Seawall or Managed Retreat for a Coastal Settlement
A small coastal settlement of 32 homes and a surf club sits behind a road that is the only access. Coastal erosion has averaged 0.35 m a year over 20 years, but two storm events in the past five years removed about 4 m. The council’s preferred option is a $2.1 million rock seawall designed for 30 years. A university report says the wall will likely shift erosion to the neighbouring beach and cost $180,000 a year to maintain. Managed retreat - buying and relocating eight most-at-risk homes and rerouting the road - is estimated at $6.4 million over 20 years. Some residents have lived there for four generations; local hapū have identified urupā and food-gathering sites near the erosion edge. The council has a 10-year plan and a legal obligation to consult. Ratepayers outside the settlement question paying for either option.
1. Analyse the causes and effects of the erosion problem, distinguishing between natural processes and human decisions. Why is the seawall not automatically the ‘safe’ option?
Read the worked answer
Natural processes: wave action, storms, sediment supply and sea-level rise drive erosion; the 4 m lost in two storms shows that averages hide threshold events. Human decisions: building the road and homes close to the edge, and possibly earlier protection works, have put assets in the active zone; the seawall would lock in a hard edge. Why the seawall is not automatically safe: hard protection can stop erosion at the wall but remove sediment from the neighbouring beach (coastal squeeze), shift erosion downdrift, and create a maintenance liability - $180,000 a year is a recurring cost, not a one-off. It may also be designed for 30 years while sea-level rise and storm intensity are uncertain. The wall protects the road and homes in the short term but can be maladaptation if it encourages more development behind it. It does not address urupā or food-gathering sites, which are cultural losses, not just ‘assets’.
2. Evaluate the seawall and managed retreat using futures thinking and intergenerational equity. Recommend a decision process, not just an option.
Read the worked answer
Use adaptive pathways rather than a single bet. Define triggers (for example, erosion scarp within 10 m of the road, or a storm that removes another 2 m), and stage decisions. Criteria: safety, total 20-year cost, ecological effects, cultural harm, distribution of cost, reversibility. Seawall: $2.1m plus $3.6m maintenance over 20 years; protects the road but likely harms the neighbouring beach and is hard to reverse. Managed retreat: $6.4m, high upfront and social cost, but removes exposure and allows dune restoration; it must include relocation support and co-design with hapū for urupā and sites. Intergenerational equity: a seawall may pass maintenance costs to future ratepayers; retreat may impose costs now but reduce future risk. Process: council and hapū co-design options; consult ratepayers on who pays; fund independent review; monitor shoreline and water table. My recommendation is a staged retreat for the eight most-at-risk homes, with short-term road realignment and no new seawall unless triggers show retreat is impossible - because the wall shifts harm and creates a long financial tail.