Hold the line or retreat? A coastal community after the storm
A west coast settlement of 340 permanent residents and 900 holiday homes sits on a 2.5 km sand spit. A storm in 2023 eroded 12 m in one night, damaging six properties and the only access road. Long-term shoreline data show mean erosion of 0.4 m per year since 1990, but sediment supply from the river has fallen by 30% after upstream dams. A seawall for the full frontage would cost $22 million, have a 40-year design life, and would likely cause downdrift erosion and remove the beach at high tide. Managed retreat would cost $31 million over 30 years for property purchase, road relocation, and dune restoration, but it would displace residents and affect holiday-home owners. The district plan still allows new dwellings. Sea-level rise projections are 0.3–0.6 m by 2100. The community is divided: some want protection, others want a planned transition. The council has a $25 million coastal adaptation budget.
1. Analyse the natural processes and human decisions that have produced the current risk.
Read the worked answer
The risk is a product of physical exposure and social vulnerability. The sand spit is a dynamic coastal landform built by sediment delivered from the river and moved by longshore drift. Upstream dams have cut sediment supply by 30%, so the spit has less material to rebuild after storms. Mean erosion of 0.4 m/yr since 1990 is long-term; the 2023 storm shows that episodic events can remove 12 m in one night. Sea-level rise of 0.3–0.6 m by 2100 will raise the base on which waves act, increasing overwash and undermining the spit. Human decisions have increased exposure: 340 permanent residents and 900 holiday homes are located on the spit; the only access road is on the same landform; and the district plan still allows new dwellings. That is a 'development on dynamic land' problem, not a purely natural one. The seawall proposal would try to fix the shoreline, but it would interrupt sediment movement, cause downdrift erosion, and remove the beach at high tide-transferring risk to neighbours and reducing amenity. The community division reflects different attachments and interests: permanent residents depend on the road and services; holiday-home owners have financial assets but may not live with daily risk. The council's $25m budget is below the $31m retreat estimate and close to the $22m seawall, which creates a false sense that the seawall is the affordable option.
2. Evaluate seawall vs managed retreat, and recommend an adaptation strategy that considers uncertainty and equity.
Read the worked answer
The seawall is a 'hold the line' response. It has a 40-year design life and protects existing assets, but it is expensive, high-maintenance, and likely to fail or require upgrading as sea level rises. It also externalises erosion and removes the beach, which is both an ecological and economic asset. Its apparent affordability ($22m vs $31m) ignores the cost of periodic renewal and the loss of beach amenity. Managed retreat is a 'planned transition'. It is more expensive upfront but can reduce long-term risk, restore dunes, and relocate the road. Its weakness is equity: buying properties and displacing residents raises questions about compensation, cultural attachment, and whether holiday-home owners are treated the same as permanent residents. It also requires lead time and community consent. Given uncertainty, the best strategy is an adaptive pathway, not a single bet. I would recommend: (1) stop new dwellings on the spit immediately (no-regret); (2) restore dunes and set a vegetation buffer; (3) commission a detailed risk and compensation assessment; (4) use triggers-e.g., after the next overwash or when erosion reaches a set line-to relocate the road and begin voluntary buyouts of the highest-risk properties; (5) reserve the seawall as a limited, temporary option only if a cost-benefit analysis shows it protects life safety and does not harm neighbours. This uses the $25m for buyouts, road relocation, and dune restoration rather than a 40-year wall. Equity requires distinguishing permanent residents from holiday owners and providing relocation support; iwi and community should co-design the triggers.