Density-driven underflows with suspended solids in Lake Constance

Abstract

In Lake Constance, on average, underflows occur several times a year due to flood events in the River Rhine. During these events, large amounts of sediments as well as oxygenated riverine water are transported into the hypolimnion of the lake. The purpose of this study was to quantify the effect on deep water renewal and study the factors that influence the occurrence of underflows. A three-dimensional hydrodynamic model was set up. To calibrate the model, the flood event of August 2005, whose peak discharge was slightly below the discharge with a return period of 100 years, was simulated and results compared to measurements. The model was then validated simulating several further, smaller underflow events. In a model study of the flood event of June 2012, the influence of underflows to deep water renewal was investigated by using different tracers to mark the inflowing water as well as the epilimnic water. By varying the discharge, suspended solid concentration, and water temperature of the River Rhine in the model, the influence of these factors on the occurrence and spreading of underflows was studied. The simulations show that the model was able to reproduce the occurrence and spreading of under-flows satisfactorily. The tracer study indicates that a fair amount of epilimnic water was entrained into the underflow and therefore enhances deep water renewal. It was shown that the main influence factor on the occurrence of underflows was the concentration of suspended solids, followed by the discharge. Water temperatures have a relatively small influence. These results are confirmed by two underflow events in 2013 which occurred due to extreme high concentrations of suspended sediments without any increase in discharge. Although not part of the model study, it was suspected that the grain size distribution of the suspended solids also exerts a significant effect on the underflow. The hydrodynamic model was able to reproduce the main processes involved in the occurrence of underflows and therefore is a suitable tool for the investigation of influence factors on the build-up of underflows as well their effects on the lake’s sedimentation and erosion processes and water quality.