Long waves

Long waves have a large wavelength in relation to the depth. The wave height is also low in relation to the wave length. In a long wave the water particles mostly move backwards and forwards with very little up and down movement in relation to the wave’s length. Examples of long waves are tidal waves, standing waves (water oscillation, seiches) and seismic waves (tsunami).

These waves are mainly observed as variations in the sea level along the coast or when they meet the coast (tsunami). The speed of a long wave is governed by the depth, and they move faster over deep water. In a depth is 100 m, as found parts of the Baltic Sea, a long wave such as a tsunami would travel at a speed of 31 m/s or 112 km/hr.

A seiche is a standing wave that occurs in lakes and semi-enclosed seas like the Baltic. The wave length for the oscillation in the Baltic Sea between the Bothnian Bay and the Southern Baltic is twice the distance, i.e. 4000 km.

The average depth of the global open ocean is 3700 m and at this depth a long wave travels at a speed of 191 m/s or 687 km/h. This is why there is very little time to warn coastal populations about impending tsunamis.