Case study:Blanice River
Project overview
| Status | Complete |
|---|---|
| Project web site | |
| Themes | Hydromorphology |
| Country | Czech Republic |
| Main contact forename | Petr |
| Main contact surname | Hartvich |
| Main contact user ID | User:PHartvich |
| Contact organisation | University of South Bohemia |
| Contact organisation web site | http://www.prf.jcu.cz/en/ |
| Partner organisations | |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
Initial conditions
The Blanice River springs at 972 m above sea level in the Šumava Mountains and joins the Otava River at an elevation of 362 m, where it is characterised as a lowland river with remaining oxbows. The gradient of the 93.3 km long river is 5.15% and the average flow is 4.23 m3.s-1 at its lower end.
Many damming-up devices have been built for water mills, hammer mills and sawmills, increasing the need for water. The river was fragmented into parts with still water and parts where the flow was regulated. The character of the river ecosystem has changed, affecting the natural development of fish populations (Hartvich et al. 2004). A high dam works as a migration barrier. It cannot be overcome by fish moving upstream and so the long-term loss of upstream migration negatively influences the exchange of genetic information during reproduction. Separated fish populations become smaller as well as less resilient. Fish which are flushed downstream by the flow cannot get back to their habitat (Peter 1998, Lucas & Baras 2001). Therefore fish passes are built where damming-up devices (weirs etc.) are located. They allow fish and other aquatic animals to pass the barriers and move freely along the river. Fish passes transfer the backwater to the stream below the barrier and are either a part of the migration barrier or placed on the grounds next to the barrier. In this case the fish pass functions as the bypass of a barrier. These fish passes are built in such a way that their character, structure and stream flow are similar to the conditions of natural rivers (Kubečka et al. 1997, Cowx & Welcomme 1998, Gebler 2009, Lusk et al. 2011).
In total 17 fixed or mobile barriers (weirs, dams) are placed across the Blanice River. These barriers are not migration-permeable, with one exception. The river continuity is disrupted mainly by the Husinec Dam-lake (area 61 ha, backwater 3.5 km long, maximum 25.5 m deep). Below the dam, the river has a weir impassable for migrating aquatic animals. On the right bank a ground overgrown with deciduous trees and a part of a former oxbow connected to the river below the weir were available. Because of these conditions, a near-natural bypass was proposed as the most convenient solution.
Objectives Restoring and preserving healthy populations and diversity of the original fish species in Blanice River by means of building a bypass.
Restoration measures In 2002, a 35 m long bypass was built at the weir to allow upstream migration. It runs from the upper weir through natural terrain around the body of the weir and joins the river 20 metres downstream of the weir. The average gradient is 5%. Fig. 3 shows the placement of this near-natural bypass. At a medium flow rate (Q180), up to 250 l.s-1 flows through the bypass. The 2.5 m wide upper part of the bypass is a torrent fish pass with an inlet device placed upstream of the weir. The construction includes 9 stone sills for the necessary backwater, in which 7 to 16 cm wide gaps between the stones (boulders) enable fish to swim through either at the bottom or below the water surface. Gravel and smaller stones on the bottom decrease the flow in the lower water layers. The sills differ no more than 15 cm in height and their depth ranges from 0.3 to 0.5 m. The lower part of the bypass is formed by the oxbow (which was first cleaned) with slowly flowing water. The width of the lower part ranges from 3 to 5 m and the gradient is only 2%, but a few stone sills form up to 1 m deep pools. In places over sand and gravel banks shoals have been formed by high-water flows.
Management measures
- Bypass maintenance after floods, in spring and autumn.
- Removal of sediments to keep the bypass clear.
- Seasonal monitoring of local fish fauna diversity in the bypass passable for migration.
Conclusions The presence of fish in the bypass was monitored once a month during the year 2002 (except during ice cover in winter and high-water) to assess species diversity (Hartvich et al. 2004). This was done by damming up the inlet profile with a board to stop the stream, so that the fish present could be collected and the remaining ones caught with electric current. A small net was placed in the lower part to prevent the fish from escaping. The fish were measured using common ichthyologic methods and returned immediately.
The critically endangered Brook Lamprey (Lampetra planeri) and 13 species of six families were detected during the first monitoring period. According to ecological preference rheophilous (living in fast streams) species (8) were the most abundant, followed by eurytopic (5) and one limnophilous (living in standing water) species, namely Doctor-fish (Tinca tinca). The total fish fauna counted 610 individuals weighing 8,939 g in total. The most abundant species were Pseudorasbora parva, Leuciscus leuciscus, and Perca perca. In the lower part of the bypass, a few individuals of the critically endangered European Crayfish (Astacus astacus) were found. In the following period(January to November 2003), the number of species grew to 18, including the newcomers Alburnus alburnus, Barbus barbus, Scardinius erythrophthalmus and Anguilla anguilla (Hartvich et al. 2004). The annual fish fauna abundance was 993 individuals and their biomass 7,876 g. The detected species assemblage corresponds, except for Cottus gobio, to the results given by Krupauer (1984) for the Blanice River upstream of the Husinec Dam-lake, and later mentioned by Křížek et al. (2004) for the upper and central part of the Blanice River.
Other lessons learned and new perspectives
- Grassland and self-seeded trees (willows and aspen) permanently reinforce and protect the banks of the bypass against erosion. The open inlet device passes water level fluctuations into the bypass. High-water flows do not endanger the bypass construction. Loosely placed stones on the bypass banks slow down the flow, prevent lateral erosion and create shelter for fish. Coarse gravel on the bottom is an appropriate substrate for the settling of benthos.
- Monitoring results show that fish not only migrate through the bypass but also settle there for a certain period of time. The 18 species of fish and lamprey detected in the bypass correspond to the composition found in ichthyologic research conducted in the upper and central part of the Blanice River. Fish migration in the bypass takes place during the whole year, except when there is ice cover.
- Monitoring of bypass pass-ability not only provides ichthyologists, nature conservationists, water authorities, and designers and builders of fish passes with a lot of new information, but it also shows the real state of the fish fauna in river districts, especially in the case of functional passes such as the one at Bavorov. Public support
The bypass on the Blanice River was co-supported by the town council of Bavorov and by the Bavorov branch of the Czech Fishing Association. The bypass is open to everybody who is interested in it.
Monitoring surveys and results
Lessons learnt
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Supplementary InformationEdit Supplementary Information Acknowledgements The authors are grateful for support from the projects CENAKAVA CZ.1.05/2.1.00/01.0024 and GA JU 047/2010/Z. Further our thanks for valuable advice go out to fish pass designer and builder Zdeněk Linhart. We also thank Vladimír Šámal for providing important data and information, and Miroslav Fenc for extensive assistance during field work.
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