The study site at the Austrian Drava River is located in the South of Austria near Klebach-Lind in Carinthia. This area is part of the Austrian Alps, with elevations from around 550 m up to more than 3000 m. The catchment basin drains around 2445 km2 and it covers part of the limestone and east crystalline Alps. The dominant sediment is gravel with a d50 diameter of 25 mm. Sediment input mainly results from upstream active torrents in the catchment. The channel slope at the study site is approximately 0.002. The mean discharge is 74 m3/s and one-year flood reaches 320 m3/s. Flood occurs mainly in spring when snow and glaciers melt or in summer after thunderstorms. The discharge regime can be described as strongly pronounced nivo-glacial with a maximum discharge in June (Mader, 1996). The mean annual rainfall in Sachsenburg (close to the study site) is around 982 mm/a (BMFLUW, 2014). The Drava river at the study reach, before being regulated, was a wandering river, resulting from a transition from a braided to a meandering morphology. At the study site the valley is unconfined with the presence of an alluvial forest whereas most part of the Drava River is straightened and narrowed with a thin alluvial forest zone left.
a) Location of the study site at Klebach-Lind; b) historical, regulated and restored state at the study site ( Klösch and Habersack, 2017)
In the late 19th century as well as in the 1960s the Drava river was regulated in order to decrease flooding, aggradation and uncontrolled channel shifting. The systematic river regulation comprised river straightening, narrowing and protection of the riverbanks with riprap. These modifications increased the bed shear stresses, which, together with a reduction of sediment supply due to the presence of check dams in the catchment and gravel mining and the construction of hydropower plants upstream, resulted in continued channel incision that induced alterations of aquatic and terrestrial habitats.
To reverse the trend of ongoing riverbed incision countermeasures were implemented. Starting in the 1990s several riverbed widenings were applied to stabilize the riverbed itself and to obtain of adequate channel width to improve flood protection and to re-establish the ecological integrity of the river.
The period between 1991 and 1998 was showing a degrading trend, while analysis of the period between 1998 and 2008 showed aggrading trends, as a result of the implementation of major restoration measures such as the one near Klebach-Lind.
The 1.8 km-long restoration intervention at the study site in Klebach-Lind has been implemented in 2002. In the reach a riverbed widening was performed including a reconnection of a side channel to provide space for self dynamic widening. Groynes were embedded every 60 m to 110 m in the hinterland of the outer bank to prevent uncontrolled bank retreat.
Aerial image taken from a drone of the downstram part of the study site (2017)