Fishways are structures placed on or around constructed barriers (in our case, dams) to give fish the opportunity to migrate. In the 17th century, fishways were used to create steps in steep channels, allowing fish to bypass osbtructions. Nowadays, most fishways follow a simialr basic concept, allowing fish to pass around the barrier by swimming through a series of gap or slots that control the speed of water.
Each dam on a river that will be targeted for fishway construction represents a unique situation. Moreover, there are a several aspects which have to be taken into account within the design of a fishway. For instance, according to the species and size of the migrating fish communities, fishways will be designed with more and less faster flows. Thus, the hydraulic conditions within a fishway need to provide both enough depth for large fish whilst ensuring the velocity is suitable for smaller fish.
It exists several types of fishways but we will focus only on 2 types as the other ones are inappropriate for our project and doesn't really suit the expectations of the associations.
In this first type, rock and wood are used to create pools and small falls that imitate natural structures. Due to the length of channel needed for the ladder, such structures are most appropriate for relatively short barriers.
Steps to construct the fishway
Bypass fishways are low-gradient earthen or rocky channels that imitate the structure of natural streams and are often described as 'nature-like' fishways. Some flood bypass have been built or occur naturally, but bypass fishways have yet been built in Australia. They have been used because they may provide a cheaper alternative to the more technical fishway designs.
Turella Weir fishway, Wolli Creek - Australia
The creation of a rock ramp fishway on Turella Weir at Wolli Creek means that fish can now access more usptream habitat which is an important improvement in this inner Sydney Area. Wolli Creek is a tributary of the Cooks River, which flows through Sydney in Botany Bay. The weir nad causeway were built in the early 1900s to privde access and water for adjacent Chinese market gardens. Turella Reserve and Wolli Creek Regional Park have now replaced the old market gardens, meaning the weir no longer serves its original purpose but still prevents fish from moving up- and downstream for most of the time.
This video is made by Gavin Gatenby of the Wolli Creek Preservation Society.
Billnäs and Åminnefors are both historically and culturally important areas which are protected by the National Board of Antiques and Historical Monuments. Also the dams are protected. Especially Billnäs is nationnaly a very valuable sight. Nevertheless, the National Board of Antiques and Historical Monuments has stated that the construction of fishways is an important and well reasoned project. Therefore, the construction can be done if the design follows the historical s tyle of the area (Kärki 1997). Demolishing the dams would not be possible.
Because of the sharp topography around the dam of Åminnefors a technical fishway could be a reasonable solution there. For example vertical slot fishway or weir fishway good be a good solution (see example above). However, in order to restore the river ecologically as well as posible, the possibilities to construct a natural fishway were studied. A natural fishway is usually a cheaper solution as well, about 20 000 € per meter of drop (Lehtinen 2010). According to Finland's environmental administration it is possible to constructur a natural bypass fishway in Åminnefors despite the slope (Jormola 2010). That was chosen to be the best approach because it considers ecological values as well as historical surroundings.
In Billnäs, the topography is much gentler. Therefore it was decided that a natural fishway would be the best solution there. Finland's environmental administration has made a design for a natural bypass fishway in Billnäs as well (Jormola 2010). There are numerous migrating fishes which would be using the fishways in Åminnefors and Billnäs: salmon, trout, eel, whitefish, asp and zarte (technical centre of the town of Raasepori 2010).