Tests of an extension to internal seiches of Defant's procedure for determination of surface seiche characteristics in real lakes1

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Abstract
The success of A. Defant's method of calculating surface seiche periods and structures in real lakes is well known. To test whether it can be adapted and applied with similar success in studies of internal seiches in stratified lakes, in which density structure is simulated by two homogeneous layers, we compare the predictions of a computerized version of Mortimer's modified Defant procedure against observations of internal seiche motion in eight lake basins and (in three examples) against the results of more elaborate models. The acceptably close agreement inspires confidence that the modified method is useful for practical predictions which, although approximate, described the principal characteristics of the often large water mass displacements and oscillations. Through their influence on mixing and dispersal, those motions profoundly affect the chemical and biological economies of many lakes. To aid in microcomputer and even pocket calculator applications, we set out steps in the calculation in tabular flow‐chart form. A comparable table for surface seiche calculation is added.The procedure is not applicable to large lakes in which effects of the earth's rotation are dominant; but for the two largest basins tested here (Léman and Ness) a modification is introduced to take approximate account of rotation.