1. General presentation of the study site
The study site at the Austrian Drau River is located in the South of Austria near Kleblach-Lind in Carinthia (Figure 1a). The catchment basin drains around 2445 km² and covers parts of the southern limestone Alps as well as east alpine crystalline. The dominant sediment is gravel with a d50 diameter of 25 mm. Sediment input mainly results from upstream active torrents in the catchment. Floods mostly occur in spring when snowmelt – and also glacier melt - is released into the basin, 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). Historically, the Drau River at the studied reach was a wandering river which results from a transition from a braided to a meandering morphology, before it was regulated and finally restored.
Figure 1: a) Location of the study site Kleblach-Lind at the Drau River, b) historic, regulated and restored state of the study site (Klösch and Habersack, 2017)
2. Hydromorphological restoration/management
In the late 19th century as well as in the 1960s the Drau 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. This increased the bed shear stresses, which together with a reduction of sediment supply given check dams in the catchment, gravel mining and the construction of hydropower plants upstream resulted in continued channel incision and accompanying problems such as alterations of aquatic and terrestrial habitats. An overview of the major hydropower plants in the upper Drau catchment affecting the sediment transport is depicted in Figure 2.
Figure 2: Location and types of hydropower plants in the upper Drau catchment (Klösch and Habersack, 2017)
To reverse the trend of ongoing riverbed incision countermeasures were implemented, starting in the 1990s. Several riverbed widenings were applied to stabilize the riverbed, improve flood protection with the new approach of providing an appropriate channel width and to re-establish the ecological integrity of the river.
Figure 3 shows the locations of the restoration measures along the Drau River as well as bedload and total load yields for the investigated area.
Figure 3: Annual bedload yield and total load along the Drau River related to restoration measures for the time periods 1991–1998, 1998–2008, and 2008–2013. The reduction of sediment loads with distance downstream reflects a shift from degradation to an aggrading trend as a consequence of restoration works (Klösch & Habersack, 2016)
The 1.8 km-long restoration measure at the study site in Kleblach-Lind was 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 110m in the hinterland of the outer bank to prevent uncontrolled bank retreat. The data shows the morphologic evolution of the site since the restoration measures in 2002 and 2003
A small flood with a peak discharge of 286 m³ /s in the first year after channel construction almost doubled the mean width of the side channel from 29 m to 55 m. Widening was largest upstream, where it was accompanied by the development of a large mid-channel bar. The groynes along the left bank were exposed quickly by bank erosion and have since then acted as constraints to the channel morphology. Due to aggradation the side-channel disconnected at low flow conditions, so that the inlet was modified in 2009 (the inlet structure - kind of a groyne - was set back a few meters), since then it is better connected.
The bed-levels rised after installation (0.35m in the area of the former regulated bed between 2003 and 2011). More diverse flow patterns were established and habitats were successfully recreated. Big bars developed, but they are immobile given the remained channel constraints and continue aggrading.
3. Monitoring activities
3.1 General objectives of the monitoring program
The restoration measures at the case study were already implemented in 2002 and 2003 and were already monitored in past projects, mainly in relation to the fulfilment of the restoration objectives defined prior to the restoration works:
Improve the ecological integrity
Mitigate channel incision
Improve flood protection
The previous monitoring activities could already attest an improvement of ecological integrity (Unfer et al., 2004) and an increase of bed levels in the restored reach (Habersack et al., 2013). The effects on flood protection were not analysed but the more complex morphology of the site can be assumed to increase the retention of flood peak levels and improve flood situations downstream. Regarding the long time passed since measure implementation, and considering that the last survey of the case study reach near Kleblach was conducted in 2011, the case study site offers an opportunity to study the longer-term effects of restoration measures. The objectives of the surveys and related analyses are:
Assessing the longer term trajectory of a restored reach
Assessing the sustainability of side channel
Assessing the longer term effects of restoration at larger scale
Obtaining data for testing new HyMoCARES tools
Derive recommendations for future planning.
3.2 Physical monitoring
For Physical monitoring we realize:
- Main channel survey
- Side channel survey
- Wolman pebble counts
- Radio tracer stone telemetry
- Bank erodibility measurements
3.3 Ecological monitoring
From earlier projects (e.g. LIFE ‘Auenverbund Obere Drau’) data on the development of aquatic and terrestric fauna and flora is available. Most data is available from one ongoing project on the succession of vegetation since the implementation of restoration measures, with the most recent survey conducted in April 2017.