What is a seiche? A seiche is a standing wave that rocks back and forth in an enclosed body of water, similar to a pendulum.
Most waves are caused by the wind. The wind pushes across the surface of a body of water and picks up water, pushing it across the body of water. If the wind is strong, the amount of water can slowly build up. Waves are also caused by disturbances under the surface of the water. Earthquakes on the seabed can cause tsunamis. Tsunamis are different from regular waves because they are caused by a sudden input of energy into the body of water, which affects the entire column of water. The column of water is lifted up and pushed outwards from the source of the energy, heading across the body of water. When it gets to shore, the raised body of water can travel up and over the land for a considerable amount of time before all of the energy is diminished. Tsunamis travel much faster than regular waves across the body of water, but they slow down and get much bigger when they reach the shore.
A seiche is different from both a tsunami and a regular wave. The word comes from Swiss French, where the phenomena were first observed by a hydrologist called Francois-Alphonse Forel. The Swiss French word came from the Latin siccus, which means dry. When a seiche wave moves in, it pulls the water off the beach and leaves it dry. We still use the Latin word in desiccate.
A seiche only happens in an enclosed body of water, such as a bath, a swimming pool, or a lake. Water anywhere is obviously pulled down by gravity. Gravity pulls on all of the water equally, and water is a liquid, so it always settles to the point where none of the water is higher than any other part of the water. That is the surface of the water. You can alter that by applying energy, but if you remove the energy, the water always eventually returns to the flat position with all of the surface at equal heights. Even if you have an unequal base under the water, the water will move until the surface is completely flat.
A seiche wave happens when the wind, or some other force, pushes all of the water to one end of the enclosed space. The water piles up in that area, and then it slides back. The water wants to go back to its equilibrium, but it has too much kinetic energy and it slides past the center, all the way to the other end, and piles up there. It keeps sliding backwards and forwards until all of the energy is spent, and it can go back to being flat and still. The easiest way to see this in action is to try it in your bath. Fill the bath with water, then push all of the water in one direction with your hand and remove your hand. The water will slosh up to one end of the bath, then rebound back and keep sloshing until all of the energy has gone. If you push the water hard enough, it may even spill over the edge of the bath. A wave machine in a swimming pool makes seiches. You can see it in your cup of coffee as well. If you blow your hot cup of coffee too hard, the coffee will be pushed over the side of the cup.
Seiches can happen in any enclosed body of water, but they are obviously easier to spot in smaller spaces. Some of the Great Lakes in North America have seiche, and they can be quite dangerous. The water on the lakes pushes a lot of the water to one end, and the resultant surge can cause a lot of flooding and damage. They are particularly common in Lake Erie because it is a relatively shallow lake, and it lines up with the prevailing winds. There was a seiche on Lake Erie in 1844 that produced an 8 m high seiche that went over the sea wall and killed 78 people. A lot of storm defenses need to be capable of dealing with seiches as well as regular waves. They can also cause problems because when the water bunches up at one end, there is less water at the other, and this can leave things like water inlets to power stations exposed. Then the water rushes back and floods the system. And this is what I learned today.
Sources
https://www.nps.gov/articles/coastal-geohazards-seiches.htm
https://en.wikipedia.org/wiki/Seiche
https://en.wikipedia.org/wiki/Standing_wave
https://oceanservice.noaa.gov/facts/seiche.html
Photo by Matt Hardy: https://www.pexels.com/photo/wavy-body-of-water-during-golden-hour-1615681/