Hydrodynamics of very shallow lakes - A study in Lake Krankesjön, Sweden

Shallow lakes have great ecological and recreational values. They are also largely affected by humans, mainly through eutrophication, and great efforts are therefore being put into the understanding of these ecosystems. The knowledge is however not complete, and more research is needed to manage these lakes in a sustainable way. The physical characteristics that are special for shallow lakes compared to deep lakes are that no stable stratification develops and that resuspension of sediments due to wave action is important for the water quality. Further, vegetation can colonize a larger part of the bottom and thereby affect water movements. In this thesis, hydrodynamics of lakes that are very shallow are examined. The special characteristics of shallow lakes are even more pronounced in these very shallow lakes. Furthermore, they lack areas deep enough to allow permanent accumulation of sediments which affects resuspension and thereby water transparency. The study aimed at determining a) the water movements induced by non-uniform heating of the water surface due to shading by vegetation, b) the wave climate and the interaction with vegetation, and c) resuspension and redeposition of fine material for different wind conditions. Most results were obtained through field measurements in the very shallow Lake Krankesjön in southern Sweden. These included temperature profiles and radiation outside and within a reed belt; wave measurements in the open water and within emergent vegetation; concentrations of suspended material and turbidity; resuspension potential of the bottom material and its organic content; and weather parameters. A model to describe wave damping in reed vegetation was derived. Historical wind data from the region were analyzed and compared to historical data on water transparency and submerged vegetation density in the lake. A significant reduction (85%) in net radiation by reed vegetation causing a temperature difference (about 0.5°C) that induced a convective current (~1 cm/s) between the open water and the water within the vegetated belt was found. The induced density current is an important driving force for exchange processes between the reed belt and the open water during sunny summer days. It was shown that linear wave theory and wave height prediction models based on these correlated well with the measured wave heights in Lake Krankesjön. The damping of waves in reed vegetation was quantified (in average 4–5% m-1) and compared to a derived expression for the wave height decay. The model predicted the measured wave damping well. The probability density function of the wave heights was not proven to significantly change within the vegetation. The resuspension in Lake Krankesjön was shown to depend not only on the wind speed, but also on the wind direction, since soft easily resuspended sediments were found to accumulate on the lee-side for dominant wind directions. Winds from unusual directions resuspended these sediments. In a longer time-scale it was shown that periods of high frequency of strong winds in the directions opposite of the prevailing directions were correlated to periods when Lake Krankesjön had shifted to a turbid state. Periods when the lake had shifted to a clear state were related to periods of low abundance of such winds. The results add more components to the understanding of shallow and especially very shallow lake ecosystems. Hopefully, this will lead to an improvement of the management of these complex and valuable natural resources