Bypass combined with other solutions

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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.

Methods, tools, and devices

During planning

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.

During implementation

Installation of bypass system requires a suite of skilled labor on civil works.

During operation

The bypass can be subject to clogging. Each bypass must be designed to allow evacuation of debris.

Relevant MTDs and test cases

Relevant MTDs
3D fish tracking system
3D sensorless, ultrasound fish tracking
Acoustic Doppler current profiler (ADCP)
Acoustic Doppler velocimetry (ADV)
Acoustic telemetry
Barotrauma detection system
Current meter
Differential pressure sensor base artificial lateral line probe, iRon
Guidelines for fishpasses numerical modelling
Radio frequency identification with passive integrated transponder (PIT tagging)
Radio telemetry
Sediment simulation in intakes with Multiblock option (SSIIM)
Visible implant elastomer
Relevant test cases Applied in test case?
Anundsjö test case Yes
Bannwil test case -
Gotein test case Yes
Guma and Vadocondes test cases -
Las Rives test case Yes
Trois Villes test case Yes

Classification table

Classification Selection
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 of migration of fish
Which physical parameter is addressed N/A
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
Cost of solution See cost table

Relevant Literature

  • Larinier, M. and Travade, F. 2002. Downstream migration: problems and facilities. Bulletin Francais De La Peche Et De La Pisciculture 364: 181-207
  • Gosset, C., Travade, F., Durif, C. M. F., Rives, J., & Elie, P. (2005). Tests of two types of bypass for downstream migration of eels at a small hydroelectric power plant. River Research and Applications, 21, 1095– 1105.
  • Travade, F., Larinier M., Subra S., Gomes P., De-Oliveira E. Behaviour and passage of European Silver Eels (Anguilla anguilla) at a small hydropower plant during their downstream migration. Knowl. Manage. Aquat. Ecosyst., 398 (2010), pp. 1-19
  • Bau, F., et al. 2013. Anguille et Ouvrages : migration de dévalaison - Suivi par radiopistage de la dévalaison de l’anguille argentée sur le Gave de Pau au niveau des ouvrages hydroélectriques d’Artix, Biron, Sapso, Castetarbe, Baigts et Puyoo (2007-2010). 2013. Rapport de synthèse.