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 8 spctr.h41.3.detr.MB spctr.h31.3.detr spctr.h.20.3.detr spctr.h16.3.detr 6 4 2 0 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.00 0.05 0.10 0.15 0.20 0.25 frequency 0.00 0.05 0.10 0.15 0.20 0.25 frequency FP5_Contract No.: EVK1-CT1999-00004 Version: 1.2 Date: 24.08.2004 File: D28.doc spctr.h50.95.detr spctr.h45.95.detr spctr.h40.95.detr.NB spctr.h35.95.detr spctr.h30.95.detr spctr.h25.95.detr spctr.h20.95.detr Page 90 of 92 Figure 63 : Frequency analysis of temperature fluctuations in mid basin (at 16 to 41 metres, picture above) and north basin (20 to 51 metres, picture below) for low pass-filtered data.
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
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D28: Internal seiche mixing study - Hydromod

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