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 | |
| Contact organisation | |
| 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 waterThe width of the low
Monitoring surveys and results
Lessons learnt