D28: Internal seiche mixing study - Hydromod

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).