Transport and propagation of fine sediment at Alpine Rhine delta and Lake Constance during floods.

Daniel Ehrbar, Magdalena Eder, David Vetsch, Ulrich Lang

Tuesday 30 june 2015

12:57 - 13:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling, Poster pitches

Parallel session: Poster pitches: 5A. Sediment - Erosion

The Alpine Rhine transports large quantities of fine sediment into Lake Constance each year. Embankments were built on the original delta depositions to avoid further aggradation at the river mouth and to convey the sediment-laden water into deeper parts of the lake. Because recent flood statistics indicate the potential of increased flood discharges, an upgrade of protection measures upstream of the river mouth is planned. In addition, the impact of future floods and sediment discharges on the river mouth and Lake Constance are investigated by numerical simulations. The mainly fluvial transport processes in the embankment channel and the interaction with the Lake are simulated using a 2D Saint-Venant-Exner model. The incorporated sediment transport model includes both bed-load and suspended-load and the exchange between both modes. A Reynolds-averaged 3D model is applied for fine sediment propagation in the lake, thereby considering density as well as temperature gradients and thus allowing for the simulation of turbidity currents and lake stratification. The fluvial and lake models are coupled by exchange of water and sediment discharge. For model calibration and validation, two past floods were simulated and compared with measurements, i.e. estuarine and lake topography as well as data of temperature and turbidity loggers. These simulations indicate that morphological patterns in the channel are sensitive to configuration of transport model exchange terms. In the lake, the sediments may be transported over large distances. Depending on the processes, interflow and underflow form and turbidity currents may occur reaching nearly the German shore. These underflows are strongly linked to resuspension processes. Based on the validated model, scenarios of future floods at different combinations of discharge and lake level are investigated. The results to be presented in the paper submitted indicate the fitness of the existing estuarine works. Furthermore, depositions and propagation of fines in the lake are also discussed.