989. SBEACH: Numerical Model for Simulating Storm-Induced Beach Change; Report 1 Empirical foundation and model development. Technical Report CERC -89- 9. Coastal Engineering Research Centre, Vicksburg, MS, USA.

MRA 2015. Point Moore Inundation & Coastal Processes Study, R656 Rev 1. Prepared for City of Greater Geraldton.

MRA 2016a. City of Joondalup Cyclone Modelling, R774 Rev 0. Prepared for City of Joondalup.

MRA 2016b. Pinnacles Holiday Park Coastal Hazard Rick Management & Adaptation Plan, R766 Rev 0. Prepared for RAC Developments.

MRA 2017. Cape Burney to Greys Beach Inundation & Coastal Processes Study, R810 Rev 0. Prepared for City of Greater Geraldton.

WAPC 2013. State Planning Policy No. 2.6 - State Coastal Planning Policy. Western Australian Planning Commission, Perth.

Wise, R. A., Smith, S. J. & Larson, M. 1996. SBEACH: Numerical Model for Simulating Storm- Induced Beach Change; Report 4, Cross shore transport under random waves and model validation with SUPERTANK and field data. Technical Report CERC-89-9 rept. 4. Coastal Engineering Research Centre, Vicksburg, MS.

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 22

7. Appendices Appendix A Coastal Hazard Maps

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 23

Appendix A Coastal Hazard Maps

m p rogers & associates pl NACC, Coastal Hazard Assessment of Islands in the NAR K1516, Report R1009 Rev 0, Page 24 0 50 100 150 200 m

LANCELIN ISLAND 1:3,000

LEGEND:

Rock Shoreline Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area 2068 Inundation Hazard Area 0 25 50 75 100 m 2118 Inundation Hazard Area WEDGE ISLAND 1:2,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in LANCELIN & WEDGE ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-01 AS SHOWN LEGEND:

Rock Shoreline NORTH GREEN ISLAND Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area 2068 Inundation Hazard Area 2118 Inundation Hazard Area

SOUTH GREEN ISLAND

0 50 100 150 200 m

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in GREEN ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-02 1:2,000 0100200300400 m

ESCAPE ISLAND 1:5,000

LEGEND:

WHITLOCK ISLAND 1:3,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in ESCAPE & WHITLOCK ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-03 AS SHOWN 0 100 200 300 400 m

BOULLANGER ISLAND 1:5,000

LEGEND:

Rock Shoreline Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area

0 50 100 150 200 m 2068 Inundation Hazard Area 2118 Inundation Hazard Area FAVORITE ISLAND 1:3,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in BOULLANGER & FAVORITE ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-04 AS SHOWN 0 200 400 600 800 m

LEGEND:

0 200 400 600 800 m

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in PELSAERT ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-05 1:12,000 0 200 400 600 800 m

PELSAERT ISLAND 1:12,000

LEGEND:

MIDDLE ISLAND 1:5,000

CLIENT PROJECT

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GUN ISLAND 1:5,000

LEGEND:

Rock Shoreline Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area

0 50 100 150 200 m 2068 Inundation Hazard Area 2118 Inundation Hazard Area STICK ISLAND 1:3,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in GUN & STICK ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-07 AS SHOWN 0 150 300 450 600 m

LEO ISLAND 1:8,000

LEGEND:

ALEXANDER ISLAND 1:5,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in LEO & ALEXANDER ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-08 AS SHOWN 0 100 200 300 400 m LEGEND:

SERVENTY ISLAND Rock Shoreline 1:5,000 Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area 2068 Inundation Hazard Area 2118 Inundation Hazard Area

0 100 200 300 400 m

CAMPBELL ISLAND 1:5,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in SERVENTY & CAMPBELL ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-09 AS SHOWN 0 100 200 300 400 m

LEGEND:

0 100 200 300 400 m

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in SUOMI ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-10 1:5,000 LEGEND:

Rock Shoreline Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area 2068 Inundation Hazard Area

0 50 100 150 200 m 2118 Inundation Hazard Area

KERU ISLAND 1:3,000

0 100 200 300 400 m

WOODED ISLAND MORLEY ISLAND 1:6,000 1:6,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in KERU, WOODED & MORLEY ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-11 AS SHOWN RAT ISLAND

LEGEND:

Rock Shoreline

LITTLE RAT ISLAND Rubble Shoreline Sand Shoreline 2043 Erosion Hazard Line 2068 Erosion Hazard Line 2118 Erosion Hazard Line Present Day 2043 Inundation Hazard Area 2068 Inundation Hazard Area 0 100 200 300 400 m 2118 Inundation Hazard Area

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE AGRICULTURAL document are the Copyright of m p rogers & associates. Use or copying of the document in RAT & LITTLE RAT ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 REV A 13.02.18 infringement of copyright. D1516-01-12 1:6,000 0 100 200 300 400 m

LONG ISLAND 1:5,000

LEGEND:

BEACON ISLAND 1:2,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in LONG & BEACON ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-13 AS SHOWN LEGEND:

0 300 600 900 1200 m

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in WEST WALLABI ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-14 1:15,000 LEGEND:

0 200 400 600 800 m

CLIENT PROJECT

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PIGEON ISLAND 1:3,000

0 50 100 150 200 m 0 10 20 30 40 m

SEAGULL ISLAND TATTLER ISLAND 1:5,000 1:1,000

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in PIGEON, SEAGULL & TATTLER ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-16 AS SHOWN LEGEND:

CLIENT PROJECT

NORTHERN The concepts and information contained in this TITLE document are the Copyright of m p rogers & AGRICULTURAL associates. Use or copying of the document in NORTH ISLAND CATCHMENT COUNCIL whole or part without the written permission of m p rogers & associates constitutes an DRAWING NUMBER SCALE AT A3 infringement of copyright. REV A 13.02.18 D1516-01-17 1:7,000