Introducing gravel management
in Alpine rivers
The tributaries of the Danube which flow down through the Alps were typical braided rivers, with broad gravel flats laced by major and minor river channels. However, engineering work to enclose these rivers in embankments and to hold back flood debris from the streams leading into them, as well as gravel quarrying, disturbed the natural dynamics of the braided rivers. Many of them changed their character altogether. Some, like the Lech river in Tyrol, were less damaged but still suffered from a lack of gravel deposition.
A LIFE-Nature project (Tiroler Lech project) set about designing and implementing a “gravel management system” to restore the Lech’s dynamics in the section of river designated as a Natura 2000 site.
The technicalities of gravel management
The supply of gravel to the main river channel will be increased gradually by opening the debris traps across the tributary streams, allowing gravel from the mountain slopes back into the Lech river. The debris trap removal works are being done gradually over five years, through step-by-step lowering of the dam.
A new “riverbed enlargement section” is being designed and laid out just above the heavily urbanised part of the Lech valley at Reutte. This river section will be broadened in such a way that all the gravel that is washed downstream will be deposited here and can be easily extracted from the riverbed. This involves the construction of an innovative type of “gravel trap”, which will closely resemble a natural gravel flat (and hence, fulfill the ecological functions of the natural habitat type). The accumulated gravel will be extracted from time to time and sold commercially – i.e. an example of sustainable gravel quarrying.
To sum up, the idea of the “gravel management system” is
Commercial gravel extraction will thus only be allowed at the gravel trap near Reutte, so that the core area for nature development (the Natura 2000 area between Stanzach and Hornberg) can from then on be left totally undisturbed.
- to restore the income of gravel into the Lech valley to its natural level,
- to increase the gravel retention capacity of the river sections in the Natura 2000 area and
- to concentrate the gravel extraction at the lowest point of the system, just above the town of Reutte.
By mid-2005, work on the gravel management system was well under way. Removal of the debris traps across the tributaries had advanced well and proved to be technically straightforward and relatively cheap. Discussion was focusing on whether the concrete rubble from the dismantled traps would have to be removed from the site and brought to a special deposition area, or whether they could simply be left on the site to be transported downstream by the river.
Work on the main river had also started – some of the groynes along its banks had been dealt with to allow the river to broaden its bed and deposit more gravel, while the major works to cap the whole project were in the final phases of the permit procedure and scheduled for execution in 2006.
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