A second order seiche. The undisturbed sea level is indicated by the broken yellow line. Five water particles are shown as an indication of water movement in the seiche.

Note the two nodes at 1/4 and 3/4 of the basin length, and that water under the nodes moves only horizontally, while water at both ends and in the centre of the basin moves vertically.

It should be straightforward from the figure to work out what higher (third, fourth etc.) order seiches look like.

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