Coastal erosion and managed retreat
A coastal township of 3,200 people sits on a low sandy shoreline. Erosion has averaged 1.2 metres per year over the past two decades, but a recent storm cut back 15 metres in a single event. Sea-level rise projections suggest erosion could accelerate to 2 metres per year. A seawall would cost $40 million and protect 400 properties for perhaps 30 years, but would likely cause 'end-effect' erosion on adjacent beaches and prevent natural beach recovery. Managed retreat-buying back and relocating 60 properties at risk-would cost $30 million, with relocation spread over fifteen years. The state government will fund one option. Residents strongly oppose retreat; the local council depends on tourism and property rates.
1. Evaluate the two options and recommend one, explaining the trade-offs.
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The seawall protects 400 properties for about 30 years at $40 million, but it locks in a hard defence against an accelerating hazard (1.2 m/yr, possibly 2 m/yr) and can worsen erosion downdrift, damaging the beach that underpins tourism. It also creates a moral hazard: more development behind the wall. Managed retreat at $30 million relocates only 60 at-risk properties but removes exposure permanently for those properties and allows the beach to adjust naturally, preserving the tourism asset. Its weakness is social: strong resident opposition, and relocation over 15 years may lag the hazard, especially after a 15 m storm cut. A defensible recommendation is phased managed retreat for the highest-risk 60 properties plus planning controls preventing new development in the hazard zone, with the seawall rejected because it transfers risk to neighbours and is not adaptive to sea-level rise. The answer should weigh cost, time horizon, equity and environmental effects, and acknowledge the political difficulty.
2. Explain why the single storm event matters when interpreting the 1.2 m/yr average.
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An average conceals variability. The 15 m storm cut is more than twelve times the annual average, showing that erosion is episodic: most change occurs in rare events. This matters for policy because a defence designed for the average will be overtopped or outflanked by the extreme, and because communities experience the storm, not the average. It also means that a seawall built to a 30-year design life may be undermined by one event. A strong answer links this to the precautionary approach: where the tail of the distribution is severe and sea-level rise is accelerating, planning should be based on extreme scenarios and adaptive pathways, not on mean rates.