Seiches are free oscillations that occur in basins of moderate size (harbour, lake, bay or even sea). They are standing waves with a frequency equal to the resonance frequency of the basin in which they occur. The oscillations may be caused by sudden changes in wind conditions. After generation the water sloshes back and forth until the wave motion is dampened out by friction. Seiches generally have half-lifes of only a few periods, but may be frequently regenerated. The difference between seiches and the earlier discussed tidal resonance phenomena in basins is that the latter are not free oscillations, but forced at a certain tidal frequency.
Seiches can have periods ranging from a few minutes up to several hours. They can cause havoc in a harbour by setting up reversing currents at the entrance or by rocking ships free of their moorings. They can also abruptly surge onto piers and beaches and sweep people away. The Great Lakes of North America and some of the large lakes in Switzerland are especially prone to seiches, because they are enclosed basins with large fetches and strong winds.
As seiches are a resonance phenomenon, it is obvious that the basin size in relation to the wavelength is an important factor. Therefore, measures against generation of seiches are usually based on size restrictions of harbour and other basins, and on the use of irregularly shaped basins.
Seiches can oscillate in a semi-enclosed (Fig. 5.77, right) and closed mode (Fig. 5.77, left) or in a combination of the two if the open end is somewhat restricted. In simple cases the wavelength is twice or four times the basin length (or a certain fraction thereof).
Usually, the vertical amplitude of a seiche, even at an antinode, is small. However, especially at a node, the horizontal displacement of the water can be significant. This can cause mooring difficulties for ships. Another related influence on large ships is the effect of the water surface slope.
In the port of Rotterdam, a seiche was observed in the morning of 1 March 1990. It appeared first as a minor fluctuation of around 10 cm at light island Goeree (an observation post some kilometres offshore) at 0:00 hours. Then, at about 01:30 hours, it appeared as a huge standing wave of 1.75 m at Rozenburg lock, a navigation lock some 15 km inland. Figure 5.78 shows a smaller seiche at the same locations observed on 27 August 2001.