Coastal defence portfolio in an Atlantic resort town
A resort town of 18,000 on the Atlantic coast has a 2.4 km sand beach that contributes an estimated €42m per year in visitor spending. Average shoreline retreat is 0.8 m per year, rising to 1.5 m per year near the promenade. About 120 properties lie within 50 m of the shoreline; 65% are holiday rentals. Three options are on the table. A rock revetment would cost €28m, last 30 years, protect 1.1 km of frontage, but narrow the beach by 15–25 m and may increase erosion downdrift. Beach nourishment would cost €3.1m every five years, maintain beach width, but depends on an uncertain offshore sediment source and has high carbon emissions. Managed realignment of the northern 600 m would compensate 14 properties, create 22 ha of saltmarsh, and meet a national biodiversity target, but would remove a popular access point. The town council wants a decision before the next storm season.
1. Assess the three options using cost-benefit and distributional criteria. Include at least one annualised cost comparison.
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Annualised capital cost: revetment €28m/30 ≈ €0.93m per year; nourishment €3.1m/5 = €0.62m per year, but recurring indefinitely; realignment is a one-off whose annual equivalent depends on compensation-say €10m over 25 years = €0.40m per year plus lost access. On narrow cost-benefit, nourishment looks cheapest per year and protects the beach asset that generates €42m in visitor spending. But it does not remove the long-term retreat problem and depends on sediment supply. The revetment protects 1.1 km but shifts erosion downdrift, so benefits are concentrated on property owners behind the wall while costs fall on neighbours and beach users. Realignment concentrates costs on 14 property owners but creates a public habitat good. Distribution matters: 65% of nearby properties are holiday rentals, so compensation may flow to absentee owners. A decision solely on annualised cost would understate the downdrift transfer and the biodiversity benefit.
2. Recommend a portfolio of measures and explain why a single option is inadequate.
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A portfolio is preferable to any single option. Use nourishment as the primary measure along the central beach because it maintains the amenity that supports €42m in visitor spending and can be triggered by shoreline monitoring. Pair it with managed realignment at the northern 600 m, where property density is lower and saltmarsh creation meets the biodiversity target; this also reduces the length of coast that needs nourishment. Do not build the full revetment: it narrows the beach and externalises erosion downdrift. A short terminal structure at the promenade may be justified if monitoring shows accelerated retreat there. The portfolio spreads risk: if sediment supply fails, realignment and terminal protection remain; if compensation talks fail, nourishment buys time. Set a budget envelope of about €3.1m every five years plus compensation, rather than €28m upfront. The council should also secure a sediment-source agreement before committing.
3. Identify one monitoring indicator and a decision trigger that would show the portfolio is failing.
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Indicator: mean shoreline position at 20 fixed transects, measured twice yearly, plus beach width at the peak summer season. Decision trigger: if mean retreat exceeds 0.5 m per year over three consecutive years, or if usable beach width falls below 25 m at the central access, the portfolio is failing. A second trigger is sediment delivery: if the offshore source fails two consecutive nourishment cycles, switch to terminal structures or accelerate realignment. These thresholds connect monitoring to action rather than producing data for its own sake.