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 FP5_Contract No.: EVK1-CT1999-00004 Version: 1.2 Date: 24.08.2004 File: D28.doc Page 46 of 92 Thorpe, S. T., U. Lemmin, et al. (1999b). Observations of the thermal structure of a lake using a submarine. Limnol. Oceanogr. 44(6): 1575-1582. Thorpe, S. A. and R. Jiang (1998). “Estimating internal waves and diapycnical mixing from conventional mooring data in a lake.” Limnol. Oceanogr. 43: 936-945. Thorpe, S. A., et al (1996) High frequency internal waves in Lake Leman. Phil. Trans. Roy. Soc. London A 354: 237-257. Thoulet, M. J. (1894) Contribution à l'étude des lacs des Vosges. Bull. Soc. Geographie 15: 557-604. Umlauf, L., H. Burchard, and K. Hutter, Extending the k-omega turbulence model towards oceanic applications, Ocean Modelling, accepted 2002. Wuest, A., G. Piepke, et al. (2000). “Turbulent kinetic energy balance as a tool for estimating vertical eddy diffusivity in wind forced stratified waters.” Limnol. Oceanogr. 45: 1388-1400. 4 MAPS AND TABLES OF FREE INTERNAL SEICHES IN UPPER LAKE CONSTANCE FOR PRACTICAL USE 4.1 OBJECTIVE The assessment of the local intensity of internal seiches due to their variable structure is of considerable concern in applications of water management, certain water constructions and other executive measures, for which essential information on transient currents and corresponding water displacements is required. For instance, the spill of harmful substances, drift of lost bodies, mixing and dispersion in various limnological and hydrological problems may depend during the period of stratification occasionally strongly on internal seiches. Their local effect in such cases can be estimated on the basis of adequately resolved graphical representations of their variable spatial intensity. As rough information it is often sufficient to get an idea on the local variation of potential activity by superposed internal seiches of different order. To enable the expert community without resort to detailed knowledge of the physics of internal waves, the oscillations have to be displayed in a form easy to grasp by inspection. This was done in a different delineation than the structures usually shown in terms of wave parameters such as lines of equal range and phase, which give a condensed overview. Instead, the resolution into momentary wave stages and local amplitude variations with time has been presented providing better imagination and at the same time more refined quantitative information on the structure. The corresponding horizontal motion is given in field representations of ellipses of the rotating current vectors with indication of the sense of rotation and the zero phase position for a defined moment of the vertical elevation. Such a description consists of several diagrams for one mode and sums up to a collection of numerous illustrations as the number of modes represented increases. Despite this inconvenience the use is nevertheless facilitated, as the corresponding diagrams of each mode have the same scale and the thematic content has been drawn in the same graphically proper forms. This description has been compiled for the first 15 modes of Upper Lake Constance. A few examples are selected in the following to give an idea for practising.
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 47 of 92 The work had been completed by Bäuerle and Ollinger (1991) in collaboration with Hollan in a project of the ISF. Later refined calculations by Bäuerle resulted in more details but confirmed the main structures of the oscillations already calculated in this early approach. Thus the compilation from 1991 is still worthwhile in the present context and may also serve as an example for the other large lakes under consideration. In particular reference to Lake Leman, the calculation with the same model has been carried out for the first 12 modes also by Bäuerle (1985). The extension to a similar presentation was included for merely two modes, thus not allowing for scanning the local potential contribution of all the 12 modes. The usefulness of the chosen graphic description of internal seiches has been demonstrated very early by Bäuerle and Hollan (1983) for the case of the fundamental and a transverse mode of Lake Tanganyika enclosed in the monography on the lakes of the warm belt by Serruya and Pollingher (1983). This reference is quoted within the preceding one. C. Serruya recommended in this context to carry out such work on other large stratified lakes as accomplished later in the advanced version for Lake Constance here. 4.2 OUTLINE OF THE CALCULATION The strength of internal seiches is most adequately described in terms of forced oscillations. However, this approach implies precise knowledge of the driving agent, which is mostly the wind field over the lake. The horizontal variation of this quantity is generally not sufficiently known for that purpose, in particular, for large lakes. Moreover, the superposed different internal modes excited during one event have to be decomposed for identification of the associated single contributions. Desisting from such a difficult description, which is practically beyond reach, the relative structures and natural periods in terms of free oscillations may already serve for essential information to quantify the phenomenon. If certain observations exist on the mean amplitude of internal seiches with respect to typical wind fields and the stratification in the lake, the determination relative to an arbitrary factor may be converted to absolute values, which often suffices for assessment.
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