Tekniska lösningars tillämpbarhet för förbättrad nedströmspassage för ål: applicerat på Göta älv och Motala ström

The aim of this report is to review measures for improved passage conditions for silver eels at hydroelectric facilities, and to discuss the applicability of these to facilities/plants in the Swedish River Göta älv and River Motala ström. The report contains drawings of potential technical solutions at five sites and their feasibility are discussed and evaluated. In the conclusions we emphasize that the implementation of such measures must be preceded by detailed technical investigations and analyses of costs and risks associated with the facilities. The literature review shows that low-sloping physical fish guiding devices and associated bypasses and/or collection facilities are the most promising techniques to provide safe downstream passage route past hydroelectric facilities to silver eels, as well as other species and life-stages. The use of physical barriers is of particular importance to successfully guide downstream migrating eels, since they are bottom-oriented and are known to pass barriers that e.g. do not cover the entire water column from surface to bottom. It should be noted, however, that examples of successful downstream passage facilities are rare in peer-review litterature, in particular for eels and large hydroelectric facilities (intake capacity >100 m3/s). The proposed strategy to increase the survival of downstream migrating silver eels from the primary rearing areas to the sea, is to construct guiding devices and collections facilities directly below the lakes that produce most of the silver eels in the systems. In the River Göta älv this concerns the Vargön hydroelectic facility, which is the first plant downstream from the Lake Vänern. In the River Motala ström this concerns the Älvås and Fiskeby hydroelectic facilities, which are the first plants downstream from Lakes Roxen and Glan, respectively. The best short-term solution is to collect eels at these three sites, and to transport and release them downstream the lowermost plant in each system. For the plants in the River motala ström, we argue that the existing knowledge is sufficient to allow for a more detailed evaluation of the proposed solutions. Important aspects to evaluate further are e.g. dam safety, debris clogging and ice formation. For the plants in the River Göta älv we primarily propose to further investigate the feasibility of installing a β-rack and a collection facility at the uppermost Vargön hydroelectric facility. Considering the large size of the river system and the plants, the feasibility of such as solution is uncertain, and the important aspects to evaluate further are several e.g. dam safety, debris clogging, ice formation, shipping, erosion, risk for landslides, etc. Low-sloping guiding devices in combination with bypasses and collections facilities are the most promising techniques to provide safe downstream passage for eels in regulated rivers, nevertheless there is a need for improved knowledge. With that in mind we recommend several future R&D activities. There is an urgent need to implement and evaluate more downstream passage solutions, in particular in large rivers. Such a project will have to be carried out using a prioritized step-wise approach, analyzing in detail technical design, risks, costs etc. The use of both theoretical and physical/hydraulic models, would probably be very useful tools in such a project. Finally, there is