Baffle fishways

Introduction

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Figure 1:

Baffle fishways, including Denil passes is also referred to as countercurrent fishways since it consists of special deflectors that lead to helical countercurrent currents, high energy conversion and reduced water velocities in the main stream. Fish species with high swimming capacity can swim right up in the water stream and a baffle fishway can be constructed relatively steep for salmon (gradient 0.2-0.25). However, the remaining current conditions are still turbulent and often water velocities are above 2 m/s. It has been shown that baffle fishways are unsuitable for most fish species and juvenile fish, including all carp fish, eels, white fish and grayling (DWA 2014). AG-FAH (2011) indicates that baffle fishways have not proven to work in practice. Armstrong (2010) writes that specially designed low gradient Denil fishways can work for several species, but they require certain hydraulic ramifications and are therefore not suitable for varying water discharges (Armstrong et al., 2010). The baffle fishway can be used at particularly steep fishway sites and may be suitable for adult salmon and trout for limited space and steep terrain. However, in most cases, other fishway types should be chosen.

Methods, tools, and devices

During planning

Planning of a baffle fishway will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier, including information about the hydropower scheme. Surveying must also be conducted in the area of the river bank where the fishway is planned, including geological surveying. Geographic data should be handled in GIS software for further planning and analyses. The design of the fishway should be conducted with conventional hydraulic- and civil engineering calculations and drawing. All material used in a fishway must be planned to withstand physical strain from water, floods and frost. Monitoring facilities should basically be planned in the upper part of the fishway.

During implementation

Physical implementation of pool-type fishways requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. Work with explosives is relevant in most cases and blasted rocks and transportation of material out from the site is common. Surplus rocks should not be disposed at site because of pollution risk. The construction phase includes construction of concrete formwork, casting of concrete and iron reinforcement work.

During operation

Injuries on baffle fishways from physical wear must be monitored and repaired in order to secure regular fish migration. Maintenance work normally require hand-tools more than heavy equipment but casting of concrete is typical. Depending of the site, removal of sediment, branches, logs and floating debris in pools and fishway entrance is common. Monitoring systems require regular inspection, depending on product and system.

Classification table

Classification	Selection (multiple)
Fish species measure designed for	All, but to the largest extent the stronger swimmers
Does the measure require loss of power production?	Operational (requires flow release outside turbine)
Recurrence of maintenance	Irregular at events
Which life-stage of fish is measure aimed at?	Upstream movement and migration Adult fish
Which physical parameter mitigated?	All
Hydropower type the measure is suitable for	Plant in dam Plant with bypass section
Dam height [m] the measure is suitable for	0-20 meter
Section in the regulated system measure is designed for	Bypass section
River type implemented	All
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 8 TRL 9	operational environment system complete and qualified actual system proven in operational environment