Seawall or Mangroves on Abaiang
A village on Abaiang has 40 households and one road running parallel to the lagoon. A survey team finds the shoreline retreating about 0.6 m per year, and the road is now 25 m from the vegetation line. Spring tides already flood three houses. The community has $60,000 from remittances. Two proposals are on the table. A 200 m rock seawall would cost about $180,000, last 10–15 years, and engineers warn it will worsen erosion at both ends. A mangrove replanting project along the same 200 m would cost $15,000, but the seedlings need a muddy, low-wave site and may take 5–8 years to protect the shore. The village also needs a new rain tank. The meeting must choose a first step, not a final solution.
1. Compare the two proposals using cost per metre, affordability and time. What do the numbers not tell you?
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Seawall: $180,000 ÷ 200 m = $900 per metre. It costs three times the available $60,000, so the village cannot build the full length without outside funding. It offers protection quickly, but only for 10–15 years, and the engineers warn of end-scour, so the effective protected length may be shorter than 200 m.
Mangroves: $15,000 ÷ 200 m = $75 per metre, twelve times cheaper per metre and affordable within the budget. But the seedlings may take 5–8 years to protect the shore, and they need a muddy, low-wave site. During those years the road continues to erode.
At the current rate, 25 m ÷ 0.6 m/year ≈ 42 years before the road is reached, which sounds slow. But erosion is not steady: one storm or a change in sand movement could remove several metres in a night. The numbers do not tell you wave energy, whether a mudflat exists, sediment supply, land tenure, community capacity to maintain a seawall, or how often storms occur. They also do not price the rain tank, which is a separate need. Cost per metre is a starting point, not a decision.
2. Recommend a first step within the $60,000 budget, with a monitoring plan.
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Do not build the full seawall; it is unaffordable and may shift erosion elsewhere. A defensible first step is a hybrid, evidence-building package. Allocate about $15,000 to a mangrove trial along the lower-energy section, but only if the site is muddy and sheltered; if it is not, do not plant. Allocate about $8,000 to monitoring: fixed shoreline markers, repeat photographs, seedling survival counts, and a simple log of spring-tide flooding. Allocate about $10,000 to a technical design for a short rock toe or groyne at the most vulnerable 30–40 m, not a continuous wall. Keep at least $15,000 for the rain tank and contingency, because water security is an immediate need and the shoreline work will take years. Set a decision rule: if monitoring shows the road edge losing more than 0.6 m per year, or if a storm removes more than 3 m, seek outside engineering funds for the vulnerable section. Review after 12 months.
3. What evidence would make you recommend a seawall for a section, and what evidence would make you reject it?
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Recommend a seawall for a section if there is measured high wave energy and frequent overwash, if a house or the road will be lost within 5–10 years, if there is no suitable mudflat for mangroves, and if the community can fund maintenance and has secure land tenure. The design must include end-scour protection, drainage and a plan for sand movement; otherwise the wall will fail or move the problem. Reject a seawall if it would be built with sand mined from the beach, if there is no maintenance budget, if it would cut off the community's access to the lagoon, or if the erosion is mainly caused by local sand mining that could be stopped. Also reject it if the same money could protect more households through mangrove rehabilitation and managed retreat. The decision should be based on measured wave and erosion data, not on the seawall looking stronger. A short rock toe at a critical section can be sensible even when a continuous wall is not.