The Bushfire Wind Shift
A rural town sits in a valley with one sealed road out to the east and a dirt track to the north. A bushfire is 12 km west of town and moving east at 15 km/h. The wind is forecast to shift 90 degrees to the north within the next hour. The town has 400 residents. The east road can carry 300 cars per hour, and about 70 percent of households own a car. The north track is narrow and becomes impassable after 5 mm of rain, which is forecast for later that evening. The fire service must decide whether to order evacuation now, wait for the wind shift, or shelter in place.
1. Estimate how long the town has before the fire arrives at current speed, and explain why that number is misleading.
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At 15 km/h over 12 km, the fire arrives in 12 ÷ 15 = 0.8 hours, or 48 minutes.
Why it is misleading:
- The wind shift to the north changes the fire's direction, so the 48-minute figure assumes no change.
- Fire speed is not constant; it accelerates uphill, in dry fuel and with wind gusts.
- Embers can start spot fires ahead of the main front, so the 'arrival' time may be earlier than the front itself.
The 48 minutes is a lower-bound planning figure, not a reliable countdown. A good answer uses it to justify acting now rather than waiting.
2. Compare evacuating east versus north, and recommend an action with reasoning.
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East road: 70% of 400 residents is about 280 car-owning residents, roughly 280 cars if one car per household. At 300 cars per hour, clearing them takes about 280 ÷ 300 ≈ 0.93 hours, close to an hour. That is longer than the 48-minute lower bound, so the east road is tight but usable if started immediately.
North track: The forecast rain makes it impassable later, and a narrow dirt track carries far fewer cars per hour. It is a trap, not an escape route.
Shelter in place: Only defensible if the town is built to a defendable standard and residents cannot leave in time. With a sealed road available, sheltering is the riskier choice.
Recommendation: Order evacuation east now. Start cars moving immediately, prioritise households without cars by arranging lifts or a bus, and close the north track to traffic. Waiting for the wind shift risks losing the window before the fire or the rain arrives.