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Designing a New Type of Fish Bypass

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A new variation on the standard vertical slot bypass facilitates fish passage at dams and hydro projects while minimizing water diverted from the powerhouse. This design, called the Maba MS fishpass, features lower velocity flow than standard designs and allows fish to swim at their preferred water depth.

By Helmut Mader and Michael-Marcus Tauber

Current directives regarding water resources in Europe contain contradictory demands with regard to fish passage at hydroelectric projects.

For example, one of the goals of the EU Water Framework Directive 2000/60/EG, adopted in December 2000, is to restore water bodies to good ecological status or potential by 2015 through removal of barriers to migration of fish and other aquatic organisms. In particular, a focus is set on restoring the river continuum at existing barriers to fish migration, such as dams. However, for dams that lack fish bypass systems and also impound water for hydroelectric generation, the flow required to operate these systems once they are installed results in a reduction in water available for power production.

On the other hand, EU Renewable Energy Directive 2001/77/EC, adopted in 2001, as well as the 2009 Renewable Energy Directive, sets ambitious targets for all member states. The goal of these targets is to allow the EU to reach a 20% share of energy from renewable sources by 2020. Ultimately, this means maximizing the efficiency of existing renewable facilities, like hydro.

These contradictory demands prompted the authors to focus on developing a new method for fish passage. This new fish bypass system, called Maba MS (multistructure slot) fishpass, was designed to maintain or even enhance migration acceptance while reducing the flow required for operation. Research work began in 2008. A first prototype was built and has been tested in operation since November 2009. Functioning of the Maba MS fishpass was approved by the University of Applied Life Sciences in Vienna and the design was completed in 2010.

Designing the system

The optimization of parameters in the design of a fish bypass depends on the interaction of hydraulic and biological variables. These include minimum acceptable swimming depth, maximum pool size, and energy dissipation and swimming capabilities of the target species and life stages. The Maba MS fishpass is a vertical slot design, which allows fish to migrate upstream without leaping over obstacles.

In designing the Maba MS fishpass system, the main focus was to obtain isolated roughness over artificial surface obstacles similar to a very rough natural wall. Basically, the bypass consists of a rectangular channel with a variable width and depth and a sloping floor. The concrete channel is divided into a number of pools of regular lengths by multi-slot structures and guiding walls within baffles. A multi-slot structure consists of two narrow slots separated by a local widening extending vertically over the full height of the baffle. These structures are alternating. Water flows through the slots, creating two small drops, and disperses into the pool.

With this design, fish are able to ascend from pool to pool by swimming though the slot at a depth they prefer. Several fish species (such as bullhead and barbell) prefer to migrate close to the bottom where flow velocity is low, while other species (such as trout or nase) migrate at all different water levels and in a wide range of flow velocities. Energy is dissipated by widening and narrowing effects within the multi-slot structure, the rough natural-like floor, and water circulation within the pools.

Once the design was complete, physical models with scales of 1:5 and 1:1 were built in the hydraulic laboratory of the Institute for Water Management, Hydrology and Hydraulic Engineering. Data gathered from testing of these models showed a significant decrease in flow velocities within the bypass as well as the necessary flow rate compared with standard vertical slot bypass systems with similar dimensions. The guide walls lead to a continuous steady flow following an omega shape. Because migrating fish avoid turbulent zones in fish bypasses, one of the main goals of the work was to reduce turbulence and turbulent zones to a minimum. Compared with standard vertical slot bypasses, the formation of turbulences on the upstream end of the slots is decreased significantly.

The multi-slot structures induce an intended energy loss that leads to a significant reduction in water velocity in both the structures and pools. In fact, compared with a standard vertical slot bypass with the same slot widths, pool depth, water volume, and water level differences, the Maba MS fishpass shows a decrease in slots water velocity of more than 10%. The velocity distribution in the multi-slot structures is even, and the same velocity prevails from close to the bottom of the slot to the water surface and also in the two consecutive slots at each baffle. The flow patterns inside the pools show low turbulence at all observed levels.

Figure 1, above left, shows results from the level close to the ground. The mean velocity in the Maba MS fishpass at about 400 observed points was 25% – 30% lower than in a standard vertical slot bypass. Furthermore, energy dissipation within the pools and the resting areas was significantly increased. Therefore, it is much easier for fish to migrate through the Maba MS fishpass, yielding an increase in the system's ecological functioning.

In addition, the amount of water required to obtain the same water depths and water volume in the pools and equal vertical drops from one pool to the next has been reduced by more than 30% when compared with standard vertical slot bypasses.


The Maba MS fishpass is designed to dissipate the energy of the flowing water in order to decrease water velocities and increase water depths and water volume to assist upstream migrating fish in overcoming obstructions, such as dams. Reducing the flow rate through the fish bypass leads to more water available for hydropower generation and thus to an economical benefit for the operators. Reduction in water consumption using this fishpass was calculated at more than 30%.

The 1:5 scale physical model of the Maba MS fishpass is made of wood and plexiglass and featured five pools. With this design, fish are able to ascend from pool to pool by swimming though the slot at a depth they prefer.

The patented Maba MS fishpass is manufactured as precast concrete elements by Kirchdorfer Group of Austria. The Maba MS fishpass system is available in slot widths of 15, 20, 25, 30, 35, and 50 centimeters; pool length of 200, 300, and 400 centimeters; pool width of 145, 217, and 290 centimeters; and change in water surface elevation level of 10, 13, 15, 18 and 20 meters. Minimum water depths from 50 centimeters to 130 centimeters cover the necessary depth published in European guidelines for fishpasses.

The maximum height of dam for which this system can be employed has not been tested, but it is the same as existing vertical slot fishpasses. For example, the height of the fishpass at John Day Dam on the Columbia River in Oregon, USA, is 56 meters. In Austria, a Maba MS fishpass will be built at two dams with heights of 15 and 17 meters in 2011 and 2012.

Helmut Mader, Dipl. Ing., is a professor in the Department of Water, Atmosphere, and Environment in the Institute for Water Management, Hydrology and Hydraulic Engineering at the University of Natural Resources and Life Sciences in Vienna, Austria. Michael-Marcus Tauber, Dipl. Ing., with Kirchdorfer Group in Austria, assisted in performing physical model tests and calculations during development of the fishpass.

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