Coastal Management Trade-offs on a Retreating Shoreline
A 12 km sandy coastline fronts a town of 8,500 residents. Long-term aerial photography shows the shoreline has retreated 1.8 m per year for 40 years, but the last 5 years averaged 3.1 m per year. Two engineering options are proposed: (A) a 4 km rock seawall at $22 million plus $400,000 annual maintenance, and (B) planned retreat with dune revegetation at $3.5 million over 10 years. A seawall typically halts erosion behind it but can increase erosion on adjacent unprotected beaches by removing sediment supply and reflecting wave energy. The town's main road runs 120 m from the current shoreline. Council must decide within 18 months because a storm-surge event is forecast to threaten the road within 5-10 years.
1. Evaluate which option is more defensible, using the data provided and the spatial and temporal trade-offs of each.
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The recent acceleration (1.8 m/yr over 40 years versus 3.1 m/yr over 5 years) suggests the long-term average understates current risk. At 3.1 m/yr, the road 120 m inland is reached in roughly 39 years; at the long-term 1.8 m/yr, about 67 years. But a single storm-surge event could overtop the current buffer well before either date, so the forecast threat within 5-10 years is the binding constraint.
Option A protects the road and assets behind 4 km of coast but only shifts the problem: terminal scour at the wall's ends and reduced sediment supply can accelerate erosion on the remaining 8 km, effectively transferring risk to unprotected residents. The $22 million capital plus $400,000/yr is a recurring commitment with no endpoint and locks the council into a hard structure that is difficult to remove.
Option B costs far less and works with natural processes, but dune revegetation alone cannot hold a 3.1 m/yr retreat line if a surge arrives in 5-10 years. It buys ecological function, not immediate protection.
A defensible judgement is a hybrid: targeted protection only where the road is most exposed, paired with dune restoration and a managed realignment policy elsewhere, with relocation planning for the highest-risk properties. Neither single option resolves the trade-off between protecting the few and preserving the beach for the many.
2. Explain why a cost-benefit framing based only on capital cost would mislead the council.
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Capital cost captures only the upfront outlay, not the distribution of costs and benefits over space and time. Option A's $22 million is visible, but its $400,000 annual maintenance compounds over decades and its externalities - accelerated erosion on adjacent beaches - impose costs on residents outside the protected 4 km. Those transferred costs are real but do not appear on the council's ledger.
Option B's $3.5 million looks cheap, but if it fails to protect the road, the cost of road damage, disruption and emergency response falls on the public purse later. Deferring cost is not avoiding it.
A proper framing compares present-value costs and benefits across the whole coastline, includes non-market values such as beach amenity and ecosystem function, and weights who bears each cost. A capital-only comparison systematically favours the structure because its benefits are concentrated and visible, while its diffuse harms and long-term liabilities are hidden.
3. Identify one piece of additional evidence that would most change the recommendation, and explain why.
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The most decision-relevant evidence would be a sediment-budget study quantifying alongshore transport rates and how much sediment the seawall would intercept. If the net transport is low or the cell is relatively closed, the adjacent-erosion externality of Option A is small and the seawall becomes far more defensible; if transport is high, the transfer of erosion to the remaining 8 km is severe and Option A is hard to justify.
A close second is an asset-exposure map showing which properties and infrastructure sit within the 5-10 year surge zone. If only the road is exposed, targeted protection plus realignment is viable; if a large residential area is exposed, retreat becomes socially and politically much harder and the hybrid recommendation would need to shift toward more protection. Both pieces of evidence change the balance of the trade-off rather than merely adding detail.