For the redevelopment of the Lower Salzach in the Basin of Freilassinger (north of the city of Salzburg), the construction of so-called bottom gradations (structured ramps) is planned. One of these structures is to be built at river kilometers 51.9.
The river bed will be raised by about 2 m with this construction. This will not only prevent further deepening of the river bed, but also raise the groundwater level and improve the connection between the Salzach and the adjacent floodplains.
The planned structure (bed elevation approximate 2 m, width 140 m, slope 2 %, specific load approx. 20 m³/s and width meters) was further developed in a physical model test. The following objectives had to be considered:
- Structural stability up to the centennial flood (HQ100) with a 15 % surcharge,
- Raise the river bed upstream to the desired level,
- Possibility for fish to migrate against the direction of flow (for this purpose the structure is divided into individual steps, which are a maximum of 15 cm high along the fish path),
- Passability for boats in the direction of flow (a boat lane is planned in the middle of the building).
The model test was carried out at the experimental station in Obernach at the Technical University of Munich. We participated in this project in an advisory capacity with our experience.
The stones of the bars (steps) weigh between 2,000 and 4,000 kg. In the pictures, the stones are coloured differently according to their weight in order to be able to control the stability in the experiment more easily. The pools between the steps are secured with stones weighing between 100 and 400 kg. The stones in the boat lane (coloured blue in the pictures) weigh about 800 kg.
Entire ramp with boat (view against the flow direction)
Flow at the beginning of the ramp at about 500 m³/s (approximately double mean-water flow)
Detail of the ramp (View from above, flow direction from left to right)
Boat at the end of the tailwater bed protection