5.1 Adaptation option analysis
The analysis based on the global model suggested that high wave energy could be a contributing factor to the cliff erosion. On this basis, adaptation options are needed to protect the cliff toe from wave action to avoid destabilising the cliff through notching and undercutting. However, the fine resolution modelling indicates that wave energy does not appear to be the main reason causing the erosion of the cliff. Rather, a mix of geotechnical factors better explain the eroding cliff face. It follows that stabilisation of the top and toe of the cliff is the appropriate adaptation rather than interventions that would reduce the wave energy at the foot of the cliff. With these diverse modelling outcomes in mind, we evaluated a range of adaptation strategies and options spanning interventions that would dissipate wave energy through to those that address geotechnical factors.
5.1.1 Hold the line
Based on the estimates of high wave energy offshore from the global scale model data, coastal adaptations that could reduce the wave energy reaching the shoreline were reviewed. Hard engineering (capital works) options include seawalls, revetments, levees, breakwaters and groynes (Boetang, 2012). These options can be deployed to either shore up the cliff or prevent wave energy reaching the cliff toe. For example, concrete walls at the foot of the cliff would prevent erosion by reflecting the wave energy. This option ‘holds the line’ and does not require rebuilding of the school or realignment of the road. However, these interventions are expensive, with high capital outlays and ongoing maintenance and replacement within the envelope of the period under study. Rock breakwaters have a typical design lifetime of 30-50 years with wooden groynes having a lifetime of about 10-25 years and groynes constructed with of gabions of 1-5 years (Tomlinson and Jackson, 2019). These structures require maintenance and have limited lifetime – in the context of climate change adaptation, they are not long-lasting solutions. Results showed that a groyne or seawall placed at the entrance of the bay would reduce the wave energy and local currents at the toe of the cliff. Based on the global wave model alone, this adaptation option would have been chosen as the first option to mitigate the assumed instability at the toe of the cliff.
The fine scaled numerical coastal process model showed that the waves dissipate in the embayment and the wave energy at the toe of the cliff is low, even during a 100-year event. Reducing the wave energy reaching the toe would, therefore, not reduce the main stressors to the cliff and it would continue to erode. Port Resolution is surrounded by overhanging cliffs cut into weathered basaltic sands with a near vertical cliff face (Brothelande et al., 2016). Cliff collapse can be due to a combination of destabilisation of the cliff top from clearing the vegetation cover, groundwater pressure and general weathering from heavy rainfalls (Dawson et al., 2009).
Based on the fine-scale modelling, stabilisation of the cliff would be the more effective and therefore preferred option to prevent cliff erosion which would include maintaining and restoring vegetation growth on the top of the cliff to increase soil cohesion. Reducing the angle of the slope could help counteract future uncontrolled weathering and sliding. However, this solution requires enough space at the foot as well as at the top of the cliff (Mangor et al., 2017). Waterlogged soil also likely contributes to cliff erosion and risk of collapse, along with rainwater runoff and storm damage (Bosch et al., 2006). Therefore, it may also be appropriate to drain excess water flows during heavy rainfall events with a channel that creates a diversion away from the cliff top. An additional intervention would be to protect the toe of the cliff with revetment. Given the near vertical angle of the cliff face, the most effective interventions are likely to be at the top of the cliff, with revegetation and a diversion channel (Mangor et al., 2017).
5.1.2 Ecosystem-based approaches
An alternative to engineered interventions with structures like concrete sea walls are ecosystem-based adaptations or nature-based solutions (NbS) (Chausson et al. 2020). On the basis of the global model results which point to wave energy being the casual factor, NbS in the coastal zone could include protecting and restoring coral reefs, dunes, mangroves and saltmarsh. Coastal ecosystems stabilize shorelines by reducing wave energy and trapping sediments (Colls, in IUCN, 2009). However, their deployment is dependent on the local context including the coastal geomorphology (Chausson et al., 2020). In Port Resolution, mangroves do not naturally occur, but growth of coastal saltmarsh vegetation could potentially be encouraged to help prevent any undercutting at the base of the cliff (Bosch et al., 2006).
The benefits from the existing ecosystems and natural processes were illustrated by running the coastal process model under scenarios where the reef was removed and the embayment was dredged to increase its utility for anchoring of recreational yachts. The results showed a potential increase in wave height to the shoreline from these interventions adjacent to the road. From this perspective, protecting existing ecosystems can represent a powerful NbS. One approach for reef protection is through establishing well managed community conservation areas supported by community ranger programs and related capacity building activities (Dawson et al. 2009).
5.1.3 Limited intervention, maintaining current management options, and managed realignment
An artificial reef constructed in the embayment could act like a natural protection by dissipating the wave energy and providing some additional protection of the shoreline; these also provide a substrate for marine life provide additional ecosystem service benefits (Reguero et al., 2018). However, during high water levels, especially during storm surge events, the effects of those structures would be limited.
At the current rate of erosion (Figure 5), the main road is likely to be threatened in approximately 10 years, with the school building impacted in around 15 years. Our modelling results are equivocal as to whether coastal processes may or may not lead to continued erosion and maintaining the status-quo management provides the benefit of avoiding the costs of engineered solutions. However, this approach leaves the residual risk cost of further cliff erosion and potential sudden catastrophic loss which can be quantified annually. A 10 to 15-year timeframe allows for a reasonable buffer against sudden, large-scale loss of the cliff top. Furthermore, anecdotal evidence suggests sand and coral rubble have, until recently, been constantly removed from the bay, which may have had an impact on the rate of erosion of the cliff. Cessation of this activity therefore may slow the rate of future erosion.
Managed realignment or retreat refers to the relocation of assets landward to accommodate the predicted shoreline recession. Under this intervention, the school buildings would be pre-emptively moved and rebuilt in year 0, with the road to be realigned in year 10. Managed realignment can significantly reduce the cost of providing a level of protection and maintenance against erosion or flooding. Further cost savings can be made if realignment allows the defensive line to be shortened or completely abandoned (Nicholls et al., 2007).
5.2 Adaptations options and their cost-benefits
In addition to the efficacy of an intervention to address the cliff erosion, other factors need to be taken in account when evaluating adaptation options including: (a) the design, construction and maintenance costs; (b) how they affect or are affected by existing policy and planning such as national guidelines and local government spatial zoning; and (c) their social acceptability which reflects local social and cultural norm, needs and aspirations (Buckwell et al., 2020a). Here we focus on the direct design, construction, and maintenance costs, acknowledging that indirect costs, such as loss of income or nonmarket costs such as loss of community cultural values - were not considered. A descriptive, qualitative comparison between the candidate set of options and detailed costs are provided in Table 2.
Hard engineering structures can be built from a range of materials including gabion, geotextile, shotcrete, epoxy injection, boulder, and timber. Costs depend significantly on the design and materials used as well as the transportation, construction, and labour costs, along with any plan overlays that need to be considered. The estimated costs of rebuilding the school and realigning the road were obtained from on-site advice and phone-based estimates from reputable suppliers of each (Ware et al., 2020) (Table 2). Also included are estimates of the costs for the NbS adaptation intervention extrapolated from Buckwell et al (2020b) who estimated Community Conservation Area (CCA) implementation costs on a per hectare per year level. These estimated costs included the development of a community ranger program which is an essential capacity for CCA management practices.
Table 2 . A descriptive comparison of the candidate adaptation options given the results from both the global and local scaled models and analyses.
Adaptation
|
Impact on erosion risk
|
Cost analysis
|
Global analysis
|
Local analysis
|
Unit construction/m2 cost (US$)
|
Construction design cost (US$)
|
Annual Maintenance cost (US$)
|
Notes
|
Hold the line: Engineering Options
|
Gabion seawall
|
Absorb wave energy, reduce the scour problem.
Not very effective for high wave energy and not attractive
|
Protect the toe of the cliff but instability of the cliff could still happen due to geotechnical issues and infiltration
|
500-800
|
632,000
|
500
|
Dependent on sourcing of coral or stone rubble and local labour. Considering 3 m high
Lifetime: 25 years
|
Geotextile seawall
|
Will not protect from aggressive action of the sea but will serve to breaking up some of the wave energy and hold the soil in place
|
Protect the toe of the cliff but instability of the cliff could still happen due to geotechnical issues
It will help to stabilise and drainage
|
500
|
395,000
|
1000
|
Assumes sediment availability
For a 5m high and 790m long seawall
Lifetime: 10 years
|
Timber seawall
|
Provide barrier to wave energy, but not recommended for high waves energy
|
Protect the toe of the cliff but instability of the cliff could still happen due to geotechnical issues and infiltration
|
100-500
|
395,000
|
1000
|
Costs based on the availability of local timber (2m height seawall)
Lifetime: 10 years
|
Rock armour/
Boulder seawall
|
Effective protecting the base of the cliff, absorb wave energy
|
Push-out of slab element can occur due to uplift pressure**
|
5 000
|
3,950,000
|
0
|
Costs based on importation of suitable stone for a structure of 5m high and 790m long
Lifetime: 50 years
|
Shotcrete seawall
|
Reflect back the energy to the sea, control erosion but would need to reduce the steepness of the slope
|
Reflect back the energy to the sea, stabilise the cliff but geotechnical issues can still happen, would need to reduce the steepness of the slope, would need to understand the drainage system and groundwater depth
|
300-900
|
3,555,000
|
0
|
Lifetime: 25 years
|
Groynes
|
Reduce wave energy at the toe of the cliff
|
Reduce wave energy and local currents at the toe of the cliff but increase elsewhere and high-water levels and groundwater would still destabilise the cliff
|
15 190
|
11,850,000
|
-
|
Construction costs depend significantly on structure dimensions and availability of suitable rocks and transport. The example is for a 130m long, 5m high
Lifetime: 30-50++ years
|
Artificial reef
|
Reduce wave energy at the toe of the cliff
|
Reduce water energy and local currents at the toe of the cliff but still high water associated with geotechnical issues will destabilise cliff
|
Shed blocks: 280
Amorflex mat
Plain: 88
With transplanted coral: 130
|
-
|
0
|
Depends on the shape and dimensions for construction
Lifetime: 50 years
|
Artificial smoothing of the slope
|
Would be attacked by waves
|
Cliff stabilisation
|
Medium costs
|
-
|
-
|
|
Accommodation
|
|
Global analysis
|
Local analysis
|
US $/ha/yr. costs
|
Notes
|
Conservation reef
|
Does not make conditions worse, the reef loss would increase currents
|
Keep protecting the cliff and reduce currents
|
Medium cost
448
|
Adapted from Buckwell et al. (2020b)
|
Revegetation
|
Increase cliff stability but wave energy would impact the toe
|
Cliff stabilisation Protects against weathering and groundwater seepage and then against sliding
|
Low cost
448
|
Adapted from Buckwell et al. (2020b)
|
Managed retreat
|
Adaptation
|
Global analysis
|
Local analysis
|
Design costs (US$)
|
Construction costs (US $)
|
Notes
|
School rebuild
|
Do not need to worry about the cliff
|
Do not need to worry about the cliff
|
10 000
|
55 046
|
Cost depends on the land costs and location
|
realignment of road
|
Do not need to worry about the cliff
|
Do not need to worry about the cliff
|
1 000
|
50 000
|
+ Seawall placed at the base of the cliff per metre square.
++ A wooden groyne would have a lifetime of about 10-25 years and groynes made of gabions of 1-5 years
Based on studies from Ferrario et al., (2014) it was found that reef conservation and restoration can be cost effective for risk reduction and adaptation. They found that reef restoration was always substantially more cost effective than breakwaters across eight nations considering only coastal defence benefits. This adaptation represents low cost to develop and implement.