Shoalhaven Coastal Erosion Remediation Adaptive Works Strategy for Transitioning from “Make- Safe/Make-Good” to “End-State” Protection

Shoalhaven City Council

Final Draft 8A0386

HASKONING AUSTRALIA MARITIME & WATERWAYS

Suite 5, Level 5 100 Walker Street NORTH SYDNEY NSW 2060 +61 (0) 8854 5000 www.royalhaskoningdhv.com

Document title Shoalhaven Coastal Erosion Remediation Adaptive Works Strategy for Transitioning from “Make-Safe/Make-Good” to “End-State” Protection Status Final Draft Date July 2014 Project name Shoalhaven Coastal Erosion Remediation Implementation Strategy Project number 8A0386 Client Shoalhaven City Council Reference 8A0386gpb_17-2-14_report

Drafted by Gary Blumberg

Checked by Greg Britton Date/initials check 22/6/14 GWB Approved by Date/initials approval

CONTENTS

Page

1 INTRODUCTION 1 1.1 Background 1 1.2 Study Objectives 2 1.3 Study Area 2 1.4 Scope of Work 3 1.5 Glossary 3 1.6 Level Datum 4 1.7 Acknowledgements 4

2 DESIGN LIFE AND RISK FOR COASTAL MANAGEMENT IN THE SHOALHAVEN 5 2.1 General 5 2.2 Design Life for Coastal Developments 5 2.3 Traditional Method to Determine Degree of Coastal Erosion Hazard 6 2.4 Risk-based Determination of Coastal Erosion Hazard Lines 7

3 VULNERABILITY RANKING OF SCC BEACHES 10

4 AVAILABILITY OF SMALL SCALE SAND RESERVES FOR EROSION MANAGEMENT AT SCC BEACHES 11 4.1 Sand Quantities required for Long Term Shoreline Protection 11 4.2 Possible Sand Quantities for Short to Medium Term Protection sourced from Estuary and Entrance Sites in Shoalhaven 11

5 EXISTING “MAKE-SAFE/MAKE-GOOD” PROVISIONS 15 5.1 Introduction 15 5.2 Generic Provisions 15 5.3 Specific Provisions for Most Vulnerable SCC Beaches 15 5.3.1 Culburra 15 5.3.2 Currarong 16 5.3.3 Callala Beach 16 5.3.4 Collingwood Beach 16 5.3.5 Narrawallee Beach 16 5.3.6 Mollymook Beach 17

6 PROPOSED “END-STATE” STRUCTURAL PROVISIONS 18 6.1 Introduction 18 6.2 Shoalhaven Heads Beach 18 6.3 Culburra Beach 19 6.4 Warrain Beach 19 6.5 Currarong Beach 20 6.6 Callala Bay Beach 20 6.6.1 Long-term Management Scheme addressing Erosion and Access 21 6.6.2 Meachnical Sand Bypassing of Boat Ramp Reclamation 21 6.7 Callala Beach 22

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6.8 Collingwood Beach 25 6.1 Narrawallee Beach 27 6.2 Mollymook Beach 27

7 ADAPTIVE MANAGEMENT CONCEPTS 30 7.1 General 30 7.2 Monitoring and Trigger Response 30 7.3 Staging of Works 31 7.4 Demonstration Projects and Trials 31

8 STRATEGIES FOR TRANSITIONAL WORKS BETWEEN “MAKE- SAFE/MAKE-GOOD” AND “END-STATE” STRUCTURES 32 8.1 Introduction 32 8.2 Generic Transitional Works Schemes 32 8.2.1 Transitional Arrangements for Revetment Works 32 8.2.2 Transitional Arrangements involving Sustainable Recycling of Beach Sand 36 8.3 Suggestions for Transitional Works Schemes at Particular Beaches 43 8.3.1 Shoalhaven Heads Beach 43 8.3.2 Currarong Beach 44 8.3.1 Callala Bay Beach 48 8.3.2 Callala Beach 48 8.3.3 Collingwood Beach 50 8.3.4 Narrawallee Beach 53 8.3.5 Mollymook Beach 53

9 IMPORTANCE OF FULLY VEGETATED DUNE SYSTEMS 56

10 ADDITIONAL INVESTIGATIONS REQUIRED 57

11 REFERENCES 59

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1 INTRODUCTION

1.1 Background

Shoalhaven City Council (SCC) has recently prepared its draft Coastal Zone Management Plan covering all 35 breaches in its Local Government Area (Umwelt, 2012). Draft Coastal Emergency Action Subplans have also been prepared for the high risk beaches (Umwelt, 2011), as well as an Asset Management Plan for Council’s coastal and estuary assets (SCC, 2014).

In the event of storms, a range of “make-safe/make-good” provisions are already in place (Section 5). These can be introduced at all of the 35 beaches.

In the last two years, Council has developed conceptual designs for “end-state” protection of property at its three “authorised location” beaches; namely at Mollymook, Collingwood and Callala Beaches. These are costly to implement and are presently unaffordable.

An adaptive works strategy is required for moving from “make-safe/make-good” to “end- state” protection. Ideally this should be set in a risk context involving triggers based on distance from assets to the erosion escarpment.

The strategy is needed covering Council’s primary beaches. A largely generic response would be provided under the current investigation, but individualised at those beaches where higher vulnerability rankings occur. The programs would be staged and affordable. Listed north the south, the primary beaches are:

 Shoalhaven Heads Beach  Culburra Beach  Warrain Beach  Currawong Beach  Callala Bay Beach  Callala Beach  Collingwood Beach  Narrawallee Beach  Mollymook Beach

Haskoning Australia (HKA) has been retained by SCC to develop a suitable adaptive strategy for making the transition from “make-good/make-safe” to “end-state”. The issues need to be explained succinctly, and misconceptions addressed. Clear and logical outputs are sought to assist Council in its consultations with stakeholders.

Numerous coastal processes, management and monitoring investigations undertaken by SCC are available to inform the new strategy.

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1.2 Study Objectives

Council is seeking a coastal engineer-endorsed strategy to bridge “make-safe/make- good” provisions, to “end-state” protection arrangements.

The strategy is to identify a range of adaptive works and associated approximate costs. Works would be implemented incrementally to address progressive exposure based on triggers, and preferably contributing towards community-supported “end state” works.

The strategy would:

 highlight works at Council’s most vulnerable beach sites, but also develop generic responses and principles for its other beaches; and  be aimed at informing Council’s ongoing beach community consultation program, to recommence shortly with Mollymook and Collingwood Beaches.

Management of beach accessways is a priority for SCC. However, the type and number of accessways must be appropriate to the traffic that they support. Concepts and proposals put forward in the strategy must allow for all-ability access.

The strategy would reference relevant standards and industry accepted coastal engineering texts where appropriate.

1.3 Study Area

Coastal management by SCC is focused on nine key beaches, listed north to south as follows:

1. Shoalhaven Heads Beach 2. Culburra Beach 3. Warrain Beach 4. Currawong Beach 5. Callala Bay Beach 6. Callala Beach 7. Collingwood Beach 8. Narrawallee Beach 9. Mollymook Beach

Assets are at risk from coastal processes under current conditions at all of these beaches.

The study area covers all Shoalhaven local government area beaches of which there are 35, but retains its focus on the most vulnerable beaches in the LGA.

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1.4 Scope of Work

A scope of work was developed through discussions between SCC, OEH and HKA. These discussions culminated in a meeting held at HKA offices on 11 February 2014, and further discussions between HKA and Council to clarify the study deliverables.

The agreed scope of work covers the following:

 Overview of design life and risk for coastal management in the Shoalhaven: - Design life - Traditional coastal hazard lines - Risk-based coastal hazard lines  Vulnerability ranking of SCC beaches  Availability of small scale sand reserves for erosion management at SCC beaches  Existing “make-safe/make-good” provisions  Proposed “end-state” structural provisions  Adaptive strategies for transitional arrangements from “make-safe/make-good” to “end-state” structures - Generic ideas - Staging of works - Monitoring and triggers  Followup investigations

1.5 Glossary

“end state” engineering design for comprehensive protection of private and public assets at risk to current standards and development planning horizons within industry accepted projections for NSW wave climate and climate change scenarios. “make safe – involves assessing storm damage, closing dangerous beach access make good” ways, reshaping the beach erosion scarp to provide for reasonable public access. Responding to storm damage with suitable rationalisation of numbers of accessways, positions and design as proposed in Council’s Asset Management Plan. tombolo one or more sandbars or spits that connect an island to the mainland ZSA Zone of Slope Adjustment in accordance with Wedge Failure Plane model (Figure 1). In simple terms, an unstable zone of the dune at the back of the eroded beach which, when it dries out after the storm has passed, would slump to a more stable slope. ZRFC Zone of Reduced Foundation Capacity in accordance with Wedge Failure Plane model (Figure 1). In simple terms, a partially unstable zone of the dune during a coastal erosion event, landward of the Zone of Slope Adjustment, within which it is not acceptable to locate foundations for coastal buildings and infrastructure.

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1.6 Level Datum

All reference to Reduced Level in this report is in metres relative to Australian Height Datum (AHD). AHD is approximately current Mean Sea Level at the Australian coastline.

1.7 Acknowledgements

RHDHV would like to acknowledge the assistance provided by Ray Massie and Isabelle Ghetti in steering this investigation on behalf of Shoalhaven City Council.

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2 DESIGN LIFE AND RISK FOR COASTAL MANAGEMENT IN THE SHOALHAVEN

2.1 General

Coastal hazards are primarily related to erosion. Inundation is a further hazard, but it is more manageable and its consequences less severe.

It is also not usual to be concerned about risk to life in coastal hazard situations. Erosion does not occur without warning. Storm waves develop relatively slowly and potentially only threaten at or near the top of the tide. Warning systems for coastal storms are also in place by the Bureau of Meteorology and public announcements are made. While there are specific situations where collapse of steep sand faces have threatened beach users, it is reasonable to assume that education (including warning signage) and retreat from harm’s way can suitably manage risk to life. This report is therefore exclusively concerned with risks to property and assets.

To characterise risk for coastal management purposes there is a need to identify a planning period or design life. This then feeds into the assessment of coastal hazard and risk. It is not the ambit of this investigation to detail these matters, but a short discussion is warranted to provide context.

2.2 Design Life for Coastal Developments

It is normal practice to adopt a design life for residential development extending to 50 years, and for infrastructure up to 100 years. These periods are sometimes varied depending on whether the sites are greenfield or developed, and also depending on the scale of the project. Infill residential developments could have a 40 year life, whereas new residential subdivisions would need to be planned for at least 100 years. Important infrastructure such as sewerage, water and power would normally also have a minimum 100 year horizon.

AS 4997 Guidelines for Design of Maritime Structures recommends a design working life as 5 years or less for temporary works, 25 years for small craft facilities, 50 years for normal maritime structures and 100 years or more for special structures and residential developments (interpreted as multi-occupancy and probably greenfield).

Design life considerations for risk assessments developed by Royal HaskoningDHV (RHDHV) for Collaroy-Narrabeen (RHDHV, 2014a in prep) and Old Bar (RHDHV, 2014b) Beaches have referenced other authorised sources in Australia. The concrete structures standard (AS3600) refers to 50 years +/- 20%, that is 40 – 60 years for devising durability requirements for concrete structures. Other standards (AS2870 Residential Slabs and Footings and AS1170 Structural Design Actions) also refer to 50 years for normal, residential structures. As an indicator of extended life sought for important infrastructure, AS4678 Earth Retaining Structures, requires a 60 year life for riverine and marine structures. Finally, it is notable that the Australian Taxation Office permits the cost of new residential development to be amortised for tax purposes over 40 years (Subdivision 43-25, Income Tax Act 1997).

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Based on the above it would seem reasonable for SCC to consider say 40 years for the life of major residential alterations, 40-50 years for new houses in developed areas, and 60 to 100 years for infrastructure and subdivisions depending on scale and proximity to existing development.

2.3 Traditional Method to Determine Degree of Coastal Erosion Hazard

OEH prescribes a traditional method to determine coastal erosion hazard as originally set out in the NSW Coastline Management Manual (NSW Government, 1990) and cross-referenced in more recent guideline documents issued by OEH, particularly the Coastal Risk Management Guide (DECCW, 2010a) and Guidelines for Preparing Coastal Zone Management Plans (DECCW, 2010b).

The traditional method uses historical beach behaviour from available measurement and survey, and predictions of beach erosion and recession, to develop coastal hazard lines. The situation at the present, together with planning periods of typically 50 and 100 years, are applied with the beach receded according to sediment budget losses, plus the effects of sea level rise on recession. The approach taken is to assume that the design storm could occur today, or at any time during the planning period.. The lines are mapped in three different ways: as either the extent of wave impact during the design storm, the extent of dune slumping following the design storm, or the extent of reduced foundation capacity associated with the design storm. The Wedge Failure Plane after Nielsen et al (1992) is applied to define these mappings as shown in Figure 1.

Figure 1: Schematic representation of coastline hazard zones in Wedge Failure Plane model (after Nielsen et al , 1992)

In 2006/07, SMEC developed coastal hazard lines using the traditional method for all high risk beaches in the Shoalhaven, listed north to south, as Shoalhaven Heads Beach, Culburra Beach, Warrain Beach, Currarong Beach, Callala Beach, Collingwood Beach, Collers Beach and Mollymook Beach. Traditional coastal hazard lines were generated.

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Later, in 2010, following the adoption by SCC of sea level rise (SLR) benchmarks recommended by the NSW Government, Council set about amending the traditional coastal hazard lines to account for the recommended SLR increments of 0.4 and 0.9 m for planning dates 2050 and 2100 respectively based on 1990 levels.

SCC Interim Policy - Areas of Coastal Management (Policy No POL10/112) is an interim policy which aims to guide development in areas of coastal instability in the interim until such time as draft Development Control Plan (DCP) No 118 – Areas of Coastal Management is adopted by Council. The Interim Policy adopts the 2025 coastal hazard line, developed using the Traditional Method, as a hazard line seaward of which no new development is permitted but where minor works to existing buildings of limited area can take place subject to merit assessment. As 2025 is well within a 40 to 50 year planning horizon from today, it follows that the Interim Policy may need to be reviewed soon to permit development decisions to be made by SCC which are consistent with the assessment of coastal hazards based on the Traditional Method.

2.4 Risk-based Determination of Coastal Erosion Hazard Lines

Risk is a measure of the combination of likelihood of a deleterious event and its consequences. Recently OEH has started to accept a risk-based determination of coastal erosion hazard lines.

The risk-based methods allow for the actual design life of the asset which is being protected and look at the probability of the coastal hazard occurring over that period. The methods recognise that component hazards need not occur simultaneously and at equivalent levels of severity. For example, there is less chance that a severe prediction of sea level rise would occur at the same time as an extreme prediction of losses due to normal sediment budget, or very severe storms, and it is reasonable that this modified (reduced) likelihood is used to predict the actual hazard. The methods also consider what levels of risk are considered acceptable without the need for treatment to mitigate the risks.

The risk-based approach was initiated in NSW in qualified terms about 6 years ago for the Coffs Harbour coastline. Since then it has been considered for other sites and is incorporated into the current OEH guidelines for preparation of CZMPs. Within the past couple of years further attention has been paid to the quantification of the risks, with procedures adapted from the Australian Geomechanics Society (AGS) and their assessment of landslide risk developed in the wake of the Thredbo disaster in the late 1990s (AGS, 2007a, 2007b).

RHDHV has prepared a risk-based determination of coastal hazard lines for Collaroy Narrabeen Beach (RHDHV, 2014a in prep) and Old Bar (RHDHV, 2014b). The procedure adopts a design life for residential development, quantifies the likelihood and consequences of the coastal erosion hazard, and goes on to describe likelihood and acceptable risk lines.

It is determined that a reasonable design life for devising setbacks and controls for beachfront development is between 40 and 60 years. Since future climate is uncertain, a conservative upper-end design life of 60 years is adopted.

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Likelihood descriptors and associated probabilities are described using the AGS methodology for a design life of 60 years. The descriptors used are “almost certain”, “likely”, “possible”, “unlikely”, “rare” and “barely credible”.

Recession likelihood is then characterised as “mild case”, “best estimate” and “severe”, and attributed probabilities of exceedance of 95%, 50% and 5% respectively. Storm demand likelihood is quantified in two ways, firstly based on a storm occurring at any time over the 60 year design life, and secondly for a storm occurring at the end of the design life (the same as the traditional approach, Section 2.3). At Old Bar, where very high recession rates have been experienced in recent years, it is found that recession dominates the likelihood description of the hazard, when the storm demand contribution is relatively small.

The consequence descriptors from AGS (2007a) are applied, namely “catastrophic”, “major”, “medium”, “minor” and “insignificant”. Bay way of examples, a “major” consequence is associated with a cost of damage between 40 and 100% of the cost of a structure, whereas a “minor” consequence ranges between 1 and 10%. “Catastrophic” (>100%) has a structure completely destroyed or damaged to the extent that rectification may exceed the cost of the structure (eg. damage may include incidental damage to adjoining infrastructure).

Likelihood lines are then generated and mapped. At Old Bar, a comparison of the likelihood lines with the traditional hazard lines found that the traditional Immediate Zone of Slope Adjustment (ZSA) was comparable to the “almost certain” risk-based line, and the traditional 50 year Zone of Reduced Foundation Capacity (ZRFC) line was in places generally similar to the “likely” risk-based line.

A conventional risk matrix is then used to combine likelihood and consequence to develop the description of the risk (Table 1). So from the table, if the event was “almost certain” to happen and even if the consequences were considered to be “insignificant”, the risk would be deemed “medium”. If the event was considered “rare” and the consequences were “major”, the risk would be deemed “low”.

Table 1: Risk matrix Consequence Likelihood Catastrophic Major Medium Minor Insignificant Almost Very high Very high Very high High Medium certain Likely Very high Very high High Medium Low Possible Very high High Medium Medium Very low Unlikely High Medium Low Low Very low Rare Medium Low Low Very low Very low Barely Low Very low Very low Very low Very low credible

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An “acceptable risk” is a risk that is understood and tolerated usually because the cost or difficulty of implementing an effective countermeasure exceeds the expectation of loss. Thus society is prepared to accept this level of risk without management. Expenditure to reduce the risk is not justifiable.

Based on RHDHV’s discussions with the AGS Working Group, it was found that a “low” risk to property was an appropriate recommendation for acceptable risk for a residential dwelling (acceptable risk threshold is designated by the dark line in Table 1). Similarly, a low risk was considered to be acceptable for buildings and facilities where up to 300 people could congregate (eg. school), whereas a “very low” risk was needed to be met for buildings and structures designated as essential facilities or those with special disaster relief functions.

The application of risk-based determination of coastal hazard lines is a work in progress. RHDHV understands that OEH is supportive of the methodology which is being incorporated into new manuals and guidelines that are being prepared to support the Stage 2 NSW Coastal Reforms. More robust statistical procedures are required for combining probabilities of independent events to improve the confidence of the outcomes.

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3 VULNERABILITY RANKING OF SCC BEACHES

Vulnerability is a measure of how changes can harm a system. In relation to natural hazards, vulnerability links the relationship that people have with their environment to social and cultural values. Vulnerability gauges the extent to which a community can be affected by the impact of the hazard, either physically or emotionally.

The University of Sydney, School of Civil Engineering, has recently undertaken an assessment of the relative vulnerability to sea level rise and associated erosion processes of eight exposed beaches in the Shoalhaven (University of Sydney, 2014). These beaches, identified by SCC as part of its long-term adaptation planning, comprised Culburra, Callala, Shoalhaven Heads, Warrain, Mollymook, Collingwood, Collers and Currarong. While these beaches were selected based on geophysical hazard studies undertaken for SCC, it is appreciated that risk exposure is not the only determinant of vulnerability.

The goal of the project was to rank the vulnerability of the eight beaches to sea level rise and associated erosion processes. The investigation used a so-called “indicator-based outranking approach”, developed at the University of Sydney.

The key findings of the study were that Mollymook and Collingwood Beaches are the most vulnerable of the eight beaches. This finding was determined to be insensitive to how community preferences were assembled, hence was presented as a robust result. Both beaches have foreshore infrastructure which is at risk. For Collingwood Beach, while a significant number of private properties are at risk, the beach residents indicated better adaptive capacity. The next most vulnerable beaches are Warrain, Callala and Culburra Beaches where variable scores were developed for exposure, sensitivity and adaptive capacity.

The study suggests a number of adaptation targets for SCC to consider to reduce vulnerability which are worthy of mention here:

 improve robustness of water and wastewater systems at Mollymook and Collingwood;  improve protection of Mitchell Parade and bridge (Mitchell Parade crossing over Blackwater Creek);  engage with stakeholders of Mollymook Golf Club, within the Council and the broader community to canvas ideas for protection of the golf club;  engage with residents along Warrain, Callala and Culburra Beaches to identify ways of reducing their vulnerability to SLR.

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4 AVAILABILITY OF SMALL SCALE SAND RESERVES FOR EROSION MANAGEMENT AT SCC BEACHES

4.1 Sand Quantities required for Long Term Shoreline Protection

Beach nourishment is presented as a bona-fide strategy for long-term protection of coastal property at Mollymook, Callala and Collingwood Beaches, however the significant quantities of sand required to implement such schemes are not presently available. If massive nourishment was implemented to achieve protection over 50 years without companion works, it is estimated that some 1.98 million m3 of sand is required immediately to protect private and public property and infrastructure assets at these three beaches, plus combined ongoing nourishment of approximately 260,000 m3 every 5 years. If companion protection works were constructed, namely groynes at Callala and Collingwood Beaches and selected protective structures at Mollymook Beach, then these immediate and ongoing nourishment volumes would reduce to approximately 690,000 m3 and 180,00 m3 respectively (RHDHV, 2012, 2013a, 2013b).

If only beach amenity is to be preserved without regard to protection of property and assets in storms, similar ongoing nourishment quantities are required at Mollymook and Collingwood Beaches. At Callala Beach, the existing fairweather beach widths are generous and are predicted to exceed the nominated amenity width of 15 m for in excess of 50 years, so no amenity nourishment is required (RHDHV, 2012, 2013a, and 2013b).

4.2 Possible Sand Quantities for Short to Medium Term Protection sourced from Estuary and Entrance Sites in Shoalhaven

The Shoalhaven Citywide Dredging Feasibility Study sets out a strategy to deal with increasing demand for improved navigation and boating safety at a number of estuaries in the Shoalhaven (Peter Spurway & Associates, 2014). The study targeted recent deposits assumed to be clean marine sand, minimising potential disturbance of aquatic animals and habitat. Sites and possible dredge quantities, stated in or inferred from the study, are summarised in Table 2.

The Dredging Feasibility Study considered three types of dredge sites: river and estuary entrance and shoals, lake entrances and boat ramps. Clearing around boat ramps and removal of sand for lake entrance management is potentially associated with small quantities of sand in the order of 5,000 – 10,000 m3 throughout the LGA, generated say every 2 years. Removal of sand from estuary and river entrance shoals, rather than the entrance berm, potentially releases more material, estimated to be up to between 30,000 and 70,000 m3 every 10 years. So in annual average terms, the most sand that could be provided from Council’s dredging projects is in the order of 5,000 – 10,000 m3 per year. This is less than 5% of ongoing nourishment needed to protect from coastal erosion the Shoalhaven’s three “authorised location” beaches at Mollymook, Callala and Collingwood. Clearly if Council’s other threatened beaches are to be included, then the sand shortfall is even greater.

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Table 2 – Sites and Potential Dredge Quantities identified in Shoalhaven Citywide Dredging Feasibility Study (Peter Spurway and Associates, 2014)

Nominal Site Quantity (m3) Recurrence Comments (1)

River and Shoalhaven Heads 2,000 - 15,000 10 yrs Flood relief main issue. Council attempts to maintain ‘dry notch’ to assist with breakout Estuary (2,000 m3), but opening completely for flushing also canvassed (15,000 m3). Entrance Currarong Creek 7,000 10 yrs Estimated quantity available to contribute to nourishment of Currarong Beach (SMEC, 2011). Shoals Refill in 7-15 years depending on other beach protection works at or near the entrance. Currambene Creek 7,000 – 8,000 10 yrs To achieve safe navigation at all tides to a 20-25 m width and dredge level between of 2.0 and 2.5 m below LAT. Sussex Inlet 8,000 10 yrs To restore estuary channel to 1.2 m below LAT Lake Conjola Entrance 10,000 - 30,000 10 yrs Configuration dredging mainly for flood relief (10,000 m3) or larger scale project to provide sand for beach nourishment (30,000 m3)

Lake Entrance Lake Conjola 1,000 - 2,000 1 – 2 yrs Locations to place sand specified under lake’s opening policy and/or REF, mostly to build up Management Swan Lake 1,000 - 2,000 dunes near entrances. Burrill Lake 1,000 - 2,000 Lake Tabourie 1,000 - 2,000

Boat Ramps Callala Bay 1,500 2 - 5 yrs Small volumes, returned to beach on N side of ramp. Silts and clays possible. Sanctuary Point 500 2 - 5 yrs Minor volumes, reused locally Conjola (Aney Street) Nil No dredging warranted Cunjurong Point Nil No dredging warranted. Narrawallee Inlet Nil Limited demand for dredging. Improvements not possible without excessive amounts of sand being removed.

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There are particular threatened beaches in the Shoalhaven that are within reasonable proximity of potential estuary and entrance sand sources that can be linked to possible erosion mitigation projects. These occur at Currarong, Callala, Mollymook and Collingwood Beaches.

Currarong Beach Erosion Remediation Study Options Assessment (SMEC, 2011) developed a preferred erosion mitigation strategy for Currarong Beach which entailed a groyne structure located close to the entrance of Currarong Creek, and placement of some 15,000 to 20,000 m3 of sand to “fill” the groyne. The concept is shown in Figure 2 and discussed in Section 6.5.

Beach nourishment requirements for long-term protection of Callala Beach involve substantial quantities of sand (RHDHV, 2013b). However, there could be some benefit in placing the 7,000 – 8,000 m3 of sand on the beach sourced from Currambene Creek some 4 km away (Section 8.3.2).

Mollymook and Collingwood Beaches are found to be the most vulnerable in the Shoalhaven (Section 3). Like Callala, Collingwood Beach could potentially benefit from placements of any sand sourced from the mouth of Currambene Creek which, although closer at approximately 2.5 km, may be more problematic to deliver the sand (Section 8.3.3). The Shoalhaven “Authorised Locations” Coastal Erosion Remediation Options report for Mollymook Beach presented a staged scheme for a revetment along the southern properties of Mitchell Parade. Stage 1 of that scheme included a sand placement of some 5,100 m3 to increase the dune sand reserves and achieve a uniform dune crest level as discussed in Section 8.3.5. This material could possibly be sourced from Council’s lake entrance clearing activities (4,000 – 8,000 m3 available over a two year period), or even from a dedicated dredging of the entrance shoals in Lake Conjola.

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Figure 2: Preferred coastal management scheme for Currarong Beach comprising groyne and beach nourishment developed by SMEC (2011)

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5 EXISTING “MAKE-SAFE/MAKE-GOOD” PROVISIONS

5.1 Introduction

SCC has in place a number of emergency “make-safe/make-good” provisions. These are mainly enunciated in the Coastal Emergency Action Subplans developed for individual high risk beaches (Umwelt, 2011), and in the Shoalhaven Asset Management Plan for Coastal and Estuary Assets (SCC, 2012).

A brief summary of these provisions in terms of their generic application to the Shoalhaven coastline, and additional specific provisions for the most vulnerable SCC beaches, is presented below.

5.2 Generic Provisions

In accordance with authorised Action Subplans, Council would take the following emergency actions during and after a coastal emergency event:

 Temporarily close public accessways damaged by storm waves  Monitor seawalls  Implement beach scraping if necessary

The NSW State Emergency Services (SES) is the lead agency for storm damage control in NSW. It is responsible for the emergency management of coastal erosion and inundation as a result of severe storms. The SES prepares emergency plans for coastal erosion and inundation and coordinates the emergency response to coastal erosion and inundation. The Coastal Erosion StormSafe Guide (SES, 2013) outlines the risk from coastal erosion and the differing responsibilities of the SES, councils and Office of Environment and Heritage. It also highlights some of the recent arrangements regarding permanent and temporary protection works. Most importantly, the guide outlines safety messages before, during and after storms in relation to coastal erosion. The Guide refers to weather warnings issued by the Bureau of Meteorology, and the basic measures that can be taken to prepare for coastal erosion and inundation.

5.3 Specific Provisions for Most Vulnerable SCC Beaches

The generic provisions noted in Section 5.2 would be undertaken at all beaches in the Shoalhaven. Additional specific emergency provisions identified for Council’s most vulnerable beaches are set out below.

Note that as “authorised locations” under the Coastal Protection Act 1979 Code of Practice, the placement of sand and sand-filled geotextile containers as temporary works are permissible at some parts of Callala, Collingwood and Mollymook Beaches with appropriate certification but without the need for regulatory approvals.

5.3.1 Culburra

Only damaged public accessways at the ends of streets would be closed temporarily.

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5.3.2 Currarong

Under the draft CZMP, in responding to accessway damage, Council may elect to close some unnecessary public accessways.

If erosion impinges within three (3) metres of the seaward edge of Warrain Crescent, Council would erect barriers to close off the eastern end of the road. Council would consult with the six residents who depend on this access, to ensure they are aware of the circumstances.

5.3.3 Callala Beach

Callala Beach was affected by coastal erosion in the 1974 storms.

As a pre storm measure to reduce hazard, beach nourishment to raise the height of the frontal dune and installation of sand-filled geotextile bags at the toe of constructed accessways are recommended in the SCC Coastal Emergency Action Subplan (Umwelt, 2011). As it is not feasible to implement these actions quickly before a storm hits, the practicality of identifying nourishment and geotextile containers as emergency actions is questioned by RHDHV. It would seem more appropriate to consider raising the height of the frontal dune as a transitional measure, assisting to provide a level of protection against medium term coastal risk (Section 8.3.1).

Priority accessways for inspection and repair are at Centre Street and Callala Beach Road.

5.3.4 Collingwood Beach

Collingwood Beach was severely impacted by coastal erosion in the 1974 storms.

Preparatory nourishment (where feasible) and beach scraping to enhance dune volume and raise the dune crest height is also a proposed emergency action for Collingwood Beach under the SCC Coastal Emergency Subplan. Currambene Creek is identified as the best potential source of sand for this nourishment. The feasibility and practicality of implementing this measure as an emergency action is questioned by RHDHV.

5.3.5 Narrawallee Beach

Although not listed in the eight most vulnerable beaches in the Shoalhaven (University of Sydney, 2014), the Shoalhaven Site Specific Emergency Action Plan identifies Mollymook and Narrawallee at immediate risk and recommends dune raising as an emergency measure. It may be more appropriate to treat this response as a transitional measure, assisting to provide a level of protection against medium term risk (Section 8.3.4).

Beach scraping remains for Council a valid strategy for bolstering the dune and restoring dune height at Narrawallee Beach.

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5.3.6 Mollymook Beach

As a pre storm measure to reduce hazard, dune reshaping immediately north of Blackwater Creek (also referred to as Back Creek), construction of a geotextile container “tripper wall”, and protection of stormwater outlets are recommended in the SCC Coastal Emergency Action Subplan (Umwelt, 2011). As it is not feasible to implement these actions quickly before a storm hits, the practicality of identifying these works as emergency actions is questioned by RHDHV. It would seem more appropriate to consider such works as transitional measures, assisting to provide a level of protection against medium term coastal risk (Section 8.3.5).

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6 PROPOSED “END-STATE” STRUCTURAL PROVISIONS

6.1 Introduction

SCC has undertaken a number of studies which have proposed works to provide protection from coastal hazard over a suitable development planning period, nominally 40 to 50 years for new houses and 60 to 100 years for infrastructure and subdivisions (Section 2.2). These works include seawalls, beach nourishment, dune construction and reshaping, groynes and training walls.

We summarise here the current understanding of the proposed “end-state” provisions for management of coastal hazard at SCC beaches over the longer term.

6.2 Shoalhaven Heads Beach

This beach is not a listed ‘authorised location’ beach under the Coastal Protection Act 1979. It is also not identified as of high relative vulnerability compared to the likes of Mollymook and Collingwood Beaches (Section 3). Nonetheless, the beach was severely eroded in the 1977 storms, threatening the Surf Club (Photo 1).

Photo 1 – Shoalhaven Heads Beach showing Surf Club in 1977 (left) and following rock protection in 1978 (right). Approximately 200 - 250 m3/m eroded in the March 1978 combined flood/storms in the vicinity of the Club (SMEC, 2007a)

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No formal “end-state” works have been proposed for Shoalhaven Heads Beach. The long-term management strategy in the draft CZMP refers to a number of protective actions although no details are given. These include:

 investigate capacity of Surf Club seawall and upgrade as necessary;  maintain dune vegetation;  develop design for reuse of sand from Shoalhaven River ‘flood notch’ for property protection;  relocate Surf Club outside 2050 ZSA.

6.3 Culburra Beach

Culburra Beach is not listed as an ‘authorised location’ beach under the Coastal Protection Act 1979. However, it is found to be a beach with a significant number of at- risk private properties and whose residents have the least adaptive capacity of all beaches in the Shoalhaven (shared with Callala Beach) (University of Sydney, 2014).

Also like Shoalhaven Heads Beach, no formal “end-state” works have been proposed for Culburra Beach. The long-term management strategy in the draft CZMP refers to three protective actions, although no details are given. These are:

 maintain dune vegetation;  strengthen dunes;  relocate part of Allerton Avenue outside the 2050 ZSA.

Subject to long term monitoring of the position of the erosion escarpment at Culburra, more than 80 residences in the 2050 ZSA could need to be relocated or redesigned (Umwelt, 2012). Council has indicated that further investigations are required at Culburra Beach to update the current coastal hazard assessments.

6.4 Warrain Beach

Warrain Beach is not listed as an ‘authorised location’ beach under the Coastal Protection Act 1979. However, the beach is assessed to be relatively vulnerable in relation to exposure, sensitivity and adaptive capacity, with the exposure of the Surf Club a contributing factor (University of Sydney, 2014).

No formal “end-state” works have been proposed for Warrain Beach. The long-term management strategy in the draft CZMP refers to three protective actions, although no details are given. These are:

 maintain dune vegetation;  strengthen dunes.

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6.5 Currarong Beach

Currarong Beach is not listed as an “authorised location” beach under the Coastal Protection Act 1979, and is ranked low in its vulnerability compared to other Shoalhaven Beaches. Nevertheless, the beach has attracted coastal hazard investigations which go back over 10 years, focussing on the threat to Warrain Crescent and the stability of the entrance to Currarong Creek at the eastern end of the beach.

An “end-state” coastal management scheme involving a groyne and beach nourishment was developed by SMEC (2013). This involves a 40 m long groyne located on the entrance spit, assisting to secure a 17,000 m3 placement of sand on its western side sourced from the entrance shoals (7,000 m3) and the entrance to Plutus Creek some 900 m along the beach to the west (10,000 m3). The sustainability of this arrangement is enhanced with the sand being sourced and placed within the same littoral system. The sand placement is designed to ensure that 60 m3/m is preserved on the beach and dune to withstand design storm erosion (Figure 2).

As is well known, groynes are potentially relatively high risk coastal structures. While they impound sand on their updrift side, they would lead to erosion on their leeward or downdrift side. It is possible that the groyne proposed for Currarong could lead to a reduction in the availability of sediment on its eastern side, potentially encouraging the widening and shallowing of the creek entrance. In so far as navigation of the creek entrance is concerned, this may not be so much of an issue today with the recent upgrade of the boat ramp at Yalwal Street reducing the need for movements of larger size boats in and out of the creek. Also, a relatively short and lower crest groyne can assist to permit increased downdrift transport which would mitigate any erosive impact, but at the same time may reduce the updrift impoundment such that the functionality of the erosion protection scheme is compromised. These considerations are within the remit of the Coastal Engineer who designs the scheme, and such considerations would certainly have been brought by SMEC to their proposal.

RHDHV is supportive of the scheme. However, as is recommended by SMEC and also in the draft CZMP, it would be prudent to construct a geotextile container groyne in the first instance as a trial. Both studies refer to a 5 year trial for the groyne based on the expected life of the geotextile containers. From the experience at Maroochydore on the Gold Coast, it is likely that life in excess of 5 years should be achieved (Section 0). Importantly, if there is an unexpected impact from the groyne, the structure is readily removable with no materials left on the beach or in the surf zone. If monitoring demonstrates no untoward impacts, then the geotextile containers could be replaced with rock at the end of the life of the containers.

6.6 Callala Bay Beach

Callala Bay Beach is not listed as an “authorised location” under the Coastal Protection Act 1979. The beach is exposed to ongoing erosion between the boat ramp and the entrance to Boorawine Creek. Moderate storms in recent years have threatened existing rock protection around the boat ramp and large trees fringing the reserve. The exposure of the roadhead at Sheaffe Street is also a concern.

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Two potential “end-state” erosion remediation schemes have been proposed for Callala Bay Beach:

 Long-term management scheme addressing erosion and access  Mechanical sand bypassing of boat ramp reclamation

6.6.1 Long-term Management Scheme addressing Erosion and Access

The Callala Bay Design Review reported in RHDHV (2012a) endorsed and further developed a previously proposed “end-state” protective solution for the shoreline involving a groyne and beach nourishment. Beach ramp access improvements were also developed. Council had implemented beach scraping at Callala Bay Beach in 2003 however this was short lived, with wash out by a moderate storm less than three years later.

Sand loss from Callala Bay Beach is predominantly alongshore from west to east. The destination of the sand is reported to be at the entrances to Boorawine and Wowly Creeks some 400 and 700 m along the shoreline respectively. The eastward transport potential exceeds the availability of sand on the beach which may be contributing to the long-term retreat of the shoreline (SMEC 2008, Coastal Engineering Solutions CES 2003).

Like Currarong, a groyne located at Sheaffe Street has been proposed to trap and contain sand between the boat ramp and Sheaffe Street. However, there is a concern in the community that the shoreline further to the east between Sheaffe Street and Boorawine Creek would suffer, experiencing more erosion than it does at present.

In line with findings of the earlier studies, RHDHV (2012a) proposed a trial geotextile container groyne for Callala Bay Beach plus the initial placement of some 4,200 m3 of sand trucked along the beach from the entrance to Wowly Creek. The shoreline protection project which formed Stage 2 of the works scheme developed by RHDHV was costed at $360,000 (Figure 3).

6.6.2 Mechanical Sand Bypassing of Boat Ramp Reclamation

It is clear from the earlier SMEC and CES studies, and from RHDHV (2012a), that there is a net sediment transport direction from west to east which has been interrupted over the years by the boat ramp and its reclamation. The net average longshore sediment transport rate at Callala Bay has been reported as around 1,000 – 5,000 m3/yr. The main impact probably occurred after the completion of the initial boat ramp reclamation (sometime around 1970), although subsequent boat ramp improvements could have shifted the reclamation slightly further seaward, contributing to a further impoundment of sand.

It is suggested that the small embayment on the western side of the ramp had filled by the early 2000’s with bypassing re-established (CES, 2003), however sustained recovery of Callala Bay Beach has not been observed. One mooted theory is that sand has been lost offshore, directed into a deeper area located off the beach in the vicinity of Sheaffe Street / Boorawine Creek. It should be feasible to mechanically remove the

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impounded sand from the small embayment on the western side of the ramp, and place this on the beach on the eastern side of the ramp. This would ensure that losses into the deeper areas are avoided with the sand directed to the beach where it is most needed. This idea is endorsed in the Shoalhaven Citywide Dredging Feasibility Study where a simple “cut and fill” project using a barge mounted dredge pump is proposed (Peter Spurway and Associates, 2014). While this scheme would be beneficial and may be more acceptable to the Jervis Bay Marine Park then the RHDHV (2012a) scheme above, it does not on its own offer a long term solution to the erosion problem. Reducing the sand accumulation on the western side of the ramp simply provides a sink for the future capture of sand, thereby disrupting the existing (albeit partial) sand delivery to the beach on the eastern side of the ramp. So the mechanical sand bypassing project would need to be ongoing, repeated reasonably regularly (say 3- 5 year intervals) to provide a sustainable and longer term benefit for Callala Bay Beach.

6.7 Callala Beach

Callala Beach is an “authorised location” beach under the Coastal Protection Act 1979. It is also a beach with a significant number of at-risk private properties and whose residents have the least adaptive capacity of all beaches in the Shoalhaven (shared with Culburra Beach) (University of Sydney, 2014).

RHDHV (2013b) investigated “end-state” works projects to manage coastal erosion risk at Callala Beach. This report divided the beach into 2 precincts (A and B, see Figure 4) and investigated three main protection schemes comprising revetments, beach nourishment and offshore breakwaters. Three prioritised projects were recommended, including asset relocations, as summarised in Table 3.

To address the erosion threat and remove the ZRFC hazard from all affected property or assets, it was found that the implementation of Projects 1 and 2 would be required immediately at a total works cost of some $30 million, and Project 3 from about 2025 at a further cost of $12.5 million (2013$).

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Figure 3: The Callala Bay Beach “end-state” protection scheme comprises nourishment and a groyne. A trial groyne made of geotextile containers has been proposed (RHDHV, 2012a). The rock pocket at the Sheaffe Street roadhead, the viewing platform, and the multipurpose access ramp are separate components not related to the “end-state” protection of the beach.

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Figure 4: Callala Beach precincts

Table 3 – Recommended “end-state” works to manage coastal erosion risk at Callala Beach

Project Provisional works costs and Description (2013$) (1) Priority

1 Relocation or protection of Tennis Club $650,000 (relocation) and (Community Centre) building $3 million (protection)

2 Revetment to protect all shorefront private $27.4 million properties along Quay Road (1.4 km)

3 Revetment to protect Greenway Road (0.6 km) $12.5 million

Notes (1) Additional allowance required for project preliminaries such as design development, approvals, and tendering.

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6.8 Collingwood Beach

Collingwood Beach is an “authorised location” beach under the Coastal Protection Act 1979.

Together with Mollymook, Collingwood Beach is the most vulnerable of the eight beaches investigated by Sydney University (2014). A significant proportion of private properties along the foreshore are at risk and infrastructure is also threatened. A high vulnerability is assessed even though affected residents indicate a relatively good adaptive capacity.

RHDHV (2013a) investigated “end-state” works projects to manage coastal erosion risk at Collingwood Beach. This report divided the beach into 3 precincts (A, B and C, see Figure 5) and investigated three main protection schemes comprising revetments, beach nourishment and an offshore breakwater. Six prioritised projects were recommended, including asset relocations, as summarised in Table 4.

Table 4 – Recommended “end-state” works to manage coastal erosion risk at Collingwood Beach Project Provisional Works Costs and Description (2013$) (1) Priority 1 Relocation of sewage pump station and mains $1.0 million along southern end of Elizabeth Drive

2 Relocation of mains along Susan, Montague $110,000 and Berry Streets

3 Revetment to protect Ilfracombe Avenue and $4.5 million water mains (500 m)

4 Revetment to protect all shorefront private properties between Susan Street and Albion $7.9 million Street (900 m)

5 Revetment to protect all shorefront private properties between Holden Street and Susan $3.7 – 7.7 million (2) Street (800 m)

6 Ongoing nourishment works for beach amenity $4.4 million

Notes (1) Additional allowance required for project preliminaries such as design development, approvals, and tendering. (2) Depending whether its built today or in 2050

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Figure 5: Collingwood Beach precincts

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To address the erosion threat and remove the ZRFC hazard from all affected property or assets over the next 50 years, it was found that the implementation of Projects 1 to 4 would be required immediately at a total works cost of some $14 million, Project 5 from about 2050 at $4 million (2103$) and Project 6 from about 2034 at a further $25 million (2103$).

6.1 Narrawallee Beach

Narrawallee Beach is not an “authorised location” beach under the Coastal Protection Act 1979, and is excluded from the assessment of comparative vulnerability of beaches in the Shoalhaven undertaken by the University of Sydney (2014).

The draft CZMP endorses the recommendation in SMEC (2006a) that dune crest levels are maintained to a minimum of RL 6.0, especially at the northern end of the beach where there may be potential for breakthrough of the tombolo into Narrawallee Inlet.

6.2 Mollymook Beach

Mollymook Beach is an “authorised location” beach under the Coastal Protection Act 1979.

Together with Collingwood Beach, Mollymook Beach is the most vulnerable of the eight beaches investigated by Sydney University (2014). A significant proportion of private properties along the foreshore are at risk and infrastructure is also threatened.

RHDHV (2012b) investigated “end-state” works projects to manage coastal erosion risk at Mollymook Beach. This report divided the beach into 5 precincts (A, B, C, D and E, see Figure 6) and investigated three main protection schemes comprising revetments, beach nourishment and an offshore breakwater. Ten prioritised projects were recommended, including asset relocations, as summarised in Table 5.

To address the erosion threat and remove the ZRFC hazard from all affected property and assets over the next 50 years, it was found that the implementation of Projects 1 to 8 would be required immediately at a total works cost of some $16 million, Project 9 from about 2025 at $16 million (2013$) and Project 10 from about 2042 at a further $13 m (2013$).

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Figure 6: Mollymook Beach precincts

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Table 5 – Recommended “end-state” works to manage coastal erosion risk at Mollymook Beach

Project Provisional Works and Description Costs (2013$) (1) Priority 1 Possible relocation or protection of sewer pump station $300,000 (relocation); on southern side of golf club. Elevated rock expected or nil (no works) to (~RL 0). Existing seawall protection appears $500,000 (seawall reasonable, but must be checked and upgraded if upgrade) necessary.

2 Possible relocation or protection of sewer pump station $300,000 (relocation on northern side of Mitchell Parade road reserve. only) Feasible protection relies on revetment to fully protect the reserve (see Project 5 below).

3 Possible relocation or protection of sewer pump station $300,000 (relocation) on northern side of Blackwater Creek. Protection or $1.5 million (Project achievable by training north edge of Blackwater Creek 8 with reduced dune and dune upgrade (essentially Project 8, see below). works)

4 Seawall upgrade to protect corner of Golf Ave and $500,000 Beach Street.

5 Revetment to protect Mitchell Parade road reserve $8.0 million shoreline 370 m long revetment with 2 stormwater outlets to protect road and sewer assets

6 Possible seawall upgrade to protect the Golf Club. $2.3 million Need for upgrade depends on risk-protection mix with respect to existing piled foundations, design and durability of gabion wall and life of Club premises.

7 Seawall upgrade to protect the Surf Club. Surf Club $2.6 million (includes less threatened by erosion than Golf Club, but existing 50 m stepped seawall appears highly deficient. concrete wall)

8 Training wall at Blackwater Creek and dune upgrade to Say $300,000 to protect private properties. Stage 1 of 2 stage project $1.6 million (depends for Precinct B. Protection option at Project 3 would on Project 3) largely implement Project 3.

9 Two revetments total length 680 m to protect all $16 million 29 shorefront private properties private properties along Mitchell Parade: Precinct B, Stage 2, 14 properties, 350 m long; and Precinct D, 15 properties, 330 m long)

10 Ongoing nourishment works for beach amenity $3 million/5 years following full wall and revetment protection (see Projects 1-9 above).

Notes (1) Additional allowance required for project preliminaries such as design development, approvals, and tendering.

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7 ADAPTIVE MANAGEMENT CONCEPTS

7.1 General

Adaptive management is a structured, iterative process of robust decision making in the face of uncertainty, with an aim to reducing uncertainty over time via system monitoring. The challenge in using the adaptive management lies in finding the correct balance between gaining knowledge to improve management in the future, and achieving the best short-term outcome based on current knowledge.

Adaptive management recognizes that knowledge about future conditions is uncertain. It should be linked to an appropriate temporal and spatial scale, and should be statistically relevant. Adaptive management is also useful for communication of alternatives for negotiation and selection. Because of the long timeframes over which any CZMP strategies are implemented, management objectives may change over time and new measures may need to be adopted to match changed conditions.

Changes in development patterns along a shoreline or indeed in the physical shape of the coast are possible as a consequence of changing climate or severe storms. New information may also become available over time, eg., improved estimates of sea level rise. For these reasons Council, OEH and its partners should be prepared to institute changes in specific coastal management measures.

Adaptive management ideas available to SCC to help address its transitional works strategies between “make-safe/make-good” and “end-state” protection include:

 Monitoring and trigger responses  Staging of works  Demonstration projects

7.2 Monitoring and Trigger Response

Monitoring should be a mandatory inclusion in any coastal protection works. It would typically involve regular inspections and surveys, and is also triggered by severe storm events.

SCC has recently established a program for ground survey monitoring of its beaches prone to higher erosion risk. This involves repeat surveys at pre-designated cross sections. This monitoring would provide for an improved level of accuracy for hazard mapping. The monitoring activities would inform the future Coastal DISPLAN and Draft DCP 118 – Areas of Coastal Management.

Monitoring outcomes are linked to trigger responses in the draft CZMP. The draft CZMP recommends that detailed feasibility and design studies for protection or relocation are initiated when monitoring shows that the erosion escarpment has moved to within approximately 30 m of sewerage and water infrastructure. The selected trigger distance is influenced by the type of asset, the expected asset life and the types of risks associated with failure.

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7.3 Staging of Works

Staging of works is an appropriate adaptive response. “End-state” projects often comprise multiple elements which in combination provide protection to threatened property and assets over the required planning period. Given that coastal threat increases over time as a consequence of climate change, it follows that a long-term protection project need not be fully constructed at day one.

It may also be possible to build a seawall with a crest level which is lower than the crest level required to provide protection at the end of the planning period. It may also be possible to introduce a component of the works initially such that coastal risk is reduced to an acceptable level, and then proceed later to add other components when these are needed to mitigate the increasing risk.

Staging however may not be cost effective. Mobilisation costs for infrastructure projects are high, and it would be usual to deliver more of the minimum project required to mitigate the risk to account for this overhead.

7.4 Demonstration Projects and Trials

Demonstration projects are an adaptive coastal management tool. They link back to the science and provide an opportunity for learning and feedback for improved decision making. Demonstration projects and trials can be useful to resolve areas of scientific or technical uncertainty or fill in data gaps in order to advance coastal risk reduction projects, such as new technologies for armouring embankments.

Full-scale restoration opportunities may depend upon results from demonstration projects to advance planning and analysis of alternatives. To be responsive to program needs, demonstration projects should be implemented as soon as possible and have the ability to provide meaningful results in a relatively short timeframe. Information is needed in time to feed the design and planning process to achieve both short and long- term project objectives and goals (USACE, 2009).

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8 STRATEGIES FOR TRANSITIONAL WORKS BETWEEN “MAKE-SAFE/MAKE- GOOD” AND “END-STATE” STRUCTURES

8.1 Introduction

Transitional works between “make-safe/make good” and “end-state” are of interest to Council on the basis that full protection from the effects of erosion over a planning time scale are simply unaffordable under current coastal management programs. The draft CZMP does not countenance retreat for affected shorefront private properties, so loss of property and infrastructure to erosion or protection are effectively the default alternatives. Relocation of public infrastructure is appropriate in many circumstances as has been highlighted for most beaches in the Shoalhaven (Section 6), however this report focuses on protective works.

Generic and site-specific transitional works schemes are considered below.

8.2 Generic Transitional Works Schemes

Further to the adaptive works concepts discussed above, there are a number of schemes for mitigating erosion risk in the medium term which may be of interest to SCC. These relate to revetment works, and backpass dredging or sand recycling.

8.2.1 Transitional Arrangements for Revetment Works

The construction cost of coastal revetments at Mollymook, Callala and Collingwood Beaches is high, estimated to range between approximately $10,000 and $20,000/m (RHDHV, 2012a, 2013b, and 2013c). The large price differential across these sites relates mainly to relative exposure, and variable design scour and wave runup/dune crest levels. It is not feasible to construct short lengths of revetment due to the relative influence of end effects, so full length structures are typically required which at these beaches range between 330 and 1,470 m. Clearly, revetment works for coastal protection are very expensive. Few are constructed in NSW for various reasons, one of which would be cost.

The recommended revetment sections developed for protection at Mollymook, Callala and Collingwood Beaches comprise a sloped rock structure with a rock toe. A typical section for the Mollymook concept is shown in Figure 7. It may be feasible to partly construct a revetment section such that it alleviates the erosion threat to shorefront properties, either as a complete project in its own right, or as the staged construction of the full conventional revetment section.

RHDHV has recently developed a preliminary design for a revetment structure at Old Bar Beach which is comparable to the revetment developed for Mollymook Beach, and indeed the revetments also developed for Callala and Collingwood Beaches. It was found that the present day 100 year significant wave height at the Old Bar structure was 1.8 m associated with a peak wave period of 13 s (WRL, 2013). The design still water level including wave setup was RL 2.35. The design storm erosion at Old Bar is 220 m3/m. These parameters would not be dissimilar to those at Mollymook Beach.

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Using procedures for wave attenuation over submerged rock structures set out in Coastal Engineering Manual (USACE, 2002), it is found that the transmission coefficient over the mound for the design 100 year water level (RL 2.35 including wave setup) is 50%, resulting in a transmitted significant wave height of 0.9 m. Now it is well known that wave erosion at a shoreline correlates well with rate of delivery of wave energy (c=0.93, Nanson et al 1994). Since wave energy is proportional to the square of the wave height, it follows that the design erosion demand in the lee of the transitional rock placement should reduce by approximately 75%, from 220 m3/m to approximately 55 m3/m. Thus choosing to construct only the toe of a full rock revetment would not remove the erosion risk completely, but it should significantly mitigate that risk permitting a seaward relocation of the coastal hazard lines. The scheme suggested here is depicted in Figure 7.

The calculation made above assumes that the design crest level and width of the rock toe mound is preserved for all storm events to which the structure is exposed. This is reasonable as it is constructed either to the design scour level, or is designed to deform to a lower level without destabilising the rocks at the back of the buried toe. However, with the rear wall missing in the partial construction, additional forces may develop at the back of the toe due to the severe wave overtopping and this may require the inclusion of additional rocks which do not form part of the full design section. The long term shoreline response of a toe mound rock placement at the back of the beach in the event that the full revetment was not completed, for whatever reason, should also be considered.

Enquiries of colleagues suggests that this partial construction of a rock revetment has not been done before in NSW and probably not in Australia. Low height geotextile container revetment structures have been built on Belongil Spit at Byron Bay resulting in washout behind the crest and failure of the upper portion of the wall, however it is possible that the armouring container units may have been undersized to withstand the hydrodynamic loads.

Further investigation would be required to test and develop this idea, in particular the stability of the buried toe and the reduction in erosion achieved behind the structure. The complexities of the processes involved would require that this entail a full scale trial and/or physical modelling (including modelling of sediment transport / erosion).

Another staged revetment philosophy was implemented by Wyong Shire Council in 2012 at Cabbage Tree Bay on the NSW Central Coast. Designed by Worley Parsons, the so- called Toe Drainage Structure (TDS) provides an interim solution to protect the bluff associated with a 15 year design life (Photo 2, Figure 8). The 15 year life was selected by Council to postpone a higher capital investment associated with the 50 year full life design. The adopted breaking wave height for the TDS was 2.3 m. The single armour layer solution was developed for a Kd = 2.1, a 5-10% damage in a 250 year ARI storm, and a 5% risk of failure. The TDS was installed for approximately $10,000/m or approximately $330/tonne of rock in place (Lex Nielsen, WorleyParsons, presentation to NSW Coastal Ocean and Ports Engineering Panel COPEP, September 2012).

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complete project or the staged construction of the full revetment Buried toe constructed as a first stage

“End-state” “End-state” hazard line - hazard line - no protection buried toe only “End-state” hazard line - full revetment

Storm water level and wave action

Reduced erosion with buried toe only

Figure 7: Partial or staged construction of rock revetment to potentially alleviate coastal risk. Revetment profile shown developed for Mollymook Beach in RHDHV (2012b). Scheme subject to further investigation.

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Photo 2 – Completed interim Toe Drainage Structure at Cabbage Tree Bay, Norah Head, Wyong (Photo 2012 courtesy Worley Parsons)

Figure 8: Single armour layer Toe Drainage Structure, Cabbage Tree Bay, Wyong (Source: Lex Nielsen, Worley Parsons presentation to COPEP, Sept 2012)

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8.2.2 Transitional Arrangements involving Sustainable Recycling of Beach Sand

Beach sand recycling involves the intermittent relocation of sand from a destination source and relocating it to the eroding beach. For this to be potentially feasible in the Shoalhaven, there is strong preference for the recycle borrow sand to be sourced from a downdrift subaerial beach or a downdrift entrance to a coastal creek. Attempting to dredge sand from the seabed including the Jervis Bay would be fraught with technical and approvals difficulties and is not countenanced here as a possible recycling scheme.

Recycling of beach sand is carried out periodically in NSW at Collaroy Narrabeen Beach in Sydney and at Jimmys Beach in Port Stephens. A permanent sand beach renourishment scheme also exists at Noosa Heads Beach on the Sunshine Coast in Queensland. Experience from maintenance dredging projects in Port Hacking in Sydney, with sand reuse at Cronulla Beach is also worthy of consideration.

Collaroy – Narrabeen Beach Experience

At Collaroy Narrabeen Beach, sand is excavated from the entrance to Narrabeen Lagoon approximately every 3 to 5 years, and since the 1980s has been trucked some 2 km to the south and placed onto Collaroy Beach through road heads (Photo 3 and Photo 4) . Initially sand from the lagoon entrance was not returned to the beach and since the mid 1970’s some 600,000 m3 of sand has been removed. The main purpose of this activity is to mitigate flood risk and water quality issues due to lagoon closure. At around 20,000 m3 per year, the beach replenishment benefit is modest compared to design storm erosion and recession. Present day costs to relocate the sand are approximately $22/m3.

Permits and licences obtained for Council to operate the scheme include a concurrence under the Coastal Protection Act (OEH), a licence under the Crown Lands Act (Department of Lands), a Fisheries Permit under the Fisheries Management Act ( Department of Primary Industries), and a licence under Protection of the Environment Operations Act 1997 (Environment Protection Authority) to dredge in excess of 30,000 m3. Finally an approval from Warringah Council was obtained to temporarily relax a road load limit to improve safety (Cameron et al, 2007).

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Excavator and trucks used to clear and move sand to roadheads

Coastal processes return sand northwards and refill entrance

Photo 3 –Beach sand recycling at Collaroy-Narrabeen Beach has been carried out successfully over past 20 years

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Photo 4 – Dumping and spreading sand at roadheads at Collaroy-Narrabeen Beach

At Collaroy-Narrabeen, the sand is typically transported from the entrance to the beach using bogie tipper trucks (6 wheel and 12 tonne) and placed on the beach reserve at roadheads between Jenkins Street and Devitt Street. The deposited sand is spread using excavators, loaders, backhoes and bulldozers.

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Jimmys Beach Experience

Great Lakes Council undertakes regular beach nourishment at Jimmys Beach. On average approximately 15,000 - 20,000 m3/yr is eroded from the beach with the bulk of this material understood to redeposit onto a sand shoal off Yaccabba Headland approximately 2.5 km to the south-east. In 2008 Council implemented a sustainable backpass strategy to recycle the sand back onto Jimmys Beach from Yaccabba every few years (Figure 9).

A permanent sand delivery pipeline was installed (Photo 5) and a dredge operated close to the shoreline in the lee of the Yaccabba shoal (Photo 6). However, over the past two years pressure to dredge the mouth of the Myall River further to the west has temporarily diverted the Yaccabba - Jimmys Beach strategy, although the expectation is that this would resume in the future. The cost of the initial backpass project at Jimmys Beach including the pipeline installation and involving placement of some 40,000 m3 amounted to approximately $20/m3.

Photo 5 – Installation of permanent pipeline between Yaccabba and Jimmys Beach.

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Figure 9: Jimmys Beach sand recycling scheme (Great Lakes Council, 2007). 15,000 to 20,000 m3/yr is recycled every few years. A dredge is located in the borrow area at Yaccabba shoal with sand pumped 2.5 km along the shoreline to the placement area at Jimmys Beach where houses are threatened by erosion. A booster pump is located midway along the shoreline. A permanent delivery line is installed below the beach behind the foredune. Like Jervis Bay, Port Stephens is a marine park.

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Photo 6 – Dredge operating at Yaccabba inside Port Stephens. In this initial campaign sand was pumped 2.5 km to Jimmys Beach via the permanent pipeline. Jimmys Beach is at the shoreline immediately behind the wheelhouse on the dredge in this photo.

Noosa Heads Beach Experience

The Noosa Heads scheme is designed to provide both a suitable amenity beach width and mitigate the coastal erosion hazard. A seawall is buried at the back of the beach although the integrity of this structure is unknown. The Noosa scheme involves a Sand Shifter installation designed and operated by Slurry Systems. The scheme is designed to transport between 40,000 and 60,000 m3 of sand per year pumped over a distance of 1.4 km. Sand is slurrified at depth into horizontal intakes located in the surf zone close to the Noosa River entrance breakwater. It is then pumped to the updrift delivery point at the eastern end of the beach (Photo 7 and Photo 8).

After a short trial deployment and some 5 years of project evolution and development, the permanent scheme was installed a few years ago at a capital cost of $1.8 million. Slurry Systems has an operation and maintenance contract with the Council which costs approximately $10,000 to $11,000 per month plus $3.50/m3 for sand pumping (Dave Sling, Noosa Council Senior Technical Officer, 7/2/14 pers comm). Council separately meets the electricity costs for pumping. Based on a provisional estimate of pumping requirements, it is expected that the Noosa scheme would cost Council in the order of $10/m3 to install and operate over a 50 year period if delivering 40,000 m3/yr. If delivering less, then the unit rate would be higher.

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Photo 7 – Slurry Systems pump station under construction at Noosa Heads.

Photo 8 – Pumped sand at delivery point at eastern end of Noosa Heads Beach.

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Beach sand recycling is suggested here as a transitional arrangement for erosion mitigation in the Shoalhaven and not a full “end-state” arrangement in that the sand quantities that are likely to be delivered would not fully offset the erosion needs over a project design life of 40 to 60 years. What it potentially could do however is account for a component of the sand loss from the beach, thereby permitting the hazard lines to be moved back closer to the waterline whist the renourishment scheme is operational. Like all erosion mitigation measures, a sand recycling scheme could be implemented together with a partial revetment (Section 8.2.1) to provide a higher level of certainty by adding redundancy to the erosion protection system.

Sand recycling schemes provided with or without a terminal back-beach structure may, for example, be possible at Callala Bay Beach and Collingwood Beach as discussed more specifically below in Sections 8.3.1 and 8.3.3 respectively.

Port Hacking – Cronulla Beach Experience

The navigation channels in lower Port Hacking, located between Bundeena, Gunnamatta Bay and Lilli Pilli, are located in an active tidal delta and accumulate sand at around 25,000 to 30,000 m3/yr. Dredging of these channels goes back over a 100 years with the material either sidecast to form emergent spits of sand, or pumped to deeper areas inside the port. In recent times between approximately 40,000 and 100,000 m3 is dredged every 2 to 3 years. In 1996 reuse of the dredge material at Cronulla Beach was investigated and is understood to have taken place in the subsequent maintenance dredging round (Patterson Britton & Partners, 1996).

In 2012, approximately 77,000 m3 of material was dredged with the sand deposited approximately 5 km away offshore North Cronulla in 4 to 8 m of water to form a mound to enhance surfing and provide a source for onshore sand movement and beach nourishment. This project was funded by OEH which provided Council with $2.56 million to undertake the work ($33/m3). Trailer suction hopper dredges with loaded draft 3.1 – 3.6 m, hopper capacity 320 - 450 m2, and minimum offshore dump depths of 5 - 6 m were used (Sutherland Council web site, (http://www.sutherlandshire.nsw.gov.au/Building_Development/Works_and_Projects/).

8.3 Suggestions for Transitional Works Schemes at Particular Beaches

The generic ideas for transitional works between “make-safe/make-good” and “end- state” structures developed above are considered below for specific application at particular beaches in the Shoalhaven. Recommendations for further investigations are developed around these schemes to confirm feasibility and costings and are discussed in Section 9.

8.3.1 Shoalhaven Heads Beach

The Surf Club at Shoalhaven Heads is particularly exposed in storms (Photo 1). Rock protection is buried immediately seaward of the Club but the capacity of this to protect the facility is unknown. While some protection from this rock could be expected, as the sea level rises and recession increases, this capacity would be diminished.

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A similar strategy to that adopted at Freshwater Surf Club could apply at Shoalhaven Heads. When upgraded in the mid 1980s, a detached building was constructed immediately landward of the existing threatened facility, joined by a walk through. The walk through passage buts up to the existing building but there is no structural connection. As the shoreline recedes in time and when the older building is lost, it would be removed and the new building behind retained as the primary facility (Photo 9).

Photo 9 – Freshwater Surf Club in Sydney was upgraded in the mid 1980s by adding a detached building immediately landward of the original building. It is not proposed to protect the older building as the shoreline recedes.

8.3.2 Currarong Beach

The SMEC proposal for Currarong involving a groyne and sand nourishment is endorsed by RHDHV (Figure 2).

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Given the vagaries of coastal sedimentary processes and the uncertainties associated with sea level rise, we would urge Council to adopt the geotextile groyne as a trial project, to be adapted in due course to a permanent groyne if the scheme is demonstrated to achieve a requisite level of erosion protection by regular and quality assured monitoring. The notion of implementing a trial structure is a well-accepted adaptive management concept (Section 7.4) and it should allay those reasonable concerns regarding commitments to long-term projects which may not perform to expectations and can be associated with untoward legacy impacts.

Clearing of the entrance to Currarong Creek has been undertaken on at least two previous occasions and it is our understanding that this activity has not been environmentally detrimental over the longer-term. Nevertheless it would seem reasonable to take a precautionary view and delay the nourishment component, save upfront capital costs, and build into the monitoring task a review of the need for nourishment following observed trends in the natural establishment of the profile updrift of the trial groyne. It is noteworthy that the expected beach profile without nourishment complements the fully nourished solution, so there is no technical disadvantage to delaying the nourishment component.

Geotextile containers are a suitable product to investigate for construction of a trial groyne. According to physical model testing carried out by the Water Research Laboratory of the UNSW, if the design breaking wave height is less than 1.5 m (T=15 s) to 1.75 m (T=10 s), then 2.5m3 geotextile containers should be acceptable (0-2% damage in design storm).

A similar groyne trial at Maroochydore on the Sunshine Coast was commenced in 2001. Four geotextile groynes have now been constructed at Maroochydore using 2.5 m3 containers. Although slightly damaged in storms, it is fair to say that their performance has met and probably exceeded expectations with the groynes continuing to be serviceable today (Photo 10 to Photo 14). It is expected that Maroochydore would be more exposed than Currarong Beach, so breaking wave height requirement should be satisfied.

A trial geotextile groyne structure could be designed for a relatively short life (say 10 years), possibly avoiding the need to bury the bottom container at the design scour depth. For example, it may be possible to adopt a trial groyne base level of RL 0 rather than RL -1 say (Figure 2), and allow for some settlement.

Eminent Physical Geographer Professor Enzo Pranzini, University of Florence, is to deliver a keynote address to the NSW Coastal Conference in Ulladulla in November 2014. Professor Pranzini has referred to submerged groynes that permit bypassing to reduce downdrift impacts (see http://vimeo.com/23667389 about 10 mins into this presentation). If monitoring of a trial scheme at Currarong showed downdrift impacts, it would be reasonable to cut away and remove the crest or outer containers to manage this, and confirm the response with the subsequent monitoring.

A groyne length of approximately 40 m is required according to the SMEC concept. For a 1.8 m wide crest at RL 2.0, with 1:1.5 side slopes to RL 0, approximately 160 x 2.5 m3 containers would be required. Allowing say $1,000 per container designed and

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installed, the trial groyne should cost in the order of $160,000 excluding any supplementary nourishment.

N

4

3 2 1

500m

Photo 10 – Maroochydore Beach groynes (constructed 2001 – 2003)

Photo 11 – Maroochydore Beach Groyne No 1 constructed November 2001

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10m

Photo 12 - Maroochydore Beach Groyne No 1 (25/1/10)

10m

Photo 13 - Maroochydore Beach Groyne No 1 (21/7/10)

10m

Photo 14 - Maroochydore Beach Groyne No 1 (1/5/11)

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If monitoring indicates that the trial groyne is not assisting to manage erosion at Currarong Beach, it should then be removed. This should be readily achieved by cutting the containers to release the sand, hooking the cut containers to a dragline and pulling these back to the beach. The activity would require divers and an excavator. If monitoring demonstrated that the trial groyne was assisting to retain larger volumes of sand on Currarong Beach as is predicted, then the geotextile container structure could be left in place as long as possible before damage and repair of containers was becoming cost prohibitive, and the structure could then be replaced with a permanent groyne expected to comprise rock. It is of interest to note that this very method of trial and conversion to a permanent coastal structure was applied at an offshore breakwater at Semephore Park in Adelaide. The offshore breakwater was initially constructed using geotextile tubes in 2004, and by 2013 the structure had demonstrated acceptable performance and it was replaced with rock (Photo 15).

While there was a dollar cost premium to implementing the project in this way, it is understood that the community was not prepared to accept a rock breakwater upfront and wanted to be satisfied that it would function as intended without unforeseen environmental impact (Dr Murray Townsend, Manager Coastal Branch, Department of Environment and Natural Resources, South Australia).

8.3.1 Callala Bay Beach

The “end-state” scheme developed for Callala Bay Beach is relatively inexpensive compared to the “end-state” projects identified elsewhere. The scheme includes a trial groyne so already incorporates an adaptive or transitional element. The application of a trial project with monitoring and performance review to inform any decision regarding permanent works would follow the same rationale and methodology described above for Currarong Beach (Section 0).

There is consensus in the coastal engineering reports prepared for Callala Bay Beach that the predominant pathway for sediment movement is alongshore to the east (Section 6.6). Recycling this material back to the western end of the bay would be a sustainable option to increase the level of coastal protection to the reserve both west and east of Sheaffe Street.

8.3.2 Callala Beach

Callala Beach is burdened with a narrow beach and low beach escarpment and foredune. Implementing a long-term seawall solution would almost certainly require that part of the works occupy existing private property. Subject to confirmation by further investigation, it may be possible to initiate a seawall project at Callala Beach by construction of the buried toe only, located fully on public land, and then in later years if the coastal threat increases as it is expected to do, return to construct the back slope of the wall.

As an adjunct to a rock transitional scheme, it would be possible to reuse dredge material from the entrance of Currambene Creek as suggested in the Shoalhaven Citywide Dredging Feasibility Study (Section 4.2), to build up the foredune at Callala Beach. In accordance with the draft CZMP, it is appropriate to consider raising the

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height of the frontal dune at Callala Beach as a transitional measure to assist with protection against medium term coastal risk. The sustainability of any such sand placement would be influenced by the ultimate destination of the sand, after it has been eroded from Callala Beach in later years. There is no information at this time on the littoral interconnection between Currambene Creek and Callala Beach for which further investigation would be required. This could be expected to include sediment sampling and numerical modelling, preferably calibrated and verified against field data.

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2004

2013

Photo 15 - Semaphore Park, Adelaide. Geotextile containers placed initially (2004) then replaced with rock (by 2013). Community would not have accepted rock up front. The tubes were cut and the rock placed directly on the tubes. The tubes contributed to form the geotextile underlay for the rock.

8.3.3 Collingwood Beach

A 500 m long revetment to protect Ilfracombe Avenue and its 100 mm water main is a Category 1 project recommended in the Authorised Locations Coastal Erosion Remediation Options report for Collingwood Beach. Lower priority Category 2 revetments extend between Susan Street and Albion Street (RHDHV, 2013a).

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Subject to confirmation by further investigation, as for Callala Beach it may be possible to initiate a seawall project at Collingwood Beach by construction of the buried toe only, located fully on public land, and then in later years if the coastal threat increases as it is expected to do, return to construct the back slope of the wall.

Moona Moona Creek at the northern end of Collingwood Beach exhibits an entrance area filled with a large quantity of sand. It may be possible to sustainably recycle a proportion of this material onto Collingwood Beach through a beach sand recycling scheme. However, Moona Moona Creek is not mentioned in the Shoalhaven Citywide Dredging Feasibility Study as a prospective source of sand in the Shoalhaven (Section 4.2), the nearest being that at the entrance shoals of Currambene Creek. While sand could feasibly be transported (probably by pumping) the 2.5 km from Currambene Creek to Collingwood Beach, it is less likely that Currambene Creek would be a sustainable source for Collingwood with these two sites less likely to share a coastal littoral compartment within Jervis Bay compared to Currambene Creek and Callala Beach.

SMEC (2009) indicates that Collingwood Beach would recede by 4.1 m for every 0.1 m of sea level rise (SLR). If the beach and dune is effectively raised by 0.1 m1 through sand nourishment, then the effect of this SLR is nullified. Since the coastal hazard lines include SLR recession as a component, it follows that sand nourishment of 0.1 m would effectively relocate the hazard lines seaward by 4.1 m. If the sand is only placed onto part of the subaerial beach, the coastal processes would act to ‘smear’ that sand alongshore and offshore. Some sand could be lost from the beach system, either leaking offshore into deeper water or blown landward. If it can be established that a significant proportion of sand which is say sourced from the entrance of Moona Moona Creek is transported alongshore back to the entrance of the creek (say 60% or more), then that sand could potentially be sustainably recycled and a backpass scheme established. A schematic depicting this scheme is shown in Figure 10.

Further investigations would be required to assess the feasibility of a beach sand recycling scheme at Collingwood Beach. In baseline terms, this would depend on the assumed predominance of a south-to-north longshore sediment transport regime on the beach, and the quantity of sand that could be sustainably removed from the entrance to the creek. It has been suggested that short temporary “tripper walls”/stub groynes (eg. geotextile containers) could be provided at stormwater outlets along the beach as “trial projects”, to test the sediment pathway and at the same time provide security against outlet migration in storms. Although much of this sand should eventually return to the creek entrance, progressive backpass placements must effectively occur before this return in order to permit sustained sand residence on the beach and thus some protection from erosion. The importance of Moona Moona Creek as a relatively undisturbed ecological asset in Jervis Bay was identified by a resident attendee at a public meeting to discuss the Adaptive Works Strategy in June 2014, and clearly, further coastal hydrodynamic, sedimentological and ecological investigations would be necessary to test the value and sustainability of any sand recycling scheme for Collingwood Beach.

1 Assumed that beach and dune is raised by 0.1 m over the full active profile

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Recycled sand source dredged or excavated

Sand trucked along beach or pumped

Predominant transport expected to be along shore back to creek, with Extent of some losses backpass seaward (?) scheme depends on recycled sand volumes and placement frequency

Figure 10: Schematic showing possible beach sand recycling scheme at Collingwood Beach. As an example of the protection benefit, a provisional baseline estimate suggests that some 5 - 8 m3/yr of sand for each one metre length of shoreline would need to be recycled from the creek entrance to provide interim protection with the effect of permitting a redefinition of the 2025 hazard line as a 2050 hazard line.

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8.3.4 Narrawallee Beach

The dune located along Matron Porter Drive at Narrawallee is relatively low and currently allows wave run-up to overtop into the coastal reserve. Although less of a concern than erosion risk, raising the dune crest to 6 m along the northern half of the beach would reduce the flood risk due to wave run-up and reduce the landward movement due to storm erosion (SMEC, 2006a).

Sand potentially available from the entrance shoals of Lake Conjola would satisfy the requirements for any dune crest raising at nearby Narrawallee Beach.

8.3.5 Mollymook Beach

The “end-state” works solutions proposed for Mollymook Beach included seawalls and beach nourishment. Like the other beaches investigated, these are multimillion dollar projects which may be difficult to justify and fund at this time (Section 6.2). However, while there may be a number of higher priority projects identified for Mollymook, Project 8 identified in RHDHV (2012b), which concerns protection of private properties along Mitchell Parade between Blackwater Creek and the southern end of Donlan Road, is particularly suited to a transitional works project.

Project 8 involves a training wall at Blackwater Creek and a dune upgrade to protect the sewer pump station beside Blackwater Creek and the 11 southern-most shorefront private properties between and including Nos. 2 to 22 Mitchell Parade. Project 8 forms the first stage of a two stage project to protect, using revetments, all 29 shorefront private properties at Mollymook Beach (Figure 11).

Risk exposure to private properties at Mollymook Beach is highest at those low-lying shorefront properties immediately north of the entrance to Blackwater Creek. Project 8 is designed to equalise the risk exposure across the 14 shorefront properties south of the southern end of Dolan Road by protecting the southern-most properties from the effects of creek breakout, and by bolstering the volume of sand in the dune such as to equalise the volume per metre run of beach length between Nos. 2 and 28 Mitchell Parade.

A concept design for Project 8 was estimated to cost $1.7 million in RHDHV (2012b). This includes the major works items of earthworks, entrance training works and sand placement for dune upgrade, plus all preliminaries associated with project approvals, environmental assessment, detailed design and tendering, and project supervision and administration.

Council has previously consructed a gabion “tripper wall” at the entrance to the Mollymook Creek near the northern end of Mollymook Beach, to stabilise the location of the entrance and to manage erosion due to entrance breakout in storms. Constructed in about 1994, it is understood that this wall at Mollymook Creek has proven successful and a comparable scheme has been recommended for Blackwater Creek (Umwelt, 2011; RHDHV, 2012). It is noted that entrance stabilisation to Blackwater Creek has been considered previously by Public Works, but has never been implemented for reasons unknown.

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Design drawings for the “tripper wall” at Mollymook Creek entrance show it to be constructed using gabions underlain with reneo-mattresses to protect against scour. Umwelt (2011) suggested that a “tripper wall” for Blackwater Creek could be constructed using geotextile containers.

Gabions may be appropriate for protecting creek banks but where these works extend onto beaches they are not generally preferred. Gabion baskets corrode, particularly in the marine environment, and safety issues do occur with protruding rusty wire. There is also the matter of smaller sized rocks which are released from the ruptured baskets onto the beach and into the swash zone. These issues have emerged at the existing gabion “tripper wall” at Mollymook Creek and repairs to this installation are currently required (Ray Massie SCC, pers comm).

Geotextile containers would not withstand wave loads on an open coast beach, let also at a creek entrance where storm scour would be enhanced and larger wave loads encountered. In respect of their application as a trial structure, there is little need to trial an installation at the entrance to Blackwater Creek. In essence the works at the entrance to Mollymook Creek have proved the application of a tripper or entrance training structure at Blackwater Creek and a similar beneficial functional performance would be expected at Blackwater Creek.

This report endorses RHDHV (2012b) and recommends that any entrance training works at Blackwater Creek comprise rock. The design of such would also form the southern end of the “end-state” revetment proposed to provide long-term protection of shorefront properties and assets at Mollymook Beach (Project 9, RHDHV 2012).

A major cost item for Project 8 is the sand placement for the dune upgrade. Some 5,000 m3 of sand is required to bolster the dune, costed at $75/m3 for supply and place. The Shoalhaven Citywide Dredging Feasibility Study (Section 4.2) identifies 4,000 – 8,000 m3 as immediately available over a two year period from lake entrance clearing in the Shoalhaven, and substantially more sand potentially available from a dedicated dredging project to clear the entrance shoals in Lake Conjola.

Council has also reiterated its concern for the threat to public infrastructure in Precinct C (Figure 6). It has been suggested that the stormwater outlets in this section of the beach could be extended or “trained” to preserve beach sand volumes at times of coastal storms. This would need to be approached carefully, requiring further investigation including borehole drilling to confirm the presence or otherwise of bedrock.

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Figure 11: Project 8 in RHDHV (2012b) is shown here as Stage 1 of a two stage project to protect the sewer pumping station and 14 shorefront properties immediately north of Blackwater Creek.

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9 IMPORTANCE OF FULLY VEGETATED DUNE SYSTEMS

A vegetated frontal dune serves as a natural protection to the shoreline against wave action. It acts a buffer, providing an erodible reservoir of sand which is capable of circulating between the dune, beach, surf zone and sea bed according to sea and wind conditions. In Australia, the breakdown of frontal dunes, reducing their effectiveness as erosional buffers, has occurred over the period of European coastal settlement (Chapman, et al, 1982).

Dune construction and stabilisation programs were well supported by the State Government following the coastal storms of the mid to late 1970s. All beaches in the Shoalhaven generally have a demarcated dune stabilisation area which is fenced and reasonably vegetated, mostly implemented with the assistance of the NSW Soil Conservation Service and the NSW Coastal Management Program at that time.

Erosion resilience is enhanced by a fully vegetated dune system. While it is accepted that this is not an “end state” option, vegetated dunes certainly protect the dune system from minor to moderate storm events and enhance the amenity value of the beach. There is a view that vegetated dunes underpin the beach experience and tourism economy in the Shoalhaven (Ray Massie SCC, pers comm).

This report unreservedly endorses the preservation and enhancement of dunal systems and vegetation to augment all available erosion remediation strategies for the coast.

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10 ADDITIONAL INVESTIGATIONS REQUIRED

This investigation was prompted by the absence of strategies for remediating coastal erosion risk which are of more remedial benefit than existing “make safe/make good” strategies, but are not as costly as “end-state” structural provisions. Adaptive, transitional arrangements have been considered to fill this breach, some of which are better understood than others. A number of generic and beach-specific strategies require further investigation and research.

Council has developed coastal erosion hazard lines for the Sholahaven LGA based on the “traditional method” of hazard definition. A risk based determination of coastal erosion hazard lines is available to review and potentially refine the current measure of the erosion hazard, although the technology is still developing (Section 2.4). Council has indicated that further investigations are required to update the coastal hazard assessment at Culburra Beach. This beach could provide Council with a demonstration beach site to develop a risk based assessment, to evaluate the advantages of the methodology against the “traditional approach” and consider its application to other beaches in the LGA.

SCC instigated some five years ago a program of ground survey monitoring of its beaches prone to higher erosion risk, carried out on a regular basis and also triggered by storm events. This is a valuable data collection task, the extent of which should be reviewed and enhanced as required. In baseline terms, it would be essential for the ground survey monitoring to continue at least at annual intervals. The potential for data overlap with any LiDAR coastal surveying programs should however be considered (Section 7.2).

Two types of generic transitional works schemes have been discussed; transitional arrangements for revetment works and transitional arrangements involving sustainable beach sand recycling or nourishment. While beach sand recycling could potentially take place only at particular sites, transitional arrangements for revetment works have application at all sites where revetments are being considered (Section 8.2.1).

Based on a typical open coast rock revetment design, this report finds that potentially significant reductions in design storm erosion are available with the construction of a rock toe mound as a first stage in a rock revetment construction project. However, there are uncertainties regarding the development of additional forces, particularly at the back of the toe, which could only be properly assessed by physical modelling. Given the substantial potential life cycle costs savings available, further detailed investigations are warranted to explore this adaptive works idea including detailed desk-top literature review, cost-benefit assessment and physical modelling.

Further investigations should be considered at Council’s “authorised location” beaches at Callala, Collingwood and Mollymook, and design development take place for the groyne and sand nourishment project at Currarong Beach.

The rock toe mound concept can be explored for Projects 2 and 3 at Callala Beach (Table 3), for Projects 3, 4 and 5 at Collingwood Beach (Table 4) and Projects 5 and 9 at Mollymook Beach (Table 5). It is appropriate that bedrock investigations be

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undertaken involving bona-fide and borehole drilling at these three beaches, under the direction of a Geotechnical Engineer, to confirm the assumptions regarding fully erodible material to design wave scour levels implicit in the coastal erosion hazard definition. Sand sourcing investigations to facilitate beach sand recycling or nourishment schemes, particularly at Collingwood and Mollymook Beaches, are also required. These investigations should focus on Lake Conjola for dune enhancement works at Mollymook Beach, and Moona Moona Creek and Currambene Creek for adaptive erosion hazard mitigation schemes at Collingwood Beach and possibly Callala Beaches.

This report endorses the application of adaptive trial or demonstration projects involving the installation of geotextile containers to impound beach fills, as have been proposed specifically at Currarong Beach (Section 6.5) and Callala Bay Beach (Section 6.6.1).

To support to the various investigations canvassed here, Council is interested to develop Trigger Based Action Plans aligned to beach monitoring program. This is a sensible proposition. Such plans would be useful in further prioritising remedial actions, not just for design and implementation of remedial works but also the precursor investigations given the high competition for funding coastal management across the Shoalhaven LGA.

Finally , it is also appropriate that Council instigates reviews of its Emergency Action Subplans for all of its high risk beaches.

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11 REFERENCES

Australian Geomechanics Society Landslide Taskforce, Landslide Practice Note Working Group [AGS] (2007a) Practice Note Guidelines for Landslide Risk Management 2007 Australian Geomechanics, Volume 42, No. 1, March, pp. 63 114

Australian Geomechanics Society Landslide Taskforce, Landslide Practice Note Working Group [AGS] (2007b) Commentary on Practice Note Guidelines for Landslide Risk Management 2007 Australian Geomechanics, Volume 42, No. 1, March, pp. 115 158

Cameron DW, Morris BD, Collier L, and Mackenzie T (2007) Management and Monitoring of an ICOLL Entrance Clearance NSW Coastal Conference 2007 Coastal Hazard Definition Addendum

Chapman DM, Geary M, Roy PS and Thom BG (1982) Coastal Evolution and Coastal Erosion in New South Wales Report for the Coastal Council of NSW, ISBN 0724065822

Coastal Engineering Solutions (2003) Callala Bay Shoreline Erosion Study Prepared for Shoalhaven City Council, 02-0191nsw-hprrp, Final, 25/8/03

DECCW (2010a) Coastal Risk Management Guide Incorporating sea level rise benchmarks in coastal risk assessments ISBN 978 1 74232 922 2, DECCW 2010/760, August 2010

DECCW (2010b) Guidelines for Preparing Coastal Zone Management Plans ISBN 978-1-74293-051-0, DECCW 2010/1019, December 2010

Great Lakes Council (2007) Tender Document for Jimmys Beach Sand Renourishment Project Stage 1 Contract No 16/07 – PR-PRO-TUN10, May 2007

Nielsen AF, Lord DB, Poulos HG (1992) Dune Stability Considerations for Building Foundations Aust. Civ. Eng. Trans., IEAust. Vol. CE 34, 2

NSW Government (1990) Coastline Management Manual ISBN 0730575063, September 1990

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Patterson Britton & Partners (1996) Port Hacking Maintenance Dredging - Pollution Reduction Program Alternatives to Disposal of Dredged Material Prepared for DLWC, Issue No 3, April 1996

Peter Spurway and Associates (2014) Shoalhaven Citywide Dredging Feasibility Study Prepared for Shoalhaven City Council, March 2014

RHDHV (2014a in prep) Collaroy-Narrabeen Beach CZMP Peter Horton RHDHV pers comm

RHDHV (2014b) Risk Assessment to Define Appropriate Development Setbacks and Controls in relation to Coastline Hazards at Old Bar Prepared for Greater Taree City Council, 8A0271prh-Old Bar, April 2014

RHDHV (2013a) Shoalhaven ‘Authorised Locations’ Coastal Erosion Remediation Options – Collingwood Beach Prepared for Shoalhaven City Council, September 2013

RHDHV (2013b) Shoalhaven ‘Authorised Locations’ Coastal Erosion Remediation Options – Callala Beach Prepared for Shoalhaven City Council, March 2013

RHDHV (2012a) Callala Bay Design Review Management of Shoreline Erosion and Beach Access Draft report for SCC, rp8A0092_gpb_060812, November 20912

RHDHV (2012b) Shoalhaven ‘Authorised Locations’ Coastal Erosion Remediation Options – Mollymook Beach Prepared for Shoalhaven City Council, October 2012

SCC (2012) Asset Management Plan Coastal & Estuary Assets Policy Number: POL12/58, adopted: 26/05/2009, amended: 15/04/2014 Produced By: Assets and Works Group, review Date: 01/12/2016

SCC(2009) Beach Hazard Baseline Survey Project Developed in consultation with SMEC, 19/11/09

SMEC (2011) Currarong Beach Erosion Design Study Report for: Shoalhaven City Council, Project Number: 3001859, Rev 4, May 2011

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SMEC (2008) Shoalhaven Coastal Zone Management Plan Callala Bay Coastal Management Scheme Prepared for SCC, Project No 30011209, Rev 1, 31/1/08

SMEC (2007a) Shoalhaven City Council Coastal Zone Management Study and Plan - Shoalhaven Heads, Coastal Hazard Study Draft report for SCC, Document Number 3001209-017, August 2007

SMEC (2006a) Shoalhaven City Council Coastal Zone Management Study and Plan – Narrawallee Beach, Coastal Hazard Study Draft report for SCC, Document Number 3001209-008, October 2006

State Emergency Services (2013) Coastal Erosion StormSafe Guide http://www.ses.nsw.gov.au/news/20131/update3245, October 2013

Umwelt (2012) Draft Coastal Zone Management Plan for Shoalhaven Coastline Report 2239/R04/V1 prepared for SCC in association with SMEC, May 2012

Umwelt (2011) Draft Coastal Emergency Action Subplans for Beaches in Shoalhaven City Council Report No 2239/RO4/V1, February 2011

University of Sydney (2014) Assessment of Relative Vulnerability to Sea-Level Rise and Associated Erosion Processes of 8 Exposed Beaches in Shoalhaven Prepared by Faculty of Engineering and Information Technologies, School of Civil Engineering, for SCC, February 2014

USACE (2009) Louisiana Coastal Protection and Restoration (LACPR) New Orleans District, Mississippi Valley Division - Final Technical Report June 2009

USACE (2002) Coastal Engineering Manual EM 1110-2-1100 (Part VI), 2002 to 2006

Water Research Laboratory (2013) Old Bar Revetment: Design Wave Heights and Scour Levels WL2013107 LR20131031, prepared for OEH, 5 November 2013

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