The Delta County Evacuation Plan
Delta County has 80,000 residents on a low-lying coastal plain. A Category 3 hurricane is forecast to make landfall in 36 hours with a 15-foot storm surge. The county's evacuation plan sends all residents west on two bridges. Bridge A is 14 feet above sea level and carries 1,800 vehicles per hour; Bridge B is 22 feet above sea level and carries 1,200 vehicles per hour. About 12% of households have no vehicle. County planners assume 2.5 people per vehicle. The county has one public shelter, a high school in Zone B, which is 8 feet above sea level and has capacity for 2,000 people. Zone A, closest to the coast, has 30,000 residents; Zone B has 25,000; Zone C, inland, has 25,000. The county plans to order a mandatory evacuation of Zone A first, then Zones B and C. Past evacuations show that 70% of residents comply when told to leave. The county has no agreements with neighboring counties for shelters.
1. Using the 70% compliance rate and 2.5 people per vehicle, estimate the number of evacuees and vehicles. Then assess whether the two bridges can handle the load if Bridge A is closed by a 15-foot surge.
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Expected evacuees: 80,000 times 0.70 equals 56,000 people. Vehicles: 56,000 divided by 2.5 equals 22,400 vehicles. Bridge B alone can carry 1,200 vehicles per hour; over 36 hours that is 1,200 times 36, or 43,200 vehicles, which is more than 22,400. So total capacity is sufficient in theory.
But Bridge A is only 14 feet above sea level and the forecast surge is 15 feet, so it is likely to be inundated and unusable. If all traffic funnels onto Bridge B, the plan must manage timing, because evacuating Zone A first could create a queue that blocks Zones B and C. The 36-hour window also shrinks if tropical-storm winds arrive early.
2. Identify two assumptions in the county's plan that are risky. For each, explain what could go wrong and suggest one change.
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Assumption 1: Bridge A will remain usable. A 15-foot surge exceeds its 14-foot elevation, so the western route may fail. Change: plan for Bridge B only, pre-position signs and staff, and request state contraflow or high-clearance routes.
Assumption 2: the single shelter can absorb people without vehicles. 12% of 80,000 is 9,600 people without vehicles, far above the 2,000-person shelter capacity, before considering others. Change: open additional shelters in Zone C, arrange buses for carless households, and make agreements with neighboring counties. A third risky assumption is 70% compliance; if compliance rises to 90%, vehicles increase to 28,800 and congestion worsens.
3. Should the county evacuate Zone A first? Explain the trade-offs using the geography and the shelter locations.
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Zone A is closest to the coast and has 30,000 residents, so it faces the greatest surge risk and should be warned first. But 'Zone A first' can be misleading if it means other zones wait: evacuating 30,000 people at 70% compliance means about 21,000 people and 8,400 vehicles, which could saturate Bridge B. If Zones B and C are told to wait, they may later compete for the same route, and the shelter in Zone B is only 8 feet above sea level, so it may itself be at risk.
A better plan is phased but overlapping: warn Zone A earliest, open inland shelters immediately, and tell Zones B and C to leave on assigned routes before the surge. The county should not rely on a single shelter in a low-lying zone.