The Fenwick Shoreline Decision
Fenwick sits on a low peninsula of 1,240 residents. A tide gauge records relative sea-level rise of 4.2 mm per year over 40 years. Groundwater extraction for the local bottling plant has caused subsidence of about 3.1 mm per year over the same period, measured by repeated levelling surveys. The 1971 seawall crests at 2.9 m above chart datum and was designed for a 1-in-50-year storm. A 2019 coastal study projects that by 2060 the 1-in-50-year still-water level will reach 3.4 m, and that wave run-up adds a further 0.5 m during the largest events. Thirty-eight per cent of dwellings stand within 1 m of mean high tide. Three options are costed: raising the wall to 3.9 m ($41 million, 25-year design life, narrows the beach by about 30 m); managed retreat with buyouts ($28 million, relocates 210 households); dune and wetland restoration ($11 million, cuts wave energy by 40 per cent but does not stop overtopping). The council must submit a plan in 18 months to qualify for a provincial grant covering 60 per cent of capital cost.
1. Using the figures given, estimate the total relative sea-level trend the council should plan for, and explain why the two rates are combined rather than chosen between.
Read the worked answer
Combine them: 4.2 + 3.1 = 7.3 mm per year. That is about 0.73 m per century, and roughly 29 cm over the 40-year measurement period already experienced.
The reason is that both processes move the same water surface relative to the same wall. Eustatic and regional ocean rise raises the water; subsidence lowers the land. What floods a doorstep is the difference, so 'relative' sea level is the only figure that matters locally.
Two caveats belong in the council report. First, extrapolating linearly to 2060 assumes no acceleration; most projections show acceleration, so 7.3 mm/yr is a floor, not a forecast. Second, subsidence is not fate - it is driven by local extraction, so halting or reducing pumping is a policy lever that ocean rise does not offer. Note also that the proposed 3.9 m crest equals the projected 2060 still-water level plus run-up, leaving effectively zero freeboard for a larger event.
2. Set out the criteria you would use to choose among the three options, and evaluate each against them.
Read the worked answer
Criteria: level of protection and residual risk; capital cost and cost per protected household; reversibility; distributional effects (who is protected, who pays, who is displaced); ecological impact; speed of implementation against the 18-month deadline; and leverage of the 60 per cent grant.
- Raise the wall ($41 m). Highest protection but only to a 25-year design life, so it buys time rather than solves the problem. It destroys 30 m of beach - likely the tourism base - and creates a 'levee effect' that encourages more building behind it. The grant makes it roughly $16.4 m locally.
- Managed retreat ($28 m). Addresses residual risk permanently and is the only option that removes exposure, but relocates 210 households, raises compensation and consent questions, and is politically hard to reverse once done.
- Restoration ($11 m). Best value per dollar and provides co-benefits, but cuts wave energy by only 40 per cent and does not stop overtopping, so it cannot stand alone.
The honest answer is a hybrid: restoration plus a lower wall, with buyouts for the most exposed properties. That is more complex but matches the data better than any single line item.
3. A councillor says: "Sea-level rise is a global problem, so Fenwick's subsidence is irrelevant." Respond.
Read the worked answer
The premise is right and the conclusion does not follow. Global sea-level rise is indeed driven by climate processes Fenwick cannot influence. But the 3.1 mm/yr subsidence is local, recent, and caused by extraction for a plant that operates in the town - it is roughly 42 per cent of the total relative trend.
Ignoring it produces two errors. Analytically, the council would understate the design height and under-protect residents. Politically, it would obscure a local cause and a local remedy: reducing or relocating extraction could slow subsidence, something no climate policy can do at village scale.
There is also an equity point worth raising. The bottling plant generates the subsidence; the 470-odd residents within 1 m of high tide bear the risk. That is an externality, and whether the plant contributes to the defence cost is a legitimate question for the council, not a distraction from it.