Numerical Modelling of the Sediment Balance of the River Thur

In the past, large bed level changes of the River Thur have been observed. Vertical erosion of 3.6 m within 60 years occured in the upstream region, whereas downstream aggradation of more than 1 m in the same period could be observed.

Sediment balance of the River Thur was numerically simulated with the model MORMO developped at the Laboratory of Hydraulics, Hydrology and Glaciology. MORMO is a 1-dimensional, quasi-stationary model which allows to calculate the transport of bed load and suspended load and to simulate bed movements as well as grain sorting processes on the bed surface. An important result of the calculation is the amount of transported sediments along the river and its developement by time.

In the case of the River Thur, a reach of 76 km length was modelled. The bed movements of the last 12 years were reproduced in order to calibrate the model. The following extrapolation showed that the River Thur will not reach a dynamic equilibrium within the next 80 years (period of extrapolation). Whithout counter-measures, erosion of the river bed of 2 to 3 m will occur in ancient aggradation zones.

The numerical model is now a reliable instrument to predict long term influence of training measures. Bed level changes normally occuring very slowly, can be simulated and visualized in little time.