Observation and Analysis of Internal Seiches in the Southern Basin of Lake of Lugano

Abstract

As mentioned already earlier in Chap. 15, Lake of Lugano is a lake system consisting of two large basins and a pond of much smaller size, all connected to one another. In fact, the discharge of the water masses is from the 15 km long Northern basin through the channel of Melide into the roughly S-shaped 17-km long Southern basin and from there through the 500-m long channel of Lavena into the small pond at Ponte Tresa, see Fig. 18.1. The barotropic response of the two large basins has been separately studied as has this response of the lake system as a whole. In the Southern basin, three limnigraphs, positioned at Riva San Vitale, Morcote and Agno, recorded in February 1982 water elevation oscillations with periods of 28 min and less, that could be identified with the eigenperiods of the surface seiches with amplitudes of less than 5 cm. In a further campaign in 1984, current meters were installed in the Channels of Melide and Lavena and it was found that two further longer periodic eigenoscillations were excited which were not discernible in the limnigraph records and could be interpreted as the eigenvalues of the barotropic oscillations of the lake system acting as a coupled (Helmholtz-type) resonator. The structure of the eigenmodes, i.e. the distribution of the surface elevation was relatively simple. As the eigenfrequencies (periods) increased (decreased) the eigenmodes went from simple to complex with the number of nodal lines increasing by one with each higher order mode. Qualitatively this behaviour is akin to that of a rectangular basin with constant depth, so that interpretation of the data by means of theoretical modelling is easy. Deviations of the eigenperiods and structures of the eigenfunctions from those of the rectangle are due to the bathymetry and nothing else.