A seiche (pronounced “saysh”) is a standing wave that oscillates within a confined body of water, such as a lake, bay, or reservoir. This phenomenon is often described as water sloshing back and forth, similar to water vibrating in a bathtub. Unlike a typical traveling wave, a seiche appears to stand still while the water level rises and falls. Because of its long period and sudden, significant water level changes, it is sometimes mistaken for a tide or a small tsunami.
Understanding Seiche Movement
A seiche is governed by the physics of a standing wave, resulting from two traveling waves of the same frequency interfering as they move in opposite directions. Once triggered, the water in the basin begins to oscillate at its natural frequency, a concept known as resonance. This natural frequency is determined by the specific length and depth of the water body.
The sloshing motion centers on specific points in the basin. The areas where the vertical water level displacement is at its absolute minimum are called nodes; at these points, the water surface remains relatively constant. Conversely, anti-nodes are the locations at the ends of the basin where the wave reaches its maximum vertical displacement, creating the highest crests and lowest troughs.
The period, or the time it takes for a seiche to complete one full back-and-forth oscillation, is directly related to the size of the water body. In smaller enclosed areas, the period might be only a few minutes. However, in large basins like the Great Lakes, the wave period can range from four to seven hours, which is why the effect can sometimes be confused with oceanic tides.
Forces That Initiate Seiches
A seiche requires an initial force to displace the water from its equilibrium position and set the standing wave in motion. The most frequent triggers are meteorological events that rapidly alter the water surface pressure. This includes the sudden passage of a strong pressure front, where a rapid change in atmospheric pressure pushes down on the water unevenly.
Strong winds are also a common initiator, causing a temporary “wind set-up” where water is piled up at one end of the basin. When the wind ceases, the accumulated water rebounds to the opposite side, initiating the oscillating motion.
Less common triggers include seismic activity and certain hydrodynamic forces. Ground movement from an earthquake can displace a large volume of water, starting the seiche. In harbors, sudden changes in flow or the presence of long-period infragravity waves can also provide the necessary impulse.
Documented Occurrences and Hazards
Seiches have been documented across the globe in any sufficiently enclosed body of water, with the North American Great Lakes being a particularly active area. Lake Erie, the shallowest of the Great Lakes, is known for producing the most frequent and largest seiches, sometimes reaching heights of eight feet or more. A historic event on Lake Erie in 1844 created a massive surge that resulted in significant loss of life and property damage.
The primary hazard posed by a large seiche is the rapid, unexpected fluctuation of water levels along the shoreline. Coastal areas can experience sudden flooding as the water surges in, followed quickly by a rapid drawdown that leaves docks and shallow areas exposed. This rapid change can damage coastal infrastructure, disrupt shipping, and pose a danger to people near the water.
A seiche must be distinguished from a tsunami; while both are large waves, a seiche is localized, confined by the basin’s edges, and usually generated by atmospheric forces. A tsunami, in contrast, is a deep-ocean wave caused by a massive seismic event, such as a submarine earthquake, propagating across entire ocean basins.