Vízügyi Közlemények, 1987 (69. évfolyam)

1. füzet - Rákóczi László: A tavi üledék felkeveredése

100 Rákóczi László Resuspension of lake sediments by DR. L. RÁKÓCZI, С. E., candidate of the technical sciences The paper is about the resuspension of the sediment of shallow lakes by wind and wave propagation from the point of view of hydraulics. A best possible information about the phenomenon of mixing is indispensable if a proper forecasting of water quality is needed. The different pollutants bound to the sediment particles are stored temporarily over the bottom but would reach again and again the water body due to agitation. The process of resuspension influences the life-processes of the different organisms through nutrient emission and altered optical characteristics. Finally, the process of eutrofication will also be influenced. To estimate the stirring effect of wind-induced wave-motion, first, the wind itself must be characterized by reliable measurements (temporal changes of wind-direction and velocity), then the parameters of wave-motion are to be determined on site, or by the use of suitable nomograms (Fig. 1.). In case of the second method, the needed so-called "effective fetch length" must be determined on the basis of Fig. 2. and Table I. Further tasks are: a fast, informative mapping of the bottom structure in the lake by aid of a penetrating instrument seen in Fig. 3., then the taking of undisturbed soil-samples from different points of the bottom to determine the changes in dry-material density as a function of depth measured from the level of the bottom. On the basis of Fig. 5. constructed by aid of Eqs. 6 and 7 and Fig. 4. estimates may be worked out to determine the expected beginning of resuspension due to given wind-directions and velocities. With available long-term time series on the wind-regime and with a topographical map of the lake-basin, one may determine the annual frequencies of such wind-events. For the estimations of the volume of stirred up sediment, or of the flux of the sediment stirred up from a unit of the bottom surface, measurement of the temporal changes in the density of the suspended load (Fig. 6.), and of the volume of sediment material trapped in the catches set up at different points of the lake bottom are needed beyond the data listed earlier. The author presents his results obtained in the lakes of Erken and Tämnaren in Sweden, fn the first lake, also the shear-stresses were calculated by aid of a three-dimensional numerical model induced by streaming due to windblow. By comparing these values to those caused by wave-motion it could be stated that in the coastal zones where the depth was less than 5 m, the values of the latter were always greater that those originating from the former. This should not misinform us to neglect the smaller value but rather to take into consideration both shear-stresses, at the same time. The procedures presented by the author, and especially Fig. 5. are used not only in lakes-but also in flatland reservoirs with extended free water surfaces. They may play a role in determining the location of a planned reservoir because it can be decided in advance-based on a map with an adequate scale, on available wind-regime data, and on the planned operational water level of the reservoir-which of the sites would be most suitable from the point of the lowest frequencies in stirring up. Forecasting of the magnitudes of changes in water quality in the period of planning may save substantial money later in maintenance and operation. * * * Aufwirbelung des Seesedimentes von Dr.-Ing. László RÁKÓCZI, Kandidat der technischen Wissenschaften Der Verfasser befaßt sich aus hydraulischem Gesichtspunkt mit der durch Wind-Wellengang ausgelösten Aufwirbelung des Sedimentes von Seen. Eine möglichst gute Kenntnis des Phänomens ist für die Vorhersage der Wasserbeschaffenheit erforderlich, da die verschiedenen Schmutzstoffe an die Sedimentpartikeln gebunden zeitweilig im Seebett gespeichert werden, jedoch infolge Aufwirbelung immer wieder in den Wasserkörper gelangen können. Die Aufwirbelung beeinflußt den