Physical processes in lakes and in man-made water reservoirs, such as wave erosion and resuspension of sediment, the formation of turbidity currents, the accumulation and loss of heat, and the effect of variations in water temperature on the density stratification

Turbid water layers caused by wave erosion and resuspension of sediment in the Nam Ngum reservoir in Laos.

The void ratio of newly deposited, fine-grained sediments is often high, indicating that they may be eroded and resuspended by rather weak waves and currents. Sediment focusing, as a result of erosion and resuspension and the transportation of sediment from shallower to deeper areas, is therefore of great importance for the spatial variation in sedimentation rate in many lakes and water reservoirs.

Section of reservoir showing beginning of underflow. (Modified after Knapp, 1943.)

The beginning of a turbidity current underflow is illustrated above. The turbid flow leaves the surface at the plunge point and continues as an underflow in the reservoir or basin. On account of the shear at the interface between the underflow and the overlying basin water, some of the basin water is dragged down and a corresponding counterflow is induced in the upper portion of the basin. In nature (as in Lake Laitaure) the plunge point is therefore sometimes marked by floating debris, since surface circulation flows towards this point and then dives below.

The density currents that have a higher content of suspended sediment than the enclosing fluid are called turbidity currents or suspension currents. These density currents also differ from other density currents, because the density is not uniform but increases from top to bottom. According to measurements in Lake Laitaure in northern Sweden a density contrast as low as 0.0003 between the inflowing river water and the impounded lake water was sometimes enough for the formation of turbid density currents of type underflows. As may be seen from the figure below, showing effect of differences in temperature, dissolved salts, and suspended sediment upon density differences between adjoining water layers, a difference in suspended sediment of 500 g/m3, in dissolved salts of 400 g/m3, or in temperature of 1 - 2 degrees at high water temperatures may thus be enough for the formation of density currents.

 

Effect of differences in temperature, dissolved salts and suspended sediment upon density differences between adjoining water layers. From Axelsson 1967 (Geogr. Ann. 49 A).

The density of water varies with the temperature, the content of dissolved and suspended material and with the pressure. One and the same difference in temperature causes a greater density difference at high than at low temperatures. The density difference caused by a temperature difference of 1 degree, for instance, is about three times greater at 25 degrees than at 10 degrees. At high temperatures, therefore, even small temperature differences may cause considerable damping of the turbulent exchange. This is an important reason to the frequent occurrence of more or less turbid density currents in tropical and subtropical water reservoirs.

Water temperatures in the Swedish lake Skärsjön 18/1 - 9/5 1948. From Axelsson 1957, Geographica 31 (Naturgeografiska uppsatser. Redigerade av Filip Hjulström och Valter Axelsson).

In the winter of 1948 the temperature of the bottom water in lake Skärsjön was higher than the thermical density maximum, +4oC. The greater density of the bottom water was due to its higher content of dissolved and colloidal matter. The onset of freezing occurred at a relatively early date, and the maximum thickness of the ice was 50 cm. A great amount of melt-water could therefore be obtained from the ice during the later part of the winter, and the chemical stratification of the upper water layers could become stable. The winter stratification passed directly into summer stratification without any preceding period of circulation. The ice break-up took place on 14. April, but the water did not begin to circulate until 2. May.

Accumulation and loss of heat in lake Skärsjön 4/11 1948 - 4/12 1949. From Axelsson 1957, Geographica 31. (Also in Fizikogeograficheskiy Atlas Mira, 1965.)

If the onset of freezing begins early, the mean temperature of a temperate lake is usually relatively high, and a great deal of heat is conducted from the bottom. This is an especially significant factor for the temperature conditions in shallow, temperate lakes during the winter. If, on the other hand, the onset of freezing occurs later, the water body and the bottom deposits have probably lost a great deal of heat, and the mean temperature of the lake is therefore low during the winter.

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