Vela Delta: Choosing Adaptation Under Budget and Political Constraints
Vela Delta is a low-lying coastal city of 1.2 million people. About 62% of its area lies below 3 m above sea level, and land is subsiding at roughly 2 cm per year while local sea level has risen about 4 mm per year. The city's mangrove belt has narrowed by 40% since 1990 because of aquaculture and informal housing. A 2019 storm surge flooded 18,000 homes and the industrial zone, causing an estimated $310 million in damage. The national government offers three options: (A) a 14 km hard sea wall and pumping stations costing $480 million; (B) managed retreat from the most exposed 900 hectares, compensating 12,000 households and restoring mangroves, costing $260 million; (C) a hybrid programme of mangrove restoration, raised embankments and building codes costing $150 million, but leaving some exposed areas protected only to a 1-in-25-year standard. The city has $200 million available and a national election in three years.
1. Assess Vela Delta's flood risk using the relationship between hazard, exposure and vulnerability. [8]
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Hazard: storm surges, sea-level rise (4 mm/yr) and subsidence (2 cm/yr) combine. Relative sea-level rise is therefore about 2.4 cm/yr, much faster than global sea level alone. A 1-in-100-year surge today may become more frequent. Exposure: 62% of the city below 3 m, 1.2 million people, 18,000 homes flooded in 2019, industrial zone. Vulnerability: mangrove loss (40%) removes natural buffering; informal housing on exposed land; dependence on a single standard of protection; economic losses of $310m. Risk is high because a moderate hazard meets large exposure and high vulnerability. The 2019 flood is evidence of existing risk, not just a future climate scenario. The city's capacity to recover is also uneven; informal households may lack insurance. Therefore risk is not reducible by one sea wall alone.
2. Evaluate which option best addresses both immediate risk and long-term sustainability within the budget. [12]
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Option A: $480m exceeds the $200m budget; a hard wall may protect the industrial zone but can increase erosion and does not address subsidence or drainage; maintenance costs are high. Option B: $260m also exceeds the budget, but managed retreat reduces exposure permanently and restores mangroves. It is equitable only if compensation is fair; moving 12,000 households is politically difficult. Option C: $150m fits the budget and combines ecosystem and engineering approaches, but the 1-in-25-year standard leaves significant residual risk. Judgement: C is the best fit for the budget and long-term sustainability, but it is not sufficient alone. It should be paired with land-use controls that stop new development in the most exposed areas and a phased, voluntary retreat where the standard is lowest. This uses the $150m to buy time and reduce vulnerability while addressing exposure. A and B are better on some criteria but fail the budget or political feasibility test. The recommendation should state the residual risk explicitly.
3. Explain why climate change mitigation alone would not solve Vela Delta's problems. [6]
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Mitigation reduces greenhouse gas emissions and therefore the long-term hazard. But Vela Delta already faces committed sea-level rise because of past emissions, and subsidence is largely local. Even with rapid mitigation, the city cannot avoid adaptation: 62% of its area is low-lying, mangroves are degraded, and 1.2 million people are exposed. Mitigation does not protect homes during the next storm surge or stop land subsidence. Adaptation is needed to manage current and near-term risk, while mitigation reduces the speed and scale of future risk. The two are complements, not substitutes. A city that relies only on mitigation would be unprepared for a 2019-style flood.