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 91 of 92
6 CONCLUSIONS
The better understanding and knowledge of the internal gravity waves (seiches) in deep
large lakes were the main objectives of this study. We have focused the work principally
on the determination of the waves properties through measurements analysis and numerical
simulation. The seiches qualification has been done all over the four studied
lakes. The influence of the seiches on mixing processes was also studied.
Several approaches of the internal gravity wave in large deep lakes have been presented:
• Analysis of existing measurement on Loch Lomond, Lac du Bourget, Lac
Léman,
• Linear analysis (eigen-value method): Lake Constance,
• Numerical 3D model analysis: Lac du Bourget, Lac Léman
6.1 LAC DU BOURGET
The analysed measurements are shown the existence of internal gravity wave in the
Lac du Bourget with a period between 40hours to 80 hours depending on the vertical
stratification structure.
The simulation using the TELEMAC-3D model is used to reproduce one seiche event
on the Lac du Bourget. The numerical results are in a quite good qualitative agreement
with the measurements.
The numerical results are given some interesting new information about the hydraulic
respond of the lake under wind forcing. After the generation of the internal seiches, the
wave propagation is showing a rotating structure, due to the Coriolis influence, with an
amphidromic point at the middle of the lake (Figure 16).
6.2 LAC LÉMAN
An analysis of internal seiches dynamics was carried out for Lac Léman combining field
measurements and numerical modeling. Using field data of temperature, currents and
surface elevation, it has been shown that only two modes of internals seiches are sufficiently
excited in Lac Léman to be considered significant. The first one is a Kelvin wave
and the second one is a Poincaré wave. Model calculations have indicated that other
seiche modes can only be excited by winds from certain directions. However, due to the
topographic constraints particularly in the eastern part of the lake basin the wind field
over the lake is strongly canalized and these winds do not exist in nature.
From our analysis of field studies of the longterm mean conditions of mixing it is indicated
that internal seiches are important in providing vertical mixing. Recently, it has
been shown though that most of this mixing is actually generated in the near shore
zone and then propagates into the open waters (Wuest et al., 2000). Thus, the interaction
between near shore zones and the open water is also important for mixing. Furthermore,
we have pointed to the importance of the interaction with the sloping sides of
the lake and short progressive internal waves (Thorpe and Lemmin, 1999a, Lemmin et