Belmont: Funding a Flood Scheme for a Fast-Growing Coastal Town
Belmont is a coastal town of 18,500 people on the leeward side of a small Caribbean island. Between 2015 and 2024 its population grew at about 2.3% a year, and the planning authority approved 640 new house lots on former sugarcane land lying 8–12 m above sea level, close to the Belmont River's lower course. The town's main drainage canal was built in 1974 and is rated to carry runoff from a storm of 25 mm per hour. Recent 30-minute storms have delivered up to 40 mm per hour. In October 2019, 210 homes, three schools and the health centre flooded to a depth of 0.6 m, and the main road was closed for nine hours. The council has $14 million. Option A is to widen the canal to a 40 mm/h standard at a cost of $11 million, with $300,000 a year maintenance. Option B is a detention pond plus a 30 m building setback along the river at a cost of $9 million, with $150,000 a year maintenance. Residents downstream of the town have objected to both proposals.
1. Using the figures given, explain why Belmont's flood risk has increased since 2015.
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Flood risk is the product of hazard, exposure and vulnerability, and all three have moved the wrong way.
- Hazard: storms now deliver up to 40 mm/h against a canal designed for 25 mm/h, so the channel is asked to carry about 60% more than its capacity. Overload spills onto streets and into houses.
- Exposure: growth of 2.3% a year on 18,500 people adds roughly 425 residents annually. The 640 approved lots convert former cane land - which infiltrates rainfall - into roofs, drives and roads. Hard surfaces generate far more runoff, so a bigger share of each storm reaches the canal within minutes.
- Vulnerability: the lots sit at 8–12 m and close to the lower course, so they occupy floodplain storage. Filling and building there displaces water onto neighbouring plots, which helps explain the 0.6 m depth and the downstream objections.
- Age of infrastructure: a 1974 canal is also likely to be silted and partly obstructed, reducing capacity below its design figure.
So the 2019 flood is not simply 'more rain'. A 25 mm/h canal plus 640 sealed lots plus houses in the floodplain converts an ordinary storm into a 210-home event.
2. Recommend which option the council should fund. Justify your choice with reference to capital cost, maintenance and effectiveness.
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Recommendation: fund Option B, and use the $5 million saved on targeted canal improvements and a maintenance fund.
Reasoning:
- Cost: over 20 years Option A costs about $11m + (20 × $0.3m) = $17m. Option B costs about $9m + (20 × $0.15m) = $12m. B delivers protection for roughly 70% of A's lifetime cost.
- Effectiveness: A only lifts capacity to 40 mm/h - the level storms already reach - so it buys a shrinking margin and invites the same problem again in a decade. B attacks the cause: a detention pond stores the peak and releases it slowly, while the setback stops new development from adding to exposure.
- Downstream effects: a wider canal moves water downstream faster, which can worsen flooding for the residents who have already objected. A pond flattens the peak instead.
- Sequencing: B does not protect the 210 existing homes, so the remaining $5m should fund localised canal clearing and upgrade at the worst bottlenecks, plus a sinking fund for maintenance, since a pond that silts up stops working.
- Policy: the setback must be enforced through planning conditions, or it will be eroded by exemptions.
A is the politically easy 'big dig', but B is cheaper, more durable and removes future risk rather than relocating it.
3. State one important limitation of the evidence on which this decision is based, and say what data would reduce the uncertainty.
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The evidence rests on one flood event and one design figure. The 0.6 m depth comes from a single storm in 2019, with no continuous river gauge or rain-gauge record given, so we cannot say whether 40 mm/h is a rare extreme or the new normal. Without that, the council cannot know what standard to build to.
Other gaps:
- We are not told how many of the 640 lots are actually built on, so the sealed area - and therefore the runoff increase - is unknown.
- Canal siltation and obstructions are not surveyed, so the real capacity may be well below 25 mm/h.
- The downstream objection is unquantified: no downstream flood levels are given.
- No flood-extent map exists, so it is unclear which homes a pond or setback would actually protect.
To reduce uncertainty: install a river level recorder and two rain gauges, run a rainfall–runoff model calibrated on the 2019 event, survey the canal cross-sections, and map flood extent for 25, 40 and 60 mm/h storms. A longer record would also show whether the 2019 storm was exceptional.