How are seiches produced in lagoons?

A seiche is a rhythmic oscillation of water that occurs in partially enclosed coastal inlets, such as lagoons, as well as in lakes. These oscillations are produced when the water surface is disturbed, causing gravity to attempt to restore a horizontal level, which results in simple vertical harmonic motion.1

The primary causes of this phenomenon include:

• Atmospheric and Wind Forcing: Persistent wind stress or gradients in atmospheric pressure can cause a denivellation, or tilting, of the water surface. When this external force ceases, the water flows back to restore its level, initiating a periodic oscillation.2
• Seismic Activity and Tsunamis: In coastal environments like lagoons, seiches can be triggered by the motions of earthquakes or by tsunamis.1
• Wave Reflection: Once the water is disturbed, the impulse travels the length of the basin and reflects off the boundaries. Repeated reflections generate standing waves with one or more nodes where no vertical motion occurs.1

The oscillation continues until damping effects, such as bottom friction and internal turbulence, level the water or until new external forces create another tilt.2