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Flood frequency matters: Why climate change degrades deep-water quality of peri-alpine lakes
Sediment-laden riverine floods transport large quantities of dissolved oxygen into the receiving deep layers of lakes. Hence, the water quality of deep lakes is strongly influenced by the frequency of riverine floods. Although flood frequency reflects climate conditions, the effects of climate variability on the water quality of deep lakes is largely unknown. We quantified the effects of climate variability on the potential shifts in the flood regime of the Alpine Rhine, the main catchment of Lake Constance, and determined the intrusion depths of riverine density-driven underflows and the subsequent effects on water exchange rates in the lake. A simplified hydrodynamic underflow model was developed and validated with observed river inflow and underflow events. The model was implemented to estimate underflow statistics for different river inflow scenarios. Using this approach, we integrated present and possible future flood frequencies to underflow occurrences and intrusion depths in Lake Constance. The results indicate that more floods will increase the number of underflows and the intensity of deep-water renewal – and consequently will cause higher deep-water dissolved oxygen concentrations. Vice versa, fewer floods weaken deep-water renewal and lead to lower deep-water dissolved oxygen concentrations. Meanwhile, a change from glacial nival regime (present) to a nival pluvial regime (future) is expected to decrease deep-water renewal. While flood frequencies are not expected to change noticeably for the next decades, it is most likely that increased winter discharge and decreased summer discharge will reduce the number of deep density-driven underflows by 10% and favour shallower riverine interflows in the upper hypolimnion. The renewal in the deepest layers is expected to be reduced by nearly 27%. This study underlines potential consequences of climate change on the occurrence of deep river underflows and water residence times in deep lakes.
High-Resolution Measurements of Turbulent Flow Close to the Sediment–Water Interface Using a Bistatic Acoustic Profiler
Velocity profile measurements at high spatial and temporal resolution are required for the detailed study of solute and momentum transfer close to the sediment–water interface. Still, not many devices allow such measurements in natural systems. Recently, a bistatic acoustic current profiler has become commercially available that allows the recording of profiles at down to 1-mm resolution with a maximum frequency of 100 Hz and a profile length of 3.5 cm. This study tested the ability to characterize the turbulent flow of this profiler in a laboratory flume and in a run of the river reservoir. The tests showed that average velocities were reliably measured in the upper 2.5 cm, while the flow statistics were affected by Doppler noise and signal decorrelation. The latter is caused by the decreasing overlap between the individual beam signals. Doppler noise can be estimated and accounted for by established correction procedures, but currently there is no method to quantify the influence of signal decorrelation. Both error sources mainly affect the measured variances of the velocities, while the Reynolds stresses are reliable as long as there is no interference with the solid bottom. In the field application, most problems arise because of the necessity of coordinate system rotation, since a perfect alignment of the profiler with the current is not possible. Also, because of the coordinate system rotation, the Reynolds stresses become contaminated by noise, which can be removed by low-pass filtering. Still, this filtering results in loss of the turbulent signal, which was estimated in this study to be between 2% and 10%.
Brand,A.; Noss,C.; Dinkel,C.; Holzner,M. (2016) High-Resolution Measurements of Turbulent Flow Close to the Sediment–Water Interface Using a Bistatic Acoustic Profiler, Journal of Atmospheric and Oceanic Technology, 33(4), 769-788, doi:10.1175/JTECH-D-15-0152.1, Institutional Repository
Der faszinierende Kivusee und das gelöste Methan in seinem Tiefenwasser könnten eine ganze Forschungsanstalt auf Trab halten. Mit unseren Forschungsprojekten versuchen wir, Grundlagen für eine nachhaltige und sichere Nutzung des Methans zu schaffen.
Abwasser ist mit antibiotikaresistenten Bakterien belastet.
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Seen sind grosse Wärmespeicher. Inwiefern kann diese Wärme genutzt werden, um den Verbrauch von Brennstoffen und Elektrizität zum Heizen oder Kühlen zu vermindern?