Crucial question: how far inland should people go?

Many survivors of the storm surge brought about by Supertyphoon “Yolanda” heeded the warning and promptly evacuated. Unfortunately, their evacuation centers were not far enough from the shore. The situation was aggravated by their lack of understanding of a storm surge.

Indeed, a warning on something about the sea is confusing when the disturbance is supposed to be up there in the sky. Even a “clarification” that a storm surge is not a tsunami would not be helpful.

Although before Yolanda hit land there was information on the height of the storm surge, the people were left to deduce the implications on their own. There must be a guideline explicitly describing the actions to be done for a given condition. A good example is the Department of Education’s guideline specifying which classes must be suspended at a given storm signal.

Thus, this question must be answered: If there is a typhoon, how far inland should people go? The principle relating typhoon intensity to storm surge is the Bernoulli’s Principle, which relates the atmospheric pressure to the wind speed. Crudely, it states that the faster the wind is in a particular location, the lower is the air pressure in that location. If that low pressure location is above a body of water, the water is sucked upward by the swirling wind. The volume of water “lifted” and “carried” inland is the storm surge. The area of land that will be smashed and flooded by the surge is directly related to the volume of the surging water. In other words, determining the distance that the people need to clear from the coast for a given typhoon intensity is a tractable problem.

Thus, it is feasible to make a guideline specifying the area of a storm-surge danger zone corresponding to typhoon intensity. The government should convene a technical panel to work out the details. Since a typhoon is a complex moving structure, it is inevitable that the guideline will only be a “rule of thumb.” Nonetheless, this guideline will  be a significant improvement in making the evacuation plans of coastal communities systematic and rational.

—J.A.F. BALISTA,

(in personal capacity)

Physics Division,

Institute of Mathematical

Sciences and Physics,

University of the Philippines Los Baños,

[email protected]