Circulating channels to help rivers and fish

Wodne Sprawy kanaly

Dams and reservoirs are facilities that contribute to the loss of ecological continuity of rivers. Of the 12 million kilometers of rivers analyzed by Grill et al. only 37% of the watercourses over 1,000 kilometers in length were characterized by free flow along their entire length. The devices that restore the patency of the watercourse are fish ladders. The general principle of their work is to spread the water drop over a longer section of the watercourse. This reduces the velocity of the water flow to values derived from the swimming abilities of fish and allows aquatic organisms to migrate upstream.

Biological basis for designing fish migration devices

The biological basis for the design of fish migration devices is due to the speed of fish swimming, and this depends on the size (total length) of individuals, the predisposition of individual species and water temperature. There are three basic swimming speeds of fish, with three different levels of activity associated with them:

  • Physiological speed – can be maintained by the fish for many hours without fatigue or physiological changes in the body;
  • Maximum speed (spurt speed) – occurs at a single effort of the fish caused by a scare, attack on prey or overcoming an obstacle. The maximum speed, which is 8 to 10 fish lengths per second, can be maintained for a maximum of approx. 6 seconds, after which the fish needs the rest necessary for recovery;
  • useful speed – includes periods when the fish swims once slower, once faster, and the ability to maintain speed is always limited by time (occurs, for example, when overcoming a fish migration device).

For example, for salmon, the useful rate is 1.30 – 3.20 ms-1, and the maximum rate is 6.00 ms-1. For brook trout, these values are 0.80 – 1.80 ms-1 and 4.00 ms-1, respectively. The swimming speed of the fish changes as the water temperature changes. A 20-cm-long trout at 18⁰C can swim at a maximum speed of 3 ms-1, and at 5⁰C only 1.5 ms-1.

Structural solutions for fish ladders

Biological criteria translate into the hydraulic conditions of fish ladders, m. in. on filling in the chambers and slots of the fish ladder, water velocities along its length and energy dissipation. In addition, a decoy current must be generated at the entrance to the fish ladder. Another requirement is to ensure year-round unimpaired flow through the hydraulic structure. From a hydraulic point of view, the biggest challenge to achieving an effectively operating fish ladder is to ensure that it functions over a range of flows from low to medium levels.

Among the solutions used are technical fish ladders, such as chambered, slotted, reverse current (Denil), rigid or dual-function (brush); seminatural fish ladders (close to nature), such as bottom ramps and bottom ramps (structures with a rough surface and a slight slope covering the entire width of the channel), ramps at water stages (they are an integral part of the structure and occupy only a portion of the width of the channel), circulation channels in the form of bypasses (shaped like a natural stream bypassing an obstacle) and special constructions, such as fish elevators and sluices, and eel ladders.

Seminatural passes are made of irregularly spaced stones or stiles, or as pool forms. A number of construction solutions are available, among which we can distinguish honeycomb stone bull bars, ramps and bottom ramps – cascade bull bars (bull bars) with a layer of stones attached to the ground, with a stone erratic or with a transom construction, creating small hollows.

Due to the individual nature of the facilities, the factors that affect the choice of the optimal solution are the local conditions related to the fish species present in the study area, the size of the damming, the mutual arrangement of the elements of the hydrotechnical structure, the water and legal permits issued, as well as what is not always obvious, the availability of land for investment.

Design of fish circulation channels

One of the seminatural types of fish ladders are circulation channels. These are culverts that bypass the obstacle, offering fish an alternative migration route, outside the main channel. With their design and flow conditions, they should resemble a natural watercourse. The consequence of such a solution is their considerable length, which, however, is necessary to restore ecological continuity at existing hydraulic structures without interfering with their construction. Hence, when choosing their location, it is essential to take into account the considerable space required for construction. Passages in the form of recirculating channels mimic a natural stream with a mosaic of habitats. Thus, the design of the circulation channel not only provides opportunities for fish to migrate, but also allows the creation of favorable habitat conditions for various groups of aquatic organisms. In order for fish ladders to fulfill their role, spaces are constructed to allow all stages of aquatic life.

The parameters of fish migration devices should correlate with the size of these organisms. The minimum values of the basic parameters are the same for both near-nature devices and those typically technical. These requirements include the minimum depth of the migration passage, the depth and width of the slot or overflow, and the minimum length, width and depth of the pool or chamber. For example, for salmon, the minimum pool dimensions are 3.3×2.2×0.9 m, and for brook trout, 2.1×1.40×0.6 m. For sturgeon, which can be up to 3 meters long, the minimum pool dimensions should be 6.0×4.0×1.8 meters.

Design criteria for a fish circulation channel are contained in the 1996 guidelines of the German Association for Water Management and Agricultural Construction. [Deutscher Verband für Wasserwirtschaft und Kulturbau, DVWK 1996; Polish-language edition: WWF-Poland, FAO and DVWK, 2016]. These include parameters such as bottom slope (1:100 to max. 1:20), minimum bottom width of 0.8 m, average water depth >0.2 m, average water velocity of 0.4-0.6 ms-1 and maximum flow velocity of 1.6-2.0 ms-1. The bottom should be made of natural, locally available material, arranged in a natural way and characterized by high roughness. Horizontal circulation channels should be sinuous, meandering or flowing in bends. The cross-section should be diversified, and the banks should be reinforced with bioengineering methods.

A form of circulation channel for fish is the so-called. A pool pass, shaped into a cascade of pools connected by stone thresholds. Changing the gradient of the various sections of the circulation channel makes it possible to create zones of gentler flow, habitat elements that provide a resting place or hiding place, or, when required, to create an attraction current.

In sections with significant bottom or bank slopes, it is necessary to use fortifications. In such situations, the principles that apply to river restoration should be implemented, i.e. use natural materials such as geotextile, wood, branches, stones, gravel, plantings of plants. In recirculating channels, natural processes that shape the morphology of river channels are allowed to take place and artificial bottom reinforcements are dispensed with, provided that the bottom is stable and there is no danger to adjacent areas. This is important when using oversized boulders, placed on the bottom to reduce the allowable speed of water flow. Boulders of this type are placed in an irregular manner, in the form of transoms or submerged stone thresholds. The distances between them should be two to three times their diameter (not less than 0.3-0.4 m).

Rods are formed from large boulders, arranged so that they form pools. The most common is the use of loosely adjoining stone blocks. In the case of submerged thresholds (baffles, chicanes), individual boulders form pools that “lean against each other” to form a cascade. The difference in the level of the water table at successive thresholds should not exceed 0.20 m in upland sections and 0.10-0.15 m in lowland sections, and the dispersion (dissipation) of energy must not exceed 200 Wm-3 in upland sections and 150 Wm-3 in lowland sections.

The entrance to the fish ladder should ensure the flow of water, especially at low levels. In order to control the output, they are equipped with solutions, allowing the use of traps and/or electronic devices for monitoring the operation of the fish ladder. Finally, do not forget to shade the channel with medium and tall vegetation (shrubs and trees), which has a beneficial effect on the functioning of the whole.

Effectiveness of the operation of the fish ladders

Evaluation of the effectiveness of the fish migration device is carried out after its construction through two-stage monitoring. The first stage involves assessing the location of the fish ladder in relation to the baffle, the distribution of water flows, the direction and strength of the attraction current from the lower water and from the upper water, etc. This stage is expected to provide answers to questions about whether fish will reach the fish ladder from both the upper and lower waters, with different water levels and management options. The next step evaluates the fish migration facility itself, the size of the chambers and the gradients between them (if any), the water velocity at the overflows, turbulence in the fish ladder (volumetric energy dispersion) in terms of migrating fish with the identification of priority species. The second stage is a biological assessment, that is, an actual evaluation of migration based on observations of fish crossing the barrier. It can be conducted by directly catching fish passing through the fish ladder or by telemetry methods.

The effectiveness of fish migration devices is expressed by two parameters: the proportion of the number of fish of a given species that have overcome the obstacle to the total number of fish attempting to overcome the obstacle (expressed as a percentage) or the delay in migration, i.e. the time it takes the fish to overcome the obstacle (measured in hours or days).

The scale for evaluating the performance of fish migration facilities ranges from very good with 100% of fish overcoming the obstacle with a migration delay of no more than a few hours to poor when less than 70% of fish overcome the obstacle and the delay exceeds several days.

The recommended rating, or very good and good, is given to devices that cover 95% of the fish of priority species in less than a few days. Devices rated as poor can often be improved through minor reconstruction or the construction of additional guidance devices (e.g., additional decoy current). On the other hand, if facilities are judged to be bad, they usually require demolition, followed by design and construction from scratch.

Passing close to nature on the Jasiolka River

An example of a close-to-nature fish ladder in the form of a bypass is the one in Szczepańcowa on the Jasiołka River (km 27+960) in the Upper Vistula River basin. The weir was made in the 1930s. In the 1970s. It is currently being used to capture water for municipal purposes. The structure’s headroom is 5.5 m, and the sill width is 45 m. The weir was a barrier to the migration of aquatic organisms, and the fish ladder built there did not meet the conditions for migration. Its decongestion was necessary to restore the historical migration routes of bi-environmental fish, which was one of the main goals of the project “Restoring the permeability of the ecological corridor of the Vistula River and its tributaries.” A close-to-nature fish ladder in the form of a bypass mimicking a natural section of the river, routed along a winding route on the left bank of the river, was adopted as the optimal solution. The culvert is 165 meters long and consists of 40 pools measuring 3.3×4.5×0.9 meters, and has a slope of 3.3%. The width of the main gap is a minimum of 0.3 m. In order to meet the requirements for close-to-nature fish ladders, additional studies were needed as an additional requirement emerged, namely. The need to eliminate or minimize the water vortex. Subsequent pools of the fish ladder are separated by a row of transoms with a main gap, which has the effect of concentrating the flow entering the pool. This results in the occurrence of swirling water movement also when the slots in successive rows of transoms are placed alternately, that is. On opposite sides of the fish ladder.

Laboratory measurements and numerical modeling were carried out to develop an optimal solution. The velocity distribution was tested on the physical model for variants including a row of rafters with one fixed main gap of 0.12 m and gaps of variable widths of 0.02 m, 0.01 m and 0 m (no gaps). The main slot has been placed alternately so that the main current, overcoming the pool, flows from one side to the other. The introduction of additional slots in the transom resulted in the disappearance of turbulence and the equalization of velocity distribution in the pool. Numerical modeling of the water flow conditions in the fish ladder was carried out next. A two-dimensional (2D) numerical model of the fish ladder for the actual dimensions was developed and calibrated. The modeling was carried out for the following flows: average low flow SNQ=0.33 m3s-1, flow lasting more than 300 days in a year Q300=0.95 m3s-1 and 250% of the annual average flow Q2.5xSSQ=11.04 m3s-1. The modeling involved three stages, in which calculations were carried out for the main channel with the weir and the inlet and outlet of the fish ladder, then the hydraulic flow conditions along the length of the fish ladder were calculated, and finally the decoy current.

Depending on the flow rate, it takes place only through the gaps in the fish ladder or through and above the gaps. In the main fissure, the average water velocity locally reaches 1.50 – 1.80 ms-1, and in the pools 0.1 ms-1. The energy dispersion coefficient varies from 80 to 150 Wm-3. Numerical simulations also made it possible to determine the flow required to generate the decoy current at the entrance to the fish ladder.

The fish ladder in Szczepańcowa on the Jasiołka River is one example of a streamlined water structure. The cumulative effect of project implementation is the creation of a 75 km long ecological corridor of the Wislok River (free of migration barriers), the reduction of the fragmentation of the catchment area, and the restoration of the integration of the NATURA 2000 areas “Wislok River tributaries”.

The concept of making rivers passable with optimal use of structures close to nature is correct. Such activities were recognized at the Fish Passage 2022 conference in Richland, USA, where an award was given to PGW Wody Polskie for its activities in two projects, including the elimination of migration barriers for aquatic organisms on the Wisłoka River and its tributaries – the Ropa and Jasiołka – and the restoration of ecological continuity and improvement of the functioning of the free migration corridor of the Biała Tarnowska River.


Circulating channels have a number of advantages that place them high among environmentally friendly hydrotechnical solutions. These structures blend well into the landscape and create habitats for fish and macrozoobenthos in their successive stages of development. They are more reliable and require less maintenance, as they are less prone to clogging than engineered fish ladders. They bypass the obstacle at a distance, which is advantageous due to the lack of interference with the structure and preservation of the integrity of the hydrotechnical structure, and if they are made in such a way that they connect the lower position and the area outside the backwater zone, they allow the canopy of the reservoir to be bypassed.

Unfortunately, such solutions are not without drawbacks. Due to their design, they need a lot of space, which limits the possibility of building them in many locations. Their operation is dependent on changes in the level of the upper water and the connection to the lower water, which may require additional structures at the inlet to the fish ladder (exit) and/or at the bottom position. In addition, the circulation channel may require a deep cut into the surrounding areas, which means, among other things. The need to carry out a wide range of earthworks and slope protection.

Despite some drawbacks and thanks to their undeniable advantages, solutions close to nature are gaining widespread acceptance as ways to dam rivers. It is hoped that existing examples of this type of fish ladder will be widely used in improving the permeability of rivers in Poland.

Dr. Eng. Leszek Ksiazek is a professor at the University of Agriculture in Krakow and works in the Department of Water Engineering and Geotechnics. He conducts research in the field of engineering and technical sciences. He is involved in water management, specializing in open channel hydraulics, flood control, river channel morphology and also ecohydraulics, combining natural and engineering issues. He is a member of the Water Management Committee of the Polish Academy of Sciences and the Environmental Engineering Committee of the Polish Academy of Sciences. E-mail: [email protected]

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