Free oscillations have periods that are dependent upon the horizontal 

 and vertical dimensions of the basin, the number of nodes of the standing 

 wave, that is, lines where deviation of the free surface from its undis- 

 turbed value is zero, and friction, ihe period of a true forced-wave 

 oscillation is the same as the period of the causative force. Forced 

 oscillations, however, are usually generated by intermittent external 

 forces, and the period of the oscillation is determined partly by the 

 period of the external force and partly by the dimensions of the water 

 basin and the mode of oscillation. Oscillations of this type have been 

 called forced seiches (Chrystal, 1905) to distinguish them from free 

 seiches in which the oscillations are free. 



(b) Water Motion 



(A) STANDING WAVES 



(B) CLOSED BASIN 



(I). Fundomentol Mode 

 (First Harmonic) 



Node jf 

 -Antinodes 



(2). Second Mode 



(Second Hormonic) 



(3)Third Mode 



(Third Harmonic) 







(C) OPEN-ENDED BASIN 



(I). Fundamental Mode 

 (First Harmonic) 



L„ 



h 



(2). Second Mode 



(Third Harmonic) 



(3).Third Mode 



(Fifth Harmonic) 



Surface profiles for oscllloting waves 



(after CQrr,l953) 

 Figure 3-40. Long- Wave Surface Profiles 



For the simplest form of a standing one-dimensional wave in a closed 

 rectangular basin with vertical sides and uniform depth (Fig. 3-40(B)), 

 wave antinodes, that is, lines where deviation of the free surface from 

 its undisturbed value is a relative maxima or minima, are situated at the 

 ends (longitudinal seiche) or sides (transverse seiche). The number of 

 nodes and antinodes in a basin depends on which mode or modes of oscilla- 

 tion are present. If n = number of nodes along a given basin axis, d = 

 basin depth, and £g = basin length along that axis, then Tn, the natural 

 free oscillating period is given by 



2f 



T„ = 



B 



nVgdT 



(3-42) 



3-79 



