In Feldkirch/Vorarlberg (in the West of Austria) at the border to Switzerland the river Ill flows into the river Rhine. 1965 a ramp was built at the mouth of the Ill. Since then the Rhine has deepened considerably in this area. As a result, the difference in height between the bed of the Ill and the bed of the Rhine became bigger and the existing ramp had to be enlarged, which endangered its stability and the stability of the opposite bank of the Rhine on the Swiss side of the river.
For this reason, the state government of Vorarlberg decided to carry out a renovation for the mouth of the Ill. In this context, a physical model was required, which the Institute for Hydraulic Engineering and Calibration of Hydrometrical Current-Meters was commissioned with.
In the course of the study the following investigations were carried out:
- Investigation of the existing ramp and the surrounding area to assess the current hydraulic situation (flow conditions, bed degradation, bed and bank stresses)
- Study of the possibility of redesigning the existing ramp (widening, changing the angle of the river mouth)
- Design, investigation and optimisation of a new mouth variant in the form of two block ramps and a flatter mouth angle into the Rhine
After the complete destruction of the existing ramp during a great flood in 1999, the Institute's renovation variant of the river mouth was constructed from 2000 to 2002.
Physical model of the existing ramp
Formation of the riverbed after the construction of the ramp in 1965.
The difference in height between the riverbed of the Rhine and the white area in the picture is more than 5 meters (!) in nature.
Physical model of the new mouth of the river Ill (initial state)
Physical model of the new mouth of the river Ill with two block ramps and a flatter angle of the river mouth entering the Rhine.
The positive change in the bed formation compared to the existing bed is clearly visible in the pictures.
Aerial photo of the old river mouth of the Ill with the sketched new mouth
Construction of the new river mouth of the Ill, 2000-2002
Revetment of the ramp base
Technical data (source: state government of Vorarlberg):
Catchment area Ill: approximately 1,200 km²
Catchment area Rhine upstream the Ill mouth: approximately 4,000 km²
MQ Ill: approximately 65 m³/s
HQ100 Ill: approximately 700 m³/s
HQ1 Rhine upstream the Ill mouth: approximately 430 m³/s
HQ100 Rhine upstream the Ill mouth: approximately 2,200 m³/s
HQ100 Rhine downstream the Ill mouth: approximately 2,700 m³/s
Length of river mouth: approximately 730 m
Length of the construction site: approximately 760 m
Total excavation volume: approximately 400,000 m³
Required stone mass: approximately 80,000 t