Ever experienced a Seiche?

Maritime Topics
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Standing gangway watch in Manzanillo Mexico early one quiet morning the ship suddenly and without warning starting surging ahead, just as if it someone had given it a dead slow bell. Once all the slack came out of the mooring lines they took a big strain but held and after a few seconds the ship eased back into position.

Later that day I mentioned it to someone and they told me it was a seiche, the Coast Pilot specifically mentioned that they were common in Manzanillo.

#What is a seiche? - Short answer is it’s a wave:

Longer answer:

If you have observed water sloshing back and forth in a swimming pool, bathtub, or cup of water, you may have witnessed a small-scale seiche (pronounced saysh). On a much grander scale, the same phenomenon occurs in large bodies of water such as bays and lakes. A seiche may occur in any semi- or fully-enclosed body of water.

Seiches are typically caused when strong winds and rapid changes in atmospheric pressure push water from one end of a body of water to the other. When the wind stops, the water rebounds to the other side of the enclosed area. The water then continues to oscillate back and forth for hours or even days. In a similar fashion, earthquakes, tsunamis, or severe storm fronts may also cause seiches along ocean shelves and ocean harbors.

Lake Erie is known for seiches, especially when strong winds blow from southwest to northeast. In 1844, a 22-foot seiche breached a 14-foot-high sea wall killing 78 people and damming the ice to the extent that Niagara Falls temporarily stopped flowing. As recently as 2008, strong winds created waves 12 to 16 feet high in Lake Erie, leading to flooding near Buffalo, New York. Lake Pontchartrain, Louisiana, is also known to routinely form small seiches after the passage of afternoon squall lines during summer months.

In some of the Great Lakes and other large bodies of water, the time period between the “high” and “low” of a seiche can be as much as four to seven hours. This is very similar to the time period between a high and low tide in the oceans, and is often mistaken as a tide.

A 22 foot seiche on Lake Erie! Never realized the Lakes sloshed around that much.

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I used to spend a lot of time in Point Loma San Diego on a MSC tanker. Out there the ship would surge 15-20 feet routinely; luckily we have CT winches. Getting back onboard via the gangway you had to time it just right and hop on as the ship stopped before surging back the other way. We would get the warnings via Navtex.

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Here’s one I have never heard of till this morning:

What is a meteotsunami?

Meteotsunamis are large waves caused by storms.

You’ve heard of tsunamis—those giant oceanic waves triggered primarily by earthquakes that can roll ashore, causing loss of life and disaster. But have you heard of meteotsunamis?

Meteotsunamis are large waves that scientists are just beginning to better understand. Unlike tsunamis triggered by seismic activity, meteotsunamis are driven by air-pressure disturbances often associated with fast-moving weather events, such as severe thunderstorms, squalls, and other storm fronts. The storm generates a wave that moves towards the shore, and is amplified by a shallow continental shelf and inlet, bay, or other coastal feature.

Meteotsunamis have been observed to reach heights of 6 feet or more. They occur in many places around the world, including the Great Lakes, Gulf of Mexico, Atlantic Coast, and the Mediterranean and Adriatic Seas.

Identifying a meteotsunami is a challenge because its characteristics are almost indistinguishable from a seismic tsunami. It can also be confused with wind-driven storm surge or a seiche. These uncertainties make it difficult to predict a meteotsunami and warn the public of a potential event. However, NOAA scientists have identified atmospheric conditions that are likely to generate a meteotsunami and continue to work on ways to forecast them.

Seiches and meteotsunamis. What’s the difference?

Seiches and meteotsunamis are often grouped together, but they are two different events. Winds and atmospheric pressure can contribute to the formation of both seiches and meteotsunamis; however, winds are typically more important to a seiche motion, while pressure often plays a substantial role in meteotsunami formation. Sometimes a seiche and a meteotsunami can even occur at the same time. Seiches are standing waves with longer periods of water-level oscillations (typically exceeding periods of three or more hours), whereas meteotsunamis are progressive waves limited to the tsunami frequency band of wave periods (two minutes to two hours). Seiches are usually limited to partially or fully enclosed basins, such as Lake Erie. Meteotsunamis can occur in such basins but are also prevalent on the open coast. A single meteotsunami can travel long distances and influence a very large range of the coastline.

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We see seiches @6ft or more at either end of Lake Erie throughout the year. More often we deal with lesser events that efffect how much we can load due to the shallow ports. I have to check the water gauges for every load and guess what it will be when we arrive.

A couple of years ago we left a barge for a couple of days and found it half ashore when we returned, due to a seiche.

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