A seiche is a standing wave that forms in an enclosed or partially enclosed body of water. Unlike traveling waves, a seiche appears as a rhythmic oscillation, where the water level rises and falls at opposite ends of a basin. This phenomenon occurs when water is displaced from its equilibrium position and then attempts to return, setting up a continuous, sloshing motion. The term “seiche” originates from a Swiss French dialect, aptly describing this swaying movement of water.
How Seiches Develop
Seiches begin when a force displaces water within a basin. Strong, sustained winds can push water towards one shore, creating a temporary imbalance in water levels. Rapid shifts in atmospheric pressure, such as those associated with severe thunderstorms, can exert uneven forces on the water surface. Seismic events, like nearby earthquakes, can also generate sudden jolts that set the water into motion. The basin’s dimensions play a role in determining its natural oscillation period, which is the time it takes for a full slosh.
Once the initial force subsides, gravity acts to restore the water to a level surface. However, momentum causes it to overshoot the equilibrium point, piling up on the opposite shore. This continuous back-and-forth movement, driven by gravity and momentum, creates a rhythmic oscillation known as a standing wave. The period of this oscillation, or the time it takes for one complete slosh, depends on the size, depth, and shape of the water body. If the external forcing matches the natural period of the basin, the seiche can grow in amplitude through resonance, similar to pushing a swing at the right moment.
Bodies of Water Where Seiches Occur
Seiches can occur in a variety of enclosed or partially enclosed water bodies, ranging from large freshwater lakes to man-made reservoirs. Large lakes, particularly those that are long and narrow, provide ideal conditions for these standing waves to develop due to their confined geometry, which supports the resonant oscillation. Bays and harbors, partially protected from the open ocean, can also experience seiches. Even smaller bodies of water like swimming pools and bathtubs can exhibit miniature seiche-like movements, although they are typically less noticeable and dissipate quickly. The common factor is the presence of boundaries that allow the water to slosh back and forth.
Visual Signs and Potential Effects
Observing a seiche often involves noticing a distinct and rhythmic rise and fall of the water level, particularly at the ends of the basin. While the center of the water body may appear relatively calm, the water at the shores can visibly recede and then surge back in a repeating pattern. The amplitude, or height, of this water level change can vary significantly, from a few centimeters in small seiches to several meters in more pronounced events. This visual effect is a direct manifestation of the water oscillating back and forth within its confined space.
Larger seiches can generate substantial consequences for coastal areas and infrastructure. The sudden changes in water level can create strong, swift currents that pose hazards to swimmers and small boats. These powerful surges can also damage docks, moorings, and other waterfront structures. In extreme cases, prolonged or high-amplitude seiches can lead to localized flooding of low-lying areas or contribute to shoreline erosion. Such events underscore the importance of understanding water dynamics in enclosed basins.
Seiches Compared to Other Water Phenomena
Seiches are distinct from other water phenomena such as tsunamis and tides. Tsunamis are powerful ocean waves generated by large-scale seafloor disturbances, such as earthquakes, traveling across vast open oceans. Conversely, a seiche is a localized standing wave confined to an enclosed or semi-enclosed basin. Seiches also differ from tides, which are the regular rise and fall of ocean levels driven by the gravitational forces of the Moon and Sun. Tides are a global, predictable astronomical phenomenon, while seiches result from internal oscillations within a basin, often triggered by meteorological or seismic events.