A Storm Surge Prediction Model for the Northern Bay of Bengal with Application to the Cyclone Disaster in April 1991

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  • 1 Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, England
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Abstract

A numerical model for simulating and predicting tides and storm surges in regions that include areas of open sea combined with estuarine channels and intertidal banks is described. The model makes use of modified depth-averaged equations with a numerical scheme in which the solution of 1D equations for narrow channels, 2D equations for the open sea, and an approach to modeling inundation are combined within a unified framework. This simplifies the construction of realistic models for complex coastal areas and deltas.

The model is applied to the northern shelf of the Bay of Bengal and the Ganges Delta, including the coast of Bangladesh devastated by a cyclone surge in April 1991 in which about 140 000 people died. Hindcast and “forecast” simulations of this event, using forcing derived from a semianalytical cyclone model with data supplied by the Joint Typhoon Warning Center (JTWC), are described. It is shown that the timing of cyclone landfall and its coincidence with high tide determine the area worst affected by flooding. The “forecast” was based on a standard JTWC warning issued 12–18 hours ahead of the event. The predicted landfall was located accurately but its timing was about 5 hours late. As a result, the area of highest water levels shifted north. Improvements in the model and procedures required to produce useful warnings are discussed.

Finally, results for the April 1991 storm are compared with those from a simulation of the great cyclone in November 1970. Peak surges and water levels were similar in magnitude in the two events, but differences in track and tidal conditions are again shown to be important, producing flooding in a large area of the southern delta in 1970 but along the mainland coast south of Chittagong in 1991.

Abstract

A numerical model for simulating and predicting tides and storm surges in regions that include areas of open sea combined with estuarine channels and intertidal banks is described. The model makes use of modified depth-averaged equations with a numerical scheme in which the solution of 1D equations for narrow channels, 2D equations for the open sea, and an approach to modeling inundation are combined within a unified framework. This simplifies the construction of realistic models for complex coastal areas and deltas.

The model is applied to the northern shelf of the Bay of Bengal and the Ganges Delta, including the coast of Bangladesh devastated by a cyclone surge in April 1991 in which about 140 000 people died. Hindcast and “forecast” simulations of this event, using forcing derived from a semianalytical cyclone model with data supplied by the Joint Typhoon Warning Center (JTWC), are described. It is shown that the timing of cyclone landfall and its coincidence with high tide determine the area worst affected by flooding. The “forecast” was based on a standard JTWC warning issued 12–18 hours ahead of the event. The predicted landfall was located accurately but its timing was about 5 hours late. As a result, the area of highest water levels shifted north. Improvements in the model and procedures required to produce useful warnings are discussed.

Finally, results for the April 1991 storm are compared with those from a simulation of the great cyclone in November 1970. Peak surges and water levels were similar in magnitude in the two events, but differences in track and tidal conditions are again shown to be important, producing flooding in a large area of the southern delta in 1970 but along the mainland coast south of Chittagong in 1991.

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