Sustainable river basin management for the Lower Salzach

Objectives

In the 19th and 20th centuries, river engineering works were carried out to improve navigability and create a more clearly defined border; the river was straightened and confined to a narrower channel, and debris retaining dams were erected on the upper reaches. The measures led to severe degradation of the riverbed; the ground water level fell, and the riverine woodlands were cut off from the river with negative consequences for the riverine ecosystem. As the process of degradation continues, there is a growing risk to the engineered structures and the local population. This could be exacerbated by an increase in heavy precipitation events as a result of climate change.  
Rehabilitation measures are needed to stop the riverbed degradation process, offer protection from flooding, improve the ecology of the river basin and return the regime to a natural equilibrium with the stability needed to cope with climate change.

Activities

An overall concept based on various modules with open armour layers and near-natural riverbed slopes in combination with “soft banks” and restored backwater continuity was developed in co-operation with scientists, civil engineers and the authorities in Bavaria and Austria. This programme of interdisciplinary cross-border co-operation was highly effective in terms of knowledge-sharing and state-of-the-art solutions.
Engineering works began at the end of 2006, when local protection measures were implemented with coarse-grained material and measures taken to protect the base of the dykes in the area of Laufen/Oberndorf. The first main module, in the form of an open armour layer, is now under construction and work is due to start shortly on the first near-natural riverbed slope, a soft bank with a length of more than 3 km and restoration of a backwater link. The structures are designed for a flood with a 100-year recurrence interval plus a 15% allowance for climate change.

Results (CO2 savings)

Knowledge sharing and improvements in economic and ecological terms to proven hydrological structures. The open armour layers constitute a near-natural solution for gravel bed stabilisation. The river drops are to be re-engineered as near-natural slopes with sills and pools plus a boat channel. This design is the key to ecological continuity through the structures.
The riverbed slopes also permit water to be diverted into the channels that once fed the riverine woodlands and thus restore the old backwaters. That will increase the retention capacity of these areas and improve biodiversity.

Organisation

Free State of Bavaria, represented by the Traunstein Water Resources Management Office, in collaboration with the Federal Water Engineering Administration in Salzburg