Tsunamis: A rare but real marine hazardFeb 28, 2007 12:00 AM
This massive earthquake, the fourth largest since 1900 and largest since the 1964 Good Friday earthquake in Alaska, was centered off the west coast of northern Sumatra, and was strong enough not only to generate deadly tsunamis but also to cause a slight disruption in Earth's rotation. The death and destruction spread throughout the basin, from Indonesia to India and Africa, where coastal villages were leveled and fishing fleets destroyed. The official number of dead and missing has risen continuously since the tsunami and now is near 300,000.
Also caught in the path of the destructive tsunami waves were merchant vessels, such as M/V Maersk Virginia, which was transiting the Gulf of Oman bound for the Port of Salalah. When the tsunami arrived at its location, the vessel encountered rapid sea-level fluctuations and unusual currents, which eventually pushed it aground. Instances such as these don't occur that often, but they do serve as a reminder that tsunamis pose a real threat to the mariner.
What warning do mariners have of this approaching danger, and what can they do to avoid damage to their vessel? These are important questions that require an understanding of what tsunamis are, what causes them and what warning systems are available.
In Japanese, tsunami means "great wave in harbor." The word has evolved to describe the series of waves generated by any rapid, large-scale disturbance of the ocean. Earthquakes are the most common cause of tsunamis, but they also can be triggered by surface and subsurface landslides, and volcanic activity. Once generated, they travel outward from the epicenter at speeds up to 500 knots in a pattern similar to the ripples on a pond caused by a thrown stone. In the open ocean, these waves can have wavelengths on the order of 100 miles with a height of only 2 to 3 feet, meaning the mariner will hardly notice their passage.
However, unlike wind-driven waves, which only cause water movement in the upper few hundred feet, tsunamis are felt all the way to the bottom. The speed of advance (c = square root of 9.8H m/s, H is bottom depth) and pattern of spreading are controlled by the water depth and bottom topography. For example, the wave in 4,000 meters of water travels at 198 meters per second, but when the bottom shoals to 40 meters, the speed is reduced to 19.8 meters per second.
For small islands and atolls, such as Midway and Kwajalein in the Pacific, tsunamis tend to be small due to very steep bottom topography, and pose a minimal threat. However, in areas where shoaling takes place over a greater distance, such as along a continental shelf or in harbors, the full destructive force of these waves can be realized. As the waves approach the coast and the bottom begins to shoal, the energy contained within the lower, faster-moving waves is contained within the higher, slower-moving waves.
When the waves reach the shore, they often have the appearance of a very rapid and unusual tidal fluctuation. For this reason, many people mistakenly refer to them as tidal waves, though they are not tidally generated. It is important to remember that the tsunami is not a single wave but rather a series of waves (up to 10 or more) that arrive one after the other, each capable of destroying anything in its path.
Throughout history, tsunamis have had their impact on shipping. For example, in 1868, USS Wateree was in Arica, Peru, when an earthquake struck. Crewmembers reported the entire ship shaking during the quake, and when the tsunami arrived shortly thereafter, they observed vessels in the harbor being thrown about at their anchorages. When it was over, Wateree and the Peruvian cruiser Americana were hard aground on the shore.
When Krakatau erupted in August 1883, tsunamis carried the Dutch warship Berouw up a narrow valley, depositing her nearly two miles inland and about 30 feet above sea level.
In 1923, an earthquake off the Aleutians generated waves that stranded SS Mahukona on a shoal in the middle of Kahului Harbor in the Hawaiian Islands. During Alaska's 1964 Good Friday earthquake and tsunami, the tanker American Standard was in Seward transferring diesel fuel. The earthquake caused the portion of the waterfront where the ship was moored to sink into the bay, severing the hoses and igniting a fire. This collapse caused a localized tsunami that carried the burning fuel toward a refinery, setting it ablaze. Luckily, as this wave carried American Standard away from the dock, the captain was able to maneuver the vessel safely out into the harbor and away from the burning refinery, averting further disaster.
Tsunamis are not limited to the Pacific Ocean basin. There have been similar occurrences in the Atlantic basin as well. In November 1867, vessels anchored in the Virgin Islands fell victim to a tsunami triggered by an earthquake just off the northern coast of Puerto Rico, with several vessels becoming stranded on shore. Vessels near the Grand Banks on Nov. 18, 1929, had encounters with earthquake-generated tsunamis produced by a massive undersea landslide down the Laurentian Channel in the Cabot trench. Shockwaves from this slide were transmitted through the water column and shook SS Caledonia so violently that the captain thought he had run aground in 700 feet of water.
Two midshipmen from the U.S. Merchant Marine Academy were aboard Maersk Virginia when the ship first encountered the tsunamis about 20 miles from port. The ship waited seven hours before entering the harbor on a higher tide and with the hope that the threat had passed. Upon entering the harbor, the midshipmen reported seeing extremely strange water action near the vessel and observed the sea level drop about 15 feet in about five minutes due to the lingering effects of the tsunami. It was at this point that the waves took the ship for a ride and forced it to ground. Could things have been different had there been adequate warning of the tsunamis?
In 1949, the Pacific Tsunami Warning Center was established in Hawaii to provide warnings for Hawaii as well as most countries bordering the Pacific. Today, along with the West Coast & Alaska Tsunami Warning Center, they continuously monitor data from seismological and tidal stations (tide gauges) as well as recently deployed deep-ocean buoys capable of detecting these waves. The centers evaluate earthquakes for the potential to generate tsunamis and disseminate this information throughout the Pacific Basin to emergency officials, the media and the public via special communication lines, satellite and the Internet. This information reaches the mariner via urgent marine broadcasts, Inmarsat-C SafetyNet messages, and communications with harbor/port control operations.
Unfortunately, similar coverage does not exist in the Indian Ocean, and warnings are limited. However, as a result of the Sumatra disaster, there are plans within the international community to expand this observation and warning network to include the entire Pacific and Caribbean basins and ultimately the Indian Ocean.
Mariners' planning guides
At any given time, a mariner can be faced with tropical and extra-tropical storm systems, squalls, heavy seas, poor visibility and other weather-related phenomena, though it is rare for a mariner to be caught by surprise by them. Experience and training allow mariners to observe the changing weather conditions and determine the best course of action to ensure the safety of the vessel, crew and cargo. But how do you prepare for something that is as unpredictable as a tsunami? One way is to develop a list of pre-event safety measures. Those measures should include:
Â· Ensuring that each watch officer knows and monitors the warning networks in place in each port of call.
Â· Putting ship-to-home-office emergency communication protocols in place and checking to see that they are understood and practiced.
Â· Developing and exercising a shipboard action plan in the event that a warning is received.
Â· Periodically refreshing the plan and crew awareness of it.
A ship safety action plan has to consider the nearness of the ship to the origin of the tsunami. Vessels near the epicenter of the Sumatra quake had little or no time to take action, while vessels on the other side of the Indian Ocean had a few hours. Those precious hours could allow vessels in port or entering shallow waterways to make their way to deeper water where the tsunami effects are greatly reduced.
It is important to remember that earthquakes are unpredictable, and not all generate deadly tsunamis. Having shipboard emergency procedures similar to those used in heavy weather and other emergencies will aid in determining the best course of action. Regardless of how much warning you have, always rely on your training and experience, and exercise good judgment and seamanship.
Capt. Timothy D. Tisch teaches navigation, meteorology and bridge watchstanding at the U.S. Merchant Marine Academy.Edit Module