Fishways at the hydroelectric power plants
Also known as fish passes or aquatic organism passage facilities, fishways represent mitigation measures which facilitate the passage of fish and other aquatic organisms over various barriers on watercourses, including dam structures of hydroelectric power plants (HPPs). These means of passage allow fish to reach upriver sections where they can feed and reproduce or establish their habitat. At the same time, it prevents fish populations from breaking up and declining in watercourses.
In terms of fish, two factors play a key part in ensuring proper functioning of fish passes and suitable passability. First, fish need to be drawn in to a certain spot at the foot of the barrier where the entrance to the pass is located, and second, it needs to be made possible for them to use their natural instinct to swim against the current and through the pass. There are many different fish pass designs out there. What they all have in common is the use of technical or nature-compatible measures for providing conditions that allow fish to negotiate the height of obstacles created by barriers built on watercourses. For this purpose, passes are built in the form of nature-compatible side (or bypass) channels featuring elements like chutes, series of pools, and baffles that deflect water, or incorporating a series of pools with water spilling over weirs or through slots in the weirs. Also used are fish locks, where fish swim from the lower inlet chamber into the upper outlet chamber once the sluice is filled with water, allowing them to navigate the barrier. Some fish passes of this type are also fitted with movable nets which force fish to leave the pass. Or one can simply use fish lifts, where fish are first drawn into the lift by using attraction current, then they are carried up and released into the headwaters above the barrier.
Krešimir Kvaternik, BSc in Construction Engineering, design engineer at IBE: "Watercourse barriers such as hydroelectric power plants block well-established migration paths of aquatic organisms, causing long-term changes in the species richness and abundance of individual species inside the watercourse. One of the measures for mitigating this impact is to build aquatic organism passage facilities, which allow species migrating up and down the watercourse to move past the barrier. Passage facilities come in many designs, but in every individual case the design needs to be adapted to specific local conditions: organism species for which passage needs to be provided, the watercourse's flow rate and grade, structure of the barrier, terrain configuration, and many others. In designing a passage facility, a design engineer takes into account previously conducted studies and surveys, particularly the ichthyological survey, and then, based on the assessed condition of the watercourse, determines which fish species the passage facility needs to be adapted for primarily. This data serves as the basis for determining what the conditions inside the passage should be in order to allow uninterrupted migration of aquatic organisms (such as the volume, speed and depth of water), and provides a foundation for any other configurations (resting spots, spawning grounds, river pools). In the planning we also follow examples of good practice and take into account past experience with similar structures, in particular the performance and effectiveness of any previously built passage facilities on the same watercourse. So when planning passage facilities, it is essential to look at the results of observations of the performance of the existing similar structures – based on these results, we are able to determine which solutions work and which don't. A need for any additional measures is also identified based on observations, by which I mean measures for improving the performance of the passage facility, as well as other measures for preserving the stocks of organisms in the watercourse, like making additional modifications along the watercourse, fish stocking, and other."
Fishways on the lower course of the Sava
On the lower course of the Sava, these hydroelectric power plants have fish passes: HPP Arto-Blanca, HPP Krško and HPP Brežice. The fish passes at HPP Arto-Blanca and HPP Brežice are designed as nature-compatible side channels with some technical elements, such as concrete inlet structures with sluice gates that provide a suitable flow rate through the pass. The nature-compatible side channel at HPP Arto-Blanca features alternating stone steps and resting spots. The nature-compatible section of the HPP Brežice passage facility features a series of stone steps, pools, resting spots, and spawning grounds lined with rocks. The terrain configuration at HPP Krško was unsuitable for the construction of a nature-compatible passage, which is why only a technical pass could be built there. A large section of the pass features slots for passage.
Ichthyological monitoring at all three fish passes on the lower course of the Sava is carried out in accordance with the applicable environmental protection approvals for individual HPPs. The monitoring at HPP Arto-Blanca began in 2009, at the HPP Krško fishway in 2014, and at the HPP Brežice fishway the monitoring started this year. Ichthyological monitoring for the company HESS is conducted by the Fisheries Research Institute of Slovenia, a qualified and competent national authority for carrying out research and technical tasks in the areas of freshwater fishing and fish biology. As part of the monitoring programme, the passability and populousness of the passes is monitored by carrying out multiple checks of special-purpose nets installed over the slotted passes during the year and by conducting multiple electrofishing removals in individual sections of the passes.
Monitoring results show that the passes are serving their purpose and doing their job well by allowing the fish species inhabiting this stretch of the Sava to move along the entire passes, thereby navigating past the HPP dam structures. The passes allow the passage for potamodromous fish species and fish species of European significance such as the common nase, barbel, Danube roach, asp and chub, among others. Furthermore, ichthyological monitoring has confirmed the spawning of the common nase, chub, bleak and barbel in the nature-compatible section of the HPP Arto-Blanca fish pass.
Cooperation with fishing clubs
In recent years, there has been a growing outcry by anglers and conservationists against building barriers on watercourses on account of their adverse impact, arguing that migration is an essential component of the natural life cycle of fish. Naturally, the multipurpose project to build a chain of hydroelectric power plants on the lower course of the Sava River, with its impact on the landscape, has also caused the living environment for fish to change, which in turn calls for making adjustments and additional efforts in managing the altered fishing districts. Since hydroelectric power plants also change the living conditions for fish, during construction a strong focus was placed on designing and building state-of-the-art aquatic organism passage facilities, or fishways. To ensure optimal performance of these passage facilities after the construction of the multipurpose structures, it is absolutely essential to foster constructive cooperation with the local fishing clubs, whose members are active in the field and keep track of fish populations on an ongoing basis. The members of the Posavje region's fishing clubs are the ones who "live with the fish" and observe the changes in fish populations first-hand. In accordance with the provisions of the Law on the conditions of the concession (ZPKEPS-1) and the obligations of the concessionaire under the concession contract to exploit the energy potential of the lower Sava, to ensure the protection of biological diversity and in accordance with the issued environmental approvals, reports on environmental effects, other documents and monitorings in relation to the implementation of mitigation measures as a consequence of the construction of HPPs, the HESS company actively cooperates with the Radeče Fishing Club in the HPP Boštanj area, the Sevnica Fishing Club in the HPP Arto-Blanca area, the Brestanica-Krško Fishing Club in the HPP Krško area, and the Brežice Fishing Club in the HPP Brežice area.
The cooperation between the Sevnica and Brestanica-Krško fishing clubs and the company HESS primarily involves care and maintenance of the respective aquatic organism passage facilities, including regular monthly inspections, regular inspection tours with removal of floating debris and checking the stream flow through the fishway channel, emergency interventions (complete removal of fish), and so on; in the HPP Brežice area, the Brežice Fishing Club sees to the care and maintenance of the fishing competition pegs (removal of driftwood, foliage and other debris).
Sandi Kosmač, Sevnica Fishing Club chairman: "The HPP Arto-Blanca fishway, opened in 2009, was the first such structure in the country and is made up of three parts: bisectional two-level inlet concrete channel with chambers and vertical slots, nature-compatible section, and outlet section. The upper section of the channel has gentle bends, the lower one has a winding configuration. When the water is clear, we can see fish travelling by, many of them heading to one of the largest spawning grounds in the Mirna River near Boštanj. The Sevnica Fishing Club has been entrusted with maintaining the fishway, and our job is to clean it on a regular basis as well. We spend around 30 hours a week removing debris from the pools so that the fish are able to freely migrate towards spawning grounds. Trees have been planted around the fishway to provide shade and maintain a suitable water temperature. We often mount a portable camera inside the fishway and watch the life in it, which fills us with optimism. Together with HESS, we are planning to install fixed cameras inside the fishway to allow us to precisely monitor the movement, number and species of fish. We are pleased to be working together with schools, which increasingly take pupils to see the fishway as part of nature outings. In the past, children would be taken abroad to see such structures, to Austria in particular. Our experience with the fishway so far is positive since it allows fish to freely migrate in order to survive by giving them access to spawning and nursery grounds."
Herman Kerin, Brestanica-Krško Fishing Club manager: "Ever since Krško Nuclear Power Plant (NPP) was built, the dam at the power plant has prevented fish from migrating up the Sava, the common nase in particular, which used to go spawn in the Mirna River, where one of the best-known spawning grounds used to be. The fish living upstream from Krško NPP continued to spawn in the Mirna, while the remaining population was unable to do so because of the barrier. This situation has changed with the filling up of the HPP Brežice reservoir – fish can now move freely both through the nature-compatible fishway at HPP Brežice and past Krško NPP, where the sluice gates are now raised and no longer block the passage. What's more, spawning grounds have been provided inside the fishway itself, so fish can now spawn there during migration, if necessary. At HPP Krško, due to tight space, the fishway is artificial, made of concrete, yet it still serves its purpose. And this is extremely important for the HPP Krško reservoir. The fish population there has changed. Previously, in terms of abundance, the common nase and other salmonid species prevailed, now cyprinid or warm-water species are the most abundant, such as the carp, wels catfish, Vimba bream, Danube roach, barbel, chub, tench, pike, pikeperch, and others. With fishing conditions now the same throughout the reservoir area, the fishing system there has changed as well. Previously, our fishing club didn't have any competition pegs due to unsuitable conditions, now we have them at Rožno. We host several competitions every year, much to our delight and the delight of the participants. For us anglers, the configuration of the reservoirs and fishways is a very positive one."
Petar Dimitrovski, Brežice Fishing Club chairman: "No other fishway in the country can match the one at HPP Brežice – and I've seen quite a few. It is a fine example for the rest to follow. Before the reservoir was built, this area along the Sava was completely desolate. The operation has created a Brežice promenade, and we anglers got a huge area of water where we're going to be able to host major competitions or anglers' gatherings both on the European and global scale. In two years' time, we are already planning to stage a big angling competition like the sport fishing world championship, meaning that the town of Brežice would accommodate around 500 people for a period of ten days, which is sure to have a positive financial effect. After the construction of HPP Brežice, the membership of the Brežice Fishing Club has grown (currently around 350 members), and the feedback we get from anglers is truly positive. The reservoir is home to a surprisingly large number of fish, the Sava no longer has a distinctive foul smell, it's a nice river for fishing, you don't haul garbage and whatnot out of the water. A large area of water means an increased fish capacity, now we anglers can have everything under control, and the abundance of fish is sure to increase in the future. I'm also confident that a change in the richness of fish species is bound to happen, to the benefit of the native species, of course, not non-native ones, which is only going to make the situation even better. While carrying out an extensive removal of fish due to the scheduled maintenance outage of Krško NPP, we've found the HPP Brežice fishway to serve its purpose remarkably well."