| Fish passage
Level of water – what size should the fish passage facilities be, or more specifically, how much water should pass through them?
Option – what type of fish passage facilities should be used? This is mainly a question of the type of fish pass to use.
Window – what is the option window (e.g time of year, or during which season(s)) should the fish passage facilities be designed to operate in, and when should their operation be optimised for? All of the above considerations need to have at their foundation a solid understanding of the fish populations being considered at a particular site. This understanding should ideally include the fish species present, their abundance, behaviour (particularly in relation to migration), conservation status, ecological and commercial importance. How the answers to these questions play out in
reality shapes the fish passage facilities that we have recommended and designed at different sites. For example, we have provided advice and produced designs for the developers of several large hydropower schemes on the Mekong, including Xayaburi hydropower plant. At a high level, for all of the schemes in question our input to the design process started by ensuring that entrances to the fish passage facilities were placed close (ideally <50m) to the ‘competing flow’ to migration (typically the powerhouse, from both an upstream and downstream perspective) and the facilities had a significant flow through them. The specifics of the latter recommendation are
often difficult to determine as guidance varies widely depending on the who, where and when of publication. For example, on smaller rivers in Europe, a typical recommendation for the flow through a fish pass is often that the facilities should have a minimum flow equal to 5 – 10% of the maximum competing flow. In contrast, for schemes on very large rivers, this figure could be closer to 1% of the competing flow. The technical end product from our work are the designs for a fish passage facility, with additional support during construction and post-construction monitoring on occasion. Due to the inherent variability in river systems, the fish populations they support and the schemes we have advised, we have designed fish passage facilities that range from simple ‘baffle passes’ 10m long, up to very large, multi-faceted systems over 1km long and taking over 100m3
/sec.
Holistic approach Increasingly, a broader range of species are being
considered in the design of fish passage facilities. In the UK, this has manifested as a gradual change from a ‘salmonid-centric’ approach to fish passes in the recent past, to a more holistic approach that often considers other species such as lamprey, shad (a type of anadromous clupeid, or herring) and cyprinids (such as barbel and carp). The facilities to pass this wider range of fish species are typically larger than those required for only salmon (Salmo salar) and more costly. They are often a pass type like a deep vertical slot, natural bypass channel, rock-ramp or brush-furnished pass. These type of fish passes create the slower velocities, heterogeneity of flow and resting areas that these fish species require. The most significant example of this in the UK is the ‘Unlocking the Severn’ project. This was a significant £22 million project in the UK aimed at restoring shad to the River Severn, which were extirpated from their historic range by the creation of several navigation weirs or dams in the late 19th century. Fishtek delivered all of the early optioneering and design work for the fish passes and provided further support throughout the construction of the fish passes. Similarly, the fish passage facilities we have
recommended or designed as part of hydropower schemes elsewhere in the world have been of the form of deep vertical slots, bypass channels or in the case of higher dams, fish lift-locks (in some cases with a ‘lead in’ section of a deep vertical slot style pass). These fish passes have been demonstrated to provide effective fish passage for a wide range of species, which is particularly important in tropical or sub-tropical river systems such as the Mekong, where there is a diverse fish fauna.
Guiding principle A final guiding principle or takeaway message from all
of our work is that, like dams or hydropower schemes themselves, every site is different. The challenges presented by the morphology and topography of a site, the river size and flow, the dam design (including whether it is run of river, storage or has a ‘depleted reach’) and the fish fauna present mean that there is no easy, repeatable blueprint for a fish passage facility that can be bolted onto a new hydropower scheme. The challenge to fish migration presented by a scheme must be assessed and appropriately mitigated afresh each time.
www.waterpowermagazine.com | June 2025 | 37
Above left: Toby Coe standing in the fish pass section of the fish passage facilities at Xayaburi hydropower plant
Above right: Overhead view of the fish pass section of the fish passage facilities at Xayaburi
Below: The most downstream vertical slot fish pass on the Unlocking the Severn project
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