Bypass combined with other solutions
From the 1980s to the early 2000s, research was conducted, mainly in the USA, Canada and France, to assess the efficiency of surface bypasses combined with conventional bar racks existing at HPPs for turbine protection (solution not too expensive and cumbersome). Most studies focussed on salmonids. The experiments have shown that the efficiency of these systems is heavily dependent on 3 factors (Larinier, et al., 2002):
- The repulsive effect of the bar racks on fish, depending on both the spacing of the bars in relation to fish size, and on the presence of a tangential flow velocity component creating a more or less marked ‘louver’ effect.
- The velocity pattern in the canal intake must enable fish to remain for a sufficiently long time in front of the bar rack in order for them to be guided to the bypass entrance.
- The design of the bypass entrance itself, including its position with respect to the flow organization (Figure 43), its dimensions (large and deep enough to limit repelling effect), and its entrance velocity and discharge.
In brief, for Atlantic salmon smolts, the guidance efficiency of surface bypasses combined with existing bar racks varies between 10-20% to 80-90% depending on the following parameters (Larinier, et al., 2002):
- Very low for bar spacing > 50 mm (10-20% efficiency)
- 60-70% efficiency for bar spacing of 30-40 mm and good hydraulic conditions and well-designed bypasses
- 80-90% efficiency with bar spacing of 25 mm and good hydraulic conditions and well-designed bypasses
Studies conducted on eels revealed that the efficiencies of surface or bottom bypasses combined with existing bar racks were much lower than for smolts, as eels do not show strong behavioural repulsion and are therefore likely to pass through the racks. Thus, it appeared necessary to implement physical barriers to reach high efficiency for eels ( (Gosset, et al., 2005); (Travade, et al., 2010) ; (Bau, et al., 2013)).
In France, it is considered that the association of an outlet with an existing bar rack can be a solution in some cases where the rack surface is already sufficiently large to respect the criteria on the maximum normal velocity (cf. following subsection) and to allow a reduction of the bar spacing (if not already low enough). But in many cases, the achievement of good efficiencies for smolts and/or eels will require reconfiguring the bar racks, or even the water intake, to implement fish guidance bar racks and louvers.
This mitigation measure consists in adding a bypass to an existing conventional bar rack. The configuration of the water intake must be well known (measurement, physical or numerical modelling). The physical installations of the bypasses must be planned according to the power plant geometry and construction works must be adapted to physical forces and the hydropower scheme.
Installation of bypass system requires a suite of skilled labor on civil works.
The bypass can be subject to clogging. Each bypass must be designed to allow evacuation of debris.
Relevant MTDs and test cases
|Bypass combined with other solutions|
|Fish species for the measure||All|
|Does the measure require loss of power production||Operational (requires flow release outside turbine)|
|Recurrence of maintenance||Weekly|
|Which life-stage of fish is measure aimed at||-|
|Movements and migration of fish|
|Which physical parameter is addressed||Barriers|
|Hydropower type the measure is suitable for||Plant in dam|
|Plant with bypass section|
|Dam height (m) the measure is suitable for||All|
|Section in the regulated system measure is designed for||In dam/power plant|
|River type implemented||Steep gradient (up to 0.4 %)|
|Fairly steep with rocks, boulders (from 0.4 to 0.05 %)|
|Slow flowing, lowland, sandy (less than 0.05 %)|
|Level of certainty in effect||Moderately certain|
|Technology readiness level||TRL 9: actual system proven in operational environment|