D28: Internal seiche mixing study - Hydromod

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Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D28: Internal seiche mixing study 3.2 FIELD STUDIES FP5_Contract No.: EVK1-CT1999-00004 Version: 1.2 Date: 24.08.2004 File: D28.doc Page 22 of 92 Field measurements concerning temperature and currents have been carried out in the Lac Léman over the last twenty years using moored instruments. The recording interval was 30 min. or 60 min. and instruments were deployed in campaigns lasting for several months. Most of these data were collected along the northern shore of the lake at mooring stations covering the slope down to about 175 m during fall stratification. However, some measurements were also made during summer periods. The data were analyzed for the presence of internal seiche modes using spectral analysis. For spectral analysis of the records, a standard Fast Fourier Transform with segment overlap was used. 3.2.1 Modes detected Modes were identified by comparing the periods P corresponding to prominent spectral peaks with those predicted for a two-layered approximation fitted to the lake dimensions and density distribution using the Merian formula P n =2L b (h1 +h2) nh1h2 g(ρ2 − ρ1) ρ2 where Lb is the basin length, h1 and h2 are the depth of the epilimnon and the hypolimnion and ρ1 and ρ2 are the respective densities. n is the mode number. In the calculations we assumed, for the summer (values for the fall in parenthesis) a top layer h1 = 15 (25) m; a bottom layer h2= 175 (165) m; a top layer temperature T1 = 19 (8) °C; and a bottom layer temperature T2= 5.5 °C. The analysis of all the data indicates that, independent of season and station location, only certain modes are excited. Differences were found between the number and type of modes observed at different locations in the lake in particular with distance from the shore.
Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D28: Internal seiche mixing study FP5_Contract No.: EVK1-CT1999-00004 Version: 1.2 Date: 24.08.2004 File: D28.doc Page 23 of 92 Figure 20 : Cumulative energy spectra of alongshore current component at stations P1 and P2 (for positions see Figure 19). The maximum at the nearshore station P1 is at the Kelvin wave period while that for the offshore station P2 is at the Poincaré period. (cpd = cycles per day; modified from Bohle-Carbonell, 1986) At the stations closest to shore, the most prominent seiche is the first mode (n = 1; called L1 hereinafter; Figure 20). Its period is near 81.5 h in summer, increasing to 131 h in fall, as the density structure of the water column changes and ci decreases. In the spectra from the narrow western end of the lake (not shown here) the L1 mode response is always seen most clearly. Recently, timeseries measurements were carried out during summer stratification with a current meter placed about 5 m above the lake bottom on the southern side of the deep central plateau at station S (Figure 19). Spectra from this station (Figure 21) show again a broad peak at the Kelvin seiche period. Seiche modes n = 2 to 9 were not observed in any spectra. The first cross basin or transverse mode (called T1 hereinafter), with summer and fall periods of 10.7 and 13.5 hours respectively. It is seen most clearly in the spectra at offshore stations in the central part of the lake basin (Figure 20) and is often found in the eastern part. Modes higher than 10 cannot be detected with certainty because of the cut-off imposed by the time step of the data records. The T1 period is also clearly seen in spectra from the recent deep measurements at S (Figure 21).
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D28: Internal seiche mixing study - Hydromod

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