An analysis of water oscillation in the several sea-gulf systems on the base of shallow water theory and the actual bottom relief data was provided. Computation was carried out for each part of every system independently, and their amplitude responses were compared. The numerical analysis manifests that the sea oscillations have an influence on the gulf oscillations, and the inverse influence takes place too. Harmonic locking of oscillations in both part of the sea-gulf system is established. Tuning of the sea-gulf system oscillations is effected by the depth distribution only, but a correlation between gulf narrowness places and nodes of the main seiche modes is found. The amplitude responses and modes of three coupled sea systems are presented.

The influence of the large-scale engineering activity in the gulf of the Finland (a dam construction at its head near St.-Petersburg) on the gulf amplitude response are estimated. The results of the numerical analysis show the amplitude response changes as the dam effects. Besides, the nodes of the modes have displaced. Nowadays the dam effect introduces an artificial instability, but harmonic locking must lead to a restoration of the gulf amplitude response, with an according change of the bottom relief and of the shoreline as the long-term environmental effect of the dam construction. This change is a large-scale, long-term mass transfer in the environment caused by engineering.

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