Differentiating Storm Surge from Tsunami

Storm surges and tsunamis are natural calamities that both occur in the bodies of water that result in flooding that can lead to the loss of lives and causing extensive damages. These two is both classified as long gravity waves including the ocean tides. Long gravity waves are the only waves that can climb lands.

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© Met Office

Storm surges are caused primarily by tropical storm. The low pressure has only a minimal contribution and it is more on the wind.

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© Sanibel Sea School

The occurrence of tsunami generally caused by tsunamigenic earthquakes which are earthquakes happened under the ocean whose epicenters are dislocated. With the continuous oscillating motion of crust blocks, in an area whose size is much greater than the thickness of the water layer, a rise in the water surface occur. The rise continues for the whole oscillation period, which is approximately 1 minute and 40 seconds for a large earthquake. The moment that the earthquake stops, the elevated surface was induced by seismic energy begins to spread over the surface with the release of gravity water waves form the source. Although this is only a rare case, but sometimes volcanic eruption can also cause disturbance in the ocean.

Storm tide is the height of a storm surge. Example in a location where high tide is 2 ft. higher than the mean sea level (MSL) and low tide is 2 ft. lower than the MSL, a 15 ft. storm surge will cause a 17 ft. storm tide. The length of the waves of the tsunami depends on the form of source and the magnitude of the earthquake that happened. If it is in the ocean, wave length comprises about 100-200 km. A powerful energy can travel as fast as a commercial jet. Most cases, tsunami rises not more than 10 feet (3 meters).The deeper the water, the bigger the tsunami will be. Remember that tsunamis are not just a single wave but it is a series of wave and take note that the first wave may not be the most dangerous. Do not return to low-lying places after the first wave because there will be following waves which can last up to 30 minutes. The danger may last for hours but in some cases, more than 24 hours.

People can be informed ahead of time when there is a possible storm surge. Once a tropical cyclone is spotted over the ocean before it will make a landfall, the prediction of storm surge can be made several days in advance even if there is no exact place and time of the landfall. Unlike tsunami, storm surges don’t require a travel time charts because storm surges mainly develop at the coastline itself. On the other hand, tsunamis can be predicted too. Seismologists use the travel time chart.

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© Mother Jones

The travel time charts are prepared ahead of time because the travelling speed of tsunami is determined by the ocean bathymetry ( where g=acceleration due to gravity, and D=water depth). In most cases, the earthquake serves as a warning of the tsunami. Take note that no tsunami will occur unless there is an earthquake or a volcanic eruption.It takes about a day for a tsunami to cross the Pacific Ocean while it only takes a few hours for an Indian Ocean tsunami to arrive in the most populated areas.

 

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© FFE Magazine

In the Philippines, the super typhoon Yolanda (with an international name Haiyan) made the most devastating storm surge that is responsible for the 6,190 dead people and 1,785 missing and 28, 626 injured.

1976-tsunami
© Philippines Central Info Negros

The most devastating tsunami incident that ever recorded in the country was the 1976 Moro Gulf tsunami that killed around 8000 people.

 

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