Comparative scoping study of Nadiža, Sava and Težka voda weirs in Slovenia

Selecting the most promising weir removal sites on the three different rivers in Slovenia.

Introduction

Nadiža weir is (presumably) the only barrier on the main river stem in Slovenia, which lies within the Triglav National Park and is home to endemic marble trout. Nadiža River is a pristine river with a Natura 2000 status supporting massive tourism, thus ensuring economic survival for many locals.

The Sava River is the longest tributary of the Danube. It is 990 km long and it has its source in Zelenci, Kranjska Gora in Slovenia. Removal of the Cajhnov Dam would open 35 km of the largest Slovenian River with benefits for the most iconic Slovenian fish, the Danube salmon, of which the Slovenian populations are currently the most important Huchen population in the Danube.

Težka voda is a stream that originates in Gornja Težka Voda and joins the Krka in Kandia as its right tributary. The stream is home to many fish species and is protected as a natural heritage site. The local landowners strongly support the removal.

At a glance

CountrySlovenia
RiversNadiža, Sava, Težka Voda
Key speciesMarble trout, Danube salmon, Danube whitefin gudgeon
Project typeBarrier identification and prioritisation
Project statusLive
Project websitewww.ozivimo.si/scoping_study.html

Project context and opportunity

The Nadiža River barrier is a 2-meter tall, 30m-wide weir made of wood, stone and concrete. It was built for a gauging station but without a permit. Damaged and needing repair, it causes high risk to a downstream local swimming site. Removal of presumably the only weir on the main river stem would open up the entire river in Slovenia, allowing full habitat reconnection. It would increase safety for one of the local swimming sites downstream of the weir.

A small hydropower plant dam is still in operation on the Sava River. The dam is unpassable for fish, and the fish pass is old and not functioning. The removal would open up more than 30 km of Danube salmon habitat, reconnect the populations from the Sava Bohinjka with those in the Sava Dolinka and the Sava river, enabling the Danube salmon to restore its population in this area. The site is also of huge ecological value for other organisms, as the confluence of the two main stems of the largest river in Slovenia meets the Danube River in Belgrade, Serbia. The confluence promises to be the main spawning ground for fish if the dam is successfully removed.

Težka voda has a series of obsolete dams from 2 to 3 meters tall and 10 to 30 meters wide built from concrete and rocks that fragment a tributary to the Krka River, the largest right tributary to the Sava River in Slovenia. The series of weirs is close to the confluence with the Krka river, so no migration is possible, although the tributary used to be an important spawning and nursery area for the Krka river fish. The disconnected populations in the Težka voda are dwindling and at high risk of extinction.

Project aims

The project team aims to select the most promising weir removal sites for a feasibility study based on the data collection and comparison of the three potential barrier removal sites on the three different rivers in Slovenia.

They will subcontract a policy expert to perform stakeholder mapping in each study area to identify the power relationships important for dam removal and the key players and tactics for engagement. They will develop, implement, analyse and report on five semi-structured interviews and two workshops (WS) in each case study (15 interviews and six workshops). The first WS will raise awareness about dam removal with the stakeholders identified through stakeholder mapping. The second WS will define the removal project goals, map issues, understand objectives, build a core group and plan a feasibility study proposal. These steps are crucial to building support among all levels of stakeholders.

In addition to stakeholder engagement, the team will review and compile existing technical information about the three removal sites: ownership, engineering, social, biological, use, statuses, infrastructure etc. Biological data will be collected through fish and macroinvertebrate surveys for a general habitat assessment. An outsourced sediment survey will indicate potential pollution issues. Based on this data, the team will estimate costs and compare the three sites along the three lines of benefits: ecological, economic and social, following the SWAT concept. The site that shows the highest potential for implementation will be proposed for a feasibility study and later removal.

Partners

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