Thesis – Coastal Erosion and Adaptation Planning on the New South Wales Coast

Abstract

Sandy open-coast shorelines along New South Wales are increasingly exposed to storm-driven erosion and the compounding influence of sea-level rise, yet site-specific recession rates and community adaptation preferences remain unevenly documented. This thesis combined a quantitative shoreline-change analysis with a stakeholder survey to inform coastal adaptation planning at six eroding open-coast localities. Historical aerial photography and airborne LiDAR were analysed within a geographic information system using 292 shore-normal transects to derive long-term recession rates, while a survey of 118 residents, planners, engineers and community members captured risk perceptions and adaptation preferences. Mean long-term recession across the six sites was 0.48 metres per year, ranging from 0.11 to 0.86 metres per year. Residents favoured protection works, whereas planners and engineers favoured planned retreat, revealing a persistent implementation gap. The findings support trigger-based adaptation pathways delivered through the New South Wales coastal management framework rather than reliance on hard protection alone.

Introduction

The New South Wales coast comprises a series of embayed and open sandy beaches that are among the most heavily settled parts of Australia. More than four in five residents of the state live within fifty kilometres of the coastline, and coastal local government areas continue to record strong population growth (Australian Bureau of Statistics, 2023). This concentration of people and assets sits directly in the path of two interacting hazards: acute erosion driven by high-energy storms, and the slower, cumulative shoreline recession associated with rising sea levels.

Storm erosion on this coast is dominated by east coast lows, intense synoptic systems that generate large, long-period waves and elevated water levels capable of removing substantial volumes of sand from the subaerial beach within a single event. The June 2016 storm that undermined beachfront properties at Collaroy on Sydney’s northern beaches illustrated how quickly episodic storm demand can translate into property loss where development sits close to an eroding shoreline. Superimposed on this episodic behaviour is a longer-term signal. Global mean sea level rose by roughly 0.2 metres over the twentieth century and the rate of rise has since accelerated (Church & White, 2011). Regional projections for the Australian coast indicate continued rise through the twenty-first century under all plausible emissions pathways (CSIRO & Bureau of Meteorology, 2022; Intergovernmental Panel on Climate Change, 2021; McInnes et al., 2015). Higher mean water levels raise the base from which storms operate and, over decades, drive a landward translation of the shoreline.

Despite a mature policy framework, quantified recession rates and evidence on community adaptation preferences are unevenly available across individual localities, which constrains the preparation of coastal management programs by councils. This thesis addresses that gap through three research questions. First, what are the long-term shoreline recession rates at a set of eroding New South Wales open-coast sites? Second, how do residents and coastal professionals perceive erosion risk and rank the available adaptation options? Third, what are the principal barriers to implementing adaptation, and managed retreat in particular, within the current planning framework?

Literature Review

Shoreline behaviour is conventionally understood through the sediment budget of a coastal compartment, in which the position of the beach reflects the balance between sediment supply, losses and cross-shore exchange. Thom et al. (2018) formalised this thinking for Australia through a national sediment compartment framework that is now embedded in New South Wales hazard assessment. Within a compartment, the response of a sandy shoreline to rising sea level has often been estimated using the Bruun rule, which predicts landward retreat proportional to the rise (Bruun, 1962). The rule remains influential but has been criticised for treating the profile as closed and ignoring sediment supply, alongshore transport and geological constraints; Cooper and Pilkey (2004) argued that it is frequently applied beyond the conditions for which it is valid. Contemporary assessments therefore favour probabilistic, compartment-scale methods that separate short-term storm demand from underlying recession (Kinsela et al., 2017).

Adaptation options are commonly organised as a hierarchy of protect, accommodate and retreat. Protection includes hard structures such as seawalls and revetments and softer measures such as beach nourishment. Accommodation adjusts development to tolerate hazard, for example through raised floor levels or relocatable buildings, while retreat progressively removes assets from the hazard zone. Australian vulnerability research has repeatedly noted that each option carries distributional consequences, and that hard protection of private assets can transfer costs to the public beach through passive erosion and narrowing (Harvey & Woodroffe, 2008).

Although managed retreat is widely regarded by coastal scientists as the most durable response to a persistently receding shoreline, it is the least implemented. Gibbs (2016) attributed this to political risk, arguing that retreat concentrates visible losses on identifiable property owners in the present while diffusing benefits across future users, an asymmetry that discourages decision-makers. This literature frames the empirical work that follows: technical recession rates establish the scale of the problem, while stakeholder preferences reveal the social feasibility of the responses.

Methodology

A mixed-methods design was adopted. A quantitative shoreline-change analysis established the physical rate of the hazard at each site, and a survey characterised how the hazard and its potential responses are perceived. Six open-coast localities with documented erosion were selected to span the coast: Belongil Beach in the Northern Rivers, Old Bar Beach on the Mid North Coast, Stockton Beach in the Hunter, Jimmys Beach at Port Stephens, Wamberal Beach on the Central Coast, and the Collaroy to Narrabeen embayment in metropolitan Sydney.

For the shoreline-change analysis, historical vertical aerial photographs spanning the mid-twentieth century to the present were sourced alongside recent airborne LiDAR surveys and coastal hazard datasets published by the New South Wales coastal science program (Office of Environment and Heritage [OEH], 2018). Imagery was orthorectified and georeferenced to the Map Grid of Australia, and elevations were referenced to the Australian Height Datum. The vegetation line was digitised as a stable shoreline proxy on each image, with the wet-dry line used as a supplementary indicator where vegetation was absent. Within the geographic information system, 292 shore-normal transects were cast at fifty-metre spacing across the six beaches. For each transect, the net shoreline movement between the earliest and latest surveys was measured, and a long-term recession rate was derived by linear regression through all available shoreline positions, with the standard error retained as an uncertainty estimate. Positional error from georeferencing and digitising was propagated into the reported uncertainties.

The survey used a structured questionnaire combining Likert-scaled items on risk perception with a ranking task in which respondents ordered five adaptation options. A total of 118 valid responses were obtained across the six council areas: 76 resident landholders, 18 council planners and officers, 12 coastal engineers and consultants, and 12 members of community and environmental groups. Responses were analysed using descriptive statistics and cross-tabulation by stakeholder group, and the quantitative and survey strands were then integrated to appraise adaptation options against the measured hazard. Figure 1 illustrates the overall workflow.

Results

Long-term recession was recorded at every site, but the rates varied by almost an order of magnitude, as shown in Table 1. The highest rate, 0.86 metres per year, occurred at Old Bar Beach, an unprotected and sediment-starved compartment. The lowest, 0.11 metres per year, occurred within the Collaroy to Narrabeen embayment, where the long-term average is modest but where episodic storm demand is severe. Net recession over each record was consistent with the derived rates; for Old Bar, the net movement was calculated as 0.86 metres per year multiplied by the 78-year record, giving 67 metres. Averaging the six site rates produced a mean long-term recession of 2.85 divided by 6, equal to 0.48 metres per year.

The survey revealed a clear awareness of the hazard alongside a divided view on the response. Across the whole sample, 82 per cent of respondents agreed or strongly agreed that coastal erosion poses a serious risk to their locality over the next fifty years. Preferences over adaptation options, however, diverged sharply by stakeholder group. When respondents nominated their single most-preferred option, 54 per cent of the whole sample chose some form of protection works, 24 per cent chose planned retreat, and 22 per cent chose accommodation. Among resident landholders the preference for protection was stronger still, at 63 per cent, with only 17 per cent supporting planned retreat. Among the combined group of planners and engineers, by contrast, 60 per cent nominated planned retreat or a hybrid pathway as most appropriate. A notable inconsistency emerged within the resident group: 58 per cent agreed that retreat is appropriate in principle for the highest-risk properties, yet far fewer accepted it for their own land, indicating that support for retreat weakens as it approaches the individual respondent.

Table 2 sets out the appraisal of the five adaptation options against the measured hazard and the survey evidence, summarising the trade-offs that frame the discussion.

Discussion

The recession rates confirm that the sites face a genuine and, in several cases, rapid landward translation of the shoreline that will be sustained and probably amplified by projected sea-level rise (Intergovernmental Panel on Climate Change, 2021; McInnes et al., 2015). The order-of-magnitude spread in Table 1 also reinforces the compartment-scale reasoning of Kinsela et al. (2017) and Thom et al. (2018): rates reflect local sediment supply and structural constraints rather than a single regional value, so a uniform application of the Bruun rule would misstate the hazard at both the fast-eroding and the more stable ends of the range (Cooper & Pilkey, 2004).

The central tension exposed by the survey is between protection and retreat. Residents overwhelmingly favoured holding the line, a preference that is understandable given the immediate value of beachfront property, yet hard protection carries a well-documented public cost. A seawall fixes the back of the beach while the shoreline continues to recede, so the intertidal zone narrows and can ultimately be lost through passive erosion, transferring amenity from the public to the protected private asset (Harvey & Woodroffe, 2008). The privately funded seawall constructed at Collaroy after the 2016 storm exemplifies this dynamic, protecting individual properties while raising unresolved questions about the future of the public beach in front of it. Table 2 makes the trade-off explicit: the options that best preserve the beach as a public asset are precisely those that residents were least willing to accept for their own land.

Barriers to managed retreat

The reluctance to accept retreat, even where it is acknowledged as appropriate in principle, is consistent with the political-risk explanation advanced by Gibbs (2016). Three barriers were particularly evident. First, the distribution of costs and benefits is asymmetric: retreat imposes concentrated, visible losses on present owners while its benefits accrue diffusely to future beach users, discouraging elected decision-makers. Second, questions of compensation and tenure remain unresolved, since there is no settled mechanism for acquiring or extinguishing development rights on eroding land, and the prospect of uncompensated loss hardens opposition. Third, incentives are split between levels of government, with councils responsible for preparing coastal management programs while the fiscal and legal consequences of retreat extend well beyond their capacity. These barriers explain why retreat, though favoured by the coastal professionals in the sample, rarely progresses.

Implications for New South Wales coastal planning

The New South Wales framework provides a vehicle for addressing these barriers if it is used deliberately. The Coastal Management Act 2016 and the accompanying State Environmental Planning Policy establish coastal management programs as the primary instrument for local adaptation, certified with reference to the state coastal management manual (NSW Government, 2016, 2018; Office of Environment and Heritage, 2018). Rather than forcing a binary choice between protection and retreat, the framework can accommodate trigger-based adaptation pathways in which a defined shoreline position or storm-erosion threshold activates a pre-agreed response. This approach preserves near-term amenity, defers contested decisions until the hazard materialises, and gives property owners advance certainty about the conditions under which land use will change. The measured recession rates supply the physical thresholds such pathways require, and the survey evidence indicates that framing retreat as a conditional, future trigger, rather than an immediate loss, may narrow the implementation gap identified above.

Conclusion

This thesis quantified shoreline recession at six eroding New South Wales open-coast localities and examined how residents and coastal professionals perceive the hazard and its remedies. Long-term recession averaged 0.48 metres per year and ranged widely by site, confirming a real and locally variable threat that sea-level rise will intensify. Residents favoured protection while professionals favoured retreat, and support for retreat weakened as it approached the individual, revealing a durable implementation gap rooted in the asymmetric distribution of costs, unresolved compensation and tenure, and split governance incentives. The most promising path forward is not a single option but trigger-based adaptation pathways delivered through coastal management programs, anchored to measured erosion thresholds. Future work should extend the analysis across additional compartments, incorporate probabilistic sea-level scenarios into the transect projections, and test the acceptability of specific trigger mechanisms with affected communities.

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