Cyclone landfall at high tide: timing an evacuation in Krishna district
A cyclone warning is issued for a mandal in Krishna district, Andhra Pradesh. At 12:00 the cyclone centre is 180 km east-southeast of the coast and moving west-northwest at 18 km/h. Central pressure is 968 hPa and maximum sustained winds are 130 km/h. Landfall is forecast near high tide, which is at 21:30, with an astronomical tide of 1.2 m above chart datum and a storm surge of 2.5 m. Three villages with 4,200 people sit at 1.5 m elevation. The nearest cyclone shelter is 6 km away and holds 600 people; a pucca school 2 km away can hold 400. The evacuation road crosses a canal bridge whose deck is 1.2 m above chart datum. The sub-district has 380 elderly or lactating people and no boats.
1. Calculate the landfall time and the worst total water level. Explain why the surge is greatest to the right of the track.
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Landfall time = distance ÷ speed = 180 km ÷ 18 km/h = 10 hours. From 12:00, landfall is about 22:00. High tide at 21:30 coincides with landfall, so the worst period is roughly 21:00–23:00.
Total still-water level = tide + surge = 1.2 m + 2.5 m = 3.7 m above chart datum. The villages at 1.5 m would be under about 2.2 m of water. The bridge deck at 1.2 m would be overtopped by about 2.5 m, so the road becomes unusable before the peak.
The surge is greatest to the right of the track in the Northern Hemisphere because there the onshore winds push water towards the coast, wave setup adds to the mean level, and the Coriolis effect turns the flow towards the coast. Low pressure also raises water by about 1 cm per hPa; a 42 hPa deficit contributes roughly 0.4 m, although this is already part of the forecast surge.
2. Set out an evacuation sequence and route. Use the shelter capacities and the bridge height to justify the timing.
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The shelter and school together hold only 1,000 people, far short of 4,200. The plan must therefore combine evacuation with vertical evacuation to pucca buildings and raised mounds in the villages.
Start at about 14:00 with the 380 elderly and lactating people, moving them 2 km to the school. Send the general population to the 6 km shelter in batches, but stop road evacuation by about 18:00. The bridge deck is only 1.2 m above chart datum; even the astronomical tide reaches that level, and the surge will overtop it earlier than the 22:00 peak. After 18:00, people should move to identified two-storey buildings and raised platforms, not attempt the bridge.
Record who has moved where. Keep the school and shelter from being cut off by the canal. The 600-person shelter cannot be the only destination; pre-identify at least 3,000 places of vertical refuge before the cyclone season.
3. After the cyclone, sea-surface temperature falls and chlorophyll rises. Explain these changes and their relevance to local fisheries.
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Strong cyclonic winds mix the upper ocean. Cooler water from below the mixed layer is entrained to the surface, so sea-surface temperature may fall by 2–4°C. The same mixing and upwelling bring nutrients such as nitrate into the sunlit layer, so a chlorophyll bloom often follows. Heavy rain and surge also lower surface salinity near the coast.
For fisheries, the immediate effects are damaged boats, nets and jetties. In the following weeks, the nutrient pulse may attract pelagic fish, but the low-salinity plume can stress brackish-water ponds and prawn farms. Fishermen should not be allowed to return during the calm eye, because the second half of the storm brings a sudden wind reversal. Post-storm advice should separate the offshore stock effect from the inshore water-quality risk.